Trends in Synthesis, Crystal Structure, and Thermal and Magnetic

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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

Trends in Synthesis, Crystal Structure, and Thermal and Magnetic Properties of Rare-Earth Metal Borohydrides Jakob B. Grinderslev, Kasper T. Møller, Martin Bremholm, and Torben R. Jensen* Center for Materials Crystallography, Interdisciplinary Nanoscience Center (iNANO) and Department of Chemistry, Aarhus University, Langelandsgade 140, 8000 Aarhus C, Denmark Inorg. Chem. Downloaded from pubs.acs.org by UNIV OF LOUISIANA AT LAFAYETTE on 04/24/19. For personal use only.

S Supporting Information *

ABSTRACT: Synthesis, crystal structures, and thermal and magnetic properties of the complete series of halide-free rareearth (RE) metal borohydrides are presented. A new synthesis method provides high yield and high purity products. Fifteen new metal borohydride structures are reported. The trends in crystal structures, thermal behavior, and magnetic properties for the entire series of RE(BH4)x are compared and discussed. The RE(BH4)x possess a very rich crystal chemistry, dependent on the oxidation state and the ionic size of the rare-earth ion. Due to the lanthanide contraction, there is a significant decrease in the volume of the RE3+-ion with increasing atomic number, which correlates linearly with the unit cell volume of the α- and β-RE(BH4)3 polymorphs and the solvated complexes α-RE(BH4)3·S(CH3)2. The thermal analysis reveals a one-step decomposition pathway in the temperature range from 247 to 277 °C for all RE(BH4)3 except Lu(BH4)3, which follows a three-step decomposition pathway. In contrast, the RE(BH4)2 decompose at higher temperatures in the range 306 to 390 °C due to lower charge density on the rare-earth ion. The RE(BH4)3 show increasing stability with increasing Pauling electronegativity, which contradicts other main group and transition metal borohydrides. The majority of the compounds follow Curie−Weiss paramagnetic behavior down to 3 K with weak antiferromagnetic interactions and magnetic moments in accord with those of isolated 4f ions. Some of the RE(BH4)x display varying degrees of temperature-dependent magnetic moments due to low-lying excited stated induced by crystal field effects. Additionally, a weak antiferromagnetic ordering is observed in Gd(BH4)3, indicating superexchange through a borohydride group.

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INTRODUCTION A comprehensive search for potential solid-state hydrogen storage materials, also within boron-containing compounds, has exposed several new materials exhibiting properties suitable for other applications, e.g. heat storage and batteries.1−4 The rare-earth (RE) metal borohydrides, RE(BH4)x, may possess other interesting properties, e.g. luminescence has been observed of solvates RE(BH4)2(THF)2, RE = Eu and Yb, and in the perovskite-type metal borohydride CsEu(BH4)3.5,6 Additionally, Gd(BH4)3 and the potassium and cesium derivatives have been investigated for their potential use for magnetic refrigeration.7 Furthermore, several rare-earth borohydride derivatives, e.g. RE(BH4)x-nTHF, have been widely investigated as reducing agents or catalysts within organic and polymer chemistry.8−10 Little research has been conducted on the pure RE(BH4)x. However, this class of materials may exhibit some of the interesting properties mentioned above and may serve as starting materials for rational design and synthesis of other new materials. The series of RE(BH4)3 (RE = Sm, Gd, Tb, Dy, Ho, Er, and Yb) and RE(BH4)2 (RE = Sm, Yb) has previously been synthesized by ball-milling of LiBH4 and RECl3 in a molar ratio of 6:1.11,12 Increasing the molar ratio of LiBH4 results in formation of more of the β-RE(BH4)3 polymorph.13 Using this © XXXX American Chemical Society

approach, the samples contain other compounds, e.g. LiCl, excess LiBH4, and may be halide substituted, which can influence the chemical properties. Mechanochemical treatment of LiBH4 and RECl3 (RE = La, Ce, Pr, Nd, Sm, Gd) will produce the compound LiRE(BH4)3Cl, while RE = Yb and Lu forms LiRE(BH4)4.11,14 Additionally, chloride substitution may occur on the borohydride site, e.g. in LiYb(BH4)4−xClx and Yb(BH4)2−xClx.12 Indeed, halide-free products of RE(BH4)3 (RE = La, Ce, Pr, Sm, Eu, Gd, Dy, Er) and RE(BH4)2 (RE = Sm, Eu) have been obtained via solvent-mediated synthesis using a metathesis reaction of LiBH4 and RECl3 performed in S(CH3)2 (DMS).1,15−18 This approach can provide solvated RE(BH4)3·nS(CH3)2 or halide-free RE(BH4)x (x = 2 or 3) for some of the rare-earth metals. However, only solvate complexes can be extracted and separated from the halides. Also, LiBH4 is soluble in DMS and may form an amorphous solid upon removal of the solvent, which may influence the subsequent utilization of the synthesis product.19 Recently, a synthesis method using metal hydrides and dimethyl sulfide borane, (CH3)2S·BH3, for synthesis of metal borohydrides was Received: November 21, 2018

A

DOI: 10.1021/acs.inorgchem.8b03258 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

A Note on Safety. All reagents and starting materials are air and moisture sensitive. All sample handling and preparation were performed in inert argon atmosphere using a glovebox with a circulation purifier, p(O2, H2O) < 1 ppm. Care should be exercised when handling these materials outside a protective atmosphere. Working inert, using dry glassware and protective glass is mandatory. More detailed descriptions of safety concerns are provided in literature.20 Synchrotron Radiation Powder X-ray Diffraction. High resolution in situ variable temperature synchrotron radiation powder X-ray diffraction (SR PXD) data, used for crystal structure solution and refinement, were collected at three different synchrotron facilities. In situ SR PXD data were collected at beamline I11 at the Diamond Light source, Oxford, UK on a wide-angle position sensitive detector (PSD) based on Mythen-2 Si strip modules, λ = 0.82585 Å, at the Swiss-Norwegian beamline (SNBL) BM01A at ESRF,21 Grenoble, France, with a Dectris Pilatus 2 M area detector, λ = 0.78956 Å, and at the Petra III synchrotron beamline P02.1 at DESY, Hamburg, Germany with a PerkinElmer area detector, λ = 0.207109 Å. The collected 2D diffraction images were integrated using the Bubble (SNBL) or Fit2D (P02.1) software.21,22 The powdered samples were packed in borosilicate capillaries (i.d. 0.5 mm) in an argon-filled glovebox (p(O2, H2O) < 1 ppm). Samples were rotated during data acquisition. The temperature was calibrated using a NaCl standard.23 Structure Solution and Refinement. High-resolution SR PXD data were used for structural refinement, performed using the Rietveld method as implemented in the software FullProf.24 The structural models of La(BH4)3,15 α-Y(BH4)3,25 β-Y(BH4)3,26 o-Sr(BH4)2,27 α-, α′-, γ-Ca(BH4)2,28,29 and Y(BH4)3·S(CH3)216 were used as starting points for Rietveld refinements, as these compounds are isostructural to relevant members of the series of RE(BH4)x and RE(BH4)3· S(CH3)2. In the structural models, the metal was exchanged with the corresponding RE-metal. The BH4− tetrahedra were treated as semirigid bodies, and restraints on bond lengths were used: B−H 1.2 ± 0.01 Å and H−H 1.95 ± 0.01 Å. The background was described by linear interpolation between selected points, while pseudo-Voigt profile functions were used to fit the diffraction peaks. Generally, the scale factor, unit cell parameters, zero-shift, profile parameters (U, V, W), atomic displacement parameters (B), and the atomic positions were refined. New structures were indexed using the program FOX.30 From careful inspection of systematic absences, the space groups were determined. Subsequently, the structures were solved by ab initio structure solution by global optimization in direct space, as implemented in FOX. BH4− and S(CH3)2 were treated as rigid bodies during structure solution. Subsequent Rietveld refinement were performed as described above. Fourier-Transform Infrared Spectroscopy. All samples were characterized by infrared absorption spectroscopy using a NICOLET 380 FT-IR instrument from Thermo Electron Corporation. Data were measured in the range 4000−400 cm−1, and 32 scans with a spectral resolution of 4 cm−1 were collected per sample and averaged. The sample was briefly exposed to air (∼10 s) during transfer to the instrument. Thermal Analysis. Thermogravimetric analysis and differential scanning calorimetry (TGA-DSC) of the RE(BH4)x, RE = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu, were measured on a PerkinElmer STA 6000. The sample (10−15 mg) was placed in an Al2O3 crucible under protective argon atmosphere in a glovebox and heated from 30 to 500 °C (ΔT/Δt = 5 °C min−1) in an argon flow of 40 mL min−1. Additionally, mass spectrometry data were simultaneously collected for H2 (m/z = 2), B2H6 (m/z = 26), and S(CH3)2 (m/z = 62) using a Hiden Analytical HPR-20 QMS sampling system. Sievert’s Measurement. The RE(BH4)3, RE = Tb and Lu samples were transferred to a stainless steel high-temperature autoclave and attached to a custom-made Sievert’s apparatus.31 Decomposition of the sample was performed by heating to T = 350 °C (ΔT/Δt = 3 °C min−1) at p(H2) = 1 bar and kept at 300 °C for 1 h before the sample was cooled naturally to RT. Hydrogen absorption was performed by heating the sample to T = 400 °C (ΔT/Δt = 3 °C

described, which provided additional rare-earth metal borohydrides Nd(BH4)3 and Yb(BH4)2.20 We present here an optimized approach to obtain halidefree, pure, and highly crystalline products of RE(BH4)x (x = 2 or 3) for all the rare-earth elements except the radioactive element promethium. This new approach also provides pure solvates and polymorphic selectivity of the RE(BH4)x. The compounds were investigated in detail using synchrotron radiation powder X-ray diffraction (SR PXD), thermogravimetric analysis and differential scanning calorimetry coupled with mass spectroscopy (TG-DSC-MS), Sievert’s measurements, Fourier-transform infrared spectroscopy (FT-IR), and magnetic measurements. This research provides the new compound Lu(BH4)3, crystallizing as two different polymorphs, and three new polymorphs, α-Ce(BH4)3, β2-Nd(BH4)3, and a low-temperature polymorph, m-Eu(BH4)2. Additionally, new solvate complexes RE(BH4)3·nS(CH3)2, RE = La, Ce, Pr, Tb, Dy, Ho, Er, Tm, Lu, are reported. The structural trends of the entire series of RE(BH4)x and their solvate complexes are discussed, along with thermal and magnetic properties of RE(BH4)x.

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EXPERIMENTAL SECTION

Sample Preparation. The rare-earth metal borohydrides RE(BH4)3 (RE = La, Ce, Pr, Nd, Gd, Tb, Dy, Ho, Er, Tm, Lu) and RE(BH4)2 (RE = Sm, Eu, Yb) were synthesized by a halide-free approach using the metallic RE as starting reactant. The metals were obtained from Sigma-Aldrich or Alfa Aesar with a purity of 99.9%. First, the metal was hydrogenated to form the metal hydride by heating to T = 400 °C with a heating rate of ΔT/Δt = 5 °C/min and p(H2) = 140 bar at room temperature (RT) and subsequently cooling to RT. The hydrogenation of Ce-metal was done at RT by dosing with hydrogen, i.e. p(H2) < 8 bar four times until hydrogen uptake was no longer observed. Subsequently, the pressure was increased to 150 bar and kept for 12 h at this pressure without any further measured H2 uptake observed. The lighter rare-earth metals (La, Ce, Pr) take up three equivalents of hydrogen at RT, while the rest need elevated temperatures. The resulting metal hydride, MHx, was ball milled using a Fritsch Pulverisette no. 6 to decrease the particle size and enhance the reactivity. The metal hydride was loaded under argon atmosphere in an 80 mL tungsten carbide vial together with tungsten carbide coated steel balls (diameter 10 mm) in a ball-to-powder mass ratio of 10:1. The ball-milling program was 350 rpm for 10 min, followed by a 2 min break. This sequence was repeated 10 times, i.e. 100 min of effective ball-milling time. The ball-milled metal hydride (typically 1.0 g) was added to a 100 mL round-bottomed flask with a valve outlet. Dimethyl sulfide-borane (S(CH3)2·BH3, DMS·BH3, 10 M, SigmaAldrich) was added to the powder in the molar ratio of 4.5:1 or 3:1 (50% excess of DMS·BH3) and diluted to a 5 M solution by adding toluene (Sigma-Aldrich, anhydrous). The reaction mixture was left to stir at 45 °C for 7 days. Due to the small amount of S(CH3)2 (DMS) present, RE(BH4)3·nS(CH3)2 does not dissolve. The reaction mixture was dried at 45 °C using dynamic vacuum to remove any solvent present. The dry powder was transferred to Schlenk tubes and heated to 140 °C in argon atmosphere for 2 h, followed by drying under dynamic vacuum (p ∼ 10−3 bar) for an additional 2 h. These samples are denoted “as-synthesized” samples. The as-synthesized RE(BH4)3 was then dissolved in DMS (50 mL/ g RE(BH4)3), filtrated, and extracted for comparison with the assynthesized samples. This ensures that remaining unreacted MHx, metal, or metal oxides are removed. The synthesis mixture was left to stir for a day, resulting in a colored solution in which the RE(BH4)3 was dissolved. DMS was removed from the filtrate by rotary evaporation (T = 40 °C), followed by dynamic vacuum at the Schlenk line at RT. The same drying procedure as described above was applied. These samples are referred to as the “purified” samples. B

DOI: 10.1021/acs.inorgchem.8b03258 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Table 1. Structural Parameters for the New Metal Borohydrides and Metal Borohydride Solvates Discovered in This Studya compound

space group

a (Å)

b (Å)

c (Å)

β (deg)

V (Å3)

m-Eu(BH4)2 α-Ce(BH4)3 β2-Nd(BH4)3 α-Lu(BH4)3 β-Lu(BH4)3 La(BH4)3·nS(CH3)2 α-Ce(BH4)3·S(CH3)2 β-Ce(BH4)3·S(CH3)2 β-Pr(BH4)3·S(CH3)2 Tb(BH4)3·S(CH3)2 Dy(BH4)3·S(CH3)2 Ho(BH4)3·S(CH3)2 Er(BH4)3·S(CH3)2 Tm(BH4)3·S(CH3)2 Lu(BH4)3·S(CH3)2

P21/c Pa3̅ Fm3̅c Pa3̅ Fm3̅c C2 P21/c P21/c P21/c P21/c P21/c P21/c P21/c P21/c P21/c

7.5730(4) 11.34672(4) 11.5274(4) 10.68430(7) 10.8427(1) 8.0374(3) 5.7197(1) 5.81054(9) 5.8097(3) 5.54760(3) 5.52799(3) 5.50581(4) 5.48628(5) 5.47180(7) 5.4364(3)

6.7868(4)

8.3724(4)

92.266(3)

8.2461(3) 22.9174(7) 23.7176(4) 23.690(1) 22.3266(1) 22.2569(1) 22.2170(2) 22.1755(2) 22.1098(3) 22.151(2)

9.6075(4) 8.2653(3) 8.5838(1) 8.5729(4) 8.08228(5) 8.05607(5) 8.03329(6) 8.01404(8) 7.9914(1) 7.9603(7)

106.717(3) 100.797(2) 97.2974(9) 97.280(4) 100.5192(4) 100.4874(5) 100.4766(5) 100.4359(7) 100.3924(7) 100.292(5)

429.97(4) 1460.87(1) 1531.78(8) 1219.66(2) 1274.70(3) 609.85(4) 1064.24(5) 1173.36(3) 1170.38(9) 984.24(1) 974.63(1) 966.27(1) 958.87(2) 950.94(2) 943.2(1)

All unit cell parameters are obtained at RT except m-Eu(BH4)2 (T = −173 °C).

a

min−1) at p(H2) = 150 bar and keeping it isotherm at T = 400 °C for 12 h. Subsequently, the sample was decomposed a second time, similar to the description above. Magnetic Measurement. Magnetic susceptibility was measured using a Quantum-Design Physical Property Measurement System (QD-PPMS) equipped with a vibrating sample magnetometer. Data were collected in a magnetic field of 1000 Oe (0.1 T) for temperatures from 3 to 300 K. The sample pellets of 15−30 mg were sealed in the sample holder with polyimide tape to minimize exposure to air during the transfer from the argon glovebox to the helium dewar.

can form RE(BH4)x, x = 2 or 3, i.e. RE in both oxidation state II and III.1 Using solvent-mediated synthesis, the trivalent solvent complex RE(BH4)3·nS(CH3)2 (RE = Sm, Yb) can be isolated using a lower reaction temperature for the synthesis (RT). However, a reductive solvent removal occurs upon heating, which produces RE(BH4)2, B2H6, and H2 (see eq 4).11 RE(BH4)3 ·nS(CH3)2 (s) → RE(BH4)2 (s) + nS(CH3)2 (l) +

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RESULTS AND DISCUSSION Initial Sample Analysis. The resulting powders for both the as-synthesized and the purified samples (see Experimental Section) had a broad range of different colors, see Table S1, which are in agreement with the commonly observed colors for RE-containing compounds. Generally, pure products of RE(BH4)x, RE = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu, are obtained for the as-synthesized samples following the described experimental procedure. Minor amounts of unreacted REHx (x = 2, 3) can be present, possibly due to insufficient activation by ball milling of the metal hydride. Trivalent RE borohydrides, RE(BH4)3, are readily dissolved in DMS whereas divalent, RE(BH4)2, may need a more strongly coordinating solvent like THF. This allows purification and removal by filtration of unreacted metals, hydrides and oxides that might be present after synthesis. Synthesis of Trivalent Rare-Earth Metal Borohydrides, RE(BH4)3. The samples were examined by PXD and FTIR after initial synthesis (eq 2) and DMS (S(CH3)2) removal (eq 3). This revealed that the reaction pathway for RE = La, Ce, Pr, Nd, Gd, Tb, Dy, Ho, Er and Lu is in accordance with eqs 1-3: RE(s) + 1.5H 2(g) → REH3(s)

+

(4)

Samarium(III) and ytterbium(III) borohydrides, RE(BH4)3, RE = Sm, Yb, can only be obtained using mechanochemistry and RECl3 and LiBH4 as reactants and avoiding overheating by using pauses during the treatment.11 Chloride-substituted analogues of β-Yb(BH4)2−xClx and γ-Yb(BH4)2−xClx are reported in literature, showing the importance of halide-free synthesis methods developed in this work.12 Europium appears to form the most ionic divalent hydride, EuH2,32 among the rare-earths, which reacts with DMS·BH3 in accordance with eq 5, and Eu(BH4)2 is formed directly. The reactivity of EuH2 appears to be lower compared to REH3, and unreacted EuH2 (19 wt %) remains after initial synthesis. However, the synthesis can be conducted in tetrahydrofuran (THF), resulting in full conversion. EuH 2(s) + 2S(CH3)2 − BH3(l) → Eu(BH4)2 (s) + 2S(CH3)2 (l)

(5)

Reaction Mechanism. The present work supports the previously suggested reaction mechanism based on a nucleophilic attack on the electron deficient boron of R·BH3 by ionic or polar covalent M−H bonds.20 Generally, the hydrides REH3 are ionic and REH2 are metallic. However, the hydride of europium is an exception as the dihydride, EuH2, is ionic. Another exception is ytterbium, where both hydrides, YbH2 and YbH3, are ionic.32,33 Differences in reactivity of RE hydrides are observed for CeHx. The first synthesis using this hydride was unsuccessful, possibly due to formation of a metallic hydride, CeHx, 0.3 < x < 3.32 Hence, the hydrogenation of Ce-metal was repeated using different conditions

(1)

REH3(s) + 3S(CH3)2 ·BH3(l) → RE(BH4)3 · S(CH3)2 (s) + 2S(CH3)2 (l)

1 H 2(g) 2

1 B2H6(g) 2

(2)

RE(BH4)3 · S(CH3)2 (s) → RE(BH4)3 (s) + S(CH3)2 (g) (3)

Synthesis of Divalent Rare-Earth Metal Borohydrides, RE(BH4)2. Samarium and ytterbium are different because they C

DOI: 10.1021/acs.inorgchem.8b03258 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 1. Crystal structures of (a) α-RE(BH4)3 (Pa3̅), (b) β-RE(BH4)3 (Fm3̅c), (c) r-RE(BH4)3 (R3̅c), (d) α-RE(BH4)3·S(CH3)2, and (e) βRE(BH4)3·S(CH3)2. (f) The octahedral unit of [RE(BH4)5S(CH3)2]. RE3+ (blue) are octahedrally coordinated (green octahedra) by six BH4− (a− c) or five BH4− and one S(CH3)2 (d−f). B (red), S (yellow), C (black). H (gray) is omitted for clarity (a−e).

network. All three polymorphs are related to different polymorphs of ReO3.36 Crystal Structures of RE(BH4)3·S(CH3)2. Two different polymorphs of Ce(BH4)3·S(CH3)2 are observed. A minor amount of α-Ce(BH4)3·S(CH3)2 is present (18.6(2)%, determined from Rietveld refinements, see Supporting Information), isostructural to Y(BH4)3·S(CH3)2, along with a major amount of a new polymorph, β-Ce(BH4)3·S(CH3)2, solved in a monoclinic unit cell with the same space group (P21/c) but slightly larger unit cell (ΔV = 9%). Both structures are illustrated in Figure 1 (d−f). Rietveld refinement of the new structure is shown in Figure S1. Similarly, the isostructural β-Pr(BH4)3·S(CH3)2 is present in a small amount (4.8(2)%) together with α-Pr(BH4)3·S(CH3)2 (95.2(4)%). Rietveld refinement is shown in Figure S2. The crystal structure of βRE(BH4)3·S(CH3)2 (RE = Ce, Pr) shows a close resemblance to that of α-RE(BH 4)3·S(CH3)2 (RE = Ce, Pr, Nd, Gd, Tb, Dy, Ho, Er, Tm, Lu). Both the α- and β-RE(BH 4)3·S(CH3)2 crystal structures are composed of buckled layers of distorted [RE(BH4)5S(CH3)2] octahedra connected by edge-sharing borohydride tetrahedra (η2). The main difference between the two structures is that the layers are shifted along the adirection. Additionally, a change in bond distance is observed. The Ce−B distances in α-RE(BH4)3·S(CH3)2 are 2.52 Å for the terminal borohydride group and 2.80−2.92 Å for the bridging borohydride groups. The Ce−S bond distance is 2.96 Å. In β-RE(BH4)3·S(CH 3)2, the Ce−B distances are 2.68 Å for the terminal borohydride group and 2.92−2.96 Å for the bridging borohydride groups. The Ce−S bond distance is 4.41 Å. In general, β-RE(BH4)3·S(CH3)2 shows longer Ce−B and Ce−S distances, which may explain why DMS is released at slightly lower temperature compared to α-RE(BH4)3·S(CH3) 2 due to a weaker coordination, see Figure S5. Crystal Structure of m-Eu(BH4)2. The monoclinic lowtemperature polymorph, m-Eu(BH4)2 (P21/c) forms upon

(see Experimental Section). The resulting hydride, CeH3, reacted with DMS·BH3, resulting in Ce(BH4)3·S(CH3)2. Synthesis of Sm(BH4)2 from the metallic SmH2 and THF· BH3 has been reported but with very low yields, possibly due to only small amounts of the ionic SmH3 present in the samarium hydride.34 Thus, initially a solvate, Sm(BH4)3-nTHF is formed which, upon heating, undergoes a reductive solvent elimination and forms Sm(BH4)2. This is also observed for Yb(BH4)3-nS(CH3)2, which is stable in solution and as a solvate at RT, but reductive solvate elimination readily occurs upon heating, which produces Yb(BH4)2.20 Structural Characterization of New Metal Borohydrides and Metal Borohydride Solvates. All new compounds discovered in this study are presented in Table 1, while a full list of all the compounds synthesized in this investigation is provided in Table S3. The RE(BH4)x exhibit high hydrogen content with a gravimetric hydrogen content in the range of 4.0−6.6 wt %, while the volumetric hydrogen content is in the range 110.0−131.7 kg H2/m3, see Table S2. Crystal Structures of RE(BH4)3. The crystal structures of the polymorphs r-RE(BH4)3, α-RE(BH4)3, and β-RE(BH4)3 (RE = La, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu) are shown in Figure 1 (a−c). α-RE(BH4)3 and β-RE(BH4)3 are isostructural to the reported structures of the polymorphs αand β-Y(BH4)3, while r-RE(BH4)3 are reported for RE = La, Ce.15,35 In the crystal structures, the metal cation (RE3+) is octahedrally coordinated by six borohydride groups through the edge of the BH4-tetrahedron (η2). The [RE(BH4)6] octahedron is distorted in α-RE(BH4)3 while ideal in βRE(BH4)3 and r-RE(BH4)3. The r-RE(BH4)3 may be seen as a rhombohedral deformation of β-RE(BH4)3, in which the connection between the octahedra is tilted, resulting in a more compact structure.15 In the three structures, the octahedral units are connected by corner sharing in a three-dimensional D

DOI: 10.1021/acs.inorgchem.8b03258 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry cooling of o-Eu(BH4)2 (Pbcn) to T = 100 K. The structure of m-Eu(BH4)2, see Figure 2a, consists of Eu2+ atoms coordinated

gradual change observed in the diffraction pattern during heating indicates a second-order polymorphic transition, which fits with the subgroup−supergroup relation between space group P21/c and Pbcn. In the crystal structure of m-Eu(BH4)2, all atoms are located on general sites (4e). Hence, the structure consists of one symmetry independent Eu 2+ site and two symmetry independent BH4−-sites. The Eu−B distances are in a larger range of 2.8−3.3 Å as compared to those reported for oEu(BH4)2 (3.0−3.1 Å), which shows that the new lowtemperature structure is more distorted as compared to the high-temperature ones.27 The [Eu(BH4)6] polyhedra share four edges and four vertices; hence, the connection between the coordinated units has changed compared to that of oEu(BH4)2. Structural Discussion of Rare-Earth Metal Borohydrides. The trivalent RE(BH4)3 crystallize in three different crystal structures, denoted r-, α-, and β-RE(BH4)3, respectively. La(BH4)3 crystallizes in space group R3̅c (r-RE(BH4)3), while Ce(BH4)3 shows two different polymorphs in the assynthesized sample, space group R3̅c and Pa3̅ (α-RE(BH4)3), see Table S3. An additional high-temperature polymorph of Ce(BH4)3, crystallizing in space group Fm3̅c, β-RE(BH4)3, was also discovered.15 For the as-synthesized samples, the αRE(BH4)3 polymorph is preferred, whereas purification with DMS enhances the formation of β-RE(BH4)3, see Table S4. Hence, higher temperatures are required for removal of the solvent after purification, leading to larger amounts of the hightemperature polymorph (β-RE(BH4)3). Gd(BH4)3 is the only of the trivalent RE(BH4)3 (in this work) that only crystallizes as the α-RE(BH4)3 polymorph, while β-Nd(BH4)3 shows two distinct unit cell volumes (after purification), one considerably larger than the observed trend shown in Figure 3. The Rietveld refinement of Nd(BH4)3 (purified) is shown in Figure S3. Solvent complexes of RE(BH4)3·S(CH3)2 are formed during the synthesis, in contrast to RE(BH4)2, which does not have any coordinated DMS. The series of RE(BH4)3·S(CH3)2, RE = Ce, Pr, Nd, Gd, Tb, Dy, Ho, Er, Tm, Lu, are isostructural to the reported structures of α-RE(BH4)3·S(CH3)2 (RE = Y, Gd;

Figure 2. (a) The crystal structure of m-Eu(BH4)2 (P21/c). Eu2+ (blue) are trigonally prismatically coordinated by six BH4− (green triangular prism). Boron is shown in red, and hydrogen is omitted for clarity. (b) The trigonal prismatic unit of [Eu(BH4)6]. BH4− are shown as orange tetrahedra with H (gray) in the corners.

by six BH4− units in a trigonal prismatic geometry (Figure 2b). From the PXD data, it is clear that there is some structural resemblance with the RT polymorph o-Eu(BH4)2, where Eu2+ are octahedrally coordinated, as the powder patterns of the two are very similar, but several reflections from o-Eu(BH4)2 split into two distinct reflections during the polymorphic transition. The in situ SR PXD data are discussed in a later section. A

Figure 3. Volume per formula unit (V/Z) at RT is shown as a function of the volume of the RE3+-ion for the RE(BH4)3 and RE(BH4)3·S(CH3)2. The RE(BH4)3 crystallize in three different crystal structures, the r-RE(BH4)3 (R3̅c), the α-RE(BH4)3 (Pa3̅), and the β-RE(BH4)3 (Fm3̅c) structure type. RE = Sm and Yb are obtained from ref 11 and β-Gd(BH4)3 from ref 13. The RE(BH4)3·S(CH3)2 crystallize in two different crystal structures, α- and β-RE(BH4)3·S(CH3)2 (P21/c). A linear trend line is shown for α-/β-RE(BH4)3 and α-RE(BH4)3·S(CH3)2. E

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Figure 4. Metal−boron distances in the trivalent RE(BH4)3 adopting the α- and β-RE(BH4)3 structure, shown as a function of the ionic radius of the metal. α-RE(BH4)3 contains two different M−B distances. The lines represent the trend line for the average M−B distances.

P21/c).16 RE(BH4)3·nS(CH3)2 (RE = La, Ce, Pr) form new solvate complexes, La(BH4)3·nS(CH3)2 and β-RE(BH4)3· S(CH3)2 (RE = Ce, Pr). La(BH4)3·nS(CH3)2 has been indexed in a monoclinic unit cell. Systematic absences suggest the following space groups with a C-centering: C2, Cm, or C2/ m. Le Bail fit is shown in Figure S4. The unit cell volume of 609.85(4) Å3 suggests the composition La(BH4)3·nS(CH3) 2(n = 1 or 2). Structure solution in FOX was not successful. Multiple RE(BH4)3·nS(CH3)2 complexes (RE = La, Ce) have previously been reported in the literature;15 however, the structure of these compounds has not been solved and shows no resemblance to the X-ray powder patterns observed in this work. The divalent RE(BH4)2 (RE = Sm, Eu) crystallize in space group Pcnb at room temperature and are isostructural to Sr(BH4)2 due to the similar ionic radii: Eu2+ (117 pm), Sr2+ (118 pm), and Sm2+ (119 pm).37 Several polymorphs are reported for Eu(BH4)2 at elevated temperature.27 However, in this work, a new monoclinic low-temperature polymorph, mEu(BH4)2, which has no structural analogue to any reported metal borohydride, is presented. Yb2+ is considerably smaller (102 pm) compared to RE2+ = Eu, Sr, Sm; hence, the crystal structures of Yb(BH4)2 resemble those of Ca(BH4)2 (r(Ca2+) = 100 pm), crystallizing in the space groups I4̅2d (α′Yb(BH4)2), P-4 (β-Yb(BH4)2), and Pbca (γ-Yb(BH4)2). Following our experimental procedures, almost pure γYb(BH4)2 (95.4(5)%) is obtained along with a small amount of α′-Yb(BH4)2 (3.8(1)%) in the as-synthesized samples. If Yb(BH4 )3 ·xS(CH3) 2 is purified with DMS, γ-Yb(BH4) 2 (47.7(9)%) and β-Yb(BH4)2 (52(1)%) are formed after desolvation. An additional polymorph, α-Yb(BH4)2, is recently reported, obtained from desolvation of Yb(BH4)2·2THF at T = 180 °C.20 Trends in Formula Volume of RE(BH4)3 and RE(BH4)3· S(CH3)2. Due to the lanthanide contraction, there is a significant decrease in the volume of the RE3+-ion (V(RE3+)) with increasing atomic number. The volume of the RE3+-ion is calculated assuming spherical ions and radii obtained from ref 37. The volume per formula unit (V/Z) at RT as a function of V(RE3+) for the three polymorphs of the trivalent RE metal borohydrides, RE(BH4)3, is illustrated in Figure 3. The unit cell parameters at RT for RE(BH4)3 (RE = Sm, Yb) are

obtained from ref 11, as the synthesis method presented in this article leads to formation of the divalent RE(BH4)2 (RE = Sm, Yb), while β-Gd(BH4)3 is not observed in this work, hence obtained from ref 13. The remaining results are obtained from this work, see Table S3; thus, it is halide-free RE(BH4)3. The data reveals a linear correlation between V/Z and V(RE3+) and clearly shows that the crystal structures of RE(BH4)3 (RE = La, Ce; R3̅c) are more compact compared to those of α- and βRE(BH4)3, while α-RE(BH4)3 is more compact than βRE(BH4)3 due to deformation of the [RE(BH4)6] octahedra. Similarly, a linear correlation is observed for the solvated complexes, α-RE(BH4)3·S(CH3)2, where α-RE(BH4)3·S(CH3)2 are more compact than β-RE(BH4)3·S(CH3)2. Trends in Bond Distances RE−B in RE(BH4)3. The RE− B distances for the α-RE(BH4)3 and β-RE(BH4)3 structures have been extracted after structural refinement of the atomic positions, see Table S5, and have been plotted as a function of ionic radius of the RE ion, see Figure 4. The average value of the RE−B bond distance shows a linear correlation with the ionic radii of the metal. In β-RE(BH4)3, the RE−B bond distance is equal to half the unit cell axis length, as the RE− BH4−RE form a straight bridge (angle = 180°). This contrasts α-RE(BH4)3, where RE−BH4−RE are bent at angles between 164.5° to 172.5°. Owing to the symmetry in α-RE(BH4)3, there are two unique BH4-units and hence two unique RE−B distances. The shortest RE−B distance varies from 2.88 Å in Ce(BH4)3 to 2.62 Å in Er(BH4)3. Surprisingly, Er(BH4)3 shows shorter RE−B distance than Lu(BH4)3, despite the ionic radii of Lu(BH4)3 being smaller. The longest RE−B distances vary from 2.89 Å in Nd(BH4)3 to 2.74 Å in Tm(BH4)3. In all cases except for Pr(BH4), the RE−B distance for the β-RE(BH4)3 structure is found in between the two RE− B distances observed in the α-RE(BH4)3 structure. Fourier-Transform Infrared Spectroscopy. The FT-IR spectra of RE(BH4)x, Figure 5, show similar stretching and bending modes. Stretching modes from B−H are observed in the region 2000−2550 cm−1, while B−H bending modes are observed in the region 1000−1400 cm−1, similar to the alkali and alkaline earth metal borohydrides.38 No absorption is observed for other compounds, i.e. no solvent seems to be present (e.g. S(CH3)2). From the FT-IR spectra, it is observed that RE(BH4)3 (RE = La, Ce, Pr) show similar stretch and F

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Relative Stability of RE(BH4)x Polymorphs. The different polymorphs of RE(BH4)x are found to have different stability. An in situ SR PXD plot for Ce(BH4)3 in the temperature range ΔT = RT−125 °C is shown in Figure 6. The first SR PXD pattern measured at RT shows diffraction from two polymorphs, namely α-Ce(BH4)3(82.1(2)%) and rCe(BH4)3 (17.9(1)%). Rietveld refinement is shown in Figure S6, showing sharp Bragg diffraction peaks from both compounds. However, upon heating, the Bragg diffraction peaks from α-Ce(BH4)3 decrease in intensity in the temperature range 40−120 °C, while new diffraction peaks corresponding to a second r-Ce(BH4)3 (denoted r2-Ce(BH4)3) compound start to appear at T ∼ 80 °C due to a polymorphic transition from Pa3̅ to R3̅c. r2-Ce(BH4)3 has broad diffraction peaks (full width at half-maximum, fwhm ≈ 0.136°) and a larger unit cell compared to r1-Ce(BH4)3 (fwhm ≈ 0.064°) at T = 85 °C. Upon further heating, the Bragg reflections from r1Ce(BH4)3 and r2-Ce(BH4)3 merge, and all diffraction peaks from r-Ce(BH4)3 are broad (fwhm ≈ 0.112°) at T = 125 °C. This is similar to earlier observations for r-Ce(BH4)3, where the anisotropic line broadening is explained by stacking faults in the (001) planes in the hexagonal lattice.15 However, in the present example, all Bragg reflections are broad, and the intensity decreases with increasing temperature. The line broadening of r-Ce(BH4)3 seems to be very dependent on the conditions of which DMS is removed from Ce(BH4)3·S(CH3)2. The broad reflections are present when rCe(BH4)3 is formed by a polymorphic transition from α- or βCe(BH4)3, while if r-Ce(BH4)3 is formed directly from Ce(BH4)3·S(CH3)2, no broadening of the Bragg reflections are observed, see Figure S5. Hence, the synthesis conditions applied during DMS removal are crucial in determining the polymorph and the degree of stacking faults in r-Ce(BH4)3. Similarly, the physical conditions during removal of DMS for the remaining RE(BH4)3 determine which polymorph is obtained, i.e. α- or β-RE(BH4)3. The relative amount of the different polymorphs of RE(BH 4 ) 3 may change over time, revealing different thermodynamic stability of α- and β-RE(BH4)3. A sample of Nd(BH4)3 (purified) was examined by SR PXD 1 month after purification and again after 6 months. The initial sample composition was α-Nd(BH4)3 (72.2(2) wt %) and βNd(BH4)3 (27.8(2) wt %), which changed to α-Nd(BH4)3 (6%) and β-Nd(BH4)3 (94.2(9)%) after 6 months at RT. This

Figure 5. FT-IR spectra of RE(BH4)x(x = 2, 3). RE(BH4)3 adopt the r-RE(BH4)3 for RE = La, Ce, while RE = Pr−Lu adopt the α- or βRE(BH4)3 structure. RE(BH4)2 (RE = Sm, Eu) are isostructural to Sr(BH4)2, while Yb(BH4)2 is isostructural to Ca(BH4)2. B−H stretch modes are observed between 2000 and 2550 cm−1, while B−H bend modes are observed between 1000 and 1400 cm−1.

bend modes, suggesting that their crystal structures are very alike. However, from PXD, Pr(BH4)3 is found to be isostructural to the α- and β-RE(BH4)3. Vibrational spectra give a local view; hence, the similar spectra suggest that the BH4− is coordinated in a similar fashion.39 The remaining series of RE(BH4)3 (RE = Nd−Lu) show similar stretching and bending modes and agrees with those reported for the isostructural compounds Y(BH4)3 and Gd(BH4)3.16,40 Similarly, the IR spectra of RE(BH4)2, RE = Sm, Eu, show great resemblance to that of Sr(BH4)2, while Yb(BH4)2 shows great similarity to Ca(BH4)2, owing to the structures being isostructural.38 Despite the similarities in the IR spectra of the RE(BH4)3, a shift toward higher wavenumbers is observed as the ionic radius of the metal decreases. This suggests that the B−H bond length decreases with decreasing ionic radii of the RE3+. It has been shown previously that there is a linear relation with the change in the stretching frequencies and the change in B−H bond length for the BH4− anion.39

Figure 6. In situ SR PXD of Ce(BH4)3 measured from T = RT to 125 °C, ΔT/Δt = 5 °C min−1 (λ = 0.82585 Å). α: α-Ce(BH4)3, r1: r1-Ce(BH4)3, r2: r2-Ce(BH4)3. G

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The polymorphic transition is first observed at T = −171 °C, and a gradual increase of the Bragg reflections corresponding to o-Eu(BH4)2 is observed until T = −120 °C, while no Bragg reflections from m-Eu(BH4)2 are observed above this temperature. The very low temperature needed for the polymorphic change to m-Eu(BH4)2 (T < −171 °C) suggests that a lowtemperature polymorph of the isostructural (at RT) Sr(BH4)2 and Sm(BH4)2 may also exist. However, no polymorphic change was observed for Sm(BH4)2 nor Yb(BH4)2 upon cooling to −173 °C. No low-temperature polymorph has been reported for RE(BH4)3 (RE = Y, Gd, Dy) upon cooling to −256 °C.25 Thermal Decomposition Observed by in Situ Synchrotron Radiation Powder X-ray Diffraction. The thermal decomposition of RE(BH4)3(RE = Ce, Nd, Tb, Lu) was studied by in situ SR PXD. Due to the similarities observed in TG-DSC-MS of the RE(BH4)3 (see later section), the observations from in situ SR PXD are expected to be similar for the series of RE(BH4)3 (RE = La, Ce, Pr, Nd, Gd, Tb, Dy, Ho, Er, Tm). Lu(BH4)3 shows a different decomposition pathway in TGA-DSC-MS, although the observed pathway from in situ SR PXD (see Figure S8) is similar to that of Tb(BH4)3. The decomposition pathway of the divalent RE(BH4)2 (RE = Sm, Eu, Yb) is different from that of the trivalent RE(BH4)3. The decomposition pathway observed by in situ SR PXD for halidefree RE(BH4)2 (RE = Eu, Yb) is provided in literature and thus not provided here.27 In situ SR PXD data for Tb(BH4)3 in the temperature range ΔT = RT−125 °C are shown in Figure 8. The first SR PXD

suggests that β-Nd(BH4)3 is the most stable of the two polymorphs, which is contrary to DFT calculations, which suggest that α-RE(BH4)3 is the most stable polymorph.41 Additionally, two different polymorphs were observed for βNd(BH4)3, denoted β1- and β2-Nd(BH4)3, respectively. The Rietveld refinement is shown in Figure S3. β1-Nd(BH4)3 shows the expected unit cell parameter, a = 11.40154(9) Å (V/Z = 370.5 Å3), consistent with the trend observed in Figure 3, while β2-Nd(BH4)3 shows a 3% expansion of the unit cell, a = 11.5274(4) Å (V/Z = 382.9 Å3). Furthermore, broad reflections are observed for β2-Nd(BH4)3, suggesting stacking faults in the structure. Different polymorphs of β′ and β′′RE(BH4)3 (RE = Pr, Nd) have been reported in literature, but these are more compact compared to the trend observed in this work and are induced by a high pressure of H2, showing the first example of stepwise negative thermal expansion in metal borohydrides.41 DFT calculations performed in that work suggest that the β-Pr(BH4)3 should be 12.3% larger than the experimental value. The sample of Nd(BH4)3 in this work was measured by in situ SR PXD, which provided additional information on the relative stability of the polymorphs at higher temperatures, see Figure S7. Upon heating, a sudden decrease in the intensity of β1-Nd(BH4)3 is observed at T = 210 °C, while the reflections from α-Nd(BH4)3 remain unchanged. At further heating, a polymorphic transition from α-Nd(BH4)3 to β1-Nd(BH4)3 is observed prior to decomposition. A reversible transition to the new low temperature polymorph is observed for Eu(BH4)2, see Figure 7. At T =

Figure 8. In situ SR PXD of α-Tb(BH4)3 measured from RT to 400 °C, ΔT/Δt = 5 °C min−1 (λ = 0.82585 Å).

Figure 7. In situ SR PXD of Eu(BH4)2 measured from T = −173 to −80 °C, ΔT/Δt = 5 °C min−1 (λ = 0.78956 Å). A diffractogram is shown at T = −173 °C and T = −108 °C. Four peaks are assigned to unreacted EuH2 (gray triangles). The remaining peaks are assigned to m-Eu(BH4)2 (−173 °C) and o-Eu(BH4)2 (−108 °C), respectively.

pattern for Tb(BH4)3 at RT reveals Bragg diffraction from αTb(BH4)3 as the only crystalline compound. Upon heating, the Bragg diffraction peaks start to decrease in intensity from T ∼ 240 °C and decrease fast in the temperature range 250−290 °C. No diffraction from α-Tb(BH4)3 is observed at T ≥ 320 °C. The decomposition products cannot be clearly identified from the diffraction data because only weak Bragg reflections are observed at T ≥ 320 °C, assigned to TbH2, which in literature is suggested as a possible decomposition product.42 Furthermore, an increase in the background is observed as Tb(BH4)3 decomposes, suggesting that amorphous com-

−173 °C, diffraction is observed mainly from m-Eu(BH4)2 (81(1) wt %), but peaks from unreacted EuH2 (18.9(4) wt %) are also present. Upon heating, a gradual change in the diffraction pattern is observed. This is mostly evident for the (−111) and (111) Bragg reflection in m-Eu(BH4)2 located at 2θ ∼ 10.5°, which merge to the (111) in o-Eu(BH4)2. Several Bragg reflections change in intensity, while some Bragg reflections from m-Eu(BH4)2 disappear, and new Bragg reflections in o-Eu(BH4)2 appear. H

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The MS data reveal a release of pure hydrogen with a peak temperature at T = 262 °C. No release of B2H6 or DMS (S(CH3)2) is observed. The TG-DSC-MS data are similar for Pr(BH4)3, where a first order polymorphic transition is observed at T = 196 °C, evident from the endothermic event (around 7.4 kJ/mol), see Figure S12. Similarly, a polymorphic transition from α- to β-Y(BH4)3 has been reported with a value of around 4.5 kJ/mol.43 However, a similar polymorphic transition from α- to β-RE(BH4)3 was not observed in this work. The thermal stabilities of the α- and βRE(BH4)3 are similar; therefore, only one DSC signal is observed during decomposition and also for samples containing a mixture of the two polymorphs, e.g. Er(BH4)3 (Figure S17). Similarly, α- and β-Y(BH4)3 reveal a minor difference in thermal stability of ∼8.4 °C.43 Figure 9b shows the TG-DSC-MS data of Tb(BH4)3, which reveal decomposition in a single step initiating at T = 245 °C; the thermogravimetric data reveal a mass loss of 5.2 wt % in the temperature range 250−305 °C, whereas the event is characterized from DSC data as an endotherm with a peak temperature at T = 275 °C. The MS data highlight the release of pure hydrogen with a peak temperature at 281 °C, i.e., no release of B2H6 or DMS is observed. This is similar for RE = La, Nd, Gd, Dy, Ho, Er, Tm. Figure 10a shows the TG-DSC-MS data of the Lu(BH4)3 sample. A minor hydrogen release initiates already at T = 158 °C, which corresponds to approximately 0.6 wt % up to ∼220 °C. More distinct hydrogen releases are observed to start at T ∼ 230 and 266 °C in two following endothermic events with peak temperature at T = 257 and 275 °C, respectively. The TG reveals a mass loss of 2.4 wt % in the temperature range 220− 265 °C and 2 wt % in the temperature range 265−320 °C. Hence, a total mass loss of 5.0 wt % is observed. The three decomposition steps observed for Lu(BH4)3 differ from the remainder of the RE(BH4)3. The three step decomposition path is also observed by PCT measurements (see later section). Figure 10b shows the TG-DSC-MS data of Sm(BH4)2. A minor hydrogen release initiates at T = 150 °C, which corresponds to approximately 1.5 wt % up to ∼305 °C, where the sample decomposes in a main step, releasing 2.1 wt % of hydrogen in the temperature range 300 to 335 °C. Minor hydrogen release is also observed with peak temperatures at T = 360 and 410 °C. The main decomposition step is characterized from DSC data as an endotherm with a peak temperature at T = 305 °C. The MS data highlight the release of pure hydrogen with a peak temperature at 310 °C, i.e., no release of B2H6 or DMS is observed. This is similar for RE = Eu and Yb. However, the decomposition temperature is higher for the two, see Figures S19 and S20. Additionally, the decomposition of Eu(BH4)2 and Yb(BH4)2 occurs over a broader temperature interval, and a shoulder in the MS signal suggests a multiple-step decomposition pathway, consistent with the minor hydrogen releases observed for Sm(BH4)2. The RE(BH4)3 decompose in the temperature range ΔT = 247−277 °C (DSC peak temperatures) in a one-step decomposition (except for RE = Lu), see Table S6. The decomposition is endothermic, and only pure hydrogen is observed. A possible decomposition mechanism was previously suggested for RE(BH 4 ) 3 , producing rare earth metal hexaborides, RE hydrides, and hydrogen.42 This suggests a calculated hydrogen release of 5.05−6.04 wt % for the series of RE(BH4)3, which corresponds reasonably with the observed

pounds are formed. No changes are observed upon cooling to RT. Thermal Analysis. All samples were investigated by TGDSC-MS. The RE(BH4)3 samples behave similarly in their decomposition pathway, except for Lu(BH4)3, which deviates. Additionally, Ce(BH4)3 and Pr(BH4)3 show a polymorphic transition upon heating. Thus, the Ce(BH4)3, Tb(BH4)3, and Lu(BH4)3 compounds are presented here as representatives for the series of RE(BH4)3. The RE(BH4)2 compounds also show similarities in their decomposition path; hence, Sm(BH4)2 is shown here as a general example. The new solvated compounds of La(BH4)3·nS(CH3)2 and Ce(BH4)3·S(CH3)2 were also investigated by TG-DSC-MS, see Figures S9 and S10. The remaining TG-DSC-MS data for RE(BH4)x are presented in Figures S11−S20. Figure 9a shows the TG-DSC-MS data of Ce(BH4)3. A small endothermic event (around 2.3 kJ/mol) is observed at T = 129

Figure 9. TG-DSC-MS of (a) Ce(BH4)3 and (b) Tb(BH4)3 heated from 30 to 500 °C (ΔT/Δt = 5 °C min−1).

°C, corresponding to the polymorphic transition from αCe(BH4)3 to r-Ce(BH4)3 (Figure 6), suggesting that it is a first-order polymorphic transition. The reverse polymorphic transition was not observed upon cooling. The sample decomposes in a single step initiating at T = 248 °C. Thermogravimetric data reveal a mass loss of 4.7 wt % in the temperature range 227−271 °C, whereas DSC data reveal an endothermic event with a peak temperature at T = 250 °C. I

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Figure 11. Decomposition temperature (Tdec) as a function of the Pauling electronegativity of the metal cation (χp). Lu(1) and Lu(2) refer to the two main decomposition steps of Lu(BH4)3. M(BH4)n, M = Li, Na, Mg, Ca, Mn, Sr are included for reference.27,46−49 All data are measured with very similar physical conditions. The thermal data (Tdec) were determined from the peak value of the DSC signal. The dotted linear line is added as a guide to the eye.

reference.27,46−49 For the RE(BH4)3, there is a tendency that increased Pauling electronegativity leads to an increase in the decomposition temperature. This is surprising, as the charge density of RE3+ increases as the ionic size decreases, which should destabilize the RE(BH4)3. The RE(BH4)2 are more stable than the RE(BH4)3 due to a lower charge density on the RE ion. The decomposition temperatures of RE(BH4)2, RE = Yb and Eu, correlate well with the trend for the decomposition temperature of metal borohydrides, while that for Eu(BH4)2 is significantly more stable. The decomposition temperature of RE(BH4)3, with peak temperatures between 247 and 277 °C, agrees well with previous results where an endothermic event was observed for composites of LiBH4 and RECl3 (RE = Gd, Tb, Dy, Lu) between 250 and 270 °C.11 However, the temperature range for decomposition observed in that study is much broader, and multiple endothermic events are observed during decomposition, possibly influenced by the presence of metal chlorides or LiBH4. Additionally, only one decomposition step is observed for LiLu(BH4)4,11 whereas this study reveals three decomposition reactions for the pure Lu(BH4)3. In another study, halide-free RE(BH4)3 (RE = La, Ce) show a very similar decomposition pathway to what is observed here.15 However, the weight loss observed in that study is slightly higher due to a simultaneous release of diborane, which is not observed here. The decomposition temperatures of RE(BH4)2 (RE = Sm, Eu) measured in this investigation are similar to those reported for halide-free Sm(BH4)2 and Eu(BH4)2 synthesized using a different approach.17 In that work, the authors observed multiple gas releases prior to the decomposition of the RE(BH4)2 due to release of coordinated solvent and a reduction from RE3+ to RE2+. Hence, DMS, B2H6, and H2 are released. The decomposition temperatures of Yb(BH4)2 found in this work are also very comparable to those reported for the chloride substituted Yb(BH4)2−xClx and LiYb(BH4)3Cl, suggesting that Cl substitution does not seem to have a big influence on the decomposition temperature for this system.11,12

Figure 10. TG-DSC-MS of (a) Lu(BH4)3 and (b) Sm(BH4)2 heated from 30 to 500 °C (ΔT/Δt = 5 °C min−1).

mass loss. The divalent RE(BH4)2 have higher thermal stability as compared to RE(BH4)3 and decompose in a broader temperature range, T = 306−390 °C. Most of the hydrogen is released in one step; however, minor events are also observed at higher temperatures. The hydrogen releases are endothermic. A possible decomposition pathway is suggested for M(BH4)2 (M = Ca, Sr, Ba) in literature, which may be similar for the RE(BH4)2, resulting in the same decompositions products as proposed for the RE(BH4)3.27,44,45 This suggests a calculated hydrogen release of 3.31−3.73 wt % for the RE(BH4)2, which fits reasonably with the observed weight loss. Note that the decomposition products of the RE(BH4)x were amorphous during the in situ SR PXD measurements, so the exact composition remains unknown. It is well-established that there is an empirical correlation between the decomposition temperature of metal borohydrides and the Pauling electronegativity of the metal cation.50−53Thus, the metal cation with higher electronegativity tends to destabilize the metal borohydride, resulting in lower decomposition temperatures. Additionally, metal borohydrides, which decompose below 200 °C, show a tendency to release a mixture of hydrogen and diborane upon decomposition.54,55 The decomposition temperature (DSC peak temperature) of RE(BH4)x is plotted as a function of the Pauling electronegativity of the metal cation, see Figure 11. Selected main group and transition metal borohydrides are included for J

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Inorganic Chemistry Sievert’s Measurement. Sievert’s measurement was performed on the new compound Lu(BH4)3 to support the three-step decomposition observed in TG-DSC-MS. For comparison, Tb(BH4)3 is included, which represents the remainder of the series of RE(BH4)3, as these are very similar in their decomposition pathway, as observed by TG-DSC-MS. Both are shown in Figure 12.

been made for erbium containing composites, where rehydrogenation leads to a partial hydrogen uptake, assigned to be the reversible formation of ErH2 and ErH3.56,57 Thus, this behavior is not unlikely observed for the series of RE(BH4)x Magnetic Properties of RE(BH4)x. There are few investigations of magnetism in metal borohydrides.7 The fairly large separation of magnetic ions and the ionic bonding in these materials suggests that weak exchange interactions and paramagnetism can be expected to low temperatures, similar to rare-earth di- and trihalides.58−60 The ionic size of Br− (r = 1.96 Å) is similar to that of the polyatomic BH4− (r = 2.03 Å),1 but the BH4− unit may induce stronger crystal field effects, as shown below. The magnetic properties were investigated for all RE(BH4)x compounds from RT to 3 K. Magnetic measurements are very sensitive to small amounts of magnetic impurities; hence, all samples for magnetic measurements were purified by solvent extraction to remove possible magnetic impurities such as unreacted REHx or RE oxides. The temperature dependence of the magnetic susceptibility (χ(T) and 1/χ(T)) is shown in Figures S21−S31. The Curie constants were extracted from fitting of the linear region of the inverse magnetic susceptibility, and from that, the effective magnetic moment was calculated. In a previous study, the magnetic moment and paramagnetic Weiss temperature of Gd(BH4)3 were found to be μeff = 7.39 μB and θW = −8.76 K, respectively.7 Those values are comparable to the values observed in this study with a magnetic moment of 7.94 μB and paramagnetic Weiss temperature of −6.8 K, see Figure 13a. The magnetic moment of 7.94 μB matches perfectly with the theoretical value of a free Gd3+ ion. Interestingly, a weak antiferromagnetic ordering is observed in Gd(BH4)3 with a Neel temperature of 4.5 K, which is the first report of magnetic ordering in a metal borohydride material and is the first experimental observation of magnetic ordering due to superexchange through a borohydride group. Similarly, antiferromagnetic ordering has been observed in GdBr3 at 2 K.60 A paramagnetic material is expected to show a linear correlation between the temperature and the inverse magnetic susceptibility, as observed for Ho(BH4)3, see Figure 13b. This behavior is observed to low temperature for RE(BH4)x, RE = Gd, Tb, Dy, Ho, Er, while deviations are observed for RE = Ce, Pr, Nd, Eu, Tm, Yb, and Lu at T < 100 K. La(BH4)3 and Sm(BH4)2 display low magnetic susceptibility without the 1/T dependency for a paramagnetic state; hence, the magnetic moments were not determined for those and are approximated to be zero. Theoretically, La(BH4)3 (4f0), Sm(BH4)2 (4f6), Yb(BH4)2 (4f14), and Lu(BH4)3 (4f14) are expected to be diamagnetic; however, Yb(BH4)2 and Lu(BH4)3 show a regular shape of a paramagnetic material at T > 100 K. However, the magnetic susceptibility is very low for these compounds, suggesting that the magnetic susceptibility may originate from a minor impurity, possibly containing a minor amount of other REn+, because only low levels of a magnetic ion may give rise to a weak signal. The observed magnetic data for the divalent Sm(BH 4 ) 2 and Yb(BH 4 ) 2 deviate from the expected diamagnetic state, possibly due to higher air sensitivity (samples were shortly exposed to air during sample mounting. Hence, the magnetic moment of RE(BH4)x (RE = La, Sm, Yb, Lu) is expected to be close to zero. Interestingly, the systems RE(BH4)x (RE = Ce, Pr, Nd, Eu, Tm, Yb) display Curie−Weiss behavior above 100 K but with

Figure 12. Sievert’s measurement of Tb(BH4)3 (top) and Lu(BH4)3 (bottom) from RT to 350 °C (ΔT/Δt = 3 °C min−1) at p(H2) = 1 bar.

The gas release from the single decomposition step of Tb(BH4)3 initiates with a slow release at T ∼ 220 °C, which rapidly accelerates at 260 °C. A total gas release of ∼5.0 wt % is observed. The second desorption shows a small hydrogen release of 0.2 wt % H2. For Lu(BH4)3, the first gas release of 0.5 wt % is observed between T = 160−235 °C. At T = 235 °C, the second decomposition step begins with a loss of 1.3 wt % before the final step initiates at T = 278 °C, releasing an additional 2.0 wt % of gas. Thus, the total gas release amounts to 3.8 wt %. The second desorption shows a small hydrogen release of 0.2 wt % initiating at T ∼ 238 °C. The results for the first desorption are comparable to those from the TG-DSC-MS measurements; however, the weight loss is slightly lower. Small changes in the decomposition temperature might arise from the difference in heating rate. For both measurements, it is clear that the reversibility of these systems is very poor, as the second desorption only releases 0.2 wt % in both cases. However, this suggests a rehydrogenation of REH2 to REH3, which then decomposes. Similar observations have previously K

DOI: 10.1021/acs.inorgchem.8b03258 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 13. Magnetic data for RE(BH4)x. (a) Gd(BH4)3, (b) Ho(BH4)3, (c) Nd(BH4)3. (a−c) Temperature dependence of the magnetic susceptibility is shown as red circles, while the inverse of the susceptibility is shown as blue circles. The red solid line shows the linear fit to the Curie−Weiss law, while the dashed line (c) is an extrapolation of the Curie−Weiss law. (d) Experimental (red dots) and theoretical (blue dots) magnetic moment of the RE(BH4)x as a function of the number of 4f electrons. The experimental values of RE = La, Sm, and Lu could not be determined and thus were set to zero. Note that RE = Sm, Eu, and Yb are divalent RE(BH4)2, while the remaining are trivalent RE(BH4)3.

unphysically large negative Weiss temperature, suggesting strong antiferromagnetic interactions, and in conflict with a strong gradual upturn in the magnetic susceptibility at low temperatures, which suggests a gradual onset of ferromagnetic order. This is consistent with observations for Cs2NaYbCl6 and Sr2YbMO6 (M = Nb, Ta, Sb).61,62 Below 100 K, there are deviations from Curie−Weiss behavior, e.g. as observed for Nd(BH4)3, see Figure 13c. A decrease in the 1/χ data suggests a gradually decreasing magnetic moment at low temperatures. By fitting the data to a model with temperature-dependent magnetic moment, we show that this behavior is consistent with crystal field effects (see Figures S22, S23, S25, S29, and S30), suggesting a strong hybridization between H 1s and RE 4f. Such hybridization has previously been investigated by theoretical calculations in the predicted structures of the ternary hydrides YbBeH4 and Cs3YbH6.63 The ligand field from the surrounding BH4− ligands (octahedral configuration, CN(H) = 12) introduces a subtle splitting that only becomes relevant for very low temperatures, which is well-described by the model for temperature-dependent magnetic moment, described in reference 61. Further details are provided in the Supporting Information. The experimentally and theoretically determined magnetic moments of the RE(BH4)x are illustrated in Figure 13d and show a good correlation between the two. The theoretical magnetic moments were calculated using the Landé formula, see eqs 6 and 7: μ = gJ J(J + 1) μB

gJ =

S(S + 1) − L(L + 1) 3 + 2 2J(J + 1)

(7)

The magnetic moments are in the range from 1.57 μB for Yb(BH4)2 to 10.0 μB for Ho(BH4)3, disregarding those which do not show paramagnetic behavior. For the compounds showing Curie−Weiss behavior to low temperatures, the Weiss temperatures are found to be in the range −6.8 to −11.2 K, suggesting weak antiferromagnetic interactions. The compounds that show unphysically large negative Weiss temperature in the range −31 to −339 K, i.e. RE(BH4)x (RE = Ce, Pr, Nd, Eu, Tm, Yb), generally show magnetic moments larger than those anticipated from the full spin and orbit contribution, consistent with observations in Sr2YbMO6 (M = Nb, Ta, Sb).62 An overview of the magnetic data is provided in Table S7.

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CONCLUSIONS A new synthesis method is provided for the rare-earth metal borohydrides, resulting in pure products for the full series of rare-earth metal borohydrides RE(BH4)3 and their solvates RE(BH4)3·S(CH3)2 (RE = La, Ce, Pr, Nd, Gd, Tb, Dy, Ho, Er, Tm, Lu). RE(BH4)2 are obtained from the metals with a stable oxidation state + II (RE = Sm, Eu, Yb). Rare-earth metal hydrides are formed directly from the elements and reacted with DMS·BH3, resulting in pure, halide-free products, allowing for a detailed systematic investigation of crystal structures and thermal and magnetic properties without the influence from halide-containing compounds.

(6) L

DOI: 10.1021/acs.inorgchem.8b03258 Inorg. Chem. XXXX, XXX, XXX−XXX

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Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

This research provides 15 new compounds, including Lu(BH4)3, crystallizing in the α- and β-RE(BH4)3 structure; new polymorphs of α-Ce(BH4)3, β2-Nd(BH4)3, and mEu(BH4)2; and new solvate complexes, RE(BH4)3·nS(CH3)2, RE = La, Ce, Pr, Tb, Dy, Ho, Er, Tm, Lu. This work reveals a linear correlation between the volume per formula unit (V/Z) and the volume of the RE3+ ion, V(RE3+), for the α- and βRE(BH4)3. A similar correlation is also observed for the solvate complexes α-RE(BH4)3·S(CH3)2. In situ synchrotron powder X-ray diffraction reveals that amorphous decomposition products are formed by thermolysis of RE(BH4)x. Thermal analysis, TG-DSC-MS, shows that RE(BH4)3 generally decompose in a single endothermic step, releasing hydrogen. However, Lu(BH4)3 deviates, decomposing in three distinct endothermic events. Sievert’s measurements reveal that the decomposed material from the RE(BH4)3 does not reversibly absorb H2 at the physical conditions used here. The RE(BH4)2 decompose in mainly one endothermic step, releasing hydrogen. The decomposition temperature of RE(BH4)3 is within the temperature range T = 247−277 °C, and an increase in the decomposition temperature is observed as a function of the Pauling electronegativity, which is in contrast to the empirical correlation typically observed for the metal borohydrides.50−53 The RE(BH4)2 decompose at higher temperatures, T = 306−390 °C, and no clear trend in the decomposition temperature is observed. The magnetic properties were investigated for the RE(BH4)x. The majority of the compounds follow Curie−Weiss paramagnetic behavior with weak antiferromagnetic interaction and expected magnetic moments in agreement with isolated 4f ions. However, Gd(BH4)3 shows a weak antiferromagnetic ordering with a Neel temperature of 4.5 K, being the first example of superexchange through a borohydride group. Additionally, RE(BH4)x, RE = Ce, Pr, Nd, Eu, Tm, Yb, display a temperature dependent magnetic moment below 100 K, induced by subtle crystal field effects. Further theoretical modeling could provide more information about the temperature-dependent magnetic moment, but that is beyond the scope of this paper. This study describes a versatile and interesting class of materials and enables the option to utilize polymorphic selectivity and halide-free RE(BH4)x in materials design for, e.g., ion conductors, magnetic, photo luminescent, or hydrogen storage materials. Additionally, this paper may serve as an encyclopedia in which all relevant information regarding synthesis, structural relations and thermal and magnetic properties of the halide-free rare-earth metal borohydrides can be found.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Jakob B. Grinderslev: 0000-0001-7645-1383 Kasper T. Møller: 0000-0002-1970-6703 Martin Bremholm: 0000-0003-3634-7412 Torben R. Jensen: 0000-0002-4278-3221 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The work was supported by the Danish National Research Foundation, Center for Materials Crystallography (DNRF93), the Danish Research Council for Nature and Universe (DanScatt), the Danish Council for independent research, technology and production (4181-00462), the Carlsberg Foundation, CALIPSOplus (Grant 730872) from the EU Framework programme for research and innovation HORIZON 2020 and NordForsk via the project Functional Hydrides - FunHy. The authors would like to thank Diamond Light Source for access to beamline I11, the local contacts Stephen Thompson and Sarah Day for assistance with data collection, the Deutsches Elektronen-Synchrotron (DESY) for access to beamline P02.1 at Petra III, a member of the Helmholtz Association (HGF), the local contact Jo-Chi Tseng for assistance during the experiment, the Swiss-Norwegian beamline BM01 at the European Synchrotron Radiation Facilities (ESRF), and the local contact Vadim Dyadkin for assistance during the experiment.

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REFERENCES

(1) Paskevicius, M.; Jepsen, L. H.; Schouwink, P.; Č erný, R.; Ravnsbæk, D. B.; Filinchuk, Y.; Dornheim, M.; Besenbacher, F.; Jensen, T. R. Metal Borohydrides and Derivatives - Synthesis, Structure and Properties. Chem. Soc. Rev. 2017, 46, 1565−1634. (2) Møller, K. T.; Jensen, T. R.; Akiba, E.; Li, H. Hydrogen - A Sustainable Energy Carrier. Prog. Nat. Sci. 2017, 27, 34−40. (3) Sheppard, D. A.; Paskevicius, M.; Humphries, T. D.; Felderhoff, M.; Capurso, G.; von Colbe, J. B.; Dornheim, M.; Klassen, T.; Ward, P. A.; Teprovich, J. A.; Corgnale, C.; Zidan, R.; Grant, D. M.; Buckley, C. E. Metal Hydrides for Concentrating Solar Thermal Power Energy Storage. Appl. Phys. A: Mater. Sci. Process. 2016, 122, 395. (4) Møller, K. T.; Sheppard, D.; Ravnsbæk, D. B.; Buckley, C. E.; Akiba, E.; Li, H.-W.; Jensen, T. R. Complex Metal Hydrides for Hydrogen, Thermal and Electrochemical Energy Storage. Energies 2017, 10, 1645. (5) Marks, S.; Heck, J. G.; Habicht, M. H.; Oña-Burgos, P.; Feldmann, C.; Roesky, P. W. [Ln(BH4)2(THF)2] (Ln = Eu, Yb)A Highly Luminescent Material. Synthesis, Properties, Reactivity, and NMR Studies. J. Am. Chem. Soc. 2012, 134, 16983−16986. (6) Schouwink, P.; Ley, M. B.; Tissot, A.; Hagemann, H.; Jensen, T. R.; Smrčok, L.; Č erný, R. Structure and Properties of Complex Hydride Perovskite Materials. Nat. Commun. 2014, 5, 5. (7) Schouwink, P.; Didelot, E.; Lee, Y.-S.; Mazet, T.; Č erný, R. Structural and Magnetocaloric Properties of Novel Gadolinium Borohydrides. J. Alloys Compd. 2016, 664, 378−384. (8) Visseaux, M.; Bonnet, F. Borohydride Complexes of Rare Earths, and Their Applications in Various Organic Transformations. Coord. Chem. Rev. 2011, 255, 374−420.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b03258. Crystallographic data, thermal analysis data, and magnetic susceptibility data (PDF) Accession Codes

CCDC 1873860−1873873 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, by emailing [email protected], or by contacting The M

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Inorganic Chemistry (9) Fadlallah, S.; Terrier, M.; Jones, C.; Roussel, P.; Bonnet, F.; Visseaux, M. Mixed Allyl-Borohydride Lanthanide Complexes: Synthesis of Ln(BH4)2(C3H5)(THF)3 (Ln = Nd, Sm), Characterization, and Reactivity toward Polymerization. Organometallics 2016, 35, 456−461. (10) Bünzli, J.-C. G.; Pecharsky, V. K. Handbook on the Physics and Chemistry of Rare Earths; Newnes, 2013. (11) Olsen, J. E.; Frommen, C.; Jensen, T. R.; Riktor, M. D.; Sørby, M. H.; Hauback, B. C. Structure and Thermal Properties of Composites with RE−Borohydrides (RE = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Er, Yb or Lu) and LiBH4. RSC Adv. 2014, 4, 1570−1582. (12) Olsen, J. E.; Frommen, C.; Sørby, M. H.; Hauback, B. C. Crystal Structures and Properties of Solvent-Free LiYb(BH4)4‑xClx, Yb(BH4)3 and Yb(BH4)2‑xClx. RSC Adv. 2013, 3, 10764−10774. (13) Wegner, W.; Jaroń, T.; Grochala, W. Preparation of a Series of Lanthanide Borohydrides and Their Thermal Decomposition to Refractory Lanthanide Borides. J. Alloys Compd. 2018, 744, 57−63. (14) Frommen, C.; Sørby, M. H.; Heere, M.; Humphries, T. D.; Olsen, J. E.; Hauback, B. C. Rare Earth Borohydrides - Crystal Structures and Thermal Properties. Energies 2017, 10, 2115. (15) Ley, M. B.; Jørgensen, M.; Č erný, R.; Filinchuk, Y.; Jensen, T. R. From M(BH4)3 (M = La, Ce) Borohydride Frameworks to Controllable Synthesis of Porous Hydrides and Ion Conductors. Inorg. Chem. 2016, 55, 9748−9756. (16) Ley, M. B.; Paskevicius, M.; Schouwink, P.; Richter, B.; Sheppard, D. A.; Buckley, C. E.; Jensen, T. R. Novel Solvates M(BH4)3S(CH3)2 and Properties of Halide-Free M(BH4)3 (M = Y or Gd). Dalton Trans. 2014, 43, 13333−13342. (17) Humphries, T. D.; Ley, M. B.; Frommen, C.; Munroe, K.; Jensen, T. R.; Hauback, B. C. Crystal Structure and in Situ Decomposition of Eu(BH4)2 and Sm(BH4)2. J. Mater. Chem. A 2015, 3, 691−698. (18) Heere, M.; GharibDoust, S. H. P.; Frommen, C.; Humphries, T. D.; Ley, M. B.; Sørby, M. H.; Jensen, T. R.; Hauback, B. C. The Influence of LiH on the Rehydrogenation Behavior of Halide Free Rare Earth (RE) Borohydrides (RE = Pr, Er). Phys. Chem. Chem. Phys. 2016, 18, 24387−24395. (19) GharibDoust, S. P.; Heere, M.; Sørby, M. H.; Ley, M. B.; Ravnsbæk, D. B.; Hauback, B. C.; Č erný, R.; Jensen, T. R. Synthesis, Structure and Properties of New Bimetallic Sodium and Potassium Lanthanum Borohydrides. Dalton Trans. 2016, 45, 19002−19011. (20) Richter, B.; Grinderslev, J. B.; Møller, K. T.; Paskevicius, M.; Jensen, T. R. From Metal Hydrides to Metal Borohydrides. Inorg. Chem. 2018, 57, 10768−10780. (21) Dyadkin, V.; Pattison, P.; Dmitriev, V.; Chernyshov, D. A New Multipurpose Diffractometer PILATUS@SNBL. J. Synchrotron Radiat. 2016, 23, 825−829. (22) Hammersley, A. P. FIT2D: An Introduction and Overview. ESRF Internal Report, ESRF97HA02T, 1997. (23) Hansen, B. R. S.; Møller, K. T.; Paskevicius, M.; Dippel, A.-C.; Walter, P.; Webb, C. J.; Pistidda, C.; Bergemann, N.; Dornheim, M.; Klassen, T.; Jørgensen, J.-E.; Jensen, T. R. In Situ X-Ray Diffraction Environments for High-Pressure Reactions. J. Appl. Crystallogr. 2015, 48, 1234−1241. (24) Rodriguez-Carvajal, J. Fullprof Suite; LLB Sacley & LCSIM Rennes: France, 2003. (25) Sato, T.; Miwa, K.; Nakamori, Y.; Ohoyama, K.; Li, H.-W.; Noritake, T.; Aoki, M.; Towata, S.; Orimo, S. Experimental and Computational Studies on Solvent-Free Rare-Earth Metal Borohydrides R(BH4)3 (R = Y, Dy, and Gd). Phys. Rev. B: Condens. Matter Mater. Phys. 2008, 77, 104114. (26) Lee, Y.-S.; Shim, J.-H.; Cho, Y. W. Polymorphism and Thermodynamics of Y(BH4)3 from First Principles. J. Phys. Chem. C 2010, 114, 12833−12837. (27) Sharma, M.; Didelot, E.; Spyratou, A.; Lawson Daku, L. M.; Č erný, R.; Hagemann, H. Halide Free M(BH4)2 (M = Sr, Ba, and Eu) Synthesis, Structure, and Decomposition. Inorg. Chem. 2016, 55, 7090−7097.

(28) Filinchuk, Y.; Rönnebro, E.; Chandra, D. Crystal Structures and Phase Transformations in Ca(BH4)2. Acta Mater. 2009, 57, 732−738. (29) Noritake, T.; Aoki, M.; Matsumoto, M.; Miwa, K.; Towata, S.; Li, H.-W.; Orimo, S. Crystal Structure and Charge Density Analysis of Ca(BH4)2. J. Alloys Compd. 2010, 491, 57−62. (30) Favre-Nicolin, V.; Č erný, R. FOX, free Objects for ̀ Crystallography’: A Modular Approach to Ab Initio Structure Determination from Powder Diffraction. J. Appl. Crystallogr. 2002, 35, 734−743. (31) Lee, Y.-W.; Clemens, B. M.; Gross, K. J. Novel Sievert’s Type Volumetric Measurements of Hydrogen Storage Properties for Very Small Sample Quantities. J. Alloys Compd. 2008, 452, 410−413. (32) Fukai, Y. The Metal-Hydrogen System. Springer series in Materials Science; Springer-Verlag: Berlin Heidelberg, 1993. (33) Jones, C. J. D- and f-Block Chemistry. Tutorial Chemistry Texts (Book 4), 1st ed.; Royal Society of Chemistry, 2002. (34) Møller, K. T.; Jørgensen, M.; Fogh, A. S.; Jensen, T. R. Perovskite Alkali Metal Samarium Borohydrides: Crystal Structures and Thermal Decomposition. Dalton Trans. 2017, 46, 11905−11912. (35) Frommen, C.; Aliouane, N.; Deledda, S.; Fonneløp, J. E.; Grove, H.; Lieutenant, K.; Llamas-Jansa, I.; Sartori, S.; Sørby, M. H.; Hauback, B. C. Crystal Structure, Polymorphism, and Thermal Properties of Yttrium Borohydride Y(BH4)3. J. Alloys Compd. 2010, 496, 710−716. (36) Č erný, R.; Schouwink, P. The Crystal Chemistry of Inorganic Metal Borohydrides and Their Relation to Metal Oxides. Acta Crystallogr., Sect. B: Struct. Sci., Cryst. Eng. Mater. 2015, B71, 619−640. (37) Lide, D. R. CRC Handbook of Chemistry and Physics, 89th ed.; CRC Press, 2008. (38) D’Anna, V.; Spyratou, A.; Sharma, M.; Hagemann, H. FT-IR Spectra of Inorganic Borohydrides. Spectrochim. Acta, Part A 2014, 128, 902−906. (39) D’Anna, V.; Lawson Daku, L. M.; Hagemann, H. Quantitative Spectra-Structure Relations for Borohydrides. J. Phys. Chem. C 2015, 119, 21868−21874. (40) Jaroń, T.; Grochala, W. Y(BD4)3, an Efficient Store of Deuterium, and Impact of Isotope Effects on Its Thermal Decomposition. J. Nucl. Mater. 2012, 420, 307−313. (41) GharibDoust, S. P.; Heere, M.; Nervi, C.; Sørby, M. H.; Hauback, B. C.; Jensen, T. R. Synthesis, Structure, and Polymorphic Transitions of Praseodymium(III) and Neodymium(III) Borohydride, Pr(BH4)3 and Nd(BH4)3. Dalton Trans. 2018, 47, 8307−8319. (42) Gennari, F. C.; Esquivel, M. R. Synthesis and Dehydriding Process of Crystalline Ce(BH4)3. J. Alloys Compd. 2009, 485, L47− L51. (43) Jaroń, T.; Koźmiński, W.; Grochala, W. Phase Transition Induced Improvement in H2 Desorption Kinetics: The Case of the High-Temperature Form of Y(BH4)3. Phys. Chem. Chem. Phys. 2011, 13, 8847−8851. (44) Ravnsbæk, D. B.; Nickels, E. A.; Č erný, R.; Olesen, C. H.; David, W. I. F.; Edwards, P. P.; Filinchuk, Y.; Jensen, T. R. Novel Alkali Earth Borohydride Sr(BH4)2 and Borohydride-Chloride Sr(BH4)Cl. Inorg. Chem. 2013, 52, 10877−10885. (45) Miwa, K.; Aoki, M.; Noritake, T.; Ohba, N.; Nakamori, Y.; Towata, S.; Züttel, A.; Orimo, S. Thermodynamical Stability of Calcium Borohydride Ca(BH4)2. Phys. Rev. B: Condens. Matter Mater. Phys. 2006, 74, 155122. (46) Zavorotynska, O.; El-Kharbachi, A.; Deledda, S.; Hauback, B. C. Recent Progress in Magnesium Borohydride Mg(BH 4)2 : Fundamentals and Applications for Energy Storage. Int. J. Hydrogen Energy 2016, 41, 14387−14403. (47) Rönnebro, E.; Majzoub, E. H. Calcium Borohydride for Hydrogen Storage: Catalysis and Reversibility. J. Phys. Chem. B 2007, 111, 12045−12047. (48) Paskevicius, M.; Ley, M. B.; Sheppard, D. A.; Jensen, T. R.; Buckley, C. E. Eutectic Melting in Metal Borohydrides. Phys. Chem. Chem. Phys. 2013, 15, 19774−19789. N

DOI: 10.1021/acs.inorgchem.8b03258 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry (49) Richter, B.; Ravnsbæk, D. B.; Tumanov, N.; Filinchuk, Y.; Jensen, T. R. Manganese Borohydride; Synthesis and Characterization. Dalton Trans. 2015, 44, 3988−3996. (50) Schrauzer, G. N. Ü ber ein Periodensystem der Metallboranate. Naturwissenschaften 1955, 42, 438−438. (51) Nakamori, Y.; Miwa, K.; Ninomiya, A.; Li, H.; Ohba, N.; Towata, S.; Züttel, A.; Orimo, S. Correlation between Thermodynamical Stabilities of Metal Borohydrides and Cation Electronegativites: First-Principles Calculations and Experiments. Phys. Rev. B: Condens. Matter Mater. Phys. 2006, 74, 045126. (52) Züttel, A.; Borgschulte, A.; Orimo, S.-I. Tetrahydroborates as New Hydrogen Storage Materials. Scr. Mater. 2007, 56, 823−828. (53) Li, H.-W.; Orimo, S.; Nakamori, Y.; Miwa, K.; Ohba, N.; Towata, S.; Züttel, A. Materials Designing of Metal Borohydrides: Viewpoints from Thermodynamical Stabilities. J. Alloys Compd. 2007, 446−447, 315−318. (54) Ravnsbæk, D.; Filinchuk, Y.; Cerenius, Y.; Jakobsen, H. J.; Besenbacher, F.; Skibsted, J.; Jensen, T. R. A Series of Mixed-Metal Borohydrides. Angew. Chem., Int. Ed. 2009, 48, 6659−6663. (55) Ravnsbæk, D. B.; Sørensen, L. H.; Filinchuk, Y.; Besenbacher, F.; Jensen, T. R. Screening of Metal Borohydrides by Mechanochemistry and Diffraction. Angew. Chem., Int. Ed. 2012, 51, 3582−3586. (56) Heere, M.; GharibDoust, S. H. P.; Brighi, M.; Frommen, C.; Sørby, M. H.; Č erný, R.; Jensen, T. R.; Hauback, B. C. Hydrogen Sorption in Erbium Borohydride Composite Mixtures with LiBH4 and/or LiH. Inorganics 2017, 5, 31. (57) Frommen, C.; Heere, M.; Riktor, M. D.; Sørby, M. H.; Hauback, B. C. Hydrogen Storage Properties of Rare Earth (RE) Borohydrides (RE = La, Er) in Composite Mixtures with LiBH4 and LiH. J. Alloys Compd. 2015, 645, 155−159. (58) Cotton, S. A. Scandium, Yttrium & the Lanthanides: Inorganic & Coordination Chemistry. In Encycl. Inorg. Bioinorg. Chem.; John Wiley & Sons, 2011. (59) Colwell, J. H.; Mangum, B. W.; Utton, D. B. Low-Temperature Magnetic Properties of Some Hexagonal Rare-Earth Trihalides. Phys. Rev. 1969, 181, 842−854. (60) Varsanyi, F.; Andres, K.; Marezio, M. Canted Antiferromagnetism in Gadolinium Tribromide. J. Chem. Phys. 1969, 50, 5027− 5030. (61) Karraker, D. G. Magnetic Susceptibility of Yb3+ in an Octahedral Environment. J. Chem. Phys. 1971, 55, 1084−1086. (62) Coomer, F. C.; Campbell, J.; Giordano, N.; Collins, O. M.; Cussen, E. J. Structural and Magnetic Study of Yb3+ in the Perovskites Sr2YbMO6 (M = Nb, Ta, Sb). J. Solid State Chem. 2015, 221, 411− 417. (63) Jaroń, T.; Grochala, W.; Hoffmann, R. Towards Superconductivity in Hydrides: Computational Studies of Two Hypothetical Ternary Compounds, YbIIBeH4 and Cs3YbIIIH6. J. Mol. Model. 2007, 13, 769−774.

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DOI: 10.1021/acs.inorgchem.8b03258 Inorg. Chem. XXXX, XXX, XXX−XXX