Unraveling Hidden Mg–Mn–H Phase Relations at High Pressures and

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

pubs.acs.org/IC

Unraveling Hidden Mg−Mn−H Phase Relations at High Pressures and Temperatures by in Situ Synchrotron Diffraction Kristina Spektor,*,† Wilson A. Crichton,† Sumit Konar,‡ Stanislav Filippov,§ Johan Klarbring,§ Sergei I. Simak,§ and Ulrich Haü ssermann*,∥ †

ESRF, The European Synchrotron Radiation Facility, F-38000 Grenoble, France EaStChem School of Chemistry and Centre for Science at Extreme Conditions (CSEC), University of Edinburgh, Edinburgh EH9 3FJ, United Kingdom § Theoretical Physics Division, Department of Physics, Chemistry and Biology (IFM), Linköping University, SE-581 83, Linköping, Sweden ∥ Department of Materials and Environmental Chemistry, Stockholm University, SE-10691 Stockholm, Sweden ‡

S Supporting Information *

ABSTRACT: The Mg−Mn−H system was investigated by in situ high pressure studies of reaction mixtures MgH2−Mn−H2. The formation conditions of two complex hydrides with composition Mg3MnH7 were established. Previously known hexagonal Mg3MnH7 (h-Mg3MnH7) formed at pressures 1.5−2 GPa and temperatures between 480 and 500 °C, whereas an orthorhombic form (o-Mg3MnH7) was obtained at pressures above 5 GPa and temperatures above 600 °C. The crystal structures of the polymorphs feature octahedral [Mn(I)H6]5− complexes and interstitial H−. Interstitial H− is located in trigonal bipyramidal and square pyramidal interstices formed by Mg2+ ions in h- and o-Mg3MnH7, respectively. The hexagonal form can be retained at ambient pressure, whereas the orthorhombic form upon decompression undergoes a distortion to monoclinic Mg3MnH7 (m-Mg3MnH7). The structure elucidation of o- and m-Mg3MnH7 was aided by firstprinciples density functional theory (DFT) calculations. Calculated enthalpy versus pressure relations predict m- and oMg3MnH7 to be more stable than h-Mg3MnH7 above 4.3 GPa. Phonon calculations revealed o-Mg3MnH7 to be dynamically unstable at pressures below 5 GPa, which explains its phase transition to m-Mg3MnH7 on decompression. The electronic structure of the quenchable polymorphs h- and m-Mg3MnH7 is very similar. The stable 18-electron complex [MnH6]5− is mirrored in the occupied states, and calculated band gaps are around 1.5 eV. The study underlines the significance of in situ investigations for mapping reaction conditions and understanding phase relations for hydrogen-rich complex transition metal hydrides.

■

INTRODUCTION Complex transition metal hydrides (CTMHs) are a peculiar class of solid state compounds which consist of negatively charged homoleptic hydrido complexes [THn]m− and active metal (alkali, alkaline earth, rare earth) cations.1−3 Occasionally, CTMHs also contain interstitial H which is exclusively coordinated by metal cations. Typically the transition metal T is from groups 7−10. The complex anions [THn]m−, in which hydridic H is covalently bonded to T, exhibit a perplexing range of coordination numbers and geometries, which rival those of carbonyl complexes.4,5 An interesting feature is the occurrence of low formal oxidation states of T with a ligand H− that does not afford the conventional “π-back-donation” mechanism (as for, e.g., CO).6 CTMHs are conventionally prepared from reactive sintering of mixtures of transition metal and active metal hydrides in a hydrogen atmosphere employing autoclave techniques.1−3 It has been long believed that group 7 represents a sharp © XXXX American Chemical Society

boundary between complex hydride forming late T and interstitial/ionic hydride forming early T. This boundary was attributed to the fact that early T species are less electronegative and thus would favor interstitial bonding scenarios with H, leading to metallic hydrides as opposed to semiconducting complex hydrides.1 However, recently it has been shown that the application of high pressures in the gigapascal (GPa) rangerange affords CTMHs also for group 6 and the heavier group 5 metals Nb and Ta, yielding Mg3CrH8 with pentagonal bipyramidal [CrH7]5− and Li5TH11 (T = Mo, W) and Li6TH11 (T = Nb, Ta) with tricapped trigonal prismatic complexes [TH9]3−/4−.7,8 These results not only demonstrate the shift of the previously assumed CTMH boundary, but also emphasize the capability of the H− ligand of realizing high coordination numbers and the important role of high pressures Received: November 21, 2017

A

DOI: 10.1021/acs.inorgchem.7b02968 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

this investigation only phases of Mg3MnH7 stoichiometry were observed. In particular, we report the formation conditions of hexagonal Mg3 MnH7 and, in addition, show that the monoclinic phase described by Blomqvist et al.16 actually forms upon decompression from a closely related orthorhombic polymorph. The orthorhombic phase, in turn, represents a high pressure form of hexagonal Mg3MnH7. The study underlines the significance of in situ analysis for unraveling the complex high pressure relations in CTMH systems. These are preferably performed with large volume high pressure devices (as opposed to diamond anvil cells), because of the superior control of the p, T reaction environment.

for studying the interplay of compositions, structure, and chemical bonding in hydrogen-rich materials.9 The importance of gigapascal pressures can also be recognized in the CTMH chemistry of the lightest group 7 element Mn. Whereas a conventional autoclave technique afforded K3MnH5, Rb3MnH5, and Cs3MnH5 containing tetrahedral [Mn(II)H4]2− complexes,10,11 Mg3MnH7 with a considerably higher hydrogen-to-metal ratio was obtained from high pressure synthesis by simply sintering MgH2 with Mn at 2 GPa and 800 °C.12 Mg3MnH7 crystallizes with the hexagonal Mg3ReH7 structure type13 from which a number of other Mg3THx CTMH structures (e.g., Mg3RuH6, Mg6Co2H11, and Mg6Ir2H11) can be derived.14 The structure of Mg3MnH7 is shown in Figure 1. It consists of homoleptic octahedral

■

EXPERIMENTAL METHODS

High Pressure in Situ Investigations. The samples were prepared in a glovebox under argon atmosphere using powdered MgH2 (Sigma-Aldrich, 97%) and powdered Mn metal (Sigma-Aldrich ≥99.9% trace metals basis). The components were carefully mixed at a molar ratio of 2:1 (MgH2:Mn) and compressed into pellets of 2 mm outer diameter (o.d.) and 1.5 mm height. Ammonia borane (BH3NH3, Sigma-Aldrich, 97%) was utilized as a hydrogen source, as its decomposition behavior at high p, T has been well-established in earlier studies.18 The amount of BH3NH3 used for each sample provided approximately 2.5 times molar excess of H2 during the experiment with respect to manganese. A MgH2/Mn sample pellet was sandwiched in a NaCl capsule (3.0 mm o.d., 3.8 mm height) between two pellets of BH3NH3. For the high pressure experiments a 14/8 multianvil assembly was employed. Each sample capsule was surrounded by a BN sleeve and inserted into a 14 mm OEL Cr2O3-doped MgO octahedral pressure medium, along with a carbon foil resistance furnace and ZrO2 plugs. The cross-section of the octahedral assembly is shown in Figure 2a. MgO octahedra were positioned between eight truncated 25 mm tungsten carbide cubes with 8 mm TEL equipped with pyrophyllite gaskets. Along the beam direction 2 mm o.d. cylindrical SiBCN X-ray windows and ∼4 mm wide MgO rectangles were inserted into the octahedra and gaskets, respectively (Figure 2b). The assemblies were subsequently compressed at a rate of 0.04 GPa/min and heated in a

Figure 1. Crystal structure of hexagonal Mg3MnH7 (Mg3ReH7 structure type). Octahedral [Mn(I)H6]5− complexes and the trigonal byramidal coordination environment of interstitial H− ions are highlighted as purple and green polyhedra, respectively. Mg2+ ions are depicted as gray circles.

complexes [Mn(I)H6]5− that are surrounded by a distorted cubic environment of Mg2+ counterions. These cubes, oriented along their body diagonal, are arranged into layers parallel to the hexagonal plane by sharing common edges. Such layers are connected via common corners along the hexagonal axis direction to yield a 3-D framework. Between layers, at the height of the shared corners, interstitial H− is located in trigonal bipyramidal cavities provided by Mg2+ ions. The interstitial H− species balance the charge from Mg 2+ , according to (Mg2+)3[Mn(I)H6]5− (H−). Later Blomqvist et al. reported on another, monoclinic, Mg−Mn−H compound, which was obtained from reactions of MgH2 and Mn at 6 GPa and 600 °C.15,16 This compound was tentatively assigned a composition Mg3MnH6. However, its H content and (phase) relation to hexagonal Mg3MnH7 are unknown. In this work we re-examine the Mg−Mn−H system at high pressures in the presence of hydrogen fluid. The original intention was to investigate whether Mg2Mn(II)H6 possessing octahedral d5 hydrido complexes can be obtained (recall the tetrahedral d5 [Mn(II)H4]2− complexes in A3MnH5 (A = K, Rb, Cs)11). In analogy with Mg2Fe(II)H6,17 it was anticipated that hypothetical Mg2MnH6 would crystallize with the K2PtCl6 structure. Mg2FeH6 features low spin d6 hydrido complexes which obey the 18-electron rule. Hypothetical Mg2MnH6 may display interesting magnetic properties, similar to those of antiferromagnetic A3MnH5.11 However, during the course of

Figure 2. High pressure assembly used for in situ hydrogenations at ESRF. (a) Cross-section of a MgO/Cr2O3 octahedron used as pressure medium. (b) Arrangement of 4 truncated WC cubes, gaskets with MgO windows around octahedron with respect to direction of beam (schematic drawing). B

DOI: 10.1021/acs.inorgchem.7b02968 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry Voggenreiter modified-cubic press located at the beamline ID06-LVP, ESRF.19 The heating was performed at a rate of 20 °C/min at T < 200 °C while being decreased to 5 °C/min at T > 500 °C. The temperatures were dwelled each time the release of hydrogen from BH3NH3 was expected or the growth of Mg−Mn−H materials was detected. Pressure was estimated in situ from powder X-ray diffraction (PXRD) patterns using NaCl equation of state.20 Temperature was evaluated both from the NaCl EOS and by employing type D thermocouples. Angle-dispersive PXRD patterns were continuously collected at a constant wavelength (λ = 0.22542 Å) selected with Si111 doublecrystal monochromator, from the emission of a U18 insertion device at ∼6 mm magnetic gap. During compression and decompression a pattern was typically saved each 32 s, while during the heating the data acquisition rate was 3.2 s/pattern. A Detection Technology X-Scan series1 linear pixelated detector was used for data acquisition in a 2θ range of 2.16−10.64°. LaB6-SRM660a (NIST) was used for calibrating sample-to-detector distance and the detector offset. The data was integrated and manipulated using Fit2D software.21,22 Le Bail and Rietveld refinement of the patterns obtained in situ at p, T was performed using Jana2006.23 Further details of the analysis as well as results are listed in the Supporting Information, Table S2. The product synthesized during an experiment at 5 GPa was recovered inside the glovebox as a sintered dark gray piece with large areas of bright orange inclusions. About a half of the product was sealed as a single piece inside a 2 mm glass capillary and subsequently used for the ex situ characterization. Ex Situ Powder X-ray Diffraction. A PXRD measurement of the recovered product was performed at the beamline ID15B, ESRF at a constant wavelength of 0.410884 Å. The beam was focused on the sample to a spot of 30 × 30 μm2, and the resulting pattern was acquired with a flat-panel Mar555 detector. The resulting 2-D pattern was integrated using Fit2D software.21 The analysis of PXRD patterns was performed using Jana2006.23 Further details and results of the analysis are given in the Supporting Information, Table S2. Theoretical Calculations. DFT enthalpy calculations were performed with the Vienna ab initio simulation package (VASP)24,25 in the framework of the all electron projector augmented wave (PAW) method26,27 and the generalized gradient approximation (GGA) in the form according to Perdew−Burke−Ernzerhof (PBE).28,29 The cutoff energy for the plane waves was set to 600 eV, and the cutoff energy for the plane wave representation of the augmentation charges was set to 800 eV. Structural relaxations were performed until the forces on all ions were less than 10−4 eV/Å. For the Brillouin zone integration a 8 × 8 × 8 Monkhorst−Pack k-point grid was used.30 Phonon dispersion relations at 0 K and various pressures were obtained using the small displacement method31,32 as implemented in Phonopy.33 These calculations were based on 2 × 2 × 2 supercells and a 4 × 4 × 4 Monkhorst−Pack k-point grid30 and used the same cutoff energies as for the enthalpy calculations. All the atomic positions in the supercell structures were relaxed before the phonon calculations.

internal hydrogen source for hydrogenations at gigapascal pressures.36,37 NaCl is a popular capsule material since it provides tight seals in the case of air/moisture sensitive precursors and resists hydrogen diffusion. For in situ diffraction investigations there is an additional benefit because the wellknown EOS of NaCl20 can be used for internal p, T calibration and the assessment of p, T conditions during experiments. As previously mentioned, our investigation of the Mg−Mn− H system initially targeted Mg2MnH6. Thus, we employed reaction mixtures with a molar metal ratio MgH2/Mn = 2:1. The molar ratio Mn/BH3NH3 was typically 1:0.8. Figure 3

Figure 3. Compilation of in situ diffractograms showing the formation of hexagonal Mg3MnH7 from MnH and MgH2 at 1.6 GPa and its transition to orthorhombic Mg3MnH7 at 5.35 GPa and 550 °C.

shows in situ diffraction patterns of such a reaction mixture which had been compressed to 2.3 GPa and subsequently heated. At 2.3 GPa BH3NH3 is expected to undergo its second decomposition step around 200 °C.18 During 10 min dwell at 200 °C, changes in the NaCl lattice parameter suggested the occurrence of a pressure drop to 1.6 GPa which we associated with H2 release from ammonia borane. Concomitant with the complete decomposition of BH3NH3, at around 200 °C, the formation and growth of hcp-MnH was observed. Upon further heating, hcp-MnH transformed into fcc-MnH at ∼450 °C, which is in agreement with the Mn−H phase diagram established by Antonov et al.38 At slightly higher temperatures, at 475 °C, the growth of hexagonal Mg3MnH7 (h-Mg3MnH7) was observed. After dwelling for 1 h at 480 °C the temperature was increased to 520 °C and kept constant for an additional hour. During this time diffraction patterns did not change noticeably. To conclude, with respect to the original synthesis of hMg3MnH7, from sintering of mixtures of MgH2 and Mn with a molar ratio 3:1 at 2 GPa and 800 °C,12 we observed its

■

RESULTS AND DISCUSSION Formation of Hexagonal and Observation of Orthorhombic Mg3MnH7 at High Pressure, High Temperature Conditions. We examined the Mg−Mn−H system at high pressures in the presence of hydrogen fluid. When using large volume devices hydrogen is most conveniently introduced via an internal source. Ideally this source releases hydrogen rapidly and irreversibly at slightly elevated temperatures (preferably below 300 °C), with the decomposition residual being inert toward the precursors to be hydrogenated, as well as the resulting hydride. Hitherto employed sources such as NaBH4, LiAlH4, or AlH37,34,35 all have drawbacks in this respect. We found that ammonia borane (BH3NH3), with its high hydrogen capacity and well-defined two-step decomposition behavior under pressure (BH3NH3 → (BH2NH2) + H2; (BH2NH2) → BN + 2H2) leading to inert residuals,18 represents a superior C

DOI: 10.1021/acs.inorgchem.7b02968 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry formation from MgH2 and MnH at considerably lower temperatures and also at somewhat lower pressures. Afterward the pressure was increased to 5.35 GPa (at a rate of 1 GPa/h) while keeping the power to the resistance furnace constant, during which the temperature gradually decreased to about 465 °C. At 2.7 GPa, fcc-MnH transformed back to hcpMnH. Again, this is in agreement with the Mn−H phase diagram, showing a positive slope of the p−T boundary between hcp- and fcc-MnH.39 At 5.35 GPa the temperature was increased at a rate of 5 °C/min. A change in the diffraction patterns was noticed at around 520 °C by the appearance of additional reflections. At 550 °C, and after 3 min, diffraction lines of h-Mg3MnH7 had disappeared completely. The new reflections could be indexed as a single phase in a C-centered orthorhombic unit cell and indicated the formation of a new Mg−Mn−H phase. In the following this phase is called oMg3MnH7. The volume decrease per formula unit (ΔV (Å3/Z) at the transformation was estimated as ∼6.4%. Direct Synthesis of Orthorhombic Mg3MnH7 at High Pressure, High Temperature Conditions and Recovery of Monoclinic Mg3MnH7. The next step aimed at the direct synthesis of the new, orthorhombic, Mg−Mn−H phase. A reaction mixture was compressed to 5.35 GPa at a rate of 2.5 GPa/h and subsequently heated. Figure 4a shows in situ diffraction patterns. At 5.35 GPa, BH3NH3 is expected to undergo its second decomposition step at around 250 °C.18 In this experiment, the hydrogen release occurred slightly above 260 °C which was noticed by a rapid shift of all diffraction lines in the pattern to higher 2θ angles (not shown). After this shift, diffraction lines displaced back to their original positions over a period of several minutes. A sizable pressure drop, as observed in the previous experiment described above, did not seem to occur. Upon further heating, the direct formation of o-Mg3MnH7 from MgH2 and MnH was observed at around 640 °C. In order to accelerate the growth of the phase and to achieve a complete reaction, the temperature was increased to 760 °C and subsequently held at these conditions for a period of 1 h before being cooled to 400 °C at a rate of 36 °C/min and quenched to room temperature (RT) by instantly turning off the power to the furnace. The pressure after the quench was 3.8 GPa. Prior to, or in parallel with, the growth of o-Mg3MnH7 a number of other processes took place. At around 250 °C the crystallization of MgH2 into its high pressure γ polymoprh (αPbO2 type)40 became visible, and above 360 °C, the formation of hcp-MnH was noted. The phase transition between the hcp and dhcp forms of MnH occurred rapidly at 700 °C, which is in good agreement with the Mn−H phase diagram obtained by Fukai et al.39 The reverse dhcp−hcp transformation was observed upon rapid cooling below 630 °C. During the decompression from 3.8 GPa (at an aggregate rate of ∼0.3 GPa/h) a rapid pressure drop from ∼3.1 to 0.5 GPa occurred. The sample, however, stayed intact. The pressure loss, without any mechanical failure, likely happened due to the release of unreacted hydrogen. Figure 4b shows the diffraction patterns upon pressure release and also the diffraction pattern of the recovered sample. It is clear that the latter shows a monoclinic splitting of the previously orthorhombic pattern. This splitting may already be recognizable in the patterns after the pressure drop. The diffraction peak of Mg3MnH7 at around 3.6° 2θ in the pressurized sample appeared as single and would be indexed as (130) for o-

Figure 4. Compilation of in situ diffractograms showing the formation of orthorhombic Mg3MnH7 from MnH and MgH2 at 5.3 GPa and 640 °C (a) and its recovery as monoclinic Mg3MnH7 at ambient pressure (b).

Mg3MnH7. In a monoclinically distorted structure, this peak splits into the pair (−102)/(101). Indeed, the powder pattern of the recovered sample matches the structure of the monoclinic Mg−Mn−H phase with space group P21/m reported by Blomqvist et al.16 (See Supporting Information on details of the analysis of the PXRD pattern of the recovered sample.) Because o-Mg3MnH7 was obtained from the transformation of h-Mg3MnH7 at high pressure, high temperature conditions (cf., Figure 3) and monoclinic Mg−Mn−H was obtained from the transformation of o-Mg3MnH7 upon pressure release (Figure 4b), it was assumed that the composition of the monoclinic phase is also Mg3MnH7. In the following this phase is denoted as m-Mg3MnH7. Relation between h- and o-/m-Mg3MnH7 and Their Structural Stability. A closer inspection of the monoclinic Blomqvist et al. structure (i.e., the arrangement of metal atoms)16 revealed that its monoclinic cell can be transformed into a closely related C-centered orthorhombic one. Actual D

DOI: 10.1021/acs.inorgchem.7b02968 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

structure is very similar (Figure 5b). With respect to the orthorhombic unit cell, the major differences are a slight tilt of the octahedra along the b-direction and a slight displacement of the Mg1 and Mg2 and interstitial H− ions in the ac-plane. The equilibrium volume of o-Mg3MnH7 is slightly smaller, by about 1%, than that of m-Mg3MnH7. Also, the latter is about 6.5% denser than h-Mg3MnH7, which is in good agreement with our in situ volume change estimation at the transformation. Mn−H coordination in h-Mg 3 MnH 7 is nearly regular octahedral with interatomic distances at 1.63 Å (cf. Table S1). MnH6 octahedra are increasingly distorted when going from o- to m-Mg3MnH7. Mn−H distances in both polymorphs range from 1.61 to 1.68 Å (cf. Table S1). In h-Mg3MnH7 interstitial H− possesses a trigonal bipyramidal environment of Mg ions with H−Mg distances of 1.88 Å (2×) and 2.69 Å (3×). The former distance is slightly shorter than in rutile-type MgH2 (1.95 Å (3×)), whereas the latter is substantially larger. In oand m-Mg3MnH7 interstitial H− possesses a square pyramidal environment of Mg ions. H−Mg distances in the orthorhombic structure are 1.91 Å (1×), 2.37 Å (2×), and 2.38 Å (2×), whereas they are 1.90 Å (1×), 2.05 Å (1×), 2.39 Å (2×), and 2.76 Å (1×) in the monoclinic case, in which the square pyramidal environment is significantly distorted, see Figure 6. Therefore, one may consider the coordination of interstitial H− in m-Mg3MnH7 as rather 4-fold. Figure 7 shows the DFT calculated enthalpy versus pressure relations for the Mg3MnH7 polymorphs. In very good agreement with experiment, o-Mg3MnH7 becomes more stable than h-Mg3MnH7 above 4.3 GPa. At pressures above 5 GPa, the monoclinic and orthorhombic structures attain virtually the same enthalpy. In fact, above 5 GPa the DFT-relaxed monoclinic structure can be considered merged into the orthorhombic one. At ambient pressure there is a slight enthalpy difference between the orthorhombic and the monoclinic structure, the latter being more stable by about 0.01 eV/Z. Again, this agrees with the experimentally observed monoclinic structure at ambient pressure. Figure 8 shows the phonon density of states (pDOS) for orthorhombic Mg3MnH7 and its evolution with pressure. The orthorhombic structure is dynamically unstable at ambient pressure, as seen in the contributions to the pDOS that are associated with imaginary frequency phonons. A more detailed analysis revealed that there are two important phonon modes with imaginary frequencies. These involve displacements of Mg1 and Mg2 atoms (as well as of interstitial H2) in the orthorhombic x- and z-directions, corresponding to the directions of the monoclinic distortion. With pressure, the imaginary phonons stabilize such that their contribution in the pDOS has essentially disappeared at 5.3 GPa. Thus, one can understand the phase relations between Mg3MnH7 polymorphs as follows: h-Mg3MnH7 transforms to o-Mg3MnH7 at pressures around 5 GPa. This transformation requires heating above 480 °C (cf. Figure 3). o-Mg3MnH7 can also be synthesized directly from MnH and MgH2 at pressures higher than 5 GPa for which higher temperatures are required compared to the h-to-o transition (640 °C, cf. Figure 4). o-Mg3MnH7 is not recoverable at ambient p, T. Upon decompression this phase becomes dynamically unstable and a transition to m-Mg3MnH7 occurs. Since the vibrational properties of Mg3MnH7 are not known, we briefly comment on the phonon calculations. Typically, the vibrational properties of CTMHs are rather easy to interpret:45 internal modes associated with vibrations of the complex anion,

deviations from orthorhombic symmetry, i.e., space group Cmcm, are mainly caused by the two Mg atoms Mg1 and Mg2. The relation between these two structures is detailed in Table 1. Shifting metal atoms into a higher symmetry Cmcm Table 1. Relation between the Monoclinic Blomqvist Model and an Orthorhombic C-Centered Structurea structure space group, Z lattice params, Å

V/Z (Å3 pfu) Mn Mg1 Mg2 Mg3

monoclinic (Blomqvist et al.16) P21/m, 2 a = 4.676 b = 4.657 c = 8.797 β = 105.03° 92.51 2e 0.6589 0.25 2e 0.1040 0.25 2e 0.3900 0.25 2e 0.2720 0.25

orthorhombic Cmcm, 4 a = 4.676 b = 16.9925 c = 4.657

0.2960 0.1490 0.855 0.5010

92.51 4c 0.0 0.6480 0.25 4c 0 0.0745 0.25 4c 0 0.4275 0.25 4c 0 0.2505 0.25

a The monoclinic lattice is modified by transformation matrix ⎛ 1 0 0⎞ ⎜ 1 0 2 ⎟, with origin shift 1/2 1/2 1/2. The description of the ⎝− 1 0 1 ⎠ Cmcm structure in the most similar atom arrangement of the P21/m structure was obtained by applying Euclidean normalizers as implemented in the STRUCTURE RELATIONS program at the Bilbao Crystallographic Server.41−43 Atoms Mg1 and Mg2 show highest displacements from orthorhombic symmetry (0.0295 and −0.0375 along the x-direction, respectively).

arrangement facilitated the construction of a model structure for which H atoms were placed octahedrally around Mn (thus retaining the octahedral complex motif of the h-Mg3MnH7). This resulted in a layered arrangement of octahedral complexes, which suggested the presence of additional, interstitial H−, located in square pyramidal interstices formed by Mg2+ ions. This model structure followed the overall building principle of h-Mg3MnH7 in which layers of octahedral complexes alternate with layers that host interstitial H− (cf. Figure 1) and was then also easily adapted to the original monoclinic metal arrangement. Subsequent DFT relaxation of the orthorhombic and monoclinic model structures produced very reasonable unit cell volumes and interatomic distances. Figure 5 shows the structural relation of o- and m-Mg3MnH7. Table 2 lists the structure parameters, referring to the DFT-relaxed (zero pressure) equilibrium volume. Interatomic distances are given as Supporting Information, Table S1. DFT relaxations44 included h-Mg3MnH7 as well, for which the experimental structure12 could be very well reproduced. The orthorhombic structure (Figure 5a) consists of 2-layer blocks with composition “Mg2MnH6” in which Mg2+ ions and [MnH6]5− octahedra are arranged as in the cubic K2PtCl6 structure type (space group Fm3m ̅ ). The K2PtCl6 structure corresponds to an (anti-)CaF2-type arrangement of octahedral complexes and metal cations. The latter are arranged in a cubic manner around the complex whose ligands point toward a cube’s face centers, and cubes containing octahedra share edges. The boundaries of K2PtCl6 type 2-layer building blocks are planar, square, nets of Mg2+ ions. The Cmcm structure of oMg3MnH7 results when such blocks are mutually shifted by 1/4 1/4 0 (with respect to the cubic K2PtCl6 type unit cell) so that Mg ions of neighboring boundary nets are mutually located above and below centers of square faces, thus creating a layer of pyramidal interstice which host interstitial H−. The monoclinic E

DOI: 10.1021/acs.inorgchem.7b02968 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

Figure 5. (a) Crystal structure of orthorhombic Mg3MnH7. Octahedral [Mn(I)H6]5− complexes and the square pyramidal coordination environment of interstitial H− ions are highlighted as purple and green polyhedra, respectively. Mg2+ ions are depicted as gray circles. Right: Build up of the oMg3MnH7 structure from 2-layer Mg2MnH6 (K2PtCl6 type) building blocks. Mn atoms are drawn as purple circles; one MnH6 polyhedron is depicted to show the orientation of complexes within cubes of Mg2+ ions. Interstitial H− ions are displayed as green circles. (b) Crystal structure of monoclinic Mg3MnH7 (left) and its close relation to o-Mg3MnH7 (right).

Lastly we compare the electronic density of states (DOS) of quenchable h- and m-Mg3MnH7, calculated at their equilibrium volume corresponding to ambient pressure (Figure 9). The electronic structure of h-Mg3MnH7 has been analyzed previously,47 and our results agree very well with the earlier investigation. The lower part of the DOS (up to −2.5 eV) constitutes 7 bands per formula unit. These bands have predominantly H character and relate to the bonding a1g, eg, and t1u type states of the octahedral [Mn(I)H6]5− complex. In addition, interstitial H forms a band, which has a major contribution to the states at the top of this part of the DOS, at around −2.5 eV. The second part of the DOS represents occupied states that are associated with Mn 3d (t2g) states. The t2g valence band is separated from the conduction band by a gap of about 1.5 eV. The monoclinic form has a somewhat

i.e., T−H stretching and bending, are clearly separated from the external (lattice) modes, viz. libration and translation, arising from the crystal structure. For Mg3MnH7 with octahedral MnH6 moieties, stretching and bending are in the regions 1550−1850 and 600−1000 cm−1, respectively, which are similar to the data for Mg2FeH6.46 In addition, the pDOS shown in Figure 8 reveals a sharp and isolated band at around 1200 cm−1, which is identified as a vibrational mode of interstitial H−, displacing toward the apex of the square pyramidal Mg2+ environment, i.e., along the shortest Mg−H distance (cf. Figure 6). In contrast, the two other modes of interstitial H, corresponding to displacements perpendicular to the short Mg−H distance within the square plane of the pyramid, are highly dispersed and at considerably lower wavenumbers. F

DOI: 10.1021/acs.inorgchem.7b02968 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

Table 2. Structural Parameters of h-, o-, and m-Mg3MnH7 from DFT Optimization Compared to the Experimental Data Mg3MnH7

hexagonal, Bortz et al.12

hexagonal, DFT

orthorhombic, DFT

space group Z lattice params (Å)

P63/mmc 2 a = 4.7099 c = 10.2861

P63/mmc 2 a = 4.6515 c = 10.2726

Cmcm 4 a = 4.5850 b = 16.9728 c = 4.5935

V/Z (Å3 pfu) Mn Mg1 Mg2 Mg3 H1 H2 H3 H4 H5

98.81 2a 0 0 0 2b 0 0 0.25 4f 1/3 2/3 0.0678

96.24 2a 0 0 0 2b 0 0 0.25 4f 1/3 2/3 0.0663

12k 0.1622, 2x, 0.5930 2c 1/3 2/3 0.25

12k 0.1629, 2x, 0.5929 2c 1/3 2/3 0.25

89.37 4c 0 0.6487 0.25 4c 0 0.0738 0.25 4c 0 0.4244 0.25 4c 0 0.2493 0.25 4c 0−0.0390 0.25 16h 0.2518 0.1556 0.0033 4c 0 0.5501 0.25 4c 0 0.7433 0.25

monoclinic, DFT

monoclinic, expt

P21/m 2 a = 4.6374 b = 4.5921 c = 8.7678 β = 104.6° 90.34 2e 0.6529 0.25 0.2969 2e 0.0830 0.25 0.1490 2e 0.3870 0.25 0.8498 2e 0.2545 0.25 0.4998 4f 0.0890 0.9940 0.6795 4f 0.5907 0.9999 0.7001 2e 0.0028 0.25 0.9250 2e 0.5966 0.25 0.1005 2e 0.7227 0.25 0.4858

P21/m 2 a = 4.6709 b = 4.6568 c = 8.7992 β = 105.21° 92.35 2e 0.6560 0.25 2e 0.0650 0.25 2e 0.3880 0.25 2e 0.2830 0.25

0.2910 0.1453 0.8459 0.4950

Figure 6. Coordination environments of interstitial H− in o-Mg3MnH7 (left) and m-Mg3MnH7 (right). H− and Mg2+ ions are depicted as green and gray circles, respectively. Inserted numbers refer to H−Mg (red) and Mg−Mg (black) interatomic distances (in Å).

Figure 8. Phonon density of states (pDOS) of orthorhombic Mg3MnH7 at 0, 5.3, and 10 GPa. The pDOS is partitioned into atomic contributions of H atoms attached to Mn (H1), interstitial H (H2), Mg, and Mn.

■

CONCLUSIONS The formation conditions of two complex hydrides with composition Mg3MnH7 were established from investigating reactions MnH + MgH2 by in situ high pressure diffraction experiments. The previously known hexagonal Mg3MnH7 formed at pressures 1.5−2 GPa and temperatures between 480 and 500 °C, whereas a new orthorhombic form was obtained at pressures > 5 GPa and T > 600 °C. The crystal structures of the polymorphs feature octahedral [Mn(I)H6]5− complexes and interstitial H−. Interstitial H− is located in trigonal bipyramidal and square pyramidal interstices formed by Mg2+ ions in the hexagonal and orthorhombic forms, respectively. The hexagonal form can be retained at ambient pressure whereas upon decompression or pressure quenching the orthorhombic form undergoes a second-order-like trans-

Figure 7. Computed enthalpy vs pressure relations between h-, o-, and m-Mg3MnH7. The first is taken as reference. Enthalpy differences are per formula unit.

broader t2g band but displays a band gap with virtually the same size. The calculated band gaps are certainly underestimated, as usually observed in DFT calulations using the PBE functional. However, this underestimation appears to be rather moderate since the similar, or same, reddish color of these compounds indicates an actual band gap around 2 eV. Thus, the stable 18electron complex [MnH6]5− is mirrored in the occupied states of the DOS; an additional two electrons per formula unit are hosted in the band derived from interstitial H−. G

DOI: 10.1021/acs.inorgchem.7b02968 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

for the help with measurements at beamline ID15B, and Harald Müller (ESRF) for assistance with the Chemistry Laboratory facilities at ESRF. S.I.S. acknowledges support from the Swedish Government Strategic Research Area Grant in Materials Science on Functional Materials at Linkö ping University (Faculty Grant SFO-Mat-LiU No. 2009 00971). S.F. acknowledges the financial support from Carl Tryggers Stiftelse (CTS) för Vetenskaplig Forskning through Grant 16:198. The computations were performed on resources provided by the Swedish National Infrastructure for Computing (SNIC) at High Performance Computing Center North (HPC2N).

■

Figure 9. Electronic density of states of hexagonal and monoclinic Mg3MnH7 at their equilibrium volumes corresponding to ambient pressure. EF stands for the highest occupied state.

formation in distorting to monoclinic Mg3MnH7. Both the orthorhombic and monoclinic forms of Mg3MnH7 are about 6.5% denser than the hexagonal form. Up to pressures of 5.5 GPa, o-Mg3MnH7 appears to be the most hydrogen-rich phase (i.e., the phase with highest hydrogen-to-metal ratio) in the Mg−Mn−H system. It would be interesting to extend this investigation to higher pressures to see whether more hydrogen-rich Mg2Mn(II)H6 with octahedral d5 hydrido complexes can be obtained. Hypothetical Mg2MnH6 may display remarkable magnetic properties, similar to antiferromagnetic A3MnH5 (A = K, Rb, Cs).11

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b02968. Compilation of interatomic distances for the DFTrelaxed equilibrium structures of h-, o-, and m-Mg3MnH7 at their theoretical equilibrium volumes; Le Bail/Rietveld fits to the ex situ and in situ PXRD patterns; 2-D diffraction pattern of recovered monoclinic Mg3MnH7; and table of crystallographic results containing values for the h-Mg3MnH7 observed in situ at p, T and the recovered m-Mg3MnH7 and o-Mg3MnH7 structure derived from it (PDF)

■

REFERENCES

(1) Yvon, K.; Renaudin, G. Solid State Transition Metal Complexes. In Encyclopedia of Inorganic Chemistry, 2nd ed.; King, R. B., Ed.; Wiley: Chichester, West Sussex, 2005; Vol. 3, pp 1814−1846. (2) Yvon, K. Complex Transition-Metal Hydrides. Chim. Int. J. Chem. 1998, 52, 613−619. (3) Bronger, W. Synthesis and Structure of New Metal Hydrides. J. Alloys Compd. 1995, 229, 1−9. (4) Firman, T. K.; Landis, C. R. Structure and Electron Counting in Ternary Transition Metal Hydrides. J. Am. Chem. Soc. 1998, 120, 12650−12656. (5) King, R. B. Structure and Bonding in Homoleptic Transition Metal Hydride Anions. Coord. Chem. Rev. 2000, 200 (SupplementC), 813−829. (6) Frenking, G.; Fröhlich, N. The Nature of the Bonding in Transition-Metal Compounds. Chem. Rev. 2000, 100, 717−774. (7) Takagi, S.; Iijima, Y.; Sato, T.; Saitoh, H.; Ikeda, K.; Otomo, T.; Miwa, K.; Ikeshoji, T.; Aoki, K.; Orimo, S. True Boundary for the Formation of Homoleptic Transition-Metal Hydride Complexes. Angew. Chem., Int. Ed. 2015, 54, 5650−5653. (8) Takagi, S.; Iijima, Y.; Sato, T.; Saitoh, H.; Ikeda, K.; Otomo, T.; Miwa, K.; Ikeshoji, T.; Orimo, S. Formation of Novel Transition Metal Hydride Complexes with Ninefold Hydrogen Coordination. Sci. Rep. 2017, 7, 44253. (9) Takagi, S.; Orimo, S. Recent Progress in Hydrogen-Rich Materials from the Perspective of Bonding Flexibility of Hydrogen. Scr. Mater. 2015, 109, 1−5. (10) Bronger, W.; Hasenberg, S.; Auffermann, G. K3MnH5, Das Erste Salzartige Manganhydrid. Z. Anorg. Allg. Chem. 1996, 622, 1145−1149. (11) Bronger, W.; Hasenberg, S.; Auffermann, G. Alkali-Metal Manganese Hydrides: Synthesis, Structure and Magnetic Properties. J. Alloys Compd. 1997, 257, 75−81. (12) Bortz, M.; Betheville, B.; Yvon, K.; Movlaev, E. A.; Verbetsky, V. N.; Fauth, F. Mg3MnH7, Containing the First Known Hexahydridomanganese (I) Complex. J. Alloys Compd. 1998, 279, L8−L10. (13) Huang, B.; Yvon, K.; Fischer, P. Trimagnesium Rhenium(I) Heptahydride, Mg3ReH7, Containing Octahedral [ReH6]5− Complex Anions. J. Alloys Compd. 1993, 197, 97−99. (14) Kohlmann, H. Structural Relationships in Complex Hydrides of the Late Transition Metals. Z. Kristallogr. 2009, 224, 454−460. (15) Kyoi, D.; Rönnebro, E.; Blomqvist, H.; Chen, J.; Kitamura, N.; Sakai, T.; Nagai, H. Novel Magnesium-Manganese Hydrides Prepared by the Gigapascal High Pressure Technique. Mater. Trans. 2002, 43, 1124−1126. (16) Blomqvist, H.; Rönnebro, E.; Kyoi, D.; Sakai, T.; Noréus, D. Structural Characterization of Mg3MnH6  a New High Pressure Phase Synthesized in a Multi-Anvil Cell at 6 GPa. J. Alloys Compd. 2003, 358, 82−86. (17) Didisheim, J. J.; Zolliker, P.; Yvon, K.; Fischer, P.; Schefer, J.; Gubelmann, M.; Williams, A. F. Dimagnesium iron(II) Hydride, Mg2FeH6, Containing Octahedral FeH64− Anions. Inorg. Chem. 1984, 23, 1953−1957. (18) Nylén, J.; Sato, T.; Soignard, E.; Yarger, J. L.; Stoyanov, E.; Häussermann, U. Thermal Decomposition of Ammonia Borane at High Pressures. J. Chem. Phys. 2009, 131, 104506.

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Kristina Spektor: 0000-0002-3267-9797 Ulrich Häussermann: 0000-0003-2001-4410 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was supported by the Swedish Research Council (VR) through Grants 2013-4690 and 2014-4750. The ESRF is thanked for allocating the beamtime CH-4899 at beamline ID06LVP. K.S. would like to thank Michael Hanfland (ESRF) H

DOI: 10.1021/acs.inorgchem.7b02968 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

(41) Aroyo, M. I.; Perez-Mato, J. M.; Orobengoa, D.; Tasci, E.; De La Flor, G.; Kirov, A. Crystallography Online: Bilbao Crystallographic Server. Bulg. Chem. Commun. 2011, 43, 183−97. (42) Aroyo, M. I.; Perez-Mato, J. M.; Capillas, C.; Kroumova, E.; Ivantchev, S.; Madariaga, G.; Kirov, A.; Wondratschek, H. Bilbao Crystallographic Server I: Databases and crystallographic computing programs. Z. Kristallogr. - Cryst. Mater. 2006, 221, 15−27. (43) Aroyo, M. I.; Kirov, A.; Capillas, C.; Perez-Mato, J. M.; Wondratschek, H. Bilbao Crystallographic Server. II. Representations of Crystallographic Point Groups and Space Groups. Acta Crystallogr., Sect. A: Found. Crystallogr. 2006, 62, 115−128. (44) Further details for DFT-optimized structures may be obtained from FIZ Karlsruhe, 76344 Eggenstein-Leopoldshafen, Germany (fax: (+49)7247−808−666; e-mail: crysdata@fiz-karlsruhe.de) on quoting the deposition numbers 433711, 433712, and 433713 for h-, m-, and o-Mg3MnH7, respectively. (45) Parker, S. F. Spectroscopy and Bonding in Ternary Metal Hydride complexesPotential Hydrogen Storage Media. Coord. Chem. Rev. 2010, 254, 215−234. (46) Parker, S. F.; Williams, K. P. J.; Bortz, M.; Yvon, K. Inelastic Neutron Scattering, Infrared, and Raman Spectroscopic Studies of Mg2FeH6 and Mg2FeD6. Inorg. Chem. 1997, 36, 5218−5221. (47) Gupta, M.; Singh, D. J.; Gupta, R. Origin of the 20-Electron Structure of Mg3MnH7: Density Functional Calculations. Phys. Rev. B: Condens. Matter Mater. Phys. 2005, 71, 092107.

(19) Guignard, J.; Crichton, W. A. The Large Volume Press Facility at ID06 Beamline of the European Synchrotron Radiation Facility as a High Pressure-High Temperature Deformation Apparatus. Rev. Sci. Instrum. 2015, 86, 085112. (20) Birch, F. Equation of State and Thermodynamic Parameters of NaCl to 300 Kbar in the High-Temperature Domain. J. Geophys. Res. 1986, 91, 4949−4954. (21) Hammersley, A. P.; Svensson, S. O.; Hanfland, M.; Fitch, A. N.; Hausermann, D. Two-Dimensional Detector Software: From Real Detector to Idealised Image or Two-Theta Scan. High Pressure Res. 1996, 14, 235−248. (22) Hammersley, A. P.; Svensson, S. O.; Thompson, A. Calibration and Correction of Spatial Distortions in 2D Detector Systems. Nucl. Instrum. Methods Phys. Res., Sect. A 1994, 346, 312−321. (23) Petříček, V.; Dušek, M.; Palatinus, L. Crystallographic Computing System JANA2006: General Features. Z. Kristallogr. Cryst. Mater. 2014, 229, 345−352. (24) Kresse, G.; Hafner, J. Ab Initio Molecular-Dynamics Simulation of the Liquid−Metal−Amorphous−Semiconductor Transition in Germanium. Phys. Rev. B: Condens. Matter Mater. Phys. 1993, 47, 558−561. (25) Kresse, G.; Furthmüller, J. Efficient Iterative Schemes for Ab Initio Total-Energy Calculations Using a Plane-Wave Basis Set. Phys. Rev. B: Condens. Matter Mater. Phys. 1996, 54, 11169. (26) Blöchl, P. E. Projector Augmented-Wave Method. Phys. Rev. B: Condens. Matter Mater. Phys. 1994, 50, 17953−17979. (27) Kresse, G.; Joubert, D. From Ultrasoft Pseudopotentials to the Projector Augmented-Wave Method. Phys. Rev. B: Condens. Matter Mater. Phys. 1999, 59, 1758−1775. (28) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple [Phys. Rev. Lett. 77, 3865 (1996)]. Phys. Rev. Lett. 1997, 78, 1396−1396. (29) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77, 3865−3868. (30) Monkhorst, H. J.; Pack, J. D. Special Points for Brillouin-Zone Integrations. Phys. Rev. B Solid State 1976, 13, 5188−5192. (31) Kresse, G.; Furthmüller, J.; Hafner, J. Ab Initio Force Constant Approach to Phonon Dispersion Relations of Diamond and Graphite. EPL Europhys. Lett. 1995, 32, 729−734. (32) Parlinski, K.; Li, Z. Q.; Kawazoe, Y. First-Principles Determination of the Soft Mode in Cubic ZrO2. Phys. Rev. Lett. 1997, 78, 4063−4066. (33) Togo, A.; Tanaka, I. First Principles Phonon Calculations in Materials Science. Scr. Mater. 2015, 108, 1−5. (34) Fukai, Y.; O̅ kuma, N. Evidence of Copious Vacancy Formation in Ni and Pd under a High Hydrogen Pressure. Jpn. J. Appl. Phys. 1993, 32, L1256−L1259. (35) Saitoh, H.; Takagi, S.; Matsuo, M.; Iijima, Y.; Endo, N.; Aoki, K.; Orimo, S. Li4FeH6: Iron-Containing Complex Hydride with High Gravimetric Hydrogen Density. APL Mater. 2014, 2, 076103. (36) Puhakainen, K.; Stoyanov, E.; Evans, M. J.; Leinenweber, K.; Häussermann, U. Synthesis of Li2PtH6 Using High Pressure: Completion of the Homologous Series A2PtH6 (A = Alkali Metal). J. Solid State Chem. 2010, 183, 1785−1789. (37) Puhakainen, K.; Benson, D.; Nylén, J.; Konar, S.; Stoyanov, E.; Leinenweber, K.; Häussermann, U. Hypervalent Octahedral SiH62− Species from High-Pressure Synthesis. Angew. Chem., Int. Ed. 2012, 51, 3156−3160. (38) Antonov, V. E.; Antonova, T. E.; Chirin, N. A.; Ponyatovsky, E. G.; Baier, M.; Wagner, F. E. T-P Phase Diagram of the Mn-H System at Pressures to 4.4 GPa and Temperatures to 1000 °C. Scr. Mater. 1996, 34, 1331−1336. (39) Fukai, Y.; Haraguchi, T.; Shinomiya, H.; Mori, K. Constitution of the Mn−H System at High Hydrogen Pressures. Scr. Mater. 2002, 46, 679−684. (40) Bortz, M.; Bertheville, B.; Böttger, G.; Yvon, K. Structure of the High Pressure Phase γ-MgH2 by Neutron Powder Diffraction. J. Alloys Compd. 1999, 287, L4−L6. I

DOI: 10.1021/acs.inorgchem.7b02968 Inorg. Chem. XXXX, XXX, XXX−XXX