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J. Phys. Chem. C 2010, 114, 19127–19133

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AZn2(BH4)5 (A ) Li, Na) and NaZn(BH4)3: Structural Studies ˇ erny´,*,† Ki Chul Kim,‡ Nicolas Penin,†,§ Vincenza D’Anna,| Hans Hagemann,| and Radovan C David S. Sholl*,‡ Laboratory of Crystallography and Department of Physical Chemistry, UniVersity of GeneVa, 1211 GeneVa, Switzerland, and School of Chemical and Biomolecular Engineering, Georgia Institute of Technology, 311 Ferst DriVe, Atlanta, Georgia 30332-0100, United States ReceiVed: June 28, 2010; ReVised Manuscript ReceiVed: September 21, 2010

A combination of in situ synchrotron powder diffraction, energy minimization (DFT), and Raman and infrared spectroscopy confirmed porous interpenetrated 3D-framework structures of recently discovered alkalimetal-zinc borohydrides, AZn2(BH4)5 (A ) Li, Na). In the less zinc rich NaZn(BH4)3 the 3D-framework structural model has been confirmed but with a slightly modified description giving an isolated triangular anion, [Zn(BH4)3]-, rather than a 1D anionic chain, {[Zn(BH4)3]n}n-. Another polymorph of NaZn(BH4)3, isostructural to a new compound, LiZn(BH4)3, is proposed by energy minimization. Both compounds, the new NaZn(BH4)3 polymorph and LiZn(BH4)3, are, however, not observed experimentally at ambient pressure and in the temperature range of 100-400 K. The alkali-metal-zinc borohydride NaZn(BH4)3 containing the triangular anion [Zn(BH4)3]- is an equivalent of recently characterized alkali-metal-scandium borohydrides NaSc(BH4)4 and LiSc(BH4)4 based on the tetrahedral [Sc(BH4)4]- complex anion. 2MBH4 + ZnCl2 f Zn(BH4)2 + 2MCl

Introduction Metal borohydrides are of interest for hydrogen storage in mobile applications.1,2 Borohydrides of alkali and alkaline-earth metals desorb a large quantity of hydrogen (up to 20.8% for Be(BH4)2), although the decomposition temperatures are usually high.3 On the other hand, most known borohydrides of transition metals, especially of 3d metals, are unstable.3 The thermal stability of binary metal hydrides has been inversely related to the metal electronegativity (and consequently to the standard redox potential).3 A similar relation was postulated also for borohydrides already a half-century ago,4 based on the stability ratio theory of Sanderson,5 and has been recently analyzed theoretically and experimentally.6,7 Tuning the thermodynamic properties of borohydride-based hydrogen storage materials by the preparation of bimetallic (alkali- or alkaline-earth-metal and transition-metal) borohydrides has been suggested.7 This approach resembles the consideration which led to the discovery of ternary transition-metal hydrides such as LaNi5H6.7 with ideal hydrogen storage thermodynamics (equilibrium hydrogen pressure ∼1 bar at room temperature) as a result of alloying of very stable LaH3 and extremely unstable NiH0.8.8 Preparation of zinc borohydride, Zn(BH4)2, has been frequently reported in the literature. The first report was on the synthesis from binary hydrides in ether solution9 and later on the metathesis reaction in ether solution10 or in mechanosynthesis (ball-milling):11 * To whom correspondence should be addressed. (R.C.) E-mail: [email protected]. Phone: +41 22 379 6450. Fax: +41 22 379 6108. (D.S.S.) E-mail: [email protected]. Phone: +1 404 894 2822. Fax: +1 404 894 2866. † Laboratory of Crystallography, University of Geneva. ‡ Georgia Institute of Technology. § Current address: ICMCB, CNRS, Universite´ de Bordeaux 1, 87 Avenue du Docteur Albert Schweitzer, F-33608 Pessac Cedex, France. | Department of Physical Chemistry, University of Geneva.

M ) Li, Na (1)

The zinc borohydride was reported to be stable up to 85 °C.10 The low decomposition temperature and relatively high hydrogen mass capacity (8.5 wt % hydrogen) were recognized as attractive for hydrogen storage application.12,13,6 However, no structural information on Zn(BH4)2 has become available up to now. On the contrary, already in ref 10 and later on in ref 14 the authors have reported the existence of mixed-cation (Li+ or Na+, Zn2+) and mixed-anion (BH4-, Cl-) compounds. A recent study of alkali-metal-zinc borohydrides15 shows a variety of compositions and structural topologies. It was shown that the compounds LiZn2(BH4)5 and NaZn2(BH4)5 are built of two identical interpenetrated 3D frameworks consisting of isolated complex anions, [Zn2(BH4)5]-, whereas NaZn(BH4)3 consists of a single 3D network containing polymeric anions with the composition [Zn(BH4)3]nn-. We have undertaken studies of the alkali-metal-zinc borohydrides in parallel with the authors of ref 15. Our synchrotron powder diffraction data of lithium-zinc borohydride and sodium-zinc borohydride were interpreted on the basis of LiZn(BH4)3 and NaZn(BH4)3 compounds, respectively. Later on, we were able to explain the diffraction data with published structural models of LiZn2(BH4)5 and NaZn2(BH4)5, respectively.15 Two different structural models, LiZn(BH4)3 and LiZn2(BH4)5, were difficult to distinguish from the X-ray powder diffraction data. The same is true for their Na analogues. The DFT optimization of all four structural models as well as of the correct structural model of NaZn(BH4)315 have been therefore undertaken. Here we report on the DFT-optimized crystal structures, thermal dilatation, and Raman and infrared spectra of alkali-metal-zinc borohydrides. Experimental Section Synthesis. The preparation and manipulation of all samples were performed in an argon-filled glovebox with a circulation

10.1021/jp105957r  2010 American Chemical Society Published on Web 10/20/2010

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TABLE 1: Lattice Parameters of Lithium-Zinc and Sodium-Zinc Borohydrides As Obtained by Rietveld Refinement from Synchrotron Powder Diffraction Data (293 K) and by DFT Optimization (0 K) mixture 3LiBH4/ZnCl2

structural model Li-I Li-II

2NaBH4/ZnCl2

Na-I Na-II

3NaBH4/ZnCl2 a

Na-III

method

a (Å)

b (Å)

c (Å)

β (deg)

V (Å3)

sg

Rwp, RBragg (%), energy (eV/fu)a

chemical formula

SR-PXD DFT SR-PXD DFT SR-PXD DFT SR-PXD DFT SR-PXD DFT

9.909(2) 10.59 8.6120(6) 8.3301 9.425(5) 11.42 9.440(4) 9.4059 8.271(2) 8.54

15.385(3) 14.74 17.857(2) 18.8616 16.574(6) 15.85 16.573(4) 16.4986 4.5266(8) 4.37

8.623(2) 8.66 15.380(2) 15.3453 9.123(4) 8.18 9.110(2) 9.0352 18.761(4) 18.57

115.66(2) 111.056

1184.9(5) 1261.7 2365.1(4) 2411.0 1313(1) 1380.8 1312.0(7) 1306 688.0(2) 688.6

P21/c P21/c Cmca Cmca P21/c P21/c P21/c P21/c P21/c P21/c

12.72, 5.03 -66.191 6.53, 2.25 -108.663 8.87, 1.13 -65.736 5.95, 0.64 -108.681 12.84, 3.48 -65.887

LiZn(BH4)3 LiZn(BH4)3 LiZn2(BH4)5 LiZn2(BH4)5 NaZn(BH4)3 NaZn(BH4)3 NaZn2(BH4)5 NaZn2(BH4)5 NaZn(BH4)3 NaZn(BH4)3

112.85(5) 111.245 112.99(3) 111.35 101.65(2) 97.058

Rwp and RBragg are given in the first row, and energy is given in the second row for each structural model.

purifier (p(O2, H2O) < 0.1 ppm). Anhydrous zinc chloride, ZnCl2 (Sigma-Aldrich, 99.9%), and lithium borohydride, LiBH4 (SigmaAldrich, 98%), or sodium borohydride, NaBH4 (Fluka, 96%), were combined in molar ratios of 1:2 and 1:3 to check the validity of reaction 1 and ball-milled under inert conditions (argon atmosphere) in a Fritsch Pulverisette 7 planetary mill. A 25 mL stainless steel grinding bowl sealed with a lid having a Viton O-ring and three stainless steel balls 15, 12, and 10 mm in diameter were used as the milling medium. The rotational speed of milling was set at 600 rpm. The ball mass to powder mass ratio was fixed to 25:1. The milling was stopped for 5 min (cooling brake) every 10 min to avoid heating of the system as well as agglomeration of the powder on the walls of the grinding bowl; the previous two-step process was repeated 35 times. In Situ Time-Resolved Synchrotron Powder Diffraction. In situ time-resolved synchrotron powder diffraction (SR-PXD) data were collected at the Swiss-Norwegian Beamlines (SNBL) at the European Synchrotron Radiation Facility (ESRF) in Grenoble, France. A glass capillary (o.d. ) 0.8 or 1 mm) with the sample was heated from 80 to 500 K at a rate of 1/3, 1, or 2 K/min, while synchrotron powder diffraction data (PXD) were collected. The temperature was controlled with the Oxford Cryostream 700+. The data were collected using a MAR345 image plate detector at a sample to detector distance of 250 or 400 mm and radiation with wavelength λ ) 0.66863, 0.72846, or 0.77029 Å. The capillary was oscillated by 60° during exposure to the X-ray beam for 60 s, followed by readout for ∼83 s. All obtained raw images were transformed to 2D-powder patterns using the FIT2D16 program. Structure Solution. LiBH4/ZnCl2 Mixtures. The analysis of the SR-PXD data of the 3:1 LiBH4/ZnCl2 mixture has resulted in a monoclinic structural model (P21/c) with the supposed composition LiZn(BH4)3. We will call this structural model LiI. Later on it was found that the orthorhombic structural model (Cmca) of the compound LiZn2(BH4)515 explains well our observed powder diffraction data. We will call this structural model Li-II. The SR-PXD data of the 2:1 LiBH4/ZnCl2 mixture showed the same lithium-zinc borohydride phase as the 3:1 mixture. We will not use these data here. NaBH4/ZnCl2 Mixtures. The analysis of the SR-PXD data of the 2:1 NaBH4/ZnCl2 mixture has resulted in a monoclinic structural model (P21/c) of the same type as Li-I. The composition has been supposed to be NaZn(BH4)3, and we will call the model Na-I. It was again found that the monoclinic structural model (P21/c) of the compound NaZn2(BH4)515 explains well our observed powder diffraction data. We will call this structural

TABLE 2: Essential Interatomic Distances (Å) and Angles (deg) in LiZn2(BH4)5 (Li-II) from SR-PXD (293 K) and Energy Minimization (0 K)a synchrotron powder diffraction

Zn1-B1 Zn1-B2 Zn1-B3

1× 1× 1×

Zn2-B4 Zn2-B3

2× 1×

av av 6× min max av Li-B2 1× Li-B4 2× Li-B1 1× av Li-H 8× min max av B-Zn-B 6× min max av B-Li-B 6× min max av M-B-M 4× min (M ) Zn, Li) max av Zn-H

a

DFT

293 K, ref 15

293 K, this work

0 K, this work

2.108(10) 2.164(10) 2.203(9) 2.16(4) 2.125(8) 2.312(9) 2.19(10) 1.652(15) 2.132(17) 1.81(13) 2.36(3) 2.473(11) 2.89(3) 2.55(23) 1.931(18) 2.91(4) 2.14(33) 110.4(4) 131.9(4) 120(8) 90.9(2) 150.2(3) 110(23) 169.4(4) 179.6(8) 176(4)

2.238(24) 2.259(16) 2.400(20) 2.29(8) 2.209(21) 2.287(20) 2.24(5) 1.63(11) 2.413(66) 1.91(20) 2.066(91) 2.429(46) 2.89(10) 2.45(33) 1.689(76) 2.84(23) 2.06(41) 104.7(9) 141.9(9) 120(16) 81.9(14) 135.2(25) 110(28) 153.2(16) 179.5(14) 167(10)

2.161 2.193 2.486 2.28(17) 2.182 2.286 2.22(6) 1.797 2.506 1.91(19) 2.456 2.460 2.443 2.455(8) 2.070 2.157 2.11(3) 99.1 135.3 120(12) 96.8 131.1 110(15) 156.3 178.1 170(9)

For the lattice parameters and space group see Table 1.

model Na-II. The model has the same lattice metric and space group as Na-I and Li-I but a different chemical composition. The SR-PXD data of the 3:1 NaBH4/ZnCl2 mixture has shown the presence of the monoclinic (P21/c) NaZn(BH4)3 phase.15 We will call it Na-III. The summary of all refined structural models is given in Table 1. Further details about the structure solution and refinement as well as the tables with atomic coordinates of all models are given in the Supporting Information. The selected interatomic distances are given in Tables 2-4. DFT Calculations. All DFT calculations were performed with the PW91 generalized gradient approximation (GGA) functional using the Vienna ab initio simulation package (VASP).17 The core electrons of each atom were described by the projector augmented wave (PAW) method.18 We used a conjugate gradient method for optimization of all materials. A

AZn2(BH4)5 (A ) Li, Na) and NaZn(BH4)3

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TABLE 3: Essential Interatomic Distances (Å) and Angles (deg) in NaZn2(BH4)5 (Na-II) from SR-PXD (293 K) and Energy Minimization (0 K)a synchrotron powder diffraction

Zn1-B3 Zn1-B1 Zn1-B2

1× 1× 1×

Zn2-B5 Zn2-B4 Zn2-B3

1× 1× 1×

Zn-H

6×

Na-B5 Na-B4 Na-B2 Na-B1

1× 1× 1× 1×

Na-H

8×

B-Zn-B

6×

B-Na-B

6×

M-B-M (M ) Zn, Na)

5×

av

a

av min max av

av min max av min max av min max av min max av

DFT

293 K, ref 15

293 K, this work

0 K, this work

2.142(15) 2.237(16) 2.306(18) 2.23(8) 2.054(17) 2.37(2) 2.427(14) 2.28(20)

2.302(41) 2.35(13) 2.65(13) 2.43(18) 2.124(42) 2.482(69) 2.327(49) 2.31(18) 1.64(37) 2.40(12) 2.00(26) 2.761(95) 2.602(99) 2.405(53) 3.04(14) 2.70(27) 1.80(12) 2.87(18) 2.28(39) 113.6(32) 123.6(26) 118.6(45) 76.8(39) 161.1(39) 105(29) 138.1(28) 164.8(71) 151(10)

2.164 2.178 2.263 2.20(5) 2.165 2.179 2.251 2.20(5) 1.814 2.029 1.88(7) 2.729 2.728 2.707 2.726 2.72(1) 2.310 2.407 2.35(3) 109.3 133.9 120(10) 93.7 137.2 110(20) 164.1 177.7 171(5)

2.59(7) 2.60(8) 2.68(5) 3.06(5) 2.73(22)

98.8(6) 133.9(8) 119(14) 82.2(16) 160.0(30) 109(27) 164.5(16) 176.1(12) 170(4)

For the lattice parameters and space group see Table 1.

TABLE 4: Essential Interatomic Distances (Å) and Angles (deg) in NaZn(BH4)3 (Na-III) from SR-PXD (293 K) and Energy Minimization (0 K)a synchrotron powder diffraction

Zn-B2 Zn-B1 Zn-B3 Zn-B3

1× 1× 1× 1×

Zn-H

6×

Na-B3 Na-B1 Na-B2 Na-B2 Na-B2 Na-B1

1× 1× 1× 1× 1× 1×

Na-H

12×

B-Zn-B

6× (3×)

B-Na-B

6×

M-B-M (M ) Zn, Na)

7× (3×)

a

av min max av

293 K, this work

0 K, this work

2.44(7) 2.58(6) 2.90(6) 2.94(6) 2.72(24)

2.263(57) 2.583(24) 2.836(48) 3.528(34) 2.56(28) 1.67(11) 2.55(30) 2.16(31) 2.439(53) 2.776(33) 2.697(57) 3.313(53) 3.590(43) 3.836(45) 3.11(55) 1.82(13) 3.16(23) 2.65(49) 104.2(16) 129.2(12) 115(13)

2.263 2.204 2.176 4.015 2.21(4) 1.825 1.977 1.88(5) 2.766 2.849 3.138 3.350 3.421 3.555 3.18(32) 2.228 3.381 2.70(40) 115.6 123.4 120(4)

124.8(21) 139.4(18) 133(7)

102.5 150.0 132(26)

2.43(6) 2.73(6) 2.76(7) 3.16(6)

av min max av min max av min max av min max av

DFT

293 K, ref 15

2.77(30)

95.6(18) 124.7(18) 109(10) 83.3(18) 154.0(20) 110(26) 102.0(20) 141.0(20) 121(14)

For the lattice parameters and space group see Table 1.

cutoff energy of 425 eV was used in all calculations. Geometries were relaxed until the forces on all atoms were less than 0.03 eV Å-1.

Our DFT calculations optimized the bulk crystal structures of 1 × 1 × 1 unit cells of the five solid compounds listed in Table 1. The initial structural models for these calculations were obtained from the analysis of the SR-PXD data. A Monkhorst-Pack mesh of 4 × 2 × 4 k-points was used for Li-I, Na-I, and Na-II. Monkhorst-Pack meshes of 4 × 2 × 2 and 6 × 12 × 3 k-points were used for Li-II and Na-III. The DFT energies reported in Table 1 do not include zero-point energies. Raman and Infrared Spectroscopy. Raman spectra have been obtained using a Kaiser Holospec monochromator in conjunction with a liquid nitrogen cooled CCD camera. Spectra were excited using a laser wavelength of 488 nm with a typical laser power of 50 mW. The spectral resolution of the Raman spectra is about 3 cm-1. The samples were sealed in melting point capillaries. Infrared (IR) spectra have been measured using a Biorad Excalibur instrument equipped with a Specac low-temperature Golden Gate diamond ATR system. The spectral resolution was set to 1 or 2 cm-1 for the different experiments. Samples were loaded in the glovebox in the ATR system. Results and Discussion General Comments: DFT Optimization vs SR-PXD. Our DFT optimization of experimental structures indicates (see Table 1) that Li-I, Li-II, Na-I, and Na-III have cell volumes 5%, 1.99%, 7.7%, and 0.5% larger than experimentally observed by SR-PXD, respectively. On the other hand, Na-II has a cell volume 0.46% smaller than the experimental value. It is notable that Li-I and Na-I have a relatively large deviation of the DFToptimized cell volume from the experimental value while this deviation is quite small for Li-II, Na-II, and Na-III. This indicates that the borohydride synthesized from the 3:1 LiBH4/ ZnCl2 mixture is correctly matched with the composition LiZn2(BH4)5 and structural model Li-II, in agreement with ref 15. The borohydride synthesized from the 2:1 NaBH4/ZnCl2 mixture is correctly matched with the composition NaZn2(BH4)5 and structural model Na-II, again in agreement with ref 15. Interestingly, the DFT optimization of the wrong Li-I model has changed the tetrahedral coordination of Zn by B closer to the correct triangular coordination as observed in the correct Li-II model. The same is true for Na-I and Na-II. We can compare the stability between two polymorphs of NaZn(BH4)3, Na-I and Na-III, at 0 K. The difference in DFT total energies (without zero-point energies) between two polymorphs is 0.151 eV/fu () 14.6 kJ/mol) in favor of Na-III; see Table 1. In other words, the correct structural model of NaZn(BH4)3, Na-III, obtained from the 2:1 NaBH4/ZnCl2 mixture, is more stable as characterized by these calculations than the structure wrongly determined as Na-I. It is reasonable to assume that including zero-point energies would not alter this conclusion. There is no distinct structural relation between both polymorphs Na-I and Na-III of NaZn(BH4)3. We conclude that the correct structural models are Li-II and Na-II for AZn2(BH4)5 (A ) Li, Na) and Na-III for NaZn(BH4)3. The wrong models Li-I and Na-I models result from erroneously estimated phase composition of AZn(BH4)3 instead of AZn2(BH4)5. The true orthorhombic symmetry has been also overlooked for the Li compound. Crystal Structure: AZn2(BH4)5 (A ) Li, Na). The monoclinic lattice of Na-II (and also of Li-I and Na-I) has an orthorhombic pseudosymmetry which becomes the true symmetry of Li-II. The relation between the monoclinic (m) and pseudoorthorhombic (o) cells is drawn in Figure 1. The compound LiZn2(BH4)5 stays orthorhombic within the temper-

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Figure 1. Relation between the monoclinic cell of the Na-II model and orthorhombic cell of the Li-II model.

ature range investigated (80-395 K); see the Supporting Information, Figures S4-S6. The Li-II and Na-II structures are new inorganic structure types containing two interpenetrated frameworks.15 Each framework contains an alkali-metal cation and the complex anion [Zn2(BH4)5]- built from two triangularly coordinated Zn atoms (see ref 15). Bader charge analysis19 of our DFT-optimized structures provides, for LiZn2(BH4)5, Li0.88+ and [Zn2(BH4)5]0.88and, for NaZn2(BH4)5, Na0.83+ and [Zn2(BH4)5]0.83-. It shows that the bonding within the frameworks in these compounds has a prevailing ionic character. However, as discussed in ref 15, the directional bonding metal-BH4 with covalent character cannot be neglected in these compounds and is certainly at the origin of these porous framework structures. The DFT-optimized average distances Zn-B in LiZn2(BH4)5 of 2.25 Å and the experimental average observed in this work by SR-PXD of 2.26 Å are longer than the average of 2.17 Å observed in ref 15; see Table 2. However, the Zn-B distances stay considerably shorter than in any other metal borohydride, with the exception of Be(BH4)2 and Al(BH4)3, where the metal is also triangularly coordinated. Surprisingly, in NaZn2(BH4)5 the shortest Zn-B distances result from the DFT optimization (2.20 Å) compared to the experimental average observed in this work of 2.37 Å and that observed in ref 15 of 2.24 Å; see Table 3. The DFToptimized structures indicate that the covalent part of the Zn-BH4 bonding is more important in the Na compound than in the Li compound. This conclusion, however, is not confirmed by SR-PXD. Crystal Structure: NaZn(BH4)3. The cell volume of the NaIII model of NaZn(BH4)3 as refined by the Rietveld method agrees very well with the results of DFT optimization, even if the optimized lattice parameters differ by 0.8-3.3% from the experimental value. The structure of NaZn(BH4)3 has been described in ref 15 as a 3D framework built from a 1D anionic chain, {[Zn(BH4)3]n}n-, and Na+ cations. In the Rietveld refinement using our SR-PXD data, and to an even greater extent in our DFT optimization (see the Zn-B bonds in Table 4), the anionic chain {[Zn(BH4)3]n}n- is broken. The Na-III model shows triangular ZnB3 and trigonal prismatic NaB6 coordination instead of tetrahedral ZnB4 and NaB4 as observed in ref 15. NaZn(BH4)3 can then be rationalized as a 3D framework built from Na+ cations and triangular complex anions [Zn(BH4)3]-. The complex anion [Zn(BH4)3]- has been reported in heteroleptic zinc borohydrides in refs 20 and 21. Bader charge analysis19 of our DFT-optimized structures provides, for NaZn(BH4)3, Na0.85+ and [Zn(BH4)3]0.85-, showing ionicity of the bonding within the 3D framework similar to that in NaZn2(BH4)5. The average Zn-B distances within the complex anion [Zn(BH4)3]- are shortest from the DFT optimization (2.21 Å) and longer from SR-PXD in this work (2.56 Å) and in ref 15 (2.72 Å); see Table 4. We can conclude that the DFToptimized Zn-B distances are very similar (2.20-2.25 Å) in both complex anions [Zn2(BH4)5]- in AZn2(BH4)5 and [Zn-

Figure 2. Crystal structure of NaZn(BH4)3, model Na-III, as optimized by DFT (Na, green; Zn, blue; B, red), viewed along the direction [2,0,-1]. The broken Zn-B bond (4.015 Å) is shown as a black dashed line; other Zn-B bonds (average 2.21(4) Å) within the complex anion [Zn(BH4)3]- are shown in yellow.

(BH4)3]- in NaZn(BH4)3. Even if the experimental results of SR-PXD do not fully confirm this tendency in the Zn-B distances, the presence of the isolated triangular anion [Zn(BH4)3]- rather than the 1D anionic chain {[Zn(BH4)3]n}n- in NaZn(BH4)3 seems to be proved. Breaking the {[Zn(BH4)3]n}n- chain does not necessarily break the 3D framework. However, the increase of average Na-B distances from 2.70-2.73 Å in NaZn2(BH4)5 to 3.11-3.18 Å in NaZn(BH4)3 allows the alternative description of NaZn(BH4)3 as built from strongly deformed Na cubes centered with isolated triangular [Zn(BH4)3]- anions (Figure 2). Isolated tetrahedral anions [Sc(BH4)4]- have been observed in NaSc(BH4)422 and LiSc(BH4)4.23 The closest resemblance of this new inorganic structure type is to the hp phase of CaCO3,24 where deformed Ca cubes are centered by carbonate anions [CO3]2-. However, the CaO6 polyhedra have an octahedral form and not a trigonal prismatic form as NaB6 does, and the coordinations of the O atoms differ too. Phase Composition and Thermal Decomposition. LiZn2(BH4)5. The ball-milled mixture 3LiBH4/ZnCl2 contains LiZn2(BH4)5 (41.8(4) wt %), LiCl (50.1(5) wt %), unreacted ZnCl2 (1.7(1) wt %), and excess LiBH4 (6.4(6) wt %). At 354 K the unreacted ZnCl2 and excess LiBH4 react and produce LiZn2(BH4)5, which then starts to decompose at 367 K to metallic Zn, LiBH4, diborane, and hydrogen according to the eq 2. LiBH4 then reacts with ZnCl2. The decomposition is finished at 395 K. Contrary to the Na analogue (see below) and to ref 15, we have not observed the formation of or any reaction where the ternary chloride Li2ZnCl4 is involved.

LiZn2(BH4)5 f 2Zn + LiBH4 + 2B2H6 + 2H2 alternatively: 2LiZn2(BH4)5 + ZnCl2 f 5Zn + 2LiCl + 5B2H6 + 5H2 (2) The thermal expansion shows a strong deviation from the linearity within the range of 80-395 K (Figure 3 and the Supporting Information). The cell parameter a even decreases with increasing temperature between 80 and 200 K. NaZn2(BH4)5. The ball-milled mixture 2NaBH4/ZnCl2 contains NaZn2(BH4)5 (27.6(6) wt %), NaCl (38.3(5) wt %), Na2ZnCl4 (33.2(7) wt %), and metallic Zn (0.9(3) wt %). At 342 K NaZn2(BH4)5 starts to decompose according to eq 3 to metallic Zn, diborane, hydrogen, and probably NaBH4 (observed in ref 15), which immediately reacts with NaCl. The decomposition is finished at 365 K.

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Figure 3. Cell volume of LiZn2(BH4)5 as a function of temperature.

NaZn2(BH4)5 f 2Zn + NaBH4 + 2B2H6 + 2H2 alternatively: NaZn2(BH4)5 + NaCl f 2Zn + 2Na(Cl,BH4) + 2B2H6 + 2H2

Figure 4. Cell volume of NaZn2(BH4)5 as a function of temperature. The discontinuity around room temperature is caused by a technical problem of the temperature measurement which led to the loss of data.

(3) Contrary to ref 15, we have not observed NaZn(BH4)3 as an impurity. No reaction with Na2ZnCl4, which starts to decompose only at 365 K, has been observed either. The formation of NaZn2(BH4)5 without the contamination by NaZn(BH4)3 can be probably achieved by longer ball-milling times, 350 min in our experiment, 120 min in ref 15. NaZn(BH4)3 is probably the first step of the complex reaction during the ball-milling, leading to NaZn2(BH4)5, which slowly decomposes at room temperature with time back to NaZn(BH4)3.15 The volumetric thermal expansion does not show any deviation from the linearity within the range of 100-365 K (Figure 4); however, the linear thermal expansion as seen on the lattice parameters (see the Supporting Information) is strongly anisotropic and nonlinear. The cell parameter b shows a strong anomaly around room temperature similar to that observed in o-LiBH4,25 where it signalizes the approaching phase transition to h-LiBH4. No phase transition is observed in NaZn2(BH4)5 before its decomposition. The apparent discontinuity in the volumetric thermal expansion around room temperature is caused by a two-step measurement, which is explained in the caption of Figure 4. NaZn(BH4)3. The ball-milled mixture 3NaBH4/ZnCl2 contains NaZn(BH4)3 (52.8(7) wt %), NaCl (46.0(7) wt %), and metallic Zn (1.2(1) wt %). At 365 K NaZn(BH4)3 starts to decompose according to eq 4 to metallic Zn, diborane, hydrogen, and probably NaBH4 (observed in ref 15), which immediately reacts with NaCl. The decomposition is finished at 385 K. Contrary to ref 15, we have not observed any formation of Na2ZnCl4.

Figure 5. Cell volume of NaZn(BH4)3 as a function of temperature.

(4)

Figure 6. Room temperature IR spectra of LiZn2(BH4)5 and NaZn2(BH4)5 in red and of NaZn(BH4)3 in blue, compared to the calculated spectra (program TURBOMOLE26) of isolated anions [Zn2(BH4)5]- and [Zn(BH4)3]- in black. For calculation details see the Supporting Information.

The volumetric thermal expansion does not show a strong deviation from the linearity within the range of 290-385 K (Figure 5). The lattice parameters also behave linearly (see the Supporting Information).

Raman and Infrared Spectroscopy. The IR spectra of LiZn2(BH4)5, NaZn2(BH4)5, and NaZn(BH4)3 (Figure 6) show features in the B-H stretching region similar to those observed for NaSc(BH4)422 and calculated for isolated tridentate

NaZn(BH4)3 f Zn + NaBH4 + B2H6 + H2 alternatively: NaZn(BH4)3 + NaCl f Zn + 2Na(Cl,BH4) + B2H6 + H2

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in NaSc(BH4)422 and LiSc(BH4)4.23 These bands were assigned to Sc-B stretching modes and are indicative of the presence of an isolated metal-borohydride complex. In contrast, borohydrides containing only bridging BH4- groups such as Mn(BH4)2 and Mg(BH4)2 show no similar Raman bands in this region.27,28 Conclusions

Figure 7. Room temperature Raman spectra of LiZn2(BH4)5 in red, of NaZn2(BH4)5 in black, and of NaZn(BH4)3 in blue. The two broad bands around 550 and 1050 cm-1 labeled with an asterisk arise from the capillary and do not belong to the borohydride.

Sc(BH4)4-, bidentate Mn(BH4)42-, and bidentate Al(BH4)3 (see the Supporting Information, Figure S15), with two wellseparated maxima at ca. 2078 and 2378-2450 cm-1. The large splitting of the stretching modes underscores the bidentate mode and the difference between bridging B-H-Zn and terminal B-H bonds. The relative intensity of these two maxima points clearly to the Zn(BH4)3 units in all three compounds rather than to the Zn(BH4)4 unit (see the Supporting Information, Figure S15). The IR spectra of NaZn2(BH4)5 and LiZn2(BH4)5 (Figure 6) show additional B-H stretching bands between 2200 and 2400 cm-1 which correspond to the BH4- group bridging between two bidentate Zn atoms in the complex anion [Zn2(BH4)5]-. The prominent band observed in all three compounds in the bending region at ca. 1395 cm-1 can be assigned by the comparison with Figure S15 to the bidentate bridging B-H-Zn bending mode. The IR spectra of isolated anions [Zn2(BH4)5]- and [Zn(BH4)3]- have been calculated for comparison (Figure 6, program TURBOMOLE;26 for calculation details see the Supporting Information). The calculated spectra confirm the presence of the B-H bending band at 1400 cm-1 for both anions and thus the bidentate binding mode and the presence of the B-H stretching band at 2400 cm-1 only for the isolated anions [Zn2(BH4)5]- and its absence for [Zn(BH4)3]-. The absence of this band in the IR spectrum of NaZn(BH4)3 and the relative intensity of the two maxima in the stretching mode confirm the presence of isolated anions [Zn(BH4)3]- rather than infinite chains [{Zn(BH4)3}n]n- containing bridging BH4- groups as described in ref 15. The IR spectrum of LiZn2(BH4)5 shows bands at around 1100, 1250, and 2300 cm-1 corresponding to the excess LiBH4 in the sample. All three compounds NaZn(BH4)3, NaZn2(BH4)5, and LiZn2(BH4)5 show Raman bands between 400 and 500 cm-1 (Figure 7) with intensities similar to those of the bands observed

A recently discovered series of alkali-metal-zinc borohydrides has been studied by using a combination of in situ synchrotron powder diffraction, energy minimization (DFT), and Raman and infrared spectroscopy. The interpenetrated 3Dframework structures of AZn2(BH4)5 (A ) Li, Na) have been confirmed. As for NaZn(BH4)3 the 3D-framework structural model has been confirmed but with a modified description. The structure contains isolated triangular anions [Zn(BH4)3]- rather than the 1D anionic chain {[Zn(BH4)3]n}n-. The revision of the structure relies on a better sample containing only the main phase and NaCl. A new polymorph is proposed for NaZn(BH4)3 by energy minimization, isostructural to the newly proposed compound LiZn(BH4)3. Both compounds, the new NaZn(BH4)3 polymorph and LiZn(BH4)3, are not observed experimentally at ambient pressure and in the temperature range of 100400 K. The alkali-metal-zinc borohydride NaZn(BH4)3 containing the triangular anion [Zn(BH4)3]- is an equivalent of the recently characterized alkali-metal-scandium borohydrides NaSc(BH4)4 and LiSc(BH4)4 based on the tetrahedral [Sc(BH4)4]- complex anion. The compounds we have described give an example where a wrongly estimated chemical composition of an unknown phase can lead to a structural model that has shortcomings which are difficult to detect from low-resolution powder diffraction data (weakly diffracting and badly crystallized compound). Such a structural model can have balanced interatomic forces as shown by DFT optimization, but does not correspond to a stable polymorph at the given thermodynamic conditions. Acknowledgment. This work was supported by the Swiss National Science Foundation. We acknowledge SNBL for the beamtime allocation and Ya. Filinchuk for help with the data collection. Supporting Information Available: Table of atomic positions, representative Rietveld refinement profile, thermal dilatation data, crystal data as a CIF file, details of structure solution and refinement, and details of vibration spectral calculation. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) Soloveichik, G. Mater. Matters (Aldrich) 2007, 2, 11–14. (2) Orimo, S.; Nakamori, Y.; Eliseo, J. R.; Zu¨ttel, A.; Jensen, C. M. Chem. ReV. 2007, 107, 4111–4132. (3) Grochala, W.; Edwards, P. P. Chem. ReV. 2004, 104, 1283–1315. (4) Schrauzer, G. N. Naturwissenschaften 1995, 42, 438. (5) Sanderson, R. T. Science 1951, 114, 670–672. (6) Nakamori, Y.; Miwa, K.; Ninomiya, A.; Li, H.-W.; Ohba, N.; Towata, S.; Zu¨ttel, A.; Orimo, S. Phys. ReV. B 2006, 74, 045126. (7) Li, H.-W.; Orimo, S.; Nakamori, Y.; Miwa, K.; Ohba, N.; Towata, S.; Zu¨ttel, A. J. Alloys Compd. 2007, 446-447, 315–318. (8) Schlapbach, L., Ed. Hydrogen in Intermetallic Compounds I. Electronic, Thermodynamic, and Crystallographic Properties, Preparation; Topics in Applied Physics, Vol. 63; Springer: New York, 1988. (9) Barabas, G. D.; Dillard, C.; Finholt, A. E.; Wartik, Th.; Wilzbach, K. E.; Schlesinger, H. I. J. Am. Chem. Soc. 1951, 73, 4585–4590. (10) Wiberg, E.; Henle, W. Z. Naturforsch. 1952, 7B, 579–580.

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