Lithium boro-hydride LiBH4

Journal of Alloys and Compounds 346 (2002) 200–205

L

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Lithium boro-hydride LiBH 4 I. Crystal structure ˇ ´ G. Renaudin, R. Cerny, ´ K. Yvon* J-Ph. Soulie, ` , 24, quai E. Ansermet, CH-1211 Geneva 4, Switzerland Laboratoire de Cristallographie, Universite´ de Geneve Received 25 March 2002; received in revised form 9 April 2002; accepted 9 April 2002

Abstract The crystal structure of LiBH 4 has been studied by synchrotron X-ray powder diffraction at room temperature and at 408 K. At room ˚ The tetrahedral temperature it has orthorhombic symmetry [space group Pnma, a57.17858(4), b54.43686(2), c56.80321(4) A]. 2 (BH 4 ) anions (point symmetry m) are aligned along two orthogonal directions and are strongly distorted with respect to bond lengths ˚ and bond angles [H–B–H585.1(9)8–120.1(9)8]. As the temperature is increased the structure undergoes a [B–H51.04(2)–1.28(1) A] ˚ at T5408 K). The (BH 4 )2 tetrahedra first-order transition and becomes hexagonal (space group P6 3 mc, a54.27631(5), c56.94844(8) A ˚ align along c, become more symmetric [point symmetry 3m, B–H51.27(2)–1.29(2) A, H–B–H5106.4(2)8–112.4(9)8] and show displacement amplitudes that are consistent with dynamical disorder about their trigonal axis.  2002 Elsevier Science B.V. All rights reserved. Keywords: Hydrogen storage materials; Metal hydrides; Crystal structure; Phase transition; X-ray diffraction

1. Introduction

2. Experimental

Lithium boro-hydride LiBH 4 plays an important role as a reducing agent in organic and inorganic chemistry and is a potential hydrogen storage material. Although the compound has been extensively studied with respect to thermal stability [1], heat capacity [2,3], NMR [4,5], high-pressure melting and polymorphism [6] and microwave spectra [7], little is known about its crystal structure. At ambient conditions the structure was reported to have orthorhombic symmetry and was described in terms of tetrahedral (BH 4 )2 anions and Li 1 cations [8]. However, the structure was not refined and positional coordinates were not given. At high temperature (|381 K) the compound undergoes a structural phase transformation but conflicting results have been reported concerning its symmetry that was stated to be tetragonal [6,9]. In the first part of this work a structure analysis of both LiBH 4 polymorphs by synchrotron radiation is reported. In the second part [10] a Raman spectroscopy study as a function of temperature will be presented.

Polycrystalline lithium boro-hydride LiBH 4 was purchased from Alfa Aesar (purity 95%). In view of their great sensitivity to air and humidity the samples were handled in an argon-filled glove box. For structure analysis powders were filled into sealed glass capillaries (0.5 mm diameter) and measured on synchrotron radiation facilities in Debye–Scherrer geometry to eliminate the transparency error of focusing diffraction geometries which is very disturbing for this very low absorbing compound. The room-temperature data were recorded on the Swiss–Norwegian beamline (BM1) at ESRF (Grenoble, France) at the ˚ (range of scattering angles 18# wavelength l 50.48562 A 2u #28.58, step size Du 50.0028) by using a multianalyser crystal detector. The final pattern was binned from six data sets and successfully indexed on an orthorhombic cell of refined dimensions a57.17858(4), b54.43686(2), c5 ˚ in agreement with literature data [8]. The 6.80321(4) A, systematically absent reflections suggested space group Pnma. The structure was solved by the recently developed computer programme FOX [11] and found to contain one lithium, one boron and three hydrogen sites. Structure refinement was performed by the Rietveld method using

*Corresponding author. E-mail address: [email protected] (K. Yvon).

0925-8388 / 02 / $ – see front matter  2002 Elsevier Science B.V. All rights reserved. PII: S0925-8388( 02 )00521-2

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Table 1 Refinement results for orthorhombic LiBH 4 at room temperature from synchrotron powder data Atom

Site

Symmetry

x /a

y /b

y /c

˚ 2) Biso (A

Li B H1 H2 H3

4c 4c 4c 4c 8d

m m m m 1

0.1568(4) 0.3040(3) 0.900(1) 0.404(2) 0.172(2)

0.25 0.25 0.25 0.25 0.054(2)

0.1015(6) 0.4305(1) 0.956(3) 0.280(2) 0.428(1)

5.9(1) 3.3(1) 5.3(2) 5Biso (H1) 5Biso (H1)

˚ V5216.685(3) A ˚ 3 , Z54; T5293 K, Rietveld agreement indices: Space group Pnma (No. 62), a57.17858(4), b54.43686(2), c56.80321(4) A; 2 R Bragg 53.49%, R wp 514.6% and x 51.86, e.s.d.s in parentheses.

the program FULLPROF 2000 [12]. The background was described by interpolation between 45 points, and the reflection profiles were modelled by a pseudo-Voigt function. The following 26 parameters were refined: 1 scale factor, 7 profile, 3 cell and 14 atomic parameters. The isotropic displacement parameters were constrained to be equal for atoms of the same kind. No impurity phase was found. The refinement results are summarised in Table 1, and a plot of observed, calculated and difference patterns is shown in Fig. 1. A list of interatomic distances is given in Table 2. The high-temperature data were collected at T5408 K on the Materials Science Beamline of SLS at PSI (Vil˚ that ligen, Switzerland) at a wavelength of l 50.94902 A was calibrated from the ESRF room temperature data

(range of scattering angles 68#2u #608, step size Du 5 0.0038). Four patterns were measured by using a multianalyser crystal detector and indexed on a hexagonal cell ˚ with refined dimensions a54.27631(5), c56.94844(8) A. The systematic absences suggested the space groups P31c, ] ] P31c, P6 3 mc, P62c or P6 3 /mmc. The positions of one lithium, one boron and two hydrogen sites in the structure were located by FOX [11] in space group P6 3 mc. A structure refinement was performed by the programme TOPAS [13] by simultaneously using four data sets. The lithium position (site 2b: 1 / 3, 2 / 3, z; etc) was fixed at z 5 0, and the following refinement restraints were applied: ˚ B–H distance51.1(1) H–H antibump distance51.9(1) A, ˚A. The reflection profiles were modelled by a pseudo-Voigt function. In addition to 80 nonstructure parameters (431

Fig. 1. Observed (1), calculated (2) and difference (3) synchrotron diffraction patterns and Bragg positions (4) for orthorhombic LiBH 4 at room ˚ temperature ( l 50.48562 A).

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Table 2 ˚ in orthorhombic LiBH 4 at room temperature; Interatomic distances (A) e.s.d.s in parentheses Li–H2 Li–H1 Li–H2 Li–2H3 Li–2H1 Li–2H3 Li–B Li–2B Li–B

B–H1 B–H2 B–2H3 B–Li B–2Li B–Li

1.98(1) 2.09(1) 2.15(1) 2.18(1) 2.289(4) 2.38(1) 2.475(4) 2.521(2) 2.542(4)

1.04(2) 1.25(1) 1.28(1) 2.475(4) 2.521(2) 2.542(4)

H1–B H1–H2 H1–2H3 H1–Li H1–2Li

1.04(2) 1.80(3) 2.02(1) 2.09(1) 2.289(4)

H2–B H2–H1 H2–Li H2–2H3 H2–Li

1.25(1) 1.80(3) 1.98(1) 2.13(2) 2.15(1)

H3–2B H3–H3 H3–2H1 H3–2H2 H3–2Li H3–2Li

1.28(1) 1.73(1) 2.02(1) 2.13(2) 2.18(1) 2.38(1)

zero correction, 4312 background, 431 scale factor, 436 profile) 2 cell, 4 positional and 4 isotropic displacement parameters were allowed to vary. No impurity phase was found. The refinement results are summarised in Table 3, and a plot of the observed and calculated patterns for one of the four crystal analysers is shown in Fig. 2. A list of

Table 3 Refinement results for hexagonal LiBH 4 at 408 K from synchrotron data Atom

Site

Symmetry

x /a

y /b

y /c

˚ 2) Biso (A

Li B H1 H2

2b 2b 2b 6c

3m 3m 3m m

1/3 1/3 1/3 0.172(2)

2/3 2/3 2/3 2x

0(2) 0.553(1) 0.370(2) 0.624(3)

1.0(2) 8.6(4) 3.5(5) 64(4)

˚ Space group P6 3 mc (No. 186), a54.27631(5), c56.94844(8) A; ˚ 3 , Z52; T5408 K. R Bragg 55.49%, R wp 516.63% and V5110.041(4) A x 2 51.21; e.s.d.s in parentheses.

interatomic distances is given in Table 4. Refinements in space group P31c converged towards P6 3 mc symmetry, ] ] while refinements in space groups P31c and P62c converged towards P6 3 /mmc symmetry. The three latter space groups imply an additional orientational disorder of the BH 4 tetrahedra with respect to either a mirror plane (x, y, 1 / 4) or a two-fold axis (x, 2x, 1 / 4). Refinements in space group P6 3 /mmc ( x 2 51.31 and R Bragg 511.72) did not support this type of disorder.

3. Results and discussion

3.1. Orthorhombic room temperature modification At ambient conditions LiBH 4 crystallizes with an ortho-

Fig. 2. Observed (1), calculated (2) and difference (3) synchrotron diffraction patterns for one of four crystal analysers, and Bragg positions (4) for ˚ hexagonal LiBH 4 at 408 K ( l 50.94902 A).

J-Ph. Soulie´ et al. / Journal of Alloys and Compounds 346 (2002) 200–205 Table 4 ˚ in hexagonal LiBH 4 at 408 K, e.s.d.s in Interatomic distances (A) parentheses Li–6H2 Li–3B Li–H1 Li–3H1 Li–3H2 Li–B H1–B H1–3H2 H1–Li H1–3Li H1–6H2

2.30(1) 2.496(1) 2.57(1) 2.629(5) 2.87(2) 3.106(7) 1.27(2) 2.13(2) 2.57(1) 2.629(5) 2.74(2)

B–H1 B–3H2 B–3Li B–Li

1.27(2) 1.29(2) 2.496(1) 3.106(7)

H2–3B H2–3H1 H2–6Li H2–6H1 H2–3Li

1.29(2) 2.13(2) 2.30(1) 2.74(2) 2.87(2)

rhombic structure in which each (BH 4 )2 anion is surrounded by four lithium Li 1 cations and each Li 1 by four (BH 4 )2 , both in tetrahedral configurations (Fig. 3). This

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arrangement corresponds to an orthorhombic distorted wurtzite type metal substructure in which the tetrahedral (BH 4 )2 anions point along two orthogonal directions in an ordered fashion (Fig. 4a). By comparison, all other members of the alkali metal boro-hydride series A1 (BH 4 )2 (A5Na, K, Rb, Cs) crystallize with a cubic sodium chloride type metal substructure at room temperature in which the (BH 4 )2 anions are octahedral surrounded by A1 cations and disordered with respect to orientation as shown by X-ray diffraction [14,15]. Below room temperature these compounds undergo a phase transition [16]. In LiBH 4 the (BH 4 )2 anions have point symmetry m and are strongly deformed with respect to bond lengths (B–H5 ˚ see Table 2) and bond angles (H–B–H585– 1.04–1.28 A, 1208). The lithium atoms are coordinated by nine hydrogen ˚ of which six are bonded in atoms (Li–H51.98–2.38 A) pairs to three surrounding (BH 4 ) – anions and three to the fourth (BH 4 )2 anion. This contrast with the tridentate configurations suggested previously from microwave spectra [7]. Some H–H distances within the (BH 4 )2 anions are ˚ H1–H251.80 A) ˚ but relatively short (H3–H351.73 A, still consistent with repulsive interactions.

3.2. Hexagonal high-temperature modification The structure of the hexagonal high-temperature polymorph of LiBH 4 is closely related to that of the orthorhombic room-temperature polymorph, as can be seen from the cell parameter relationships 0 Mortho→hex 5 0 1

1

Fig. 3. Boron (top) and lithium (bottom) coordinations in orthorhombic LiBH 4 at room temperature.

21 1 ] 2

0

0 2 ]12 0

2

and the preservation of the wurtzite type arrangement of the (BH 4 )2 and Li 1 ions during the phase transition. Interestingly, when going from the orthorhombic roomtemperature to the hexagonal high-temperature modification the structure contracts along orthorhombic a (hexagonal c, see Fig. 4) and expands in the orthorhombic b 3 c plane (hexagonal basal plane) such that a density increase occurs at the transition temperature (data not shown). Compared to the room-temperature structure the (BH 4 )2 tetrahedra in the high-temperature structure are more symmetric (point symmetry 3m) and less distorted with ˚ see Table 4) respect to bond lengths (B–H51.27–1.29 A, and bond angles (H–B–H5106–1128). Furthermore, they are differently oriented with respect to the room-temperature modification and all point in the same direction c (see Figs. 4b and 5). As a consequence, one of the four nearest (BH 4 ) – tetrahedra is shifted away from lithium (Li–B5 ˚ and the other three are shifted towards lithium 3.11 A) ˚ compared to the room-temperature struc(Li–B52.50 A) ˚ Thus lithium is no longer ture (Li–B52.48–2.54 A). coordinated by 9 but by up to 13 hydrogen atoms of which twelve are bonded in groups of three to the four closest

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Fig. 4. Crystal structure of LiBH 4 at room temperature (left) and at 408 K (right) as shown along two orthogonal directions (top and bottom).

˚ and one to a fifth, (BH 4 )2 anions (Li–H52.30–2.87 A), 2 ˚ The closest more distant (BH 4 ) anion (Li–H152.57 A). ˚ is distance between hydrogen atoms (H1–H352.1 A) consistent with repulsive interactions. However, given the rather large displacement amplitudes of the peripheral ˚ 2 ] and the relatively hydrogen site [Biso (H(2))564(4) A large errors of its positional parameters the corresponding ‘interatomic’ distances are not very reliable. As will be shown in the second part of this work [10] vibrational spectroscopy data suggest that the (BH 4 )2 units in the high-temperature phase are dynamically disordered about their trigonal axis.

4. Conclusion The crystal structures of the two known LiBH 4 poly-

morphs have been investigated for the first time in detail. While the symmetry of the room-temperature modification is orthorhombic that of the high-temperature modification is hexagonal and not tetragonal as reported previously. The space group symmetries (Pnma vs. P6 3 mc) suggest a first-order transition during which the tetrahedral (BH 4 )2 anions are reoriented. The data for the high-temperature structure are consistent with a rotational disorder of the tetrahedral (BH 4 )2 anions about their trigonal axis and possible ferro-electricity. Finally, to the authors’ knowledge, the present study provides the first example of a metal hydride structure for which the hydrogen atoms have been located unambiguously by X-ray synchrotron powder diffraction. Although the precision of the metal–hydrogen bond lengths obtained is lower than that usually attained by neutron diffraction, it is sufficient for a crystal chemical discussion. This underlines the potential of synchrotron

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radiation for the structure analysis of polycrystalline materials that contain large concentrations of elements having weak X-ray scattering power.

Acknowledgements The authors wish to thank W. van Beek from the Swiss–Norwegian beamline (BM1) at ESRF, Grenoble, and B. Patterson and F. Gozzo from SLS-MS beamline at PSI, Villigen, for help with the synchrotron diffraction ¨ experiments. The authors also thank A. Zuttel from University of Fribourg for having initiated this study. This work was supported by the Swiss National Science Foundation and the Swiss Federal Office of Energy.

References

Fig. 5. Boron (top) and lithium (bottom) coordinations in hexagonal LiBH 4 at 408 K. Arrows indicate rotational displacements of the (BH 4 ) – tetrahedra about their trigonal axis.

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