Vibrational Spectra of Ca(BH 4 ) 2 M. Fichtner,* ,† K. Chlopek, † M. Longhini, ‡ and H. Hagemann ‡ Forschungszentrum Karlsruhe, Institute for Nanotechnology, P.O. Box 3640, D-76021 Karlsruhe, Germany, and Departement de Chimie Physique, UniVersite ´ de Gene ` Ve, 30, quai E. Ansermet, CH-1211 GeneVa 4, Switzerland ReceiVed: February 19, 2008; ReVised Manuscript ReceiVed: May 20, 2008 IR and Raman data were obtained from R-, -, and mixed (,γ)-Ca(BH 4 ) 2 samples and from the deuterated ,γ phase mixture. The results obtained with R phase indicate that the DFT calculated values for the B-H stretching modes and the lattice vibrations are fairly close to the experimental values. The spectral behavior at temperatures around the transition to the  phase shows a continuous transition and suggests the presence of disorder caused by reorientational motions of the [BH 4 ] - ion in the  phase. The data indicate that there are more deformation bands observed for the mixed (,γ) samples than for the R phase which indicates structural variations between the  and the γ phases. Introduction In search for new hydrogen storage materials tetrahydroborate compounds, M(BH 4 ) n , (with M ) metal cation, and n ) valence of the cation) have gained considerable interest, mainly due to their high gravimetric content of H. However, only a few of the compounds exhibit thermodynamic properties that are suitable for reversible application. Among them, there are two alkaline earth compounds, Mg(BH 4 ) 2 and Ca(BH 4 ) 2 . 1–4 Recent results indicate that Ca(BH 4 ) 2 can be formed by ball milling of CaB 6 and CaH 2 and can ab- and desorb hydrogen reversibly, confirming the potential use as reversible hydrogen storage material. 5,6 R-Ca(BH 4 ) 2 , obtained by desolvation of the commercial THF solvate, crystallizes in space group Fddd. 7 The phase transforms at about 400 K to a tetragonal structural phase with space group P42/m (No. 84) called the -phase. 8 The struc- ture of the  form has recently been published 9 whereas the structure of the γ form is still not known. Table 1 summarizes the theoretical symmetry predictions for the vibrational spectra of R-Ca(BH4)2. Theoretical vibrational spectra from first principle calculations have been reported previously 7 and are compared to the experimental data in this work. Experimental Section R-Ca(BH 4 ) 2 was prepared by drying the commercial (Sigma- Aldrich) THF solvate Ca(BH 4 ) 2 · 2THF for 1 h at 433 K under a vacuum. Single phase -Ca(BH 4 ) 2 was obtained by annealing R-Ca(BH 4 ) 2 for 16 h at 200 °C and quenching, or alternatively by annealing for 90 min at 250 °C and quenching. ,γ-Ca(BH 4 ) 2 was synthesized similarly to a method described in: 2 1.920 g (0.046 mol) of CaH 2 (which was ball-milled for 1 h, at 600 rpm, in a tempered steel system) was introduced to 15.743 g (0.137 mol) of triethylamine borazane complex, Et 3 N · BH 3 (Aldrich). The mixture was heated under reflux for 2 h at 80 °C, 1 h at 95 °C, 1 h at 120 °C and finally 4 h at 140 °C. After cooling down to room temperature, 150 mL of dried and degassed n-hexane was introduced to the white solid. The latter was reduced to small pieces by means of a spatula. The suspension was stirred overnight, filtered under Ar, washed with n-hexane (3 × 20 mL) and dried at RT under vacuum, followed by drying at high temperature in vacuo (at 150 °C for 2 h, at 180 °C for 2 h, at 200 °C for 14 h). Isolated yield (with respect to CaH 2 ): 2.690 g (85%). Anal. Found: H, 10.68. Calcd: 11.56. ,γ-Ca(BD 4 ) 2 was prepared from Et 3 N · BD 3 (6.161 g, 0.0522 mol) and CaD 2 (1.046 g, 0.0237 mol) (synthesis procedure in ref 10) according to the procedure described above. The deuterated compound contained up to 5% of H due to the aminoborane used. Isolated yield (with respect to CaH 2 ): 1.432 g (78%). The reagent Et 3 N · BD 3 was synthesized from NaBD 4 (Alfa Aesar, 9.500 g, 0.227 mol) and triethylamine-hydrochloride salt (Merck, 28.399 g, 0.206 mol) in diethyl ether, according to a procedure in the literature. 11 Up to five percent of H-content was registered in the final compound on the basis of the Raman spectra. Isolated yield: 20.430 g (87%). Powder X-ray diffraction patterns (see Supporting Informa- tion) were obtained with a Philips X′PERT diffractometer, with Cu KR radiation, 2 kW, with X′Celerator RTMS detector, automatic divergence slit. The powder was measured on an Si single crystal and sealed in the glovebox by an airtight hood made of Kapton foil, the foil being mounted out of the focus of the spectrometer. IR spectra of Ca(BH 4 ) 2 in nujol mulls were recorded in the range of 4000-370 cm -1 under ambient conditions by using a Perkin-Elmer Spectrum GX FTIR spectrometer. The spectral resolution was 4 cm -1 . Evaluation of the data was done with the Perkin-Elmer Spectrum v. 2.00 Software. Measurements without nujol were obtained using a Specac “Golden Gate” ATR cell in a Perkin-Elmer Spectrum One instrument with a nominal resolution of 2 cm -1 . Several reports in the literature present IR spectra of inorganic borohydrides in KBr pellets. However, this sampling technique is not appropriate, as the BH 4 - ion and the Br - ion have quite similar ionic radii, and partial or total ion exchange may take place. Furthermore, these spectra may also present lines originating from water (at 1630 cm -1 ) present in the KBr powder. Raman spectra were obtained using 3 different set-ups: the first one is an Ar ion laser (488nm) with a Kaiser Optical * Corresponding author. E-mail: maximilian.fichtner@kit.edu. Telephone: +49-7247-82-5340. Fax: +49-7247-82-6368. † Forschungszentrum Karlsruhe, Institute for Nanotechnology. ‡ Departement de Chimie Physique, Universite ´ de Gene `ve. J. Phys. Chem. C 2008, 112, 11575–11579 11575 10.1021/jp801482b CCC: $40.75  2008 American Chemical Society Published on Web 07/02/2008