High Resolution Raman and Neutron Investigation of Mg(BH 4 ) 2 in an Extensive Temperature Range A. Giannasi,* D. Colognesi, L. Ulivi, and M. Zoppi Consiglio Nazionale delle Ricerche, Istituto dei Sistemi Complessi, Via Madonna del Piano 10, I-50019, Sesto Fiorentino, Italy A. J. Ramirez-Cuesta Rutherford Appleton Laboratory, ISIS Facility, Chilton, Didcot, Oxon, OX11 0QX, United Kingdom E. G. Bardají, E. Roehm, and M. Fichtner Karlsruhe Institute of Technology, Institute of Nanotechnology, Hermann-Von-Helmholtz-Platz 1, 76347 Eggenstein-Leopoldshafen, Germany ReceiVed: December 1, 2009; ReVised Manuscript ReceiVed: January 13, 2010 Raman spectra of Mg(BH 4 ) 2 have been measured in an extensive temperature range, from 15 to 473 K. Taking into account the high temperature conversion from the R to the  phase, we have observed evident signatures of this phase transition and determined the Raman vibrational spectrum of each phase. The neutron scattering spectra of the  phase sample were also recorded. The present experimental results have been compared to the density functional theory calculations available in the literature, and a substantial agreement has been found. I. Introduction Given its high gravimetric hydrogen content (14.9 mass %), magnesium borohydride is considered a promising material for mobile hydrogen storage applications. According to recent investigations performed by Chlopek et al. 1 and Soloveichik et al., 2 magnesium borohydride releases up to 14.4 mass % of hydrogen upon heating to 800 K, i.e., near the theoretical value. Moreover, its decomposition pathway resulted to be more complex than the simple one originally supposed by Konoplev et al., 3 passing through four different decomposition steps and forming several polyborane intermediate species. The rehydra- tion process has been recently investigated by Li et al.: 4 it has been demonstrated that the rehydrided sample could reload up to 6.1 mass % through the formation of the Mg(B 12 H 12 ) polyborane. Depending on the synthesis condition, Mg(BH 4 ) 2 crystallizes in two different phases that have been recently determined by X-ray diffraction (XRD) and neutron diffraction (ND) by Her et al. 5 and Filinchuk et al. 6 The R phase turns out to be characterized by a very complex hexagonal lattice belonging to the P6 1 22 space group, while the  phase shows an orthorhombic primitive cell with a Fddd symmetry. The R phase is formed by MgH 8 polyhedra almost linearly coordinated by H 2 BH 2 units and organized into a three-dimensional network by five-membered (-Mg-BH 4 -) n rings. The  phase results to be built up by two different MgH 8 polyhedra coordinated by (-Mg-BH 4 -) n rings, where the n indices cannot assume odd values. 6 At ambient pressure the R phase transforms into the  phase at T ) 470 K. Upon lowering the temperature, the  phase does not transform back into the R phase, due to an extremely slow kinetics, and the system remains in a  phase metastable state. 6 Room temperature Raman spectra of Mg(BH 4 ) 2 have been reported in the literature at ambient pressure 6,7 or as a function of pressure. 8 To our knowledge, no low temperature Raman spectra have been reported so far. Here we present the low temperature spectra of Mg(BH 4 ) 2 (R and  phases) in the spectral region between 150 and 2500 cm -1 . As we will show in the following, the lattice modes of Mg(BH 4 ) 2 in both crystalline phases are well resolved and distinguishable. The same applies to the bending and stretching modes of the B-H vibrations. The present Raman measurements have been complemented by an incoherent inelastic neutron scattering (IINS) experiment. In this case the measured spectrum is representative of the proton dynamics in the sample, and its interpretation turns out to be more direct. However, due to the presence of a non-negligible momentum transfer, inherent to this class of experiments, the comparison of the spectra belonging to the two different techniques can effectively be carried out in a limited energy range. II. Experimental Section A sample of Mg(BH 4 ) 2 was produced at the Karlsruhe Institute of Technology starting from magnesium hydride and triethylamine borane complex as described in ref 1. Structural characterization, using X-ray powder diffraction, was performed in a Philips X’PERT diffractometer (Cu, KR radiation). The sample powder was spread on a silicon single crystal and sealed in the glovebox with an airtight hood of kapton foil. The analysis of the XRD pattern showed a mixture of R and  phases with a dominating contribution from the R phase (see Figure 1). Elemental analysis gave a hydrogen content of 14.5 mass % (to be compared to the theoretical value of 14.9 mass %). The sample was also analyzed by Fourier transform infrared spec- * To whom correspondence should be addressed. E-mail: alessandra. giannasi@fi.isc.cnr.it. J. Phys. Chem. A 2010, 114, 2788–2793 2788 10.1021/jp911175n  2010 American Chemical Society Published on Web 02/08/2010