Lattice anharmonicity and structural evolution of LiBH₄: an insight from Raman and X-ray diffraction experiments

Abstract

New in situ Raman and synchrotron X-ray diffraction data (between 300 and 400 K) in conjunction with separate temperature-dependent Raman data (between 7 and 400 K) are presented. The low-frequency Raman spectra show good agreement with theoretical values obtained previously using periodic DFT calculations. The temperature-dependent spectra reveal the presence of significant anharmonicity of librational modes neither predicted theoretically nor noted in previous experiments. The splitting of the internal deformation mode 2 (of E symmetry in the free ion) decreases continuously with increasing temperature, but drops abruptly at the first-order orthorhombic to hexagonal phase transition observed at 381 K. The temperature dependence of the linewidth of the internal deformation mode 2 reveals coupling to reorientational motions of the borohydride ion in the orthorhombic phase. The thermal evolution of both crystal structure and vibration frequencies agree with the phase diagram suggested by the Landau theory.

Di-hydrogen contact induced lattice instabilities and structural dynamics in complex hydride perovskites

“…This relation accounts for an anharmonic coupling of a reorientational motion to the deformation mode considered. This behaviour has been studied previously in the alkali borohydrides [28,29]. The corresponding results are included in table 3.…”

“…B-H vibrations are readily studied by vibrational spectroscopy while the much lower energy transfers of stochastic reorientations are investigated by means of quasi-elastic neutron scattering. The structural dynamics of monometallic borohydrides have been extensively investigated by vibrational spectroscopy [28][29][30], solid state NMR spectroscopy [31][32][33][34][35], inelastic x-ray scattering IXS [36], inelastic neutron scattering INS [37], QENS [38][39][40] and recently by molecular dynamics assisted density functional theory (MD-DFT) [41,42]. The importance of the vibrational density of states for the stabilities of different polymorphs has also been discussed [43].…”

Differential Scanning Calorimetry (DSC) and Synchrotron X-ray Diffraction Study of Unmilled and Milled LiBH4: A Partial Release of Hydrogen at Moderate Temperatures

“…It has been shown that the order parameter in LiBH 4 can be parameterized as a shift of layers formed by Li and BH 4 , together with in-layer deformations [35]. The temperature dependence of Li-B distances has shown [15] that the layers defined in the bc plane of the orthorhombic phase become flatter as the temperature is increased: the difference between atomic coordinates for Li and B atoms decreases with temperature. Also, the distance between the layers significantly increases with temperature.…”

“…The LiBH 4 crystal structure, phase transition, and phonons modes have been investigated, mainly by X-ray diffraction [11][12][13][14][15][16][17][18], Raman spectroscopy [15,[19][20][21][22][23], NMR [18,24,25], and calorimetry [18,26].…”

Complex inorganic structures from powder diffraction:case of tetrahydroborates of light metals

“…A few light hydride systems have been studied by Rietveld methods as a function of temperature and/or pressure: these are LiBH 4 (Filinchuk et al, 2008a, c;Dmitriev et al, 2008;Hagemann et al, 2009), Mg(BH 4 ) 2 (Filinchuk et al, 2009a;Riktor et al, 2007), Ca(BH 4 ) 2 (Filinchuk et al, 2009b;Riktor et al, 2007), NaBH 4 (Kumar and Cornelius, 2005;Babanova et al, 2009), RbBH 4 , NH 3 BH 3 (Filinchuk et al, 2009c) etc. Synchrotron radiation was used in all cases, enabling rapid data collection with a fine steps in P and T. Exhaustive data analysis yielded a picture of structural evolution expressed in terms of order parameters and uncovered the P-T phase diagrams.…”