Mobility and dynamics in the complex hydrides LiAlH4 and LiBH4

Abstract

The dynamics and bonding of the complex hydrides LiBH₄ and LiAlH₄ have been investigated by vibrational spectroscopy. The combination of infrared, Raman, and inelastic neutron scattering (INS) spectroscopies on hydrided and deuterided samples reveals a complete picture of the dynamics of the BH⁻₄ and AlH⁻₄ anions respectively as well as the lattice. The straightforward interpretation of isotope effects facilitates tracer diffusion experiments revealing the diffusion coefficients of hydrogen containing species in LiBH₄, and LiAlH₄. LiBH₄ exchanges atomic hydrogen starting at 200 °C. Despite having an iso-electronic structure, the mobility of hydrogen in LiAlH₄ is different from that of LiBH₄. Upon ball-milling of LiAlH₄ and LiAlD₄, hydrogen is exchanged with deuterium even at room temperature. However, the exchange reaction competes with the decomposition of the compound. The diffusion coefficients of the alanate and borohydride have been found to be D ≃ 7 × 10⁻¹⁴ m²s⁻¹ at 473 K and D ≃ 5 × 10⁻¹⁶ m²s⁻¹ at 348 K, respectively. The BH⁻₄ ion is easily exchanged by other ions such as I⁻ or by NH⁻₂. This opens the possibility of tailoring physical properties such as the temperature of the phase transition linked to the Li-ion conductivity in LiBH₄ as measured by nuclear magnetic resonance and Raman spectroscopy. Temperature dependent Raman measurements on diffusion gradient samples Li(BH₄)_1−cI_c demonstrate that increasing temperature has a similar impact to increasing the iodide concentration c: the system is driven towards the high-temperature phase of LiBH₄. The influence of anion exchange on the hydrogen sorption properties is limited, though. For example, Li₄(BH₄)(NH₂)₃ does not exchange hydrogen easily even in the melt.