Dynamical properties and temperature induced molecular disordering ofLiBH4andLiBD4

Abstract

We report on neutron powder-diffraction experiments, inelastic incoherent neutron-scattering experiments, and density-functional calculations on dynamics, order and disorder properties of ${\text{LiBH}}_{4}$ and ${\text{LiBD}}_{4}$. From refinement of ${\text{LiBD}}_{4}$ structure at 10 and 302 K, we found an almost ideal tetrahedral geometry of ${\text{BD}}_{4}$ ions (difference between shortest and longest interatomic distances is less than 4% for B-D bond, and less than 3% for D-D bond), close to the calculated geometry. A quantitative agreement was found between experimental and calculated anisotropic temperature factors of individual atoms. For phonon energies $<15\text{ }\text{meV}$, the phonon density of states of ${\text{LiBH}}_{4}$ in the low-temperature phase depends quadratically on the phonon energy while for the high-temperature phase a linear dependence is observed, revealing a high lattice anharmonicity in the high-temperature phase. Moreover, an increased phonon density of states at low energies in the high-temperature phase compared to the low-temperature phase give a direct evidence for disorder in the high-temperature phase of ${\text{LiBH}}_{4}$ of the hydrogen sublattice which can originate from orientational disorder of ${\text{BH}}_{4}$ units. Potential energy landscape for rotation of ${\text{BH}}_{4}$ indicates that fairly localized minima and barriers higher than 0.6 eV exist in the low-temperature phase, i.e., ordered ${\text{BH}}_{4}$ ions. The high-temperature structure shows shallow barriers of $\ensuremath{\sim}0.2\text{ }\text{eV}$ without distinct energy minima, i.e., orientation of a single ${\text{BH}}_{4}$ unit cannot be precisely defined. This corroborates the large thermal displacements observed in diffraction studies and high disorder of ${\text{BH}}_{4}$ ions deduced from experimental partial phonon density of states in the high-temperature phase.