  Lithium transition metal phospho-olivines are useful electrode materials,
owing to their stability, high safety, low cost and cyclability. We report
phonon studies using neutron inelastic scattering experiments, ab-initio
density functional theory calculations and potential model calculations on
LiMPO4 (M=Mn, Fe) at ambient and high temperature to understand the microscopic
picture of Li sub-lattice. The experiments are in good agreement with
calculations. The lattice dynamics calculations indicate instability of a
zone-centre as well as zone-boundary modes along (100) at volume corresponding
to high temperature. The unstable phonon modes show mainly large vibration of
Li atoms in the x-z plane of the orthorhombic structure (space group Pbnm).
Molecular dynamics simulations with increasing temperature indicate large mean
square displacement of Li as compared to other constituent atoms. The computed
pair-correlations between various atom pairs show that there is local disorder
occurring in the lithium sub-lattice with increasing temperature, while other
pairs show minimal changes. The results find the two compounds to be thermally
stable up to high temperatures, which is a desirable trait for its battery
applications.
