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.
@article{arxiv.1312.3454,
title = {Phonons and Thermodynamics of LiMPO4 (M=Mn, Fe)},
author = {Prabhatasree Goel and M. K. Gupta and R. Mittal and S. Rols and S. J. Patwe and S. N. Achary and A. K. Tyagi and S. L. Chaplot},
journal= {arXiv preprint arXiv:1312.3454},
year = {2014}
}