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 a Li sub-lattice. The experiments are in good agreement with calculations. Here for the first time we have correlated the diffusion of lithium and dynamical instability in LiMPO₄. The lattice dynamics calculations indicate the instability of the zone-centre as well as zoneboundary phonon modes along the [100] direction at unit cell volumes corresponding to elevated temperature. Under ambient conditions, eigen vectors of Li corresponding to these modes exhibit displacement only in the x–y plane. However with initiation of phonon instability Li atoms exhibit displacement along all the three directions with the highest component along the x direction. 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.