Phonons and Thermodynamics of Unmixed and Disordered Li_(0.6)FePO_4

The lithium-storage material Li_(0.6)FePO_4 was studied by inelastic neutron scattering and differential scanning calorimetry. Li_(0.6)FePO_4 undergoes a transformation from a two-phase mixture (heterosite and triphylite) to a disordered solid-solution at 200 °C. Phonon densities of states (DOS) obtained from the inelastic neutron scattering were similar for the two-phase sample measured at 180 °C and the disordered sample measured at 220 °C. The vibrational entropy of transformation is 1.8 ±0.9 J/(K mol), which is smaller than the configurational entropy difference of approximately 3.1 J/(K mol). The measured enthalpy of the disordering transition was estimated as 2.5 kJ/mol. The phonon data show a small change in lattice dynamics upon disordering