Valence Fluctuations of ^(57)Fe in Disordered Li_(0.6)FePO_4

The local electronic structure around iron ions in Li_(0.6)FePO)4 was studied by ^(57)Fe Mössbauer spectrometry at temperatures from 25 to 240 °C. The equilibrium two-phase, triphylite plus heterosite, material was compared to a disordered solid solution that was obtained by quenching from a high temperature. Substantial electronic relaxations were found in the disordered solid solution compared to the two-phase material at temperatures of 130 °C and above. Fluctuations in the electric field gradient showed an approximately Arrhenius behavior, with an activation energy of 335 ± 25meV and a prefactor of 5 × 10^(11) Hz, whereas Arrhenius plots for the isomer shift showed activation energies of approximately 600 meV. It is suggested that these spectral relaxations are caused by the motions of Li^+ ions. A slight relaxation at 180 °C in 10% of the two-phase material can be attributed to defects in the heterosite and triphylite phases. Overall, the disordered solid solution phase shows faster electronic dynamics than the two-phase material