Polaron-ion correlations in LixFePO4 studied by x-ray nuclear resonant forward scattering at elevated pressure and temperature

Valence fluctuations of Fe2+ and Fe3+ were studied in a solid solution of LixFePO4 by nuclear resonant forward scattering of synchrotron x rays while the sample was heated in a diamond-anvil pressure cell. The spectra acquired at different temperatures and pressures were analyzed for the frequencies of valence changes using the Blume-Tjon model of a system with a fluctuating Hamiltonian. These frequencies were analyzed to obtain activation enthalpies and an activation volume for polaron hopping. There was a large suppression of hopping frequency with pressure, giving an activation volume for polaron hopping of 5.8 +/- 0.7 angstrom(3). This large, positive value is typical of ion diffusion, which indicates correlated motions of polarons and Li+ ions that alter the dynamics of both. Monte Carlo simulations were used to estimate the strength of the polaron-ion interaction energy.