Mössbauer Spectrometry Study of Thermally-Activated Electronic Processes in LixFePO₄

The solid solution phase of Li x FePO 4 with different Li concentrations, x, was investigated by Mössbauer spectrometry at temperatures between 25 and 210 °C. The Mössbauer spectra show a temperature dependence of their isomer shifts (E I S ) and electric quadrupole splittings (E Q ), typical of thermally activated, electronic relaxation processes involving 57 Fe ions. The activation energies for the fluctuations of E Q and E I S for Fe 3+ are nearly the same, 570 ± 9 meV, suggesting that both originate from charge hopping. For the Fe 2+ components of the spectra, the fluctuations of E Q occurred at lower temperatures than the fluctuations of E I S, with an activation energy of 512 ± 12 meV for E Q and one of 551 ± 7 meV for E I S . The more facile fluctuations of E Q for Fe 2+ are evidence for local motions of neighboring Li + ions. It appears that the electron hopping frequency is lower than that of Li + ions. The activation energies of relaxation did not have a measurable dependence on the concentration of lithium, x .