Mössbauer Spectrometry Study of Thermally-Activated Electronic Processes in Li_xFePO_4

The solid solution phase of Li_xFePO_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_(IS)) 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_(IS) 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_(IS), with an activation energy of 512 ± 12 meV for E_Q and one of 551 ± 7 meV for E_(IS). 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.