A Mössbauer Spectrometry Study of Electronic Processes in Li_xFePO_4

Li_xFePO_4 solid solutions with different lithium concentrations, x, were investigated by Mössbauer spectrometry at temperatures between 25° and 210°. 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) do not have a measurable dependence on the concentration of lithium, but varies between Fe^(3+) and Fe^(2+). Typically, for the Fe^(2+) components of the spectra, the fluctuations of E_Q occurred at lower temperatures than the fluctuations of E_(IS), with activation energy of 512 meV for E_Q and 551 meV for E_(IS). The more facile fluctuations of E_Q for Fe^(2+) are evidence for local motions of neighboring Li^+ ions