Mössbauer Spectrometry Study of Thermally-Activated Electronic Processes in Li_xFePO₄

Abstract

The solid solution phase of Li x FePO4 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 57Fe ions. The activation energies for the fluctuations of E Q and E I S for Fe3+ are nearly the same, 570 ± 9 meV, suggesting that both originate from charge hopping. For the Fe2+ 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 Fe2+ 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.

MAS NMR Study of the Metastable Solid Solutions Found in the LiFePO4/FePO4 System

“…11 The fact that this activation energy is much larger than that predicted for free polaron mobility, as obtained by first principles calculations, 55 was ascribed to the existence of concerted lithium and electron hopping at these temperatures. 13 report E a =700+100 meV, v-6-10 14 Hz for Fe 2+ and E a =500±100 meV, v=M0 13 Hz for Fe 3+ , while Tan et al, 14 in a later paper from the same group, report E a =560 meV, v=2-10 Hz for both Fe and Fe . The discrepancy in values, even between Fe and Fe 3+ in a process that necessarily involves both, is considered to be within error, according to the authors.…”

“…3). The electron frequency hopping rates were again calculated using the MS data reported for a similar solid solution phase (LiojFePC^), 14 [Only a slight increase in temperature is enough to reduce the resolution of the] $C0.14 spectra, and by only 75-100°C, a single, albeit broad, signal is seen (Fig. 6).…”

“…8 " 10 Mossbauer and phonon densities of states studies indicate that this phase shows structural disorder at high temperatures. 11 " 14 Rietveld refinements of neutron diffraction patterns of quenched samples containing this phase suggest that it remains disordered at room temperature, with shorter average Fe-0 bonds and longer average Ml-0 lengths (Ml refers to Li when present, and to the center of the Li site when it is vacant) compared to LiFePC^. 8 Recently, another metastable line phase, Lio.34FeP04, was shown to coexist with FePC^ in slowly cooled crystals with lower lithium content.…”

“…8 Recently, another metastable line phase, Lio.34FeP04, was shown to coexist with FePC^ in slowly cooled crystals with lower lithium content. 10 ' 14 Li x FeP04 phases, with x between 0.3 and 0.8 have also been prepared by quenching high temperature solid-solution samples. 10 ' 14 In highly crystalline samples, the line phases Lio.6FeP04 and Lio.34FeP04 may coexist with the end members for long periods of time at room temperature.…”

Polaron-ion correlations inLixFePO4studied by x-ray nuclear resonant forward scattering at elevated pressure and temperature

“…[16][17][18] A small polaron quasiparticle comprises an electron or hole localized by atomic displacements of neighboring anions. When an electron transfers between Fe 2+ and Fe 3+ sites,…”