Rapid Electron Dynamics at Fe Atoms in Nanocrystalline Li_(0.5)FePO_4 Studied by Mössbauer Spectrometry

Abstract

Two-phase mixtures of Li_(0.5)FePO_4 with crystal sizes as small as 25 nm were prepared by solid-state reaction, ball milling, and chemical delithiation. Mssbauer spectra of nanocrystalline Li_(0.5)FePO_4 found evidence for a thin layer of Fe^(3+) at the crystal surfaces. Spectra acquired at temperatures from 25 to 225 °C showed thermally driven electronic relaxations, where the electric field gradients (EFG) of the main Fe^(3+) and Fe^(2+) spectral components decreased with temperature. The isomer shifts (IS) of Fe^(3+) and Fe^(2+) showed similar thermal trends, indicative of valence fluctuations caused by small polaron hopping. The activation energies obtained from the temperature dependence of the EFG were 410 meV for Fe^(3+) and 330 meV for Fe^(2+), and an activation energy of 400 meV was obtained for the IS of both. The rapid valence electron hopping between Fe sites is intrinsic to electronic conductivity in Li_(x)FePO_4, which is calculated to be higher than most reports for bulk material.

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