Polaron Mobility and Disordering of the Sodium Sublattice in Triphylite-Na_xFePO_4

The interplay between sodium ordering and electron mobility in Na_xFePO_4 was investigated using a combination of synchrotron X-ray diffraction and Mössbauer spectrometry. Synchrotron X-ray diffraction measurements were carried out for a range of temperatures between 298 and 553 K. Rietveld analysis of the diffraction patterns was used to determine the temperature of sodium redistribution on the lattice. This diffraction analysis also gives new information about the phase stability of the system. Mössbauer spectra were collected in the same temperature range. An analysis of the temperature evolution of the spectral shapes was used to identify the onset of fast electron hopping and determine the polaron hopping rate. The temperature evolution of the iron site occupancies from the Mössbauer measurements, combined with the synchrotron diffraction results, shows a relationship between the onset of fast electron dynamics and the loss of local order on the sodium sublattice.