Fehr, K. T.; Hochleitner, Rupert; Schmidbauer, E.; Schneider, J.
Mineralogy, Mössbauer spectra and electrical conductivity of triphylite Li(Fe2+,Mn2+) PO4.
In: Physics and Chemistry of Minerals, Vol. 34, Nr. 7: S. 485-494
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The electrical conductivity of monocrystalline triphylite, Li(Fe2+,Mn2+)PO4, with the orthorhombic olivine-type structure was measured parallel (∥) to the  direction and ∥  (space group Pnma), between ∼400 and ∼700 K. Electrical measurements on triphylite are of technological interest because LiFePO4 is a promising electrode material for rechargeable Li batteries. Triphylite was examined by electron microprobe, ICP atomic emission spectroscopy, X-ray diffraction, Mössbauer spectroscopy and microscopic analysis. The DC conductivity σDC was determined from AC impedance data (20 Hz-1 MHz) extrapolating to zero frequency. Triphylite shows σDC with activated behavior measured ∥  between ∼500 and ∼700 K during the first heating up, with activation energy of EA = 1.52 eV; on cooling EA = 0.61 eV was found down to ∼400 K and extrapolated σDC (295 K) σ10-9ω-1 cm-1; ∥  EA = 0.65 eV and extrapolated σDC(295 K) ∼10-9 to 10-10 ω-1cm-1, measured during the second heating cycle. The enhanced AC conductivity relative to σDC at lower temperatures indicates a hopping-type charge transport between localized levels. Conduction during the first heating up is ascribed to ionic Li+ hopping. DC polarization experiments showed conduction after the first heating up to be electronic related to lowered activation energy. Electronic conduction appears to be coupled with the presence of Li+ vacancies and Fe3+, formed by triphylite alteration. For comparison, σDC was measured on the synthetic compound LiMgPO4 with olivine-type structure, where also an activated behavior of σDC with EA ∼1.45 eV was observed during heating and cooling due to ionic Li+ conduction; here no oxidation can occur associated with formation of trivalent cations.