Mineralogy, Mossbauer spectra and electrical conductivity of triphylite Li(Fe2+, Mn2+) PO4
Physics and Chemistry of Minerals 34(7): 485-494
The electrical conductivity of monocrystalline triphylite, Li(Fe2+,Mn2+)PO4, with the orthorhombic olivine-type structure was measured parallel (parallel to) to the  direction and parallel to  (space group Pnma), between similar to 400 and similar to 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, Mossbauer spectroscopy and microscopic analysis. The DC conductivity sigma(DC) was determined from AC impedance data (20 Hz-1 MHz) extrapolating to zero frequency. Triphylite shows sigma(DC) with activated behavior measured parallel to  between similar to 500 and similar to 700 K during the first heating up, with activation energy of E (A) = 1.52 eV; on cooling E (A) = 0.61 eV was found down to similar to 400 K and extrapolated sigma(DC) (295 K) similar to 10(-9) Omega(-1)cm(-1); parallel to  E (A) = 0.65 eV and extrapolated sigma(DC)(295 K) similar to 10(-9) 10(-10) Omega(-1)cm(-1), measured during the second heating cycle. The enhanced AC conductivity relative to sigma(DC) 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, sigma(DC) was measured on the synthetic compound LiMgPO4 with olivine-type structure, where also an activated behavior of sigma(DC) with E (A) similar to 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.