Mineralogy, Mossbauer spectra and electrical conductivity of triphylite Li(Fe2+, Mn2+) PO4

The electrical conductivity of monocrystalline triphylite, Li(Fe2+,Mn2+)PO4, with the orthorhombic olivine-type structure was measured parallel (parallel to) to the [010] direction and parallel to [001] (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 [010] 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 [001] 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.