Ionic Conductivity in Single Crystals, Amorphous and Nanocrystalline Li2Ge7O15 Doped with Cr, Mn, Cu, Al, Gd

Electrical conductivity sigma was studied in single crystals, amorphous and nanocrystalline lithium heptagermanate Li2Ge7O15 doped with Cr, Mn, Cu, Al, and Gd ions. The single crystals of Li2Ge7O15 were grown by Czhochralskii method; the glass was prepared by quenching the melt. Nanometer-sized crystals were obtained by controlled crystallization of the amorphous phase. Heterovalent doping strongly influences s in Li2Ge7O15 single crystals and makes it possible to control ionic transport in a broad range. The doping effects are discussed based on the models of the impurity ions in Li2Ge7O15 crystal structure. In undoped amorphous and nanocrystalline Li2Ge7O15, conductivity increases in about three and four orders of magnitude correspondingly as compared to a single crystal. Doping of Li2Ge7O15 glass and nanocrystals with small amounts of the impurities has a less pronounced effect than for the single crystal. It is argued that electrical conductivity in various structural states of Li2Ge7O15 is provided by mobile lithium ions which are weakly bound to the germanium-oxygen framework of the structure.