Conductivity of lithium heptagermanate glasses and single crystals doped with 3d- ions

Lithium heptagermanate Li2Ge7O15 glasses doped with Cr and Mn were prepared by the melt quenching. Electrical conductivity of pure and doped glasses was measured in AC field (f=1 kHz) within the temperature range 300-700 K. It is shown that both impurities increase conductivity of the glasses. The influence of Cr doping is very subtle whereas the maximal effect is observed for the Li2Ge7O15:Mn glass. The obtained data are compared with the results of measuring in Li2Ge7O15:Cr, Mn single crystals. It is noted that doping affects of the glasses and single crystals in different ways. In particular, the conductivity of the single crystals can be controlled by doping in a wide range: introducing Cr impurity strongly increases and vice versa doping with Mn sufficiently decreases value. Doping effect is interpreted on the basis of the previous EPR data giving information on the location of Cr and Mn centers in the Li2Ge7O15 structure. Cr ions substitute for Ge ions and are fixed within oxygen octahedra. On the other hand, Mn ions are located within the cavities formed by Ge‒O structural framework and are assumed to be mobile enough to contribute to charge transfer.