Anisotropy of the electrical conductivity of lithium heptagermanate crystals

The electrical conductivity sigma of single crystals of lithium heptagermanate Li2Ge7O15 is studied in an electric field in the frequency range 0.5-100 kHz at temperatures ranging from 300 to 700 K. Heating the crystal above 500 K gives rise to a pronounced anisotropy in the electrical conductivity, which differs in magnitude by one to two orders of magnitude for different directions of the measurement field along the crystallographic axes. It is shown that an increase in the electrical conductivity sigma with increasing temperature originates from charge transfer with an activation energy U = 1.04 eV. It is assumed that the thermally activated contribution to the electrical conductivity is governed by transport of lithium interstitial ions along channels in the structure of the Li2Ge7O15 compound.