The electrical conductivity σ of single crystals of lithium heptagermanate Li2Ge7O15 was studied in an electric field in the frequency range 0.5–100kHz at 300 to 700K. Heating the crystal above 500K 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 was shown that an increase in the electrical conductivity σ with increasing temperature originated from charge transfer with an activation energy U = 1.04eV. It was assumed that the thermally activated contribution to the electrical conductivity was governed by transport of Li interstitial ions along channels in the structure of the Li2Ge7O15 compound.

Anisotropy of the Electrical Conductivity of Lithium Heptagermanate Crystals. M.D.Volnyanskiĭ, M.P.Trubitsyn, Y.A.H.Obaidat: Physics of the Solid State, 2008, 50[3], 422-4