Metal-like temperature dependent conductivity in fast Li⁺ ionic conductor Lithium Lanthanum Titanate

Fast Li⁺ ionic conductor Lithium Lanthanum Titanate (La_0.55Li_0.35TiO₃) is synthesized by the tartaric acid assisted sol-gel synthesis and powder compacts are sintered. The electrical properties of the obtained ceramics are studied at frequencies up to 10 GHz and at temperatures up to 1280 K. The obtained results allow accurate determination of the specific conductivity of ceramic grains, which are found to reach 30 S/m in the region of high temperatures. A strong deviation from the Arrhenius law is observed with continuously decreasing activation energy of conductivity at temperatures T > 400 K, and a metal-like temperature dependent ionic conductivity (which may be also defined by positive temperature coefficient of resistivity - TCR) is observed at T > 1100 K. The phenomenon of this positive TCR may be related to the alteration of the occupancy of A positions by La³⁺ in La-enriched and La-depleted planes of crystal lattice of LLTO and to the alteration of the conditions for concerted migration of Li⁺ ions.