The ionic conductivity and the conjugate chemical diffusion of cations and electron holes in Li x Cu 2-x S (0 ≤ x ≤ 0.25) compositions are studied in the temperature range 623 -467 K. In accordance with prediction of the known "mixed mobile ions effect" the partial substitution of copper by lithium in copper sulfide leads to decreasing of the ionic conductivity and it is linked to an increase of activation energy of the ionic conductivity. In comparison with pure copper sulfide the chemical diffusion coefficients in the lithium-contained compounds are smaller too. The observed values of the activation energy of the chemical diffusion are close to those of the activation energy of the ionic conductivity.

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Phase transformation points in Li 0.25 Cu 1.75 Se mixed electronic-ionic conductor have been determined by calorimetric, conductometric and thermoelectric measurements. The phase transformation (PT) from triclinic to monoclinic occurs at 403 -413 K. At 503 -515 K the monoclinic phase is followed by a rhombohedral modification. Both of these PTs are accompanied by drops on the calorimetric curve. At about 653 K observed anomalies in the temperature dependencies of the ionic conductivity, of the chemical diffusion coefficient and the jump of the ionic Seebeck coefficient have been induced by the PT to hexagonal phase. Neutron diffraction studies reveal the cubic structure of Li 0.25 Cu 1.75 Se compound (with space group Fm3m) at 773 K. The corresponding PT causes anomalies in the electrical and diffusion properties at 703-713 K. Cu ions are statistically distributed over tetrahedral and trigonal voids in an Fm3m cage; lithium ions randomly occupy 32(f) positions.

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