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.

This review presents thermoelectric phenomena in copper chalcogenides substituted with sodium and lithium alkali metals. The results for other modern thermoelectric materials are presented for comparison. The results of the study of the crystal structure and phase transitions in the ternary systems Na-Cu-S and Li-Cu-S are presented. The main synthesis methods of nanocrystalline copper chalcogenides and its alloys are presented, as well as electrical, thermodynamic, thermal, and thermoelectric properties and practical application. The features of mixed electron–ionic conductors are discussed. In particular, in semiconductor superionic copper chalcogenides, the presence of a “liquid-like phase” inside a “solid” lattice interferes with the normal propagation of phonons; therefore, superionic copper chalcogenides have low lattice thermal conductivity, and this is a favorable factor for the formation of high thermoelectric efficiency in them.

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Synthesis, X-ray phase analysis, electron microscopy and investigations of the thermoelectric and thermal properties of nanocrystalline copper sulfide alloys contained sodium are presented. At room temperature, the alloys are a mixture of three phases of copper sulfide-the monoclinic phase of Na 2 Cu 4 S 3 , the hexagonal phase of Cu 2 S and the cubic phase of Cu 9 S 5 (digenite). The predominant phase is Na 2 Cu 4 S 3 (with content from 57 to 85 volume %). The particle sizes in the compacted samples lie in the range from 20 to 400 nm. For all samples DSC studies revealed a first-order phase transition in the (370-380) K region with enthalpies from 5234 to 11720 J/kgK. The heat capacity varies within the range (0.15-0.48) J/(gK). The electrical conductivity, Seebeck coefficient and thermal conductivity were measured in the temperature range from 290 to 590 K. A very low thermal conductivity of the samples was observed in the interval of (0.1-0.6) Wm −1 K −1. The Seebeck coefficient has a value higher than 0.2 mV/K for Na 0.15 Cu 1.85 S composition, but a low electrical conductivity about 10 S/cm limits the maximum dimensionless thermoelectric efficiency ZT of the material at 0.3 in the temperature range 290-590 K.