Thermal and electronic transport properties of ACrX ₂ superionic conductors (A=Cu, Ag and X=S, Se)

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Superionic conductors, including A Cr X 2 ( A =Ag, Cu; X = S, Se) compounds, have attracted attention due to their low lattice thermal conductivity and high ionic conductivity. These properties are driven by structural characteristics such as anharmonicity, soft bonding, and disorder, which enhance both fast ion transport and thermal resistance. In the present study, we investigate the impact of various factors (e.g. A -site disorder, microstructure, speed of sound and chemical composition) on the thermal conductivity of the compounds CuCrS 2 , CuCrSe 2 , AgCrS 2 and AgCrSe 2 . The samples were synthesized using solid state reaction, ball milling and subsequent spark plasma sintering, and thermal diffusivity, electrical resistivity, Hall coefficients and Seebeck coefficients were measured as a function of temperature. The selenides were found to behave as degenerate semiconductors, with reasonable thermoelectric figure of merit (up to 0.79 in CuCrSe 2 ), while the sulfides behaved as non-degenerate semiconductors with high electrical resistivity. At room temperature, all samples are in the ordered phase and show low lattice thermal conductivity ranging from 0.60 W m −1 -K in AgCrSe 2 to 1.1 W m −1 -K in CuCrSe 2 . Little reduction in lattice thermal conductivity was observed in the high-temperature phase, despite the increased disorder on the cation site and the onset of superionic conductivity. This suggests that the low lattice thermal conductivity in A Cr X 2 compounds is an inherent property of the crystal structure, caused by anharmonic bonding and diffuson dominated transport.