Morphology and the thermoelectric properties of $\gamma$-Gd$_{x}$Dy$_{1-x}$S$_{1.5-y}$ solid solution ceramics

The temperature dependences of the Seebeck coefficient, the electrical conductivity (T = 300–873 K) and also the heat conductivity coefficient, and the thermoelectric figure of merit ($T$ = 300–770 K) of polycrystalline samples of solid solutions based on gadolinium and dysprosium sulfides of compositions $\gamma$-Gd$_{x}$Dy$_{1-x}$S$_{1.49}$ ($x$ = 0.1, 0.2, 0.3, 0.4) have been studied. It is found that the morphological features of the samples, namely, specifically of the crystallite surface that causes a change in the number of the deformation centers, determines the thermal conductivity of $\gamma$-Gd$_{x}$Dy$_{1-x}$S$_{1.49}$ solid solutions; the thermal conductivity of the composition with $x$ = 0.2 is found to decrease anomalously. For this composition, the lowest values of the Seebeck coefficient –371 $\mu$V/K at 873 K, resistivity 880 $\mu\Omega$ m at 873 K, and the thermal conductivity coefficient 0.68 $\pm$ 0.03 W/m K at 770 K have been obtained; in this case, the thermoelectric figure of merit achieves the value $ZT$ = 0.23.