Purpose. The purpose of this research was to growth single crystals in Cu7PS6–Cu6PS5Br and Cu7PS6–Cu6PS5I systems, investigate the temperature dependences and compositional behaviour of Seebeck coefficient of Cu7−xPS6−xBrx and Cu7−xPS6−xIx mixed crystals, analyze the influence of heterovalent substitution on Seebeck coefficient values. Methods. Crystal growth was carried out using pre-synthesized Cu7PS6, Cu6PS5Br and Cu6PS5I compounds. Single crystals of initial compounds and solid solutions Cu7−xPS6−xBrx (x=0.05; 0.15; 0.2; 0.5; 0.75; 0.9), Cu7−xPS6−xIx (x=0.05; 0.1; 0.15; 0.2; 0.5; 0.75; 0.9) were growth by direct crystallization from melt. Obtained crystals were investigated by XRD technique. The temperature dependences of Seebeck coefficient were measured in the temperature range of 293-383 K at a heating rate of 30 K/h and temperature gradient which not exceed 5 K. Temperature dependences of Seebeck coefficient for individual compounds Cu7PS6, Cu6PS5Br(I) and solid solutions based on them were obtained in the heating mode. Results. Based on the measurements of thermoelectric parameters, established that for all single crystals in Cu7PS6–Cu6PS5Br and Cu7PS6–Cu6PS5I systems the values of the Seebeck coefficient in the studied temperature range are negative, which indicate the electronic type of conductivity (n-type). In the Cu7PS6–Cu6PS5Br system, with an increase of temperature in the range of 293-383 K, the absolute values of Seebeck coefficient increase monotonically for individual compounds Cu7PS6 (by 1%), Cu6PS5Br (by 10%) and solid solution with x = 0.05 (by 24%). For compositions x = 0.05; 0.15; 0.2; 0.5; 0.75 and 0.9, the absolute values of Seebeck coefficient decrease by an average of 22%. In the Cu7PS6–Cu6PS5I system, with an increase of temperature in the range of 293-383 K, the absolute values of Seebeck coefficient increase monotonically only for individual compounds Cu7PS6 (1%), Cu6PS5I (11%), and in the case of all solid solutions the absolute values of Seebeck coefficient decrease on average by 19%. Compositional dependences of Seebeck coefficient are characterized by anomalous behaviour. Thus, in the Cu7−xPS6−xBrx system the maximal absolute values of Seebeck coefficient are reached at х=0.05 (-966.8 µV/K at 373K) and for Cu7−xPS6−xIx system – at х = 0.15 (-563.2 µV/K at 373K). The anomalous behaviour of the compositional dependences of Seebeck coefficient in the investigated systems in both cases are observed for solid solutions which are crystallized in P213 space group. Conclusions. Single crystals in Cu7PS6–Cu6PS5Br and Cu7PS6–Cu6PS5I systems were grown by the method of direct crystallization from the melt. The thermoelectric parameters of Cu7−xPS6−xBrx and Cu7−xPS6−xIx solid solutions were studied in the temperature range of 293-383 K. The compositional behaviour of the Seebeck coefficient was analyzed and the influence of heterovalent substitution on the change of Seebeck coefficient in Cu7−xPS6−xBrx and Cu7−xPS6−xIx solid solutions were studied. The value of Seebeck coefficient for binary copper chalcogenides and investigated copper-containing argyrodites was compared.
Keywords: argyrodites, mixed crystals, Seebeck coefficient, compositional dependence