REGULARITIES OF CHANGES AND PREDICTION OF MELTING POINTS AND THERMOELECTRIC FIGURE OF MERIT FOR THE COMPOUNDS Tl4BIVC3 AND Tl9BVC6 (BIV – Si, Ge, Sn, Pb; BV – P, As, Sb, Bi; C – S, Se, Te)

The study is a continuation of the systematic investigation of the compounds which are the crystal chemical analogues of the Tl5Te3 structure, crystallizing in the tetragonal space groups I4/mcm or P4/ncc. The studies conducted by the staff of the Uzhhorod National University over the past decades have found that these compounds exhibit a number of physical properties that are promising for practical use in modern technology. The subject of this work was the relationship between some physicochemical properties of the ternary chalcogenide compounds, such as Tl4BIVC3 and Tl9BVC6 (where BIV = Si, Ge, Sn, Pb; BV = P, As, Sb, Bi; and C = S, Se, Te), and the average charge (Zave) of the compouns’ atomic nuclei. Some of the predicted values of the melting temperature presented in the work may correspond to the temperature of the peritectic (or other type) transformation, as the nature of the formation and properties of this type of compounds have not yet been studied. For the first time, based on the results of collecting the literature data, detailed analysis and conclusions from them, the predicted melting temperature (Tm) values for 24 chalcogenide compounds of the Tl4BIVC3 (I) and Tl9BVC6 (II) compositions were established. There are five separate groups of the ternary chalcogenides of the both aforementioned types with a linear dependence of the changes in properties in the coordinates Tm – Zave, which include the analogues with both space groups, P4/ncc and I4/mcm. The limiting range of the Zave values of transformation of the above compounds from the space group P4/ncc to the I4/mcm one has been established; the defined range is within the Zave of ~60-62. The ternary compounds Tl9P(As,Sb,Bi)S6, Tl9P(As)Se6, Tl4Si(Ge,Sn,Pb)S3 and Tl4Si(Ge,Sn)Se3 belong to the space group P4/ncc, while the chalcogenides Tl9P(As,Sb,Bi)Te6, Tl9Sb(Bi)Se6, Tl4Si(Ge,Sn,Pb)S3 and Tl4PbSe3 belong to the space group I4/mcm. There is a clear tendency to increase the value of the thermoelectric figure of merit ZT for individual thio-, seleno- and tellurium-derived compounds at the substitution of the elements Si → Ge → Sn → Pb. Similar changes in the ZT values were also found for the vast majority of the type (II) compounds at the P → As → Sb → Bi transitions. The exceptions to this trend are thio-, selenium- and tellurium bismuth derivatives.