TAGS-related indium compounds and their thermoelectric properties – the solid solution series (GeTe)xAgInySb1−yTe2 (x = 1–12; y = 0.5 and 1)
Various members of the solid solution series (GeTe)xAgInySb1−yTe2 can be obtained by quenching high-temperature phases (x = 12 for y = 1 and x > 5 for y = 0.5). In contrast, high-temperature and high-pressure conditions (2.5 GPa, 350 °C) are required for the synthesis of members with In contents >3.6 atom% (such as x < 12 for y = 1 and x < 5 for y = 0.5) in order to avoid the formation of AgInTe2. The latter exhibits tetrahedrally coordinated indium atoms under ambient conditions and therefore does not form mixed crystals with tellurides of germanium and antimony that are characterized by sixfold coordinated atom sites. Solid solutions with x ≤ 5 crystallize in rocksalt-type structures with octahedrally coordinated indium, whereas the ones with x > 5 adopt the α-GeTe structure type (3 + 3 coordination). Thus, in all samples investigated, 3 or 4 cations are disordered at one Wyckoff position. The quenched high-temperature or high-pressure phases, respectively, are almost homogeneous. Their powder X-ray diffraction patterns suggest pure phases; yet, high-resolution electron microscopy occasionally reveals a very small extent of nanoscopic precipitates as well as dislocations and twinning. (GeTe)5.5AgIn0.5Sb0.5Te2 shows a maximal ZT value of 0.75 even when (partial) decomposition into the TAGS material (GeTe)11AgSbTe2 and chalcopyrite-type AgInTe2 has occurred at 300 °C. (GeTe)5.5AgInTe2 prepared under high-pressure conditions exhibits a ZT value of 0.6 at 125 °C, i.e. far below the decomposition temperature and thus is an interesting new low-temperature thermoelectric material.