Structural and physical properties of chalcogenide materials for thermoelectric applications

Investigations of structural and thermoelectric properties of shandite-type sulphides and diamond-like quaternary chalcogenides have been carried out. Electron and hole doping is investigated in Co3Sn2S2 through the chemical substitution of cobalt by its neighbouring elements. The synthesis of two series, Co3-xNixSn2S2 (0 ≤ x ≤ 3) and Co3-xFexSn2S2 (0 ≤ x ≤ 0.6) is described. Powder neutron diffraction experiments have been carried out for both series, while 119Sn and 57Fe Mössbauer spectroscopy measurements have been conducted for Co3-xFexSn2S2 (0 ≤ x ≤ 0.6). The materials become more metallic with increasing nickel content, while the substitution of Co by Fe induces a metal-to-n-type-semiconductor transition, and an increase in the thermoelectric figure-of-merit (ZT) is achieved to a maximum of 0.2 at 525 K. Further improvement in ZT is achieved by the simultaneous substitution at the transition metal and main-group metal sites. In the series Co2.667Fe0.333Sn2-yInyS2 (0 ≤ y ≤ 0.7), the materials become more semiconducting with increasing In content and a marked increase in the Seebeck coefficient is observed. Co2.667Fe0.333Sn1.4In0.6S2 exhibits ZT = 0.28 at 473 K. The quaternary chalcogenides A2ZnCQ4 (A = Cu, Ag; C = Sn, Ge; Q = S, Se) were synthesized and investigated using powder neutron diffraction. Rietveld analysis reveals that all phases crystallize in the kesterite structure at room temperature and the Cucontaining compounds exhibit partial Cu/Zn disorder in the z = 0.25 and 0.75 planes. For Cu2ZnGeS4, an irreversible phase transition is observed at 1123 K from the kesterite to the wurtzite-stannite structure. For Cu2ZnGeSe4, the cations in z = 0.25 and 0.75 planes become fully disordered at 473 K, while simultaneously, Cu vacancies are created. Hole doping in Cu2+xZnGe1-xSe4 (0 ≤ x ≤ 0.15) results in a marked decrease in electrical resistivity, increasing the ZT to 0.18 at 573