Layered tellurides: stacking faults induce low thermal conductivity in the new In2Ge2Te6 and thermoelectric properties of related compounds

Abstract

A new ternary layered compound In₂Ge₂Te₆, belonging to the hexatellurogermanate family has been synthesized from the reaction of appropriate amounts of the pure elements at high temperature in sealed silica tubes. In₂Ge₂Te₆ crystallizes in the rhombohedral space-group R:H with lattice parameters a = 7.0863(3) Å and c = 21.206(2) Å and its structure is resolved using single crystal X-ray diffraction. The transport properties (Seebeck coefficient, resistivity and thermal conductivity) of compounds belonging to the family AMTe₃ (A = In and Cr; M = Ge and Si) are reported. All compounds are p-type semiconductors. InSiTe₃ and Cr₂Si₂Te₆ are too resistive to be good thermoelectric materials, with maximal power factors of 10⁻⁶ and 10⁻⁵ W m⁻² K⁻² at 473 K, while In₂Ge₂Te₆ and Cr₂Ge₂Te₆ exhibit maximal values of about 10⁻⁴ and 10⁻³ W m⁻² K⁻² at 673 K, respectively. All compounds exhibit thermal conductivity below 2 W m⁻¹ K⁻¹, with values dropping to 0.35 W m⁻¹ K⁻¹ at 673 K for In₂Ge₂Te₆. Transmission electron microscopy evidences stacking faults explaining such low thermal conductivities. The best ZT values are observed for Cr₂Ge₂Te₆ with 0.45 at 773 K and In₂Ge₂Te₆ with 0.18 at 673 K. Among these layered structures, a spark plasma sintered Cr₂Ge₂Te₆ sample exhibits some thermal conductivity anisotropy but only weakly due to crystallite orientations.