Layered tellurides: stacking faults induce low thermal conductivity in the new In 2 Ge 2 Te 6 and thermoelectric properties of related compounds
Résumé
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[3 with combining macron]: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.