Structure and physical properties of a new telluride Mg1.2(1)In1.2(1)Si2Te6

A new Mg-containing disordered quaternary telluride Mg_1.2(1)In_1.2(1)Si₂Te₆ is prepared at 1223 K by direct fusion of elements. This mixed metal telluride adopts a trigonal (P$\bar{3}$1m space group) structure having refined cell constants of a = b = 7.0603(3) Å and c = 7.1681(4) Å. Four unique crystallographic sites are filled in the structure: one disordered metal site (In1/Mg1), one partially filled Mg2, one Si1, and one Te1. Each metal ion (In and Mg) in the structure sits at the center of a distorted octahedron of Te1 atoms. Two Si atoms are involved in forming an ethane-like Si₂Te₆ unit with the help of a Si−Si bond. The local coordination environment around each Si atom can be described as a distorted tetrahedron comprising one Si1 and three Te1 atoms. A polycrystalline sample with the loaded composition of Mg_1.2In_1.2Si₂Te₆ shows an optical bandgap of 1.02(2) eV. The p-type semiconducting nature of the Mg_1.2In_1.2Si₂Te₆ sample was established from the positive values of the Seebeck coefficient (S) and resistivity studies. The complex structure of the Mg_1.2In_1.2Si₂Te₆ phase, which features heavy elements (In and Te), helps to achieve ultralow total thermal conductivity (k_tot) of 0.33 W/mK at 773 K.