Syntheses, crystal structures, and physical properties of non-centrosymmetric MgIn₂Te₄ and MnIn₂Te₄

In this study, we prepared two ternary tellurides, MgIn₂Te₄ and MnIn₂Te₄ (single crystals and bulk polycrystalline phases), by directly fusing the elements at high temperatures using the sealed tube synthesis method. Single crystal X-ray diffraction studies confirmed the non-centrosymmetric tetragonal structures (space group: I 4‾ 2m) of the MgIn₂Te₄ and MnIn₂Te₄ phases with two formula units (Z = 2). Interestingly, the refined unit cell lengths of a = b = 6.2149(5) Å and c = 12.4240(19) Å for the MgIn₂Te₄ are slightly longer than those of the MnIn₂Te₄ structure (a = b = 6.1901(3) Å and c = 12.3877(7) Å). The asymmetric unit of these two MIn₂Te₄ (M = Mg and Mn) isostructural compounds contains three crystallographically independent sites: two disordered metal sites (M1/In1, M2/In2) and one Te1 site. Each metal atom in these structures is tetrahedrally coordinated with four Te1 atoms. Optical absorption studies for polycrystalline MgIn₂Te₄ and MnIn₂Te₄ samples showed bandgaps of 1.1(2) eV and 0.9(2) eV, respectively, consistent with these powders' black color. Seebeck studies confirm that holes are the majority of charge carriers for the MgIn₂Te₄ sample. The thermal conductivity (k_tot) studies of the samples reveal ultralow values at 773 K: 0.37 Wm⁻¹K⁻¹ for the MgIn₂Te₄ and 0.44 Wm⁻¹K⁻¹ for the MnIn₂Te₄. Magnetic studies on the polycrystalline MnIn₂Te₄ show a paramagnetic behavior with a magnetic moment of 5.73 μ_B. Our theoretical studies indicate the possibility of achieving a high thermoelectric figure of merit (zT > 1.5) by optimizing the hole concentration in the MgIn₂Te₄ sample.