Mn Interstitialsin Layered Mn_1+<i>x</i>Sb_{2–2/3<i>x</i>}Te₄:Structural Modification and Curie Temperature Boost

Layered ternary (Mn<i>X</i>₂Te₄)­(<i>X</i>₂Te₃)_<i>n</i> (<i>X</i> = Bi or Sb, <i>n</i> = 0–3) tellurides are intensely studied as perspective magnetic topological insulators: MnBi₂Te₄ and MnBi₄Te₇ demonstrate the quantum anomalous Hall effect up to several Kelvin. To enlarge the temperature range for this quantum behavior, the materials require a net magnetization and a high Curie temperature, <i>T</i>_C. Recently, we found that Mn enrichment and Mn/Sb site intermixing increase the <i>T</i>_C from 30 K in MnSb₂Te₄ to 58 K in Mn_2.01(1)Sb_1.19(1)Te₄. Here, we synthesize the utmost manganese-rich members of this materials family, with an average Mn content of 28–32 at. % and the record <i>T</i>_C = 65–73 K nearing the liquid-nitrogen threshold. By combining single-crystal X-ray diffraction, <i>ab initio</i> modeling and bulk DC magnetization, we pinpoint the relationship between the lattice symmetry and the magnetic order. The trigonal Mn_1.90(1)Sb_1.39(1)Te₄ phase with manganese atoms in the van der Waals gap hosts the highest-<i>T</i>_C ferrimagnetic state. We get the first insights into its electronic structure and topological nature by <i>ab initio</i> modeling using density functional theory. Initiated by the filling of the van der Waals gap, the compound mimics a structural transition from a trigonal (sp. gr. <i>R</i>3̅<i>m</i>) to a cubic lattice (sp. gr. <i>Fm</i>3̅<i>m</i>), which is reminiscent of the structural polymorphism of the Ge–Sb–Te thermoelectrics.