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

Layered ternary (MnX2Te4)(X2Te3)n (X = Bi or Sb, n = 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, TC. Recently, we found that Mn enrichment and Mn/Sb site intermixing increase the TC from 30 K in MnSb2Te4 to 58 K in Mn2.01(1)Sb1.19(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 TC = 65–73 K nearing the liquid-nitrogen threshold. By combining single-crystal X-ray diffraction, ab initio modeling and bulk DC magnetization, we pinpoint the relationship between the lattice symmetry and the magnetic order. The trigonal Mn1.90(1)Sb1.39(1)Te4 phase with manganese atoms in the van der Waals gap hosts the highest-TC ferrimagnetic state. We get the first insights into its electronic structure and topological nature by ab initio 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. R3̅m) to a cubic lattice (sp. gr. Fm3̅m), which is reminiscent of the structural polymorphism of the Ge–Sb–Te thermoelectrics.