Spintronic and thermoelectric functionalities in transition-metal-based K₂MCl₆ double perovskites (M = Mn, Mo, Ta, Tc)

The vacancy-ordered double perovskites K₂MCl₆ (M = Mn, Mo, Ta, Tc) were systematically investigated using first-principles calculations to explore their structural, electronic, magnetic, and thermoelectric properties. Calculations based on the FP-LAPW + lo method within the WIEN2k framework, supported by phonon dispersion and AIMD analyses, confirm the thermodynamic and dynamical stability of all compounds in the cubic Fm-3 m phase. The transition-metal substitution strongly influences lattice parameters, elastic behavior, and electronic characteristics. Magnetic calculations reveal stable ferromagnetic ground states for all systems. K₂MnCl₆, K₂TaCl₆, and K₂TcCl₆ exhibit half-metallic behavior with complete spin polarization, whereas K₂MoCl₆ behaves as a spin-polarized semiconductor. Strong exchange interactions and spin–orbit coupling effects significantly govern the electronic structure. Thermoelectric analysis demonstrates remarkable performance, including Seebeck coefficients up to − 1150 µV/K and electronic figures of merit approaching unity over 300–1000 K, highlighting K₂MCl₆ compounds as promising materials for spintronic and thermoelectric energy applications.