Electronic Structure and Magnetic Properties of 4d and 5d Transition Metal Halide Lead-Free Perovskites for Possible Spintronic Applications

The present report aims at the detailed investigation of structural dynamics, ferromagnetism, and band profiles of cesium–molybdenum (Cs2MoX6) and cesium–tungsten (Cs2WX6)-based halide perovskites, where X = Cl and Br. A ferromagnetic ground state originates from the only 4d-states of Molybdenum and 5d-states of tungsten in both the cases, respectively. The relaxed lattice systems provide the precise atomic arrangements in agreement with the previous experimental data. Experimentally, Cs2MoCl6 and Cs2WCl6 exhibit cubic structures with semiconducting properties, while Cs2MoBr6 and Cs2WBr6 are assumed for comparison only. Present calculations of elastic constants predict these systems as stable, and positive phonon modes ensure the dynamic stability as well, where all the optical modes are similar to a considerable difference in low phonon manifolds having mixed optical and acoustic characteristics. Contrary to the previous theoretical studies on Cs2MoCl6, the spin-polarized HSE06 calculations decide the spin-up semiconducting nature of all these isostructural systems with a maximum gap in (Mo/W)Br compared to their Cl counterparts. Ferromagnetic exchange interactions between magnetic atoms via nonmagnetic halogen atoms constitute a total spin magnetic moment μ ∼ 2.0 μB for each material, where a transition metal atom only contributes the magnetic moment to the whole unit cell. The magnetic moment is primarily contributed by the transition metal atoms, and the compounds display high Curie temperatures (TC) ranging from 480 to 681 K. The magnetic properties confirm a second-order phase transition, with the derivative of magnetization (dM/dT) revealing sharp variations at TC. Magnetocaloric properties show a linear dependence of magnetic entropy change and relative cooling power on external magnetic fields. These findings highlight the potential of Cs2Mo/WX6 compounds for spintronic, optoelectronic, and energy-efficient magnetic refrigeration applications.