Study of half-metallic ferromagnetism and transport properties of Cs₂XI₆ (X = transition metals) for spintronic and energy harvesting applications

The double perovskites (DPs) are an emerging applicant for spintronic devices. In the present paper, the ferromagnetism, Curie temperature, and transport characteristics of Cs₂XI₆ (X = Ta, W, Re, Os) are controlled by the 5d-electrons of transition metal ions. The optimization curves indicate the stability of ferromagnetic states because the energy released in ferromagnetic states is greater than in antiferromagnetic states. The formation energy and phonons dispersion calculations confirmed thermodynamic and dynamic stabilities, while the Goldsmith tolerance factor ensured structural stability. The Heisenberg model and polarization density findings ensure ferromagnetism at high temperatures and 100 % spin polarization. Furthermore, the analysis of the band structures and density of states shows that half-metallic ferromagnetism has a major contribution from 5d-X and 5p-I states. The hybridization, double exchange model, exchange constants, and crystal field energies are also reported for the comprehensive study of the origin of ferromagnetism. Finally, the Seebeck coefficients, electrical and thermal conductivities, and thermoelectric power are illustrated comprehensively. The significant values of the figure of merit make these materials potential candidates for energy harvesting applications.