Multifunctionality in Lead-Free Rb₂XTaBr₆ (X = Na, Li) Double Perovskites: A First-Principles Study of Structural,...

First-principles FP-LAPW calculations were used to investigate the structural, mechanical, electronic, optical, and thermoelectric properties of the double halide perovskites Rb₂NaTaBr₆ and Rb₂LiTaBr₆. Structural optimization confirmed their stable cubic phase. Elastic analysis shows they are mechanically stable, ductile, and anisotropic. Spin-polarized calculations reveal that both compounds are half-metallic ferromagnets with an integer total magnetic moment of 2 μB per formula unit, dominated by the Ta-d states. They exhibit strong dielectric response and strong UV light absorption originating from interband electronic transitions, despite their intrinsic metallic ground state. Boltzmann transport theory revealed a temperature-dependent enhancement of electrical conductivity and power factor. Both materials, especially Rb₂LiTaBr₆, show promise for high-temperature thermoelectric and UV optoelectronic applications.