High temperature thermoelectric energy conversion in half metallic Cs2MBr6 double perovskites M=Mn, Mo, Ta, Ir from first principles

Vacancy-ordered double perovskites have recently emerged as promising multifunctional materials for energy and spin-based technologies. In this work, we present a comprehensive first-principles investigation of the structural, electronic, magnetic, mechanical, and thermoelectric properties of Cs2MBr6 (M=Mn, Mo, Ta, Ir). The compounds are found to be thermodynamically and mechanically stable, exhibiting ductile mechanical behavior suitable for device fabrication. Electronic structure analysis reveals robust half-metallic ferromagnetism with 100% spin polarization at the Fermi level, classifying Cs2MnBr6 and Cs2TaBr6 as inverted half-metals, while Cs2MoBr6 and Cs2IrBr6 show conventional half-metallic character. Remarkably high thermoelectric performance is predicted over a wide temperature range (300-1000 K). Substantial Seebeck coefficients exceeding 400-1000 μV/K at room temperature, combined with thermally activated electrical conductivity and suppressed electronic thermal conductivity, yield near-unity and thermally stable figures of merit (ZT ≈ 0.83-0.99). The outstanding thermoelectric efficiency is directly correlated with spin-selective transport, large spin-dependent band gaps, and favorable carrier effective masses.