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

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.