Weakening the spin-orbital coupling for band convergence

Band convergence is a favorable strategy that is utilized to improve the thermoelectric performance in experiment, whereas a theoretical guidance for this purposive design is rare. With high-level first-principles treatment of electronic and transport properties in the emerging ABX (A = Ca, Sr, Ba; B = Cu, Ag; X = P, As, Sb) Zintl systems, a principled scheme via weakening the spin-orbital coupling is proposed based on symmetry and orbital analysis to promote the band convergence. This given rule is numerically confirmed in BaAgX alloys. Despite the intensified intervalley electron-phonon scattering, the alloying of BaAgP in BaAgSb prominently triggers the band convergence and contributes to much larger density of state effective mass, which eventually results in a similar to 40% improvement in zT values. This work offers an applicable principle for designing band convergence, which also broads the compositions of thermoelectrics to light and earth-abundant elements.