Halogen substitution-driven phonon softening and enhanced thermoelectric performance in vacancy-ordered Rb2SeX6 (X = Br and I) perovskites

Abstract

Vacancy-ordered halide perovskites provide a promising platform for thermoelectric applications because of their soft lattices and intrinsically weak heat transport. Here, the thermoelectric properties of Rb2SeX6 (X = Br and I) were systematically investigated using first-principles calculations, temperature-dependent phonon renormalization, and AMSET carrier-transport simulations. Bonding analyses reveal a pronounced hierarchy between strong Se–X interactions and weak ionic Rb–X interactions. Replacing Br with the heavier and more polarizable I weakens chemical bonding, softens the phonon spectrum, and reduces phonon lifetimes and group velocities. Consequently, Rb2SeI6 exhibits ultralow lattice thermal conductivities of approximately 0.73 Wm−1K−1 at 300 K and 0.62 W m−1 K−1 at 600 K. Rb2SeI6 also shows favorable electronic transport and high-power factors, particularly under p-type doping. Under optimized carrier concentrations, the combined electrical and thermal transport properties yield predicted maximum p-type ZT values of approximately 1.25 at 300 K and 2.27 at 600 K. These results identify Rb2SeI6 as a promising thermoelectric material for near-room- to medium-temperature applications and highlight halogen substitution as an effective strategy for optimizing transport in vacancy-ordered halide perovskites.