High thermoelectric performance via stacking-controlled symmetry breaking in layered AZnBi (A = Rb, Cs) Zintl materials

High thermoelectric efficiency requires a high Seebeck coefficient, high electrical conductivity, and low thermal conductivity. However, strategies that suppress thermal conductivity often simultaneously degrade electrical conductivity, making effective electrical-thermal decoupling highly challenging. Here, we show that a change in atomic-layer stacking order in AZnBi (A = Rb, Cs) provides an efficient route to achieve such decoupling. Even though electronic transport coefficients and relaxation times remain largely insensitive to stacking order due to preserved Fermi-surface topology, the lattice thermal conductivity exhibits a strong stacking dependence, with AB stacking significantly suppressing it below 1 Wm −1 K −1 at temperatures above 300 K. The stacking transition from AA to AB breaks structural symmetries. It increases the three-phonon phase space and available scattering channels, substantially suppressing phonon transport by about 50% in both materials. As a result, the AB-stacked phases yield high ZT values of 1.95 (1.68) in n-type CsZnBi (RbZnBi) at 900 K, which is about 40% (30%) higher than AA stacking. These findings establish the AZnBi family as promising thermoelectric candidates and highlight stacking-order-controlled phonon transport as a robust strategy for advancing thermoelectric material design.