Cation-Driven Vibrational Hierarchy in NaCdX (X = As, Sb) Thermoelectrics: From Static Insulation to Rattling-Like Dissipation

In the current work, the crystal structure, phonon, electronic transport, and thermoelectric (TE) properties of NaCdX (X = As, Sb) compounds are systematically investigated through first-principles calculations, Boltzmann transport theory, and a two-channel model. The Na+ ion in NaCdX (X = As, Sb) compounds vibrates along different directions due to the different X (X = As, Sb) lone-pair electrons. Consequently, a pronounced anisotropy is discovered for the lattice thermal conductivity. The synergistic effect of the lone-pair electrons of X (X = As, Sb) atoms and the "static insulation to rattling-like dissipation" properties of the Na+ ion contribute to the low lattice thermal conductivities (0.52 and 0.55 W m–1K–1 @ 600 K) of NaCdX (X = As, Sb) compounds. Additionally, the TE performance of the NaCdX (X = As, Sb) compounds is evaluated by considering the multicarrier scatterings. The p-type NaCdAs compound exhibits an optimal figure of merit (ZT) of 1.2, while the n-type NaCdSb compound demonstrates a high ZT of 2.1 at 600 K. The present work not only offers a fundamental insight of the transition from the "static insulation" to "rattling-like dissipation" in suppressing lattice thermal conductivity, but also reveals the excellent TE properties of n-type NaCdSb compound.