Low lattice thermal conductivity induced by rattling-like vibration in RbCaX (X = As, Sb) compounds with excellent thermoelectric properties†

Layered alkali metal (A)–alkaline earth metal (AE)–pnictogen (Pn = N, P, As, Sb, and Bi) compounds are promising candidates for thermoelectric applications due to their thermal stability and low thermal conductivity. This study systematically investigates and compares the anisotropic thermoelectric properties of the layered RbCaAs and RbCaSb compounds using density functional theory (DFT) and semiclassical Boltzmann transport theory. The results show that the rattling thermal damping effect from weak Rb–As/Sb bonds in RbCaX (X = As, Sb) compounds leads to low lattice thermal conductivity (∼3.22/1.20 and ∼1.90/0.94 W m⁻¹ K⁻¹ at 300 K along the x-/y-direction). The n-type RbCaSb exhibits significantly optimal dimensionless thermoelectric figure of merit (ZT) of ∼3.19 (cross-plane) and ∼1.71 (in-plane) at 900 K, which are significantly higher than ∼0.54 and ∼0.80 of n-type RbCaAs and typical layered thermoelectric materials like p-type SnSe (ZT ∼2.6, 923 K) and BiCuOSe (predicted ZT ∼0.75, 900 K). This enhancement is attributed to the lower scattering rate boosting power factor, and lattice softening induced by the heavy Sb elements strengthens anharmonic phonon scattering and reduces lattice thermal conductivity. These findings highlight RbCaSb as a promising candidate in the field of thermoelectric materials.