X$_{2}$CsBi (X = Li, K, Na, Rb) Heusler alloys as the prospective materials for thermoelectric applications

The thermoelectric efficiency of X$_{2}$CsBi (X = Li, Na, K, and Rb) Heusler alloys using the density functional theory, quasiharmonic Debye approximation and Boltzmann transport equation at constant electron relaxation time is investigated. For all compounds under study, the low values of lattice thermal conductivity $\kappa_{\rm lat}$, which are below 1 Wm$^{-1}$K$^{-1}$ over the whole temperature range, are obtained. The room temperature values of $\kappa_{\rm lat}$ are calculated to be 0.57, 0.51, 0.42 and 0.28 Wm$^{-1}$K$^{-1}$ for Na$_{2}$CsBi, K$_{2}$CsBi, Li$_{2}$CsBi, and Rb$_{2}$CsBi, respectively. The ZT merit thermoelectric figure is shown to be 0.45, 1.2, and 2.46 for Li$_{2}$CsBi, K$_{2}$CsBi, and Rb$_{2}$CsBi under appropriate p-type (hole) doping. The large values of thermoelectric efficiency make these alloys prospective materials for thermoelectric applications at room temperature.