Theoretical prediction of mechanics, transport, and thermoelectric properties of full Heusler compounds Na2KSb and X2CsSb (X

Full Heusler compounds have become a research hotspot in the thermoelectric (TE) field, because they have the remarkable electronic properties and high-TE power factor. Nevertheless, the inherent high thermal conductivity ${\ensuremath{\kappa}}_{L}$ hinders their further application as TE device. Hence, we investigate the mechanical, transport, and thermoelectric properties in ${\mathrm{Na}}_{2}\mathrm{KSb}$ and ${\mathrm{X}}_{2}\mathrm{CsSb}$ (X = K, Rb) with eight valence electrons per formula unit (f.u.) by using the first-principles calculations combined with self-consistent phonon (SCP) theory, compressive sensing (CS) techniques, and Boltzmann transport equation (BTE). Due to the strong three phonon scattering combined with small phonon group velocity, we obtain the ultralow lattice thermal conductivities. An evaluation of their mechanical properties reveals that they are all brittle compounds, and ${\mathrm{Rb}}_{2}\mathrm{CsSb}$ has the strongest elastic anisotropy. To capture rational electron transport properties, we include the effects of the acoustic deformation potential (ADP) scattering, polar optical phonon (POP) scattering, and ionized impurity (IMP) scattering on the electron relaxation time. Finally, a high p-type $\mathit{ZT}$ values of 2.74 (2.48) at 600 (900) K are captured in the cubic ${\mathrm{K}}_{2}\mathrm{CsSb}$ $({\mathrm{Na}}_{2}\mathrm{KSb})$. These findings not only help us to comprehensively understand the physical properties of full Heusler compounds with eight valence electrons per f.u., but also support them as potential candidates for thermal management and thermoelectric applications.