Understanding the origin of the high thermoelectric figure of merit of Zintl-phase KCaBi.

Herein, we have investigated the unexplored thermoelectric properties of Zintl-phase KCaBi using first-principles calculation and the solution of the Boltzmann transport equation. KCaBi shows intrinsically very low lattice thermal conductivities ( κ l ) along the ( x ( y ), and z )-directions of (1.78, 0.68) and (1.15, 0.43) W m -1 K -1 at 300 and 800 K, respectively. Along with the effect of very low κ l , the high figure of merit ( ZT ) for p-type KCaBi results from the high Seebeck coefficients ( S ) and optimal electrical conductivities ( σ ), which originate from the high and steep total density of state (TDOS) at the valence band edge and the less dispersed multi-valley nature of the valence band edge in the band structure. On the other hand, large ZT for n-type KCaBi results from moderate S and high σ caused by the sloped TDOS at the conduction band edge and the highly dispersed nature of the conduction band edge in the band structure, and very low values of κ l . The highest ZT of KCaBi that we obtained at 800 K along the ( x ( y ), and z )-directions was (1.83, 0.80) for the p-type case at a hole concentration of 10 21 cm -3 and (1.36, 1.22) for the n-type case at electron concentration 7 × 10 18 cm -3 . Our study demonstrates that both p-type and n-type KCaBi have the potential to be promising thermoelectric materials.