Origin of different thermoelectric properties between Zintl compounds Ba3Al3P5 and Ba3Ga3P5: A first-principles study

Abstract

The electronic structure and the thermoelectric properties of Zintl compounds Ba3M3P5 (M =Al, Ga) were investigated by the density functional theory (DFT) combined with the semiclassical Boltzmann transport theory. It is found that the transport properties of p-type Ba3M3P5 are better than that of n-type one at optimum carrier concentration. By p-type doping, the maximum ZT of Ba3Al3P5 and p-type Ba3Ga3P5 can reach 0.49 at 500K and 0.65 at 800K, corresponding to the carrier concentration of 7.1×1019 holes per cm3 and 1.3×1020 holes per cm3, respectively. The higher thermoelectric performance of p-type Ba3M3P5 than n-type one is mainly due to the large valence band dispersion near the Fermi level. For Ba3Ga3P5, the multiple extrema on the top of valence bands will increase its electrical conductivity. The calculated partial charge density near the Fermi level of Ba3M3P5 shows that there is little charge density around the P1 atoms in Ba3Al3P5. On the contrary, the high charge density appears around all P atoms in Ba3Ga3P5, which may be the reason why Ba3Ga3P5 has multiple extrema on its top of valence bands. Meanwhile, the minimum lattice thermal conductivities of Ba3Al3P5 and Ba3Ga3P5, are small and are comparable to those of Ca5Al2Sb6 and Ca5Ga2Sb6. Compared with p-type Ba3Al3P5, p-type Ba3Ga3P5 shows better thermoelectric properties, which is mainly due to the multiple extrema on its top of the valence bands and its small band gap. Moreover, p-type Ba3Ga3P5 shows nearly isotropic transport behavior. Hence, good thermoelectric performance for p-type Ba3Ga3P5 can be predicted.