First Principles Study on the Thermoelectric Performance of CaAl2Si2-type Zintl Phase Compounds

We investigate the thermoelectric properties of CaAl2Si2-type Zintl phase compounds AB(2)X(2) (A = Mg, Ca, Sr, Ba, B = Mg, Zn, Cd, and X = P, As, Sb) using first principles band calculations within the Boltzmann transport theory assuming the constant relaxation time approximation. We introduce the effective degree of valley degeneracy n(TE) to focus on the relationship between the thermoelectric properties and the multivalley character of the electronic band structure around the Fermi level. We also introduce a quantity gamma(TE), which takes into account n(TE) and anisotropy of the valley structure, and it is found that gamma(TE) enables us to well understand the overall trend of the material dependence of the power factor. We finally suggest promising thermoelectric materials, e.g., BaMg2P2 for PF similar to 20 mu W/cm K-2 and ZT > 0.2 at 300K and SrZn2As2 for PF similar to 35 mu W/cm K-2 and ZT > 0.35 at 300K assuming a relaxation time of 10 fs and a lattice thermal conductivity value of 2 W/mK.