Electronic structure and thermoelectric performance of Zintl compound Ca5Ga2As6

Abstract

Ca₅Ga₂As₆ gives us a chance to achieve a high thermoelectric performance with a normal bulk system. This is mainly due to the fact that its complex chemical structure can not only supply an ultra-low lattice thermal conductivity but also show a high Seebeck coefficient. In this paper, we have discussed the lattice, electronic, and transport properties of Ca₅Ga₂As₆ for elucidating its high thermoelectric performance. In our calculations, density-functional theory and Boltzmann transport theory were used. For intrinsic Ca₅Ga₂As₆, the valence band effective mass is larger than the conduction band effective mass, which leads to its positive sign of S over the whole temperature range. By studying the anisotropy of its transport properties, we find that the transport properties of p-type Ca₅Ga₂As₆ are better than that of the n-type one, which mainly comes from the large anisotropy of its band structure. Moreover its transport properties along the z direction are much better than those along the x and y directions, which can be attributed to its anisotropic one-dimensional lattice structure. At different temperatures, the peak value of Z_eT appears at different carrier concentrations, which makes it possible to obtain a highest efficiency under large temperature gradients. The carrier concentrations corresponding to large Z_eT are in the range of 0.033–0.5 e per uc, which is equal to 5.19 × 10¹⁹ cm⁻³ to 7.87 × 10²⁰ cm⁻³. The largest value of Z_eT is equal to 0.95.