Optimization of ca14 mgsb11 through chemical substitutions on sb sites: Optimizing seebeck coefficient and resistivity simultaneously

Abstract

In thermoelectric materials, chemical substitutions are widely used to optimize thermoelectric properties. The Zintl phase compound, Yb14 MgSb11, has been demonstrated as a promising thermoelectric material at high temperatures. It is iso-structural with Ca14 AlSb11 with space group I41 /acd. Its iso-structural analog, Ca14 MgSb11, was discovered to be a semiconductor and have vacancies on the Sb(3) sites, although in its nominal composition it can be described as consisting of fourteen Ca2+ cations with one [MgSb4]9− tetrahedron, one Sb7−3 linear anion and four isolated Sb3− anions (Sb(3) site) in one formula unit. When Sn substitutes Sb in Ca14 MgSb11, optimized Seebeck coefficient and resistivity were achieved simultaneously although the Sn amount is small (<2%). This is difficult to achieve in thermoelectric materials as the Seebeck coefficient and resistivity are inversely related with respect to carrier concentration. Thermal conductivity of Ca14 MgSb11-x Snx remains almost the same as Ca14 MgSb11. The calculated zT value of Ca14 MgSb10.80 Sn0.20 reaches 0.49 at 1075 K, which is 53% higher than that of Ca14 MgSb11 at the same temperature. The band structure of Ca14 MgSb7 Sn4 is calculated to simulate the effect of Sn substitutions. Compared to the band structure of Ca14 MgSb11, the band gap of Ca14 MgSb7 Sn4 is smaller (0.2 eV) and the Fermi-level shifts into the valence band. The absolute values for density of states (DOS) of Ca14 MgSb7 Sn4 are smaller near the Fermi-level at the top of valence band and 5p-orbitals of Sn contribute most to the valence bands near the Fermi-level.