Optimization of Ca14MgSb11 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, Yb 14 MgSb 11 , has been demonstrated as a promising thermoelectric material at high temperatures. It is iso-structural with Ca 14 AlSb 11 with space group I4 1 /acd. Its iso-structural analog, Ca 14 MgSb 11 , 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 Ca 2+ cations with one [MgSb 4 ] 9− tetrahedron, one Sb 3 7− linear anion and four isolated Sb 3− anions (Sb(3) site) in one formula unit. When Sn substitutes Sb in Ca 14 MgSb 11 , 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 Ca 14 MgSb 11-x Sn x remains almost the same as Ca 14 MgSb 11. The calculated zT value of Ca 14 MgSb 10.80 Sn 0.20 reaches 0.49 at 1075 K, which is 53% higher than that of Ca 14 MgSb 11 at the same temperature. The band structure of Ca 14 MgSb 7 Sn 4 is calculated to simulate the effect of Sn substitutions. Compared to the band structure of Ca 14 MgSb 11 , the band gap of Ca 14 MgSb 7 Sn 4 is smaller (0.2 eV) and the Fermi-level shifts into the valence band. The absolute values for density of states (DOS) of Ca 14 MgSb 7 Sn 4 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.

The remarkable crystal chemistry of the Ca14AlSb11 structure type, magnetic and thermoelectric properties

“…The data on doping on the Pn site are limited to Ge, Te, and Sn which only substitute for a trace amount of Sb in the structure. [85][86][87] However, Bi has been shown to make a full isoelectronic solid solution. 84,124 The doping of the Sb site with Ge, Te, or Bi differently affects the carrier concentration (higher for Bi and lower for Te, the same for Ge), but in all cases leads to an increased mobility which correspond to a lower effective mass.…”

Chemical Route to Yb14MgSb11 Composites with Nanosized Iron Inclusions for the Reduction of Thermal Conductivity

“…2,29 Energy filtering is thought to be another mechanism of improving zT in nanocomposites where inclusions can separate higher energy electrons from lower energy electrons, leading to increased Seebeck coefficients such as in Sb 2 Te 3 composited with PEDOT. 30 In the YB 22 CN system, compositing with VB 2 is observed to increase the Seebeck coefficient by doping and electrical resistivity by creating a nanoweb of conductive pathways, 31 similar to what is described as the compositeassisted funneling of electrons (CAFE) effect in the Yb 14 MgSb 11 −micron iron nanocomposite where the resistivity is decreased without changing the Seebeck coefficient. 32 Furthermore, an annealed nanograin model suggests that annealing defective nanoparticles creates a nondefective core with nanoparticle grain boundaries high in ionized impurities to achieve charge balance.…”

“…The most successful in terms of increasing zT has been the solid solution containing Al, Yb 14 Mn 1– x Al x Sb 11 ( x = 0.6, 0.8) . Analogues of the structure type such as Yb 14 MgSb 11 with a max zT s of 1.2 can also provide new compositions for optimization. , Yb 14 MgSb 11 is an exciting phase because it has a higher Seebeck coefficient than Yb 14 MnSb 11 and has a higher ZT (see eq ) at 873–1273 K (1.10 vs 1.07). ,, …”

“…14 Analogues of the structure type such as Yb 14 MgSb 11 with a max zTs of 1.2 can also provide new compositions for optimization. 22,24 Yb 14 MgSb 11 is an exciting phase because it has a higher Seebeck coefficient than Yb 14 MnSb 11 and has a higher ZT (see eq 2) at 873−1273 K (1.10 vs 1.07). 9,23,24 i k j j j j j j j j j y Reducing κ l has long been a strategy for improving zT.…”

Finding the order in complexity: The electronic structure of 14-1-11 zintl compounds

“…From detailed experimental and theoretical analysis of the high zT compositions Yb 14 MnSb 11 and Yb 14 MgSb 11 and related compounds it has been shown 15,23,[25][26][27][28] that the valence bands are essentially the same for the Sb compounds. The valence bands contain a simple, light valence band with a second heavier valence band close in energy that has high valley degeneracy which provides additional conduction paths, contributing to a high electrical conductivity, while maintaining a large thermopower 5,6 .…”