No abstract

Two new zinc phosphides, LaZnP and LaMgZnP, were synthesized via transport reactions, and their crystal structures were determined by single crystal X-ray diffraction. LaZnP and LaMgZnP are built from three-dimensional Zn-P and Zn-Mg-P anionic frameworks that encapsulate lanthanum atoms. The anionic framework of LaZnP is constructed from one-dimensional ZnP, ZnP, and ZnP chains. The ZnP chains are also the main building units in LaMgZnP. In LaZnP, the displacement of a zinc atom from the origin of the unit cell causes the Zn4 position to split into two equivalent atomic sites, each with 50% occupancy. The splitting of the atomic position substantially modifies the electronic properties, as suggested by theoretical calculations. The necessity of splitting can be overcome by replacement of zinc with magnesium in LaMgZnP. Investigation of the transport properties of a densified polycrystalline sample of LaZnP demonstrates that it is an n-type semiconductor with a small bandgap of ∼0.04 eV at 300 K. LaZnP also exhibits low thermal conductivity, 1.3 Wm K at 300 K, which mainly originates from the lattice thermal conductivity. LaZnP is stable in a sealed evacuated ampule up to 1123 K as revealed by differential scanning calorimetry.

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.