Zn ₅Sb ₄In _2-δ - A ternary derivative of thermoelectric zinc antimonides

Zn ₅Sb ₄ln _2-δ (δ = 0.15(3)) was synthesized in the form of millimeter-sized crystals from reaction mixtures containing excess zinc. The ternary intermetallie compound is temperature polymorphic, and at room temperature it crystallizes with a new structure type in the orthorhombic space group Pbcn, where a = 7.1619(2), b= 17.1562(4), c=8.6887(4) Å, V = 1067.6(1) Å ³, and Z = 4. The structure features 3 ²434 nets of Sb atoms that are stacked in antiposition to yield layers of square antiprisms sharing edges plus intervening tetracapped tetrahedra (tetreadersterns). The majority of Zn atoms occupy peripheral tetrahedra of such tetraedersterns, and attain at the same time the peculiar five-coordination by one like atom and four Sb atoms typical for the structures of binary zinc antimonides. The In and remaining Zn atoms are distributed in the tetragonal channels formed by the square antiprisms and display some disorder. At temperatures below 200 K Zn ₅Sb ₄ln _2-δ undergoes a phase transition into a more ordered structure with monoclinic symmetry (P2 ₁/c) without any change of the unit cell. The thermoelectric properties of Zn ₅Sb ₄In _2-δ were measured between 10 and 350 K. Exceptionally low thermal conductivity values (1 W/mK range) were obtained in the whole temperature range. Resistivity and thermopower values are characteristic of a heavily doped or degenerate semiconductor (2.5 mΩ cm and 160 μV/K, respectively, at room temperature) and show a discontinuity around 220 K. The thermoelectric figure of merit of Zn ₅Sb ₄In _2-δ is higher than that of Zn ₄Sb ₃ in the investigated temperature range.