Effect of metal doping on the low-temperature structural behavior of thermoelectric β-Zn ₄ Sb ₃

The low-temperature structural phase transitions of Bi, Pb, In and Sn-doped samples of thermoelectric Zn ₄ Sb ₃ have been characterized on crystals grown from molten metal fluxes, using electrical resistance and single crystal X-ray diffraction measurements. Room temperature stable, disordered, β-Zn ₄ Sb ₃ undergoes two phase transitions at 254 and 235 K to the consecutively higher ordered phases α and α′, respectively. The ideal crystallographic composition of α-Zn ₄ Sb ₃ is Zn ₁₃ Sb ₁₀ . The α-α′ transformation is triggered by a slight and homogenous Zn deficiency with respect to this composition and introduces a compositional modulation in the α-Zn ₄ Sb ₃ structure. When preparing β-Zn ₄ Sb ₃ in the presence of metals with low melting points (Bi, Sn, In, Pb) the additional metal atoms are unavoidably incorporated in small concentrations (0.04-1.3 at%) and act as dopants. This incorporation alters the subtle balance between Zn disorder and Zn deficiency in Zn ₄ Sb ₃ and has dramatic consequences for its low-temperature structural behavior. From molten metal flux synthesis it is possible to obtain (doped) Zn ₄ Sb ₃ samples which (1) only display a β-α transition, (2) only display a β-α′ transition, or (3) do not display any low-temperature phase transition at all. Case (2) provided diffraction data with a sufficient quality to obtain a structural model for highly complex, compositionally modulated, α′-Zn ₄ Sb ₃ . The crystallographic composition of this phase is Zn ₈₄ Sb ₆₅ .