Metastable Cd₄Sb₃: A complex structured intermetallic compound with semiconductor properties

The metastable binary intermetallic compound Cd₄Sb₃ was obtained as polycrystalline ingot by quenching stoichiometric Cd-Sb melts and as mm-sized crystals by employing Bi or Sn fluxes. The compound crystallizes in the monoclinic space group Pn with a = 11.4975(5) Å, b = 26.126(1) Å, c = 26.122(1) Å, β = 100.77(1)°, and V = 7708.2(5) A,̊³. The actual formula unit of Cd₄Sb₃ is Cd₁₃Sb₁₀ and the unit cell contains 156 Cd and 120 Sb atoms (Z = 12). Cd₄Sb₃ displays a reversible order-disorder transition at 373 K and decomposes exothermically into a mixture of elemental Cd and CdSb at around 520 K. Disordered β-Cd ₄Sb₃ is rhombohedral (space group R3c, a ≈ 13.04 Å, c ≈ 13.03 Å) with a framework isostructural to β-Zn ₄Sb₃. The structure of monoclinic α-Cd ₄Sb₃ bears resemblance to the low-temperature modifications of Zn₄Sb₃, α- and α′- Zn4Sb3, in that randomly distributed vacancies and interstitial atoms of the high-temperature modification aggregate and order into distinct arrays. However, the nature of aggregation and distribution of aggregates is different in the two systems. Cd₄Sb₃ displays the properties of a narrow gap semiconductor. Between 10 and 350 K the resistivity of melt-quenched samples first increases with increasing temperature until a maximum value at 250 K and then decreases again. The resistivity maximum is accompanied with a discontinuity in the thermopower, which is positive and increasing from 10 to 350 K. The room temperature values of the resistivity and thermopower are about 25 mΩcm and 160 μV/K, respectively. Flux synthesized samples show altered properties due to the incorporation of small amounts of Bi or Sn (less than 1 at. %). Thermopower and resistivity appear drastically increased for Sn doped samples. Characteristic for Cd₄Sb₃ samples is their low thermal conductivity, which drops below 1 W/mK above 130 K and attains values around 0.75 W/mK at room temperature, which is comparable to vitreous materials.