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) Å3. 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-Cd4Sb3 is rhombohedral (space group R3̅c, a ≈ 13.04 Å, c ≈ 13.03 Å) with a framework isostructural to β-Zn4Sb3. The structure of monoclinic α-Cd4Sb3 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.