Structural Complexity and Tuned Thermoelectric Properties of a Polymorph of the Zintl Phase CaCdSb with a Non-centrosymmetric Monoclinic Structure.

The Zintl phase CaCdSb was found to be dimorphic. Besides the orthorhombic CaCdSb (-o), here we report on the synthesis, the structural characterization, and the thermoelectric transport properties of its monoclinic form, CaCdSb (-m), and its Lu-doped variant CaLuCdSb ( ≈ 0.02). The monoclinic structure exhibits complex structural characteristics and constitutes a new structure type with the non-centrosymmetric space group ( = 30). The electrical resistivity ρ() measured on single crystals of both phases portrays a transition from a semiconductor to a degenerate p-type semiconductor upon doping with Lu and with an attendant change in the Hall carrier concentration from 7.15 × 10 to 2.30 × 10 cm at 300 K. The Seebeck coefficient () of both phases are comparable and indicate a hole-dominated carrier transport mechanism with magnitudes of 133 and 116 μV/K at 600 K for CaCdSb (-m) and CaLuCdSb, respectively. The convoluted atomic bonding with an attendant large unit cell volume of ∼4365 Å drives a putative low thermal conductivity in these materials resulting in a power factor PF of 1.63 μW/cm K and an estimated thermoelectric figure of merit of ∼0.5 for CaLuCdSb at 600 K. Differential scanning calorimetry results reveal the stability of these phases up to about 960 K, making them candidates for moderate temperature thermoelectric materials.