Exploring the limits of the zintl concept for the A₁₄MPn₁₁ structure type with M = Zn, Cd

A₁₄ZnSb_11+x and A₁₄CdSb_11+x (A = Ca, Sr) are synthesized by reacting elements in a 14: 1:11 ratio at 1000°C. Low-temperature (130 K) single-crystal X-ray diffraction data shows that these compounds are tetragonal, space group I4₁/acd, Z = 8, and are structurally similar to previously studied A₁₄MPn₁₁ (A = Ca, Sr, Ba; M = Al, Ga, Mn; Pn = As, Sb, Bi) compounds. However, these Zn and Cd compounds with the exception of Sr₁₄ZnSb₁₁ contain additional Sb and cannot be understood according to simple Zintl electron counting rules. Low-temperature lattice parameters, R1(wR2) are Ca₁₄ZnSb_11.20, a = 16.778 (3), c = 22.088 (6) Å, R1(wR2) = 0.063 (0.115); Ca₁₄CdSb_11.43, a = 16.583 (3), c = 23.167 (7) Å, R1(wR2) = 0.0353 (0.0792); Sr₁₄ZnSb₁₁, a = 17.569 (5), c = 22.961 (6) Å, R1(wR2) = 0.0486 (0.0997); Sr₁₄CdSb_11.37, a = 17.637 (3), c = 23.163 (4) Å, R1(wR2) = 0.0627 (0.1435). Temperature-dependent resistivity measurements show that these materials are semiconducting in behavior with small activation energies: Ca₁₄ZnSb_11.20: E_a = 3.9(1) × 10^-3 eV; Ca₁₄CdSb_11.43, E_a = 7.6(1) × 10^-2 eV; Sr₁₄ZnSb₁₁, E_a = 1.5(1) × 10^-2 eV; Sr₁₄CdSb_11.37, E_a = 1.1(1) × 10^-2 eV. Several simple models are presented in order to understand the crystallographic results.