EXPLORING THE LIMITS OF THE ZINTL CONCEPT FOR THE A(14)MPN(11) STRUCTURE TYPE WITH M=ZN, CD

A(14)AnSb(11+x) and A(14)CdSb(11+x) (A = Ca, Sr) are synthesized by re acting elements in a 14: 1:11 ratio at 1000 degrees C. Low-temperature (130 K) single-crystal X-ray diffraction data shows that these compou nds are tetragonal, space group I4(1)/acd, Z = 8, and are structurally similar to previously studied A(14)MPn(11) (A = Ca, Sr, Ba; M = Al, G a, Mn; Pn = As, Sb, Bi) compounds. However, these Zn and Cd compounds with the exception of Sr14ZnSb11 contain additional Sb and cannot be u nderstood according to simple Zintl electron counting rules. Low-tempe rature lattice parameters, R1(wR2) are Ca14ZnSb11.20, a = 16.778 (3), c = 22.088 (6) Angstrom R1(wR2)= 0.063 (0.115); Ca14CdSb11.43, a = 16. 583 (3), c = 23.167 (7) Angstrom, R1(wR2)= 0.0353 (0.0792); Sr14ZnSb11 , a = 17.569 (5), c = 22.961 (6) Angstrom, R1(wR2)= 0.0486 (0.0997); S r14CdSb11.37, a = 17.637 (3), c = 23.163 (4) Angstrom, R1(wR2) = 0.062 7 (0.1435). Temperature-dependent resistivity measurements show that t hese materials are semiconducting in behavior with small activation en ergies: Ca14ZnSn11.20: E(a) = 3.9 (1) x 10(-3) eV; Ca14CdSb11.43, E(a) = 7.6 (1) x 10(-2) eV; Sr14ZnSb11, E(a) = 1.5 (1) x 10(-2) eV; Sr14Cd Sb11.37 E(a) = 1.1 (1) x 10(-2) eV. Several simple models are presente d in order to understand the crystallographic results.