New tetragonal structure type for A2Ca10Sb9 (A = Li, Mg).: Electronic variability around a zintl phase

The phases LiMgCa10Sb9 (1), Mg2Ca10Sb9 (2), and Li1.38(2)Ca10.62Sb9 (3) have been synthesized by high-temperature solid-state means and characterized by single-crystal means and property measurements. These occur in space group P4(2)/mnm, Z = 4, with a = 11.8658(5), 11.8438(6), 11.9053(7) angstrom and c = 17.181(2), 17.297(2), 17.152(2) angstrom, respectively, The two types of A atoms occupy characteristic sites in a Ca-Sb network that contains a 5:2 proportion of (formal) Sb3- and Sb-2(4-) anions and can be described in terms of two slab types stacked along [001]. Bonding appears to be strong in these salts with generally normal distances and high coordination numbers except for the 4-bonded atoms in a C-2v position for the second type of A that is occupied by Li, Mg or Ca0.62(2)Li0.38, respectively. The three compounds are, respectively, an ideal electron-precise Zintl phase, one e(-)-rich and 0.40(4) e(-)-short per formula unit. The LiMgCa10Sb9 compound is correspondingly diamagnetic and presumably a semiconductor.