New tetragonal structure type for A₂Ca₁₀Sb₉ (A = Li, Mg). Electronic variability around a Zintl Phase

Abstract

The phases LiMgCa₁₀Sb₉ (1), Mg₂Ca₁₀Sb₉ (2), and Li_1.38(2)Ca_10.62Sb₉ (3) have been synthesized by high-temperature solid-state means and characterized by single-crystal means and property measurements. These occur in space group P4₂/mnm, Z = 4, with a = 11.8658(5), 11.8438(6), 11.9053(7) Å and c = 17.181(2), 17.297(2), 17.152(2) Å, 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 Sb⁴⁻ 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 Ca_0.62(2)Li_0.38, 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.