Gallium substitutions as a means to stabilize alkaline-earth and rare-earth metal pnictides with the cubic Th3P4 type:: Synthesis and structure of A7Ga2Sb6 (A = Sr, Ba, Eu)

Three new coin pounds-Sr7.04(2)Ga1.94(2)Sb6. Ba7.02(3)Ga1.98(3)Sb6 and Eu7.04(3)Ga1.90(3)Sb6-have been synthesized from reactions of the corresponding elements using gallium as a metal flux. Their crystal structures (space group 1 (4) over bar 3d (No. 220), Z = 2 with unit cell parameters: a = 9.9147(9) angstrom for the Sr-compound; a = 10.3190(9) angstrom for the Ba-compound; and a = 9.7866(8) angstrom for the Eu-compound) have been established by single-crystal X-ray diffraction. The structures are best described as Ga-stabilized derivatives of the hypothetical Sr4Sb3, Ba4Sb3 and Eu4Sb3 phases with the cubic Th3P4 type. Such an inclusion of interstitial Ga atoms in this atomic arrangement results in the formation of isolated [Ga2Sb6](14-) fragments, isoelectronic and isostructural with the [Sn2Te6](6-) anions in the K3SnTe3 type, and allows for the attainment of a charge-balanced electron count. In that sense, the Sr4Sb3, Ba4Sb3 and Eu4Sb3 binaries, which are expected to be electron-deficient and are currently unknown, can be "turned" into Sr7Ga2Sb6, Ba7Ga2Sb6 and Eu7Ga2Sb6, whose structures are readily rationalized following the Zintl concept. 2008 Elsevier Inc. All rights reserved.