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 compounds—Sr_7.04(2)Ga_1.94(2)Sb₆, Ba_7.02(3)Ga_1.98(3)Sb₆ and Eu_7.04(3)Ga_1.90(3)Sb₆—have been synthesized from reactions of the corresponding elements using gallium as a metal flux. Their crystal structures (space group I4¯3d (No. 220), Z=2 with unit cell parameters: a=9.9147(9) Å for the Sr-compound; a=10.3190(9) Å for the Ba-compound; and a=9.7866(8) Å 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.