Structure and Bonding of Sr3In11: How Size and Electronic Effects Determine Structural Stability of Polar Intermetallic Compounds

The binary compound Sr3In (SrIn3.667) was synthesized and structurally characterized by X-ray diffraction experiments. It crystallizes in the orthorhombic LasAln structure type (space group Immm, Z= 2; a = 4.9257(6), b = 14.247(2), c = 11.212(2) A). The crystal structure of Srsinii bears features of the monoclinic Euln structure, which is adopted by SrIn4, and the prominent tetragonal BaAI4 structure. Srsinn is stable until 550 °C. At higher temperatures it decomposes peritectically into SrIn2 and In. Structural stability and bonding properties of Sr3In were investigated by first principles calculations and compared to SrIn4 in the monoclinic Euln4 and the tetragonal BaAI4 structure. All three structures consist of a three-dimensional, polyanionic, network formed by In atoms and Sr cations encapsulated in cages. For the BaAI4-type SrIn, In-In network bonding is perfectly optimized. In contrast, the networks of Euln4-type Srln4 and Srsinn appear hypo- and hyperelectronic, respectively. The formation of Sr3In11 with a composition close to 1:4 and the nonexistence of BaAI4-type Srln4 is explained by a delicate interplay of size and electronic factors governing structural stability in the In-rich part of the Sr-In system.