Tetrastrontium-digalliumoxide (Sr4Ga2O7)—synthesis and crystal structure of a mixed anion strontium gallate related to perovskite

Crystal growth experiments yielded single crystals of Sr4Ga2O7. The title compound is monoclinic, space group P1c1, image, image, image, image, image, image (image for 7155 observed reflections and 660 parameters). The crystals showed twinning by pseudo-merohedry. Furthermore, the diffraction data exhibited the typical features of a superstructure. Sr4Ga2O7 can be classified as a mixed anion gallate comprising insular [GaO4]-groups and [Ga3O10]-trimers. Both anion groups are located in layers perpendicular to [100]. 32 symmetrically independent Sr-cations crosslink between the tetrahedral anion groups. Alternatively, the compound can be described as a 3×4×4 superstructure of ABO3 perovskite, with 22.2% vacancies in the oxygen sublattice. The relationship with perovskite can be expressed in the following crystal chemical formula: Sr(Ga2/3Sr1/3)(O7/9□2/9)3. A discussion of the structure in the context with other similar defect perovskites in presented.