Sr₃Ga₄O₉ - A strontium gallate with a new type of tetrahedral layer structure

Synthetic strontium gallate Sr₃Ga₄O₉ is triclinic, space group Pl̄ (a = 6.7490(9) Å, b = 8.0494(12) Å, c = 9.7295(15) Å, α = 85.01(2)°, β=72.71(2)°, γ = 74.24(2)°, V = 485.7(3) ų, Z = 2, D_calc = 4.69 g · cm^-3). The structure was solved by direct methods using 2656 unique reflections and refined to a residual of R1 = 0.042 (146 parameters). The compound belongs to the group of tetrahedral single layer structures. Individual sheets are parallel to (100), thus producing a prominent cleavage. The stepped layers can be described as being built by the condensation of six tetrahedra-wide bands. The bands in turn consist of two open branched dreier single chains connected via common corners and they contain tertiary and quaternary tetrahedra Q³ and Q⁴ in the ratio 1: 1. The stacking of the layers containing four-, six- and eight-membered rings results in a three-dimensional structure in which the strontium cations reside in the voids between the layers and are coordinated by six to seven oxygen neighbors.