High-pressure synthesis and crystal structures of the strontium oxogallates Sr₂Ga₂O₅ and Sr₅Ga₆O₁₄

High-pressure synthesis experiments in a piston–cylinder apparatus at 1.5 GPa/3.0 GPa and 1000 °C resulted in the formation of single-crystals of Sr₂Ga₂O₅ and Sr₅Ga₆O₁₄, respectively. The structures of both compounds have been solved from single-crystal diffraction data sets using direct methods. The first compound is orthorhombic with space group type Pbca (a=10.0021(4) Å, b=9.601(4) Å, c=10.6700(4) Å, V=1024.6(4) ų, M_r=394.68 u, Z=8, D_x=5.12 g/cm³) and belongs to the group of single layer gallates. Individual sheets are parallel to (0 0 1) and can be built from the condensation of unbranched vierer single chains running along [0 1 0]. The layers are characterized by the presence of four- and strongly elliptical eight-membered rings of corner connected tetrahedra in UUDD and UUUUDDDD conformation. Strontium atoms are sandwiched between the tetrahedral layers for charge compensation and are coordinated by six and seven oxygen ligands, respectively. Sr₂Ga₂O₅ is isotypic with several other double sulfides and selenides. To the best of our knowledge, it is the first example of an oxide with this structure type. From a structural point of view, Sr₅Ga₆O₁₄ is a phyllogallate as well. The crystal structure adopts the monoclinic space group P2₁/c (a=8.1426(3) Å, b=8.1803(3) Å, c=10.8755(4) Å, β=91.970(4)° V=723.98(5) ų, M_r=1080.42 u, Z=2, D_x=4.96 g/cm³). Individual sheets extend along (0 0 1). Basic building units are unbranched dreier single chains parallel to [1 0 0]. The layers contain tertiary (Q³) und quaternary (Q⁴) connected [GaO₄]-tetrahedra in the ratio 2:1 resulting in a Ga:O ratio of 3:7 and the formation of exclusively five-membered rings. Linkage between adjacent tetrahedral sheets is provided by three symmetrically independent strontium ions which are surrounded by six to eight oxygen atoms. The layers in Sr₅Ga₆O₁₄ are similar to those observed in the melilite structure-type. Crystallochemical relationships between the present phases and other known compounds are discussed in detail. - Graphical abstract: A single tetrahedral layer and the location of the Sr-cations directly above the sheet in high-pressure Sr₅Ga₆O₁₄. - Highlights: • We report the crystal structures of two new strontium oxogallates with composition Sr₂Ga₂O₅ and Sr₅Ga₆O_14. • Both phases have been obtained in the course of high-pressure piston–cylinder experiments performed at 1.5 and 3.0 GPa. • The crystallochemical characteristics and similarities with known structure types are discussed in detail