The stuffed framework structure of BaGa₂O₄

Crystals of barium monogallate were obtained by growth from a stoichiometric melt at 1350°C. The hexagonal crystals belong to space group P6₃, a = 18.640.3(7) Å and c = 8.6801(2) Å, V=2611.9(2) ų, Z = 24, D(calc.) = 5.20 g cm^-3. The structure was solved by Patterson techniques using a single crystal diffraction data set. Merohedral twinning of the crystal (twin element 2([110])) was accounted for in the refinement calculations (wR2 = 0.056 for 4183 reflections). The main structural features of BaGa₂O₄ are layers of six-membered rings of GaO₄ tetrahedra perpendicular to the c-axis. The stacking of the layers parallel to c results in a three-dimensional framework containing tunnels, where the barium cations are located. Within a single layer two different types of ditrigonal-shaped six-membered rings are distinguished on the basis of relative orientation of up (U) and down (D) pointing apices of adjacent tetrahedra: one-fourth of the rings have an UDUDUD topology, whereas the sequences of directedness of the remaining rings is UUUDDD. The structure is isotypic with KAlGeO₄. It is the first member of this structure family containing exclusively trivalent tetrahedral ions of only one chemical species and divalent nonframework cations within the channels. (C) 2000 Academic Press.