The stuffed framework structure of BaGa2O4

Crystals of barium monogallate were obtained by growth from a stoichiometric melt at 1350 degreesC. The hexagonal crystals belong to space group P6(3), a = 18.6403(7) Angstrom and c = 8.6801(2) Angstrom, V = 2611.9(2) Angstrom (3), 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.