Phase stability and structure of alkali doped-beta-gallia rutile intergrowths

Abstract

The incorporation of alkali cations into the tunneled structure of Ga 4TiO 8 was investigated and compared to predictions based on atomistic computer simulations. Samples were prepared as A x Ga 4− x Ti 1− x O 8, A=Li, Na, and K, x ≤ 0.7, and as Na x Ga 4+ x Ti 2− x O 10 ( x = 0.7, 0.85, and 1.0) using solid-state reactions at 1050–1350 °C. The sodium-containing tunneled structure, Na x Ga 4− x Ti 1− x O 8, formed via solid-state reaction, but the potassium and lithium analogs did not. Instead, these systems formed mixed-phase assemblages, which are discussed in reference to compatibility triangles in the Li 2O–Ga 2O 3–TiO 2 and K 2O–Ga 2O 3–TiO 2 systems. Experimental results were compared to the results of energy minimization calculations using the General Utility Lattice Program (GULP). For the lithium-containing system, the computer simulations correctly predicted the formation of a mixed-phase assemblage containing LiGa 5O 8, Ga 2O 3, and TiO 2. For the sodium- and potassium-containing system, the computer simulations suggested that mixtures of the single-cation oxide components should be the stable phase assemblages, in contradiction with experimentally observed results. Energy minimization calculations conducted on structurally different Na x Ga 4+ x Ti 2− x O 10 and Na x Ga 4+ x Ti 3− x O 12 phases indicated that those based on the n = 6 and n = 7 β-gallia rutile intergrowth structures have lower lattice energies than the experimentally observed sodium titanogallate structures reported previously in literature.