Theoretical and experimental investigations into novel oxynitride discovery in the GaN-TiO₂ system at high pressure

We employed ab initio evolutionary algorithm USPEX to speed up the discovery of a novel oxynitride in the binary system of GaN-TiO₂ using high-pressure synthesis. A 1:2 mixture of GaN and nanocrystalline TiO₂ (anatase) was reacted under 1 GPa of pressure and at 1200^◦ C in a piston cylinder apparatus to produce a mixture of TiO₂ (rutile) and an unknown phase. From the initial analysis of high resolution neutron and X-ray diffraction data, it is isomorphic with monoclinic V₂ GaO₅ with a unit cell composition of Ga₁₀ Ti₈ O₂₈ N₂ with the following parameters: monoclinic, space group C2/m, a = 17.823(1) Å, b = 2.9970(1) Å, c = 9.4205(5) Å, β = 98.446(3)^◦; Volume = 497.74(3) ų. Further, a joint rietveld refinement revealed two distinct regimes—A Ti-rich block and a Ga-rich block. The Ti-rich block consists of four edge-shared octahedra and contains a site which is about 60% occupied by N; this site is bonded to four Ti. The remainder of the block consists of edge linked Ti-octahedral chains linked to the TiN/TiO fragments at octahedral corners partially occupied by nitrogen. The Ga-block contains two symmetry independent octahedral sites, occupied mostly by Ga, and a pure Ga-centered tetrahedral site bonded mostly to oxygen.