Theoretical and Experimental Investigations into Novel Oxynitride Discovery in the GaN-TiO2 System at High Pressure

Abstract

Abstract:We employed ab initio evolutionary algorithm USPEX to speed up the discovery of a novel oxynitride in the binary system of GaN-TiO 2 using high-pressure synthesis. A 1:2 mixture of GaN and nanocrystalline TiO 2 (anatase) was reacted under 1 GPa of pressure and at 1200 • C in a piston cylinder apparatus to produce a mixture of TiO 2 (rutile) and an unknown phase. From the initial analysis of high resolution neutron and X-ray diffraction data, it is isomorphic with monoclinic V 2 GaO 5 with a unit cell composition of Ga 10 Ti 8 O 28 N 2 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) Å 3 . 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.

Evolutionary Algorithm Directed Synthesis of Mixed Anion Compounds LaF2X (X=Br, I) and LaFI2

“…It has also been applied to ambient pressure phases, e.g., boride Ti 2 InB 2 , electride Sr 5 P 3 and carbides Hf‐Ta‐C [14] . There have been only a few reports that use evolutionary algorithms to predicting mixed‐anion compounds, [15] and to our knowledge none have been successful in synthesizing the predicted phases; while evolutionary algorithms predicted oxynitrides like GaTiO 2 N and GaTi 2 O 4 N, attempts to synthesize these phases resulted in the formation of Ga 10 Ti 8 O 28 N 2 , albeit a new phase [15a] . One plausible reason for this is the presence of O/N disorder, which forces one to consider impossibly large supercells together with configurational enthalpies in order to more accurately predict stability.…”

Toward Shape-Controlled Metal Oxynitride and Nitride Particles for Solar Energy Applications

“…Also, with shape-controlled oxynitride and nitride materials achieved, self-assembly methods , could be adopted to control the architecture of these materials to maximize surface area and light absorption. In addition, advanced characterization of these materials can help to better understand their electronic structures, whether it be the application of TAS to understand defect states as a function of nitridation conditions, X-ray and neutron scattering methods to elucidate anion clustering effects in oxynitride solid solutions, ,− or something new. In fact, Bartlett and Brancho noted that complete control over the nitrogen content that is incorporated during ammonolysis of oxides is limited and reliable quantification techniques to determine the nitrogen content of products are needed .…”