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

Alwin James,Lars Ehm,John Parise,Alexandra Sinclair,M Esfahani,Artem Oganov,William Woerner

doi:10.3390/cryst8020015

Abstract

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 V2GaO5 with a unit cell composition of Ga10Ti8O28N2 with the following parameters: monoclinic, space group C2/m, a = 17.823(1) A, b = 2.9970(1) A, c = 9.4205(5) A, β = 98.446(3)°; Volume = 497.74(3) A3. 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.

Highlights

The search for alternatives to fossil fuels is driven by increasing energy demands, and the desirability of limiting growth in CO2 in the atmosphere, while retaining the advantages of fuel transportability
Using state of the art evolutionary algorithm USPEX along with density functional theory calculations, we studied the pressure-driven stability of GaN-TiO2 system to better understand and evaluate the structures and stability fields of possible Ga-Ti-oxynitride compounds with chemical composition chosen from a mix of GaN and TiO2 endmembers
We constructed the convex hull, which connects the phases that are stable against decomposition into elements or binaries at a given pressure

Summary

Introduction

The search for alternatives to fossil fuels is driven by increasing energy demands, and the desirability of limiting growth in CO2 in the atmosphere, while retaining the advantages of fuel transportability In this regard, visible light [1] driven photocatalysis for overall water splitting (OWS). Members of the solid solution have bandgaps smaller than the GaN and ZnO endmembers They achieved overall water splitting at relatively high efficiency of up to 6%. Inspired by these results, we synthesized the complete solid solution (GaN)x (ZnO)1−x under pressure and demonstrated hydrogen evolution from water without the use of cocatalysts and sacrificial reagents [18]. Using state of the art evolutionary algorithm USPEX along with density functional theory calculations, we studied the pressure-driven stability of GaN-TiO2 system to better understand and evaluate the structures and stability fields of possible Ga-Ti-oxynitride compounds with chemical composition chosen from a mix of GaN and TiO2 endmembers

Theoretical Predictions Using USPEX

Crystallographic Characterization

Composition

Implications and Future Work

Theoretical Prediction Methods

High Pressure Synthesis