Prediction of the new spinel phase of ${\mathrm{Ti}}_3{\mathrm{N}}_4$ and ${\mathrm{SiTi}}_2{\mathrm{N}}_4$ and the metal-insulator transition

Titanium nitrides are important materials with many industrial applications. However, the structure and properties of nitrogen-rich compounds are not well established. Based on ab initio calculations, stochiometric titanium nitride compounds, $c-{\mathrm{Ti}}_3{\mathrm{N}}_4$ and $c-{\mathrm{SiTi}}_2{\mathrm{N}}_4,$ with a spinel structure were predicted. This result is different from the accepted model that ${\mathrm{TiN}}_x$ with $x>\mathrm{}1\mathrm{}$ has a rocksalt structure with Ti vacancies. Electronic structure calculations show that these are highly covalent superhard materials. $c-{\mathrm{Ti}}_3{\mathrm{N}}_4$ is a narrow gap semiconductor and $c-{\mathrm{SiTi}}_2{\mathrm{N}}_4$ is a metal. By doping Ti at the octahedral site of the spinel $c-{\mathrm{Si}}_3{\mathrm{N}}_4,$ the direct band gap can be adjusted. An insulator-to-metal transition in $c-{\mathrm{Si}[\mathrm{Si}}_{1-x}{\mathrm{Ti}}_x{]}_2{\mathrm{N}}_4$ is predicted to occur at $x=\mathrm{0.44.}\mathrm{}$ A process of preparing these compounds is suggested, and several promising applications contemplated.