Spinel-type gallium oxynitrides attainable at high pressure and high temperature

Abstract

We study the enthalpy of formation $\ensuremath{\Delta}H$ of spinel-type gallium oxynitrides ($\ensuremath{\gamma}$ galons) as a function of pressure. Formation of the ternary oxynitride from gallia $({\mathrm{Ga}}_{2}{\mathrm{O}}_{3})$ and gallium nitride (GaN) is endothermic at any pressure. $\ensuremath{\Delta}H$ is minimal, however, at the $\ensuremath{\beta}\ensuremath{\rightarrow}\ensuremath{\alpha}$ phase transformation of gallia, which we calculate to occur at 2.6 or $6\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$, depending on whether we treat the $d$ elctrons of Ga as core or as valence states. Hence, at this pressure the driving force to form $\ensuremath{\gamma}$-galons is maximal. The positive enthalpy of formation may be compensated by entropy contributions arising from a mixing of O and N within the anion sublattice and of Ga atoms and vacancies on octahedral sites of the cation sublattice. Since with increasing pressure the decomposition of gallium nitride is suppressed, we propose the synthesis of ternary spinel-type gallium oxynitrides at high temperature and/or high pressure conditions.