Theoretical investigation of the solid state reaction of silicon nitride and silicon dioxide forming silicon oxynitride (Si2N2O) under pressure

The high-pressure behavior of Si2N2O is studied for pressures up to 100 GPa using density functional theory calculations. The investigation of a manifold of hypothetical polymorphs leads us to propose two dense phases of Si2N2O, succeeding the orthorhombic ambient-pressure polymorph at higher pressures:a defect spinel structure at moderate pressures and a corundum-type structure at very high pressures. Taking into account the formation of silicon oxynitride from silicon dioxide and silicon nitride and its pressure dependence, we propose five pressure regions of interest for Si2N2O within the pseudo-binary phase diagram SiO2-Si3N4: (i) stability of the orthorhombic ternary phase Of Si2N2O up to 6 GPa, (ii) a phase assemblage of coesite, stishovite, and beta-Si3N4 between 6 and I I GPa, (iii) a possible defect spinel modification of Si2N2O between I I and 16 GPa, (iv) a phase assemblage of stishovite and gamma-Si3N4 above 40 GPa, and (v) a possible ternary Si2N2O phase with corundum-type structure beyond 80 GPa. The existence of both ternary high-pressure phases of Si2N2O, however, depends on the delicate influence of configurational entropy to the free energy of the solid state reaction.