Theoretical study of the relative stability of structural phases in group-III nitrides at high pressures

We present the results of a first-principles theoretical study of the relative stability of several structural phases of the group-m nitrides AlN, GaN, and InN that complements the picture of the behavior under pressure of these technologically important materials. Along with structures which have been previously considered in other theoretical studies of these materials (comprising those of the observed phases: wurtzite, zinc-blende, and rocksalt; and the d-beta -Sn, NiAs, and CsCl structures) we have also assessed the stability of several novel structures, viz., the cinnabar structure, the Cmcm structure, and the sc16 structure, which have been recently observed in high-pressure experiments on various related compounds (e.g., GaAs) and have also been reported to be either stable or close to stable in a certain range of pressures in other m-V and II-VI compounds on the basis of first-principles calculations. Our results indicate, however, that in AIN, GaN, and InN, the high-pressure rocksalt phase remains stable with respect to any other phase considered in this study up to the highest pressures investigated of similar to 200 GPa, which agrees with the available experimental data. We have further considered the effect of the semicore d orbitals of GaN and InN on the phase diagram of these compounds.