Stability of the wurtzite-type structure under high pressure: GaN and InN

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Abstract
High-pressure in situ x-ray-diffraction studies on GaN and InN have been carried out using an imaging-plate technique and a diamond-anvil cell up to about 60 GPa. The two compounds crystallize in the wurtzite-type structure at ambient conditions. The axial ratio c/a of GaN remains unchanged from the ambient value of 1.626 whereas c/a of InN is considerably decreased from 1.613 to 1.597 with increasing pressure to about 15 GPa. Equation-of-state data obtained for the wurtzite phase have yielded the bulk modulus of GaN to be 237(31) GPa and that of InN to be 125.5(4.6) GPa. Structural phase transition into the rocksalt-type structure takes place in GaN at 52.2 GPa and in InN at 12.1 GPa. The trend in the transition pressures of the III-V nitrides is discussed in terms of various ionicity scales.