Crystal structure of NiO under high pressure

The effect of pressure on the lattice parameters of NiO with a rhombohedral distorted B1 structure was investigated up to 141 GPa by in situ angle-dis persive x-ray diffraction using synchrotron radiation, on imaging plate. an d a diamond anvil cell. The lattice constants a and c, expressed in the hex agonal lattice, decrease monotonically with increasing pressure. The axial ratio cia also decreases monotonically with increasing pressure, indicating that the distortion in the rhombohedral [111] direction becomes larger as pressure increases. No phase transition was observed up to 141 GPa. a signi ficant change of pressure coefficient of c/a above 60 GPa, as suggested by a recent theoretical calculation, was net observed.