X-RAY-DIFFRACTION STUDY OF HAFNIA UNDER HIGH-PRESSURE USING SYNCHROTRON RADIATION

Hafnia, HfO2, has been studied to 20 GPa at ambient temperature using synchrotron energy-dispersive X-ray powder diffraction methods. Traces of the initial monoclinic (baddeleyite) phase could still be identified at 20 GPa, but a phase transition begins at 2.6 GPa to HfO2 II, the lines of which can be indexed on an orthorhombic cell (Pbcm, Z = 4). The relative intensities of the (111) and (-111) monoclinic reflections suggests that a further phase change may begin near 10 GPa but work at high temperatures is needed to establish this. The bulk compressibility of hafnia is 1.2 x 10(-4) kbar-1. A Rietveld analysis of the baddeleyite phase gave results in close agreement with earlier work but showed the sample to be internally strained.