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During the past decade, the radial x-ray diffraction method using a diamond anvil cell (DAC) has been developed at the X17C beamline of the National Synchrotron Light Source. The detailed experimental procedure used with energy dispersive x-ray diffraction is described. The advantages and limitations of using the energy dispersive method for DAC radial diffraction studies are also discussed. The results for FeO at 135 GPa and other radial diffraction experiments performed at X17C are discussed in this report.

Magnesiowustite (Mg0.1Fe0.9)O was studied at ambient temperature under both hydrostatic and quasihydrostatic pressures to 35 GPa. The elastic behaviour, equation of state, transition pressure, and the structure before and after the B1–rhombohedral transition were determined. K0-and K0 '-values for the B1 phase were calculated to be 155 ± 10 and 3.6 ± 0.8 GPa respectively under hydrostatic conditions. The second-order B1–rhombohedral transition occurred at 20 GPa. Quasihydrostatic conditions were used to determine the stress/strain in different crystallographic directions to elucidate the transition mechanism which is triggered by the softening of the c44 in the B1 phase.

The phase transitions and equation of state of cobalt oxide (CoO) have been studied at ambient temperature under non-hydrostatic and hydrostatic high pressures, with the diamond-anvil cell technique. We discovered that under hydrostatic pressure conditions the CoO structure transformed from the low-density rhombohedral to a high-density rhombohedral phase at 90 ± 1 GPa with the molecular volume decreasing by about 2.7%. This phase transition with volume discontinuity may be related to magnetic collapse, which was predicted by theory. The pressure of the phase transition of CoO from face-centred cubic B1 to rhombohedral structure was 43 ± 2 GPa. The decompression spectrum data of CoO under non-hydrostatic pressure showed that these phase transitions are reversible.