The phase transitions of CoO under static pressure to 104 GPa

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. (Some figures in this article are in colour only in the electronic version).