Raman spectroscopy and x-ray diffraction of phase transitions in Cr ₂O₃ to 61 GPa

Raman spectroscopy and x-ray diffraction measurements have been performed for pure Cr₂O₃ to 61 GPa at ambient temperature with and without laser heating. Several changes were found at 15-30 GPa under cold compression: splittings of two E₈ phonons, steep selective increase of some phonon intensities, softening of low-frequency phonon modes, selective broadening of some diffraction lines, systematic deviation of diffraction positions relative to the corundum structure assignment, and eventual splitting of the (110)_H diffraction line at 58 GPa. These changes are consistent with those expected for a phase transition to the monoclinic V ₂O₃-type (I2/a) structure. Another phase transition has been found above 30 GPa after laser heating. This is accompanied by considerable changes in Raman spectra and diffraction patterns, which implies that the transition is reconstructive. The diffraction patterns of this phase can be well explained by either of the orthorhombic structures, perovskite or Rh₂O₃-II type. This confirms the recent prediction of stability of an orthorhombic phase in Cr₂O₃ at high pressure by first-principles calculations. Color changes of Cr₂O ₃ were also detected: from green to red at 14 GPa during cold compression and from red to green during the phase transition to the orthorhombic phase above 30 GPa. Detailed analyses on diffraction patterns show that the color changes are relevant to Cr-O bond-length change. Furthermore, the observed variation trends of bond lengths in Cr₂O₃ during cold compression are similar to those in Al₂O₃ with much more pressure sensitivity, which is consistent with recent first-principles calculations, but are opposite to those in V₂O ₃ and Fe₂O₃ at high pressure.