Optical study of MnO under high pressure

Raman and optical absorption studies of the antiferromagnetic insulator MnO were performed at several temperatures under high pressure. In Raman scatt ering at 2 K, it was observed that the two-magnon peak shifts to higher ene rgy with increasing pressure and then disappears at around 20 GPa. This sug gests that a change in spin arrangement occurs at that pressure. At 77 K th e peak-vanishing pressure increases to 25 GPa. At room temperature, a jump of the Raman peak position was observed at 30 GPa and the peals vanishes at 89 GPa. These results are consistent with previous X-ray diffraction studi es which showed phase transitions at 30 and 90 GPa. The vanishing of the Ra man peak suggests a metallization transition. The pressure dependence of th e Mn2+ d-d transition observed in optical absorptions was consistent with t he crystal field theory up to around 25 GPa at room temperature.