Optical absorption and IR reflection measurements of MnO were performed under various pressures up to 140 GPa at room temperature. In IR reflection measurement, a drastic increase in reflectivity due to the metallization is observed from 94 GPa. On the other hand, in absorption, it is observed that the band gap does not close monotonously with pressure but accelerated extremely from 80 GPa, as indicated by an abrupt increase of an additional absorption tail located at the lower energy than the absorption edge. The absorption tail extends to 0 eV around 90 GPa, which is consistent with the IR reflection measurement. The pressure coefficient of the band gap shift is obtained to be −0.016 eV/ GPa up to 80 GPa.The manganese perovskite materials ͑RMnO 3 , where R is a rare earth metal͒ are exceptionally important Mott ͑or charge transfer͒ insulators since many interesting phenomena are observed including the colossal magnetoresistance ͑CMR͒, 1,2 half-metallic behavior, 3 and phase separation. 4 The main stage in such phenomena occurs in a MnO 6 octahedron and the actor is the d electrons in the manganese e g level split off from the t 2g level by the crystal field generated by six oxygen ions. When chemical pressure is applied by replacing the R-site ions, the transfer integral of manganese d͑e g ͒ electrons changes through the change of the Mn-O-Mn bond angle. MnO is the simplest manganese oxide and shows antiferromagnetic character at the temperature lower than T N = 118 K. The crystal has the NaCl-type ͑B1-type͒ structure above T N , and below the temperature, a slight contraction along a ͗111͘ direction occurs. It means that a manganese ion is surrounded by six oxygen ions and forms a MnO 6 octahedron. There are two methods for the metallization of the insulator: bandwidth control and band-filling control. In contrast with the filling control due to the doping or introduction of the defects, the bandwidth control by pressuring is able to conserve the system pureness. Furthermore, in MnO, the Mn-O-Mn bond angle is never changed by compression. Therefore, to study the physical properties of MnO under various pressures is regarded as that of the core of the manganese perovskites as the function of the Mn-O bond length. Recently, Kondo et al. 5 performed an x-ray analysis of MnO at room temperature and observed new phase boundaries at 30, 90, and 120 GPa. At that time, they also observed, by an optical microscope, that the sample becomes highly reflective above 90 GPa. There is a possibility that a metallic or semimetallic transition occurs near this pressure. For clarifying that a phase is true metallic, it is the best and the simplest way to show ͑i͒ high infrared ͑IR͒ reflectivity or ͑ii͒ both low specific resistance and its decreasing nature with decreasing temperature, and in high pressure study, their pressure insensitivity. Although the transition has been investigated theoretically, 6,7 we present here, experimental evidence of transition to the true metallic phase.We performed the IR reflection and absorptio...
