Electronic properties of transition-metal oxides under high pressure revealed by x-ray emission spectroscopy

摘要

The high-pressure electronic and magnetic properties of MnO, CoO and NiO have been investigated by x-ray emission spectroscopy. Both MnO and CoO show a magnetic collapse revealed by the abrupt decrease of the satellite intensity in the metal K emission lines at about 80 and 100 GPa respectively. The magnetic transition pressures agree well with the known structural transition pressures in these systems. No such magnetic transition was observed in NiO, which rather shows hints of delocalization of the d electrons. The NiO data have been further analysed within a full multiplet approach. The low- and high-pressure emission spectra could be reproduced well. The confrontation between theory and experiment gives a handle on fundamental quantities such as the electron correlation strength, hybridization and charge transfer energy, introduced in the calculations as parameters. Both theoretical and experimental clues indicate that NiO is on the verge of a metal-insulator transition. In MnO and CoO, the magnetic transition is likely to arise from the interplay between the ligand field and the O p bandwidth.