VALENCE FLUCTUATION IN ALPHA-CUVO3 STUDIED BY PHOTOEMISSION SPECTROSCOPY

We have performed a photoemission study of the mixed-valence semiconductor alpha-CuVO3 (Cu7-xV6O19-x, x similar to 0.38). From the measured spectra we have found that both Cu and V are mixed-valent between Cu+ and Cu2+ and between V4+ and V5+. The mean valences of Cu and V deduced from the spectra suggest a considerably larger number of local magnetic moments than that deduced from the Curie-Weiss magnetic susceptibility, indicating that the magnetic moments of the V4+ and Cu2+ ions are partially suppressed. Combining the present results with the NMR and ESR data, we proposed that the V4+ local moments are totally suppressed by the formation of singlet pairs and the Cu2+ local moments are reduced by the Cu+-Cu2+ valence fluctuation. The O 2p to Cu 3d charge-transfer energy deduced from the analysis of the Cu 2p core-level spectrum is found to be small, which may favor the occurrence of the valence fluctuation at the Cu sites. (C) 1994 Academic Press, Inc.