THE ELECTRONIC-STRUCTURE OF BI2-XGDXRU2O7 AND RUO2 - A STUDY BY ELECTRON-SPECTROSCOPY

The bismuth gadolinium pyrochlore ruthenates Bi//2// minus //xGd//xRu//2O//7 have been studied in relation to RuO//2 by the techniques of X-ray and ultraviolet photoelectron spectroscopy (XPS and UPS) and high-resolution electron-energy-loss spectroscopy (HREELS). The composition-dependent metal-to-semiconductor transition in the pyrochlore system is mirrored by the progressive decrease with composition parameter in (i) the density of electronic states at the Fermi energy in UPS, (ii) the plasmon frequency in HREELS, and (iii) the probability of screening a Ru:3d core hole in XPS. These changes are too gradual in themselves to pinpoint the transition, but are generally consistent with transport and X-ray diffraction data that indicate a metal-to-nonmetal transition at x equals 1. 55 mediated by an interplay between disorder and correlation-induced electron localization.