The bismuth gadolinium pyrochlore ruthenates Bi2-xGdxRu2O7 have been studied in relation to RuO2 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 = 1.55 mediated by an interplay between disorder and correlation-induced electron localization. Comparison of the results for the pyrochlores with those from RuO2 suggest that the fine structure in the Ru : 3d spectrum of the latter material, previously believed to arise from differing oxidation states at the surface, should in fact be attributed to final-state screening effects in a stoichiometric material. Our conclusion is confirmed by the signals associated with Ru : 4d electrons in XPS, UPS, and HREELS: each of these three techniques appears to probe a conduction-electron concentration essentially equal to its bulk value. In particular, UPS confirms details of the band structure of RuO2 not obvious from previous photoemission experiments. © 1986.
