The metal-to-semiconductor transition in ternary ruthenium (IV) oxides: a study by electron spectroscopy

The pyrochlore ruthenium (IV) oxides Pb2Ru2O 7-y, Bi2Ru2O7, Y2Ru 2O7 and the perovskites SrRuO3, CaRuO 3 have been studied by ultraviolet and X-ray photoelectron spectroscopy (UPS and XPS) and by high-resolution electron-energy-loss spectroscopy (HREELS). The presence of a Fermi edge in UPS confirms that, with the exception of Y2Ru2O7, these materials are metallic; the density of states at the Fermi energy g( epsilon F) decreases in the sequence Pb2Ru2O7-y>Bi 2Ru2O7>SrRuO3>CaRuO 3. The probability of reaching final states in XPS where a Ru:3d core hole is screened by conduction electrons decreases in the same sequence, and for Y2Ru2O7 only unscreened final states are observed. In parallel with these trends, the conduction-electron plasmon loss found in HREELS moves to lower energy with decreasing g( epsilon F); plasmon excitation is not evident in the energy-loss spectrum of semiconducting Y2Ru2O7. The results are consistent with a progressive decrease in the transfer-energy (resonance) integral that determines the Ru:4d bandwidth in the series Pb2Ru2O 7-y>Bi2Ru2O7>SrRuO 3>CaRuO3>Y2Ru2O7, and with a transition to non-metallic behaviour due to correlation-induced electron localisation in Y2Ru2O7. A technique for the calculation of the width of photoemission lines in UPS from HREELS data is presented.