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

The pyrochlore ruthenium (IV) oxides Pb₂Ru₂O_7-y, Bi₂Ru₂O₇, Y₂Ru₂O₇ and the perovskites SrRuO₃, CaRuO₃ 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 Pb₂Ru₂O_7-y>Bi₂Ru₂O₇>SrRuO₃>CaRuO₃. 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>SrRuO₃>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.