Optical investigations on Y2-xBixRu2O7: Electronic structure evolutions related to the metal-insulator transition

Optical conductivity spectra of cubic pyrochlore Y2-xBixRu2O7 (0.0≤x≤2.0) compounds are investigated. As a metal-insulator transition (MIT) occurs around x=0.8, large spectral changes are observed. With increase of x, the correlation-induced peak between the lower and the upper Hubbard bands seems to be suppressed, and a strong mid-infrared feature is observed. In addition, the p-d charge transfer peak shifts to the lower energies. The spectral changes cannot be explained by electronic structural evolutions in the simple bandwidth-controlled MIT picture, but are consistent with those in the filling-controlled MIT picture. In addition, they are also similar to the spectral changes of Y2-xCaxRu2O7 compounds, which is a typical filling-controlled system. This work suggests that, near the MIT, the Ru bands could be doped with the easily polarizable Bi cations.