THE NATURE OF METAL-NON-METAL TRANSITION IN THE ANION RUO6-CHAINS UNDER THE SUBSTITUTION OF METAL ATOMS OF THE CATION SUBLATTICE IN RARE-EARTH BISMUTH RUTHENATES

The electronic structure of rare earth bismuth ruthenates, Gd-x Bi2-x Ru2O7 and NdxBi2-xRu2O7, showing the metal-non-metal transition at x = 1.4-1.6 was studied by the X-ray emission and X-ray electron spectros copy. The results obtained reveal that when the bismuth atoms are subs tituted by the rare earth atoms the following changes take place: i) n on-equivalent charge states of oxygen appear, ii) valence electron den sity of ruthenium atoms becomes delocalized, iii) covalency of Ru-O bo nds increase. The metal-non-metal transition model based on the locali zation of conduction band electronic states cannot be conformed with t hese results, whereas the model based on the splitting of the t(2g)-ba nd into the completely filled and empty bands is consistent with them.