Structural and electronic factors governing the metallic and nonmetallic properties of the pyrochlores A(2)Ru(2)O(7-y)

The metal-versus-semiconductor behavior of ruthenium pyrochlores A(2)Ru(2)O(7-y) was examined by calculating their electronic band structures, This behavior is explained in terms of the Mott-Hubbard mechanism of electron localization. The width of the t(2g)-block bands of A(2)Ru(2)O(7-y) increases with increasing Ru-O-Ru bond angle, and the Ru-O-Ru angle increases with increasing size of the A cation. There is a good linear relationship between the ionic radius of the A cation and the Ru-O-Ru bond angle, This relationship makes it possible to calculate the amount of the O' atom vacancy based on the observed Ru-O-Rn angle in Tl2Ru2O7-y. (C) 1997 Academic Press.