CRYSTAL-CHEMISTRY OF BA2YCU3O7 AND RELATED-COMPOUNDS

The role of electropositive elements in stabilizing high oxidation states in crystals of transition elements such as copper is emphasized. An analysis of bond valences in the superconductor Ba//2YCu//3O//7 indicates that the valences of the two crystallographically distinct Cu atoms in the structure are intermediate between Cu (II) and Cu (III). Observed bond lengths, including the irregular coordination of Ba, can be understood when allowance is made for the differences in Cu-O bonds. A similar analysis of the insulating phase Ba//2YCu//3O//6 reveals well-defined Cu (I) and Cu (II) sites and rationalizes the presence of large displacements for two atoms in the structure. Defect structures in Ba//2YCu//3O//7 are discussed.