The influence of internal strain on the charge distribution and superconducting transition temperature in Ba2YCu3Ox

The bond lengths in Ba2YCu3Ox expected from chemical considerations alone are not compatible with the constraints imposed by the crystal structure. The resulting strain in the bond lengths can be relaxed inter alia by charge transfer between the two Cu atoms. The effect of the strain and its relaxation is examined using bond valences to model the structure at various compositions. At x = 7.0 the structure adapts by transferring charge between the two crystallographically distinct Cu atoms to give each an oxidation state of about 2.3. At x = 6.0 this mechanism is less effective and the structure contains residual internal strains (BaO bonds stretched and CuO bonds compressed). Between these limits the charge distribution is determined by crystallographic restrictions on the bond lengths. The predictions of the model agree with two recent systematic studies of the structure and are used to show that the 60 K plateau in Tc results from the ordering that occurs at x = 6.5 and that the 90 K plateau is associated with the saturation of Tc as a function of hole concentration.