A determination of the oxidation states and internal stresses in Ba sub 2 YCu sub 3 O sub x , x = 6-7 using bond valences

A bond valence analysis of the title compound indicates that Cu{sup 1+} is found only on the Cu1 site but Cu{sup 3+} is found on both sites with a 25% occupancy on Cu2 and a 50% occupancy on Cu1 for x = 7. The analysis also predicts how the electron holes would be distributed over the four crystallographically distinct O atoms if they reside on the O rather than the Cu atoms. Modeling the two end members using Kirchoff-like network equations and a distance-least-squares refinement shows that the bonds most sensitive to Cu oxidation are those formed by Ba and that the structure is subject to internal stresses that are close to a ferroelastic instability at x = 6 and a ferroelectric instability at x = 7.