Maximum transition temperatures and bond valence sums in copper oxide superconductors The influence of internal strain

The correlation between the bond valence sums around the Cu atoms in copper oxide superconductors and their maximum transition temperatures is shown to arise, not from a redistribution of holes between the Cu and O atoms, but from residual internal strain that places the CuO2 layer in a state of tension (high Tc(max)) or compression (low Tc(max)).