Abstract
Anomalies in vibrational spectra measured by neutron and Raman scattering and spatial inhomogeneities in 1618O exchange lead to a two-region micromorphological model of YBCO. This model explains why among ‘‘single-phase’’ high-temperature superconductors only YBCO exhibits a nearly zero isotope effect. This small isotope effect has been used as the justification for many exotic nonphonon models of high-temperature superconductivity, but the present discussion shows that such models are unnecessary.