Temperature dependence of YBa2Cu3O7-x local structure: A perturbed-angular-correlation study

We have used perturbed-angular-correlation (PAC) spectroscopy of dilute In111 dopants to characterize the local structure of YBa2Cu3O7-x vs T from room temperature to 950°C. We obtain highly reproducible results with fine-grain powder samples that are prepared by nitrate evaporation and cooled very slowly after firing. These samples display orthorhombic YBa2Cu3O7 x-ray diffraction patterns at room temperature and have a very sharp Meissner flux exclusion transition at Tc=92.7 K. PAC spectra, which clearly indicate nontetragonal local symmetry, do not change at temperatures where diffraction measurements indicate an orthorhombic-to-tetragonal phase transition. We interpret this observation as evidence that the tetragonal phase is in these materials retains local orthorhombic order.