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

Abstract

We have used perturbed-angular-correlation (PAC) spectroscopy of dilute ${}_{}{}^{111}{}_{}{}^{}\mathrm{In}$ dopants to characterize the local structure of ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathrm{x}}$ 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 ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_7$ x-ray diffraction patterns at room temperature and have a very sharp Meissner flux exclusion transition at $T_c$=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.