Synchrotron x-ray diffraction measurements on a high purity single crystal reveal that the absence of long-range ortho-II order in YB${\mathrm{a}}_2$C${\mathrm{u}}_3$ ${\mathrm{O}}_{6+x}$ is most likely due to random-field effects. The ortho-II superstructure peaks show three qualitative features consistent with random-field behavior: First, instead of the development of a sharp Bragg peak at the transition $T_{\mathrm{OII}}=125\mathrm{}\left(5\right)$ °C (for $x=0.5\mathrm{}$), the initial diffuse anisotropic Lorentzian (critical scattering) above $T_{\mathrm{OII}}$ transforms to a Lorentzian-squared form below $T_{\mathrm{OII}}$. Second, the equilibration time increases dramatically at the onset of ordering. And third, the domain growth is logarithmic in time below $T_{\mathrm{OII}}$.

• Received 3 October 1994

DOI:https://doi.org/10.1103/PhysRevLett.74.1446