Neutron powder diffraction and ac susceptibility data on a series of ${\mathrm{ErBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{6+\mathit{x}}$ samples prepared to achieve thermal equilibrium at low temperature provide evidence of structural inhomogeneities near the tetragonal-to-orthorhombic (T→O) transition. In the composition range 0.25≤x≤0.45, the data can best be described as a mixture of orthorhombic and tetragonal phases. This observation, together with the presence of discontinuities in the order parameters, leads to the conclusion that the T→O transition is first order in the temperature range where these samples were prepared. This interpretation provides a simple explanation for the differences between oxygen-deficient R${\mathrm{Ba}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{6+\mathit{x}}$ (R=Y, rare earths) samples prepared by different techniques.

• Received 23 October 1991

DOI:https://doi.org/10.1103/PhysRevB.45.4923