Phase constitution and thermal expansion ofYBa2Cu3O7−δsingle crystals

We present an x-ray diffraction study of $\mathrm{Y}{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-\delta }$ single crystals. High-resolution scans reveal two orthorhombic phases with different lattice constants but a common orientation indicating a phase separation into phases with $\delta \approx 0.0 \mathrm{and} 0.3$. Thermal expansions of the unit-cell parameters of both phases measured between 10 and 300 K were fit using a quasiharmonic approximation giving a best-fit Debye temperature of 370±5 K. The measured orthorhombicities were found to change smoothly through the superconducting transition temperature without any anomaly.