Anomalous variation of the c lattice parameter of a sample of YBa<SUB>2</SUB>Cu<SUB>3</SUB>O<SUB>7-δ</SUB> through the superconducting transition

Using a powder diffractometer with a continuous flow arrangement, the lattice parameters of a sample of YBa2Cu3O7-&#948; (with &#948; estimated to be between 0.10 and 0.15) were determined from 300 K down to 80 K with a temperature regulation of 0.1 K. The diffractogram showed a single phase with the orthorhombic structure. Measurements from 100-80 K were performed at close temperature intervals and showed an anomalous change in c in a narrow temperature range around Tc. This anomaly correlates with the anomaly in the nuclear quadrupole resonance frequency, positron annihilation lifetimes, and sound velocity observed by some workers. A smaller decrease in c was observed around 240 K as the sample was cooled, which again correlates with anomalies observed in sound velocity and attenuation and differential scanning calorimetry. The present observations differ from the recently published results of Horn et al