Evidence by x-ray diffraction for two apical oxygen sites in a copper-deficient YBa2Cu2.78O7 crystal.

To show that single-crystal x-ray-diffraction data can reveal the presence of two apical oxygen sites in YBa 2 Cu 3 O 7 -based samples, the structure of a copper-deficient 1:2:3 sample, YBa 2 Cu 2.78 O 7 , has been determined. Small crystals were grown under high oxygen pressure. Gandolfi and precession techniques showed that the symmetry is tetragonal with parameters: a=3.8764(4) A and c=11.625(1) A. High order reflection scans, carried out during the single-crystal intensity-data collection, revealed that some of these reflections were broadened indicating that the crystal was twinned and consequently of lower symmetry than tetragonal