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

Abstract

To show that single-crystal x-ray-diffraction data can reveal the presence of two apical oxygen sites in ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_7$-based samples, the structure of a copper-deficient 1:2:3 sample, ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_{2.78}$ ${\mathrm{O}}_7$, 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) Å and c=11.625(1) Å. 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. Except for the Cu vacancies the structure of ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_{2.78}$ ${\mathrm{O}}_7$ is similar to that of the 1:2:3 compound. All the vacancies were found to be on chain Cu(1) sites. This means that every either fourth or fifth Cu(1) site is empty and there exist two apical O(1) sites and two Cu(2)-O(1) distances. No long-range superstructure was detected either by x-ray diffraction or electron diffraction. The two O(1)-site feature is revealed qualitatively by the large thermal motion of O(1) along the c axis and quantitatively by electron density maps. The relationship between this structure and the double-well model deduced by EXAFS data for the apical oxygen is discussed.