The characteristics of orthorhombicity of YBa2Cu3O7-δ in superconducting state

The temperature dependence of the lattice constants a, b and c of YBa2Cu3O7-delta (delta = 0.5) has been determined in a range 28-300 K by high-angle double-crystal X-ray diffractometry, and examined near the superconducting transition temperature T-c = 55 K. The angle phi that is related to the orthorhombicity of the crystal as phi = tan(-1) (a/b) decreases with decreasing temperature; it is found, however, that at T-c the decrease starts to slow down. The difference between phi observed and that extrapolated from the normal state is proportional to (1 - T/T-c)(xi). This is new behaviour in connection with the onset of superconductivity, and is discussed in terms of a change in the bond strength around Cu atoms. By comparing the results with that of a specimen with delta similar to 0.2 (T-c similar to 90 K) in literature, it is shown that, owing to weaker bond strength around Cu atoms, the crystal with delta = 0.5 exhibits the behaviour more clearly. (C) 2001 Elsevier Science B.V. All rights reserved.