The characteristics of orthorhombicity of YBa₂Cu₃O_7-δ in superconducting state

The temperature dependence of the lattice constants a, b and c of YBa₂Cu₃O_7-δ (δ = 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 φ that is related to the orthorhombicity of the crystal as φ = 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 φ observed and that extrapolated from the normal state is proportional to (1 - T/T_c)^ξ. 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 δ ∼ 0.2 (T_c ∼ 90 K) in literature, it is shown that, owing to weaker bond strength around Cu atoms, the crystal with δ = 0.5 exhibits the behaviour more clearly.