Thermodynamics and kinetics of oxygen ordering in YBa2Cu3Ox

This work presents linear thermal expansion data of detwinned YBa2Cu3Ox (x = 6.3 - 7) single-crystals in the temperature range 150 - 500 K. Large contributions to the expansivity due to oxygen ordering (as a result of large Grueneisen parameters) are observed above room-temperature, which, for the first time, allows a thermodynamic study of oxygen ordering in YBa2Cu3Ox. Below room-temperature, these contributions vanish at a glass-like transition because the kinetics of oxygen ordering becomes exceedingly slow. The structural phase-diagram of YBa2Cu3Ox determined from these measurements is very simple, showing only a critical point at x ∝ 6,56, which is attributed to the 2D-Ising-like Ortho-I <-> Ortho-II (OI <-> OII) phase transition. No evidence for the existence of the OIII, OV or OVIII phases is seen. Starting from x ∝ 6,56, the 2D ordering character decreases with increasing x, and, for 6,8 ≤ x ≤ 7, the expansivity can be described with a 1D ordering model with a strongly strain depended next-nearest-neighbor interaction along the CuO chains. This large strain dependence of the ordering is shown to be responsible for the different degree of oxygen order in different REBa2Cu3Ox systems (RE = Y, Nd, Yb,..), as well as for the unusual doping-dependence of the lattice-constants of YBa2Cu3Ox. The, from the thermodynamics of O-ordering estimated, hole density depends nearly linearly on x, i.e. it is hardly influenced by the OI <-> OII phase-transition, which suggests that oxygen ordering is not responsible for the '60 K plateau' seen in Tc versus x. (orig.)