MONTE-CARLO SIMULATION OF OXYGEN DIFFUSION IN PLANAR MODEL OF 123-YBCO - LOW-TEMPERATURE REGIME AND EFFECT OF TRAPPING BARRIER

The asymmetric next-nearest neighbour lattice gas model is used for de scribing the oxygen-vacancy phase diffusion in the basal plane of 123 YBaCuO superconducting ceramics at low temperatures. A finite value Q for the trapping potential barrier is added. The Monte Carlo technique has been used to obtain the components of the (tracer) diffusion coef ficient. The character of its dependence on the coverage is shown to v ary, from convex at high temperatures to concave at lower ones. The te mperature of the transition depends on the trapping barrier height. Th e activation energy (obtained from an Arrhenius plot) is seen to be va rying as a function of coverage and the trapping barrier, and is not s ymmetrical with respect to c = 0.5 if Q is finite. The same of course is true for the diffusion coefficient. The value of the ''final-state energy'' is reported and compared to the ground-state energy. This ind icates the likely occurrence of fine structure at specific coverage va lues corresponding to a recently predicted devil's staircase distribut ion of superstructure phases. The time evolution of twins is also repo rted. A discussion of the limited validity of the model for describing the 123 structural phase diagram is presented.