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 describing 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 coefficient. The character of its dependence on the coverage is shown to vary, from convex at high temperatures to concave at lower ones. The temperature of the transition depends on the trapping barrier height. The activation energy (obtained from an Arrhenius plot) is seen to be varying as a function of coverage and the trapping barrier, and is not symmetrical 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 indicates the likely occurrence of fine structure at specific coverage values corresponding to a recently predicted devil's staircase distribution of superstructure phases. The time evolution of twins is also reported. A discussion of the limited validity of the model for describing the 123 structural phase diagram is presented.