MONTE-CARLO SIMULATION OF LOW-TEMPERATURE PHASE-DIAGRAMS OF YBA2CU3O6+X

We report the results of Monte Carlo simulation of oxygen ordering in the oxygen deficient portion (x < 0.5) of YBa2Cu3O6+x at low temperatures. We find qualitative agreement among cluster - variation, Monte Carlo and transfer matrix methods. However, low temperature and ground state simulations clearly indicate the presence of a tetragonal phase. There is also evidence for two second order phase transition lines separating the tetragonal and the "double cell" ortho II phase. The effect of decreasing the inter-chain repulsion on oxygen ordering has also been investigated.