ON THE FIRST-ORDER PHASE-TRANSITIONS IN YBA2CU3O6+X

We investigate, using the cluster-variation method (CVM), the existenc e of first-order phase transitions in the ASYNNNI model for YBa2Cu3O6- x. We find that the regions of phase separation between the tetragonal and orthorhombic phases and between the two orthorhombic phases in th e (T, x) phase diagram shrink with increasing size of the basic cluste r. The phase diagram, calculated in the 12-point approximation, demons trates the best known agreement between the CVM and the Monte Carlo re sults. The first-order phase transitions occur in the model for YBa2Cu 3O6+x when the strain of the tetragonal-to-orthorhombic transition is accounted for.