1ST-ORDER PHASE-TRANSITION BETWEEN ORTHORHOMBIC PHASES IN YBA2CU3OZ

We investigate the effects of elastic interactions and the associated orthorhombic distortion on the structural phase transitions in YBa2Cu3 Oz. Starting from a two-dimensional Ising model previously demonstrate d to describe accurately the high-temperature phase boundaries, we sho w by means of Landau theory that the square-rectangular symmetry break ing may lead to a first-order transition between the orthorhombic sing le- and double-cell phases at low temperatures. This explains experime ntal observations of phase separation between orthorhombic phases, whi le retaining the simplicity and predictive power of the original model .