THE ORDER-DISORDER PHENOMENON IN YBA2CU3O7-DELTA FOR VARYING-DELTA AND THE POSSIBLE EXISTENCE OF 2 ORDERING TEMPERATURES

We examine a model of long range atomic ordering in YBa//2Cu//3O//7// minus // delta (0 less than equivalent to delta less than equivalent to 0. 5), when the temperature T varies, in particular below the tetragonal-orthorhombic transition temperature T//o. A simple mean field description with two order parameters and interactions between first and second neighbors, is proposed for the ordering in the oxygen deficient basal CuO planes. It is the two dimensional analogue of the one given in the past for b. c. c. Fe//1// minus //xAl//x alloys and seems appropriate here according to recent experimental results. According to this scheme, a second ordering temperature T**40 //o is expected to occur below T//o and to increase with delta while T//o decreases. The observation of T** prime //o may have been missed in fast cooling experiments so far, since equilibrium requires a long time to be reached at ordering temperatures. Recently experiments performing slow annealings appear to indeed support the existence of that second transition, all the more important that it is experimentally accompanied by an increase of the superconducting temperature T//c.