SPRING-DEFECT MODEL OF STRUCTURAL PHASE-TRANSITION IN YBA2CU3O7-DELTA

A ''spring-defect model'' has been developed for describing the connec tion between elastic distortion and oxygen ordering and the concomitan t structural phase transition (from tetragonal to orthorhombic at simi lar to 900 K) in high T-c (less than or similar to 90 K) superconducto r YBa2Cu3O7-delta. The lattice statics of the CuO basal plane is treat ed in the harmonic approximation in which the mean distance between tw o copper atoms is assumed to take two different values depending on wh ether or not there is an intervening oxygen defect. The presence or th e absence of the latter is modelled in terms of a pseudo spin sigma = +1 or -1. Elimination of the lattice degrees of freedom leads to Ising interactions between oxygen atoms in a two-dimensional lattice, which turn out to be long-ranged, repulsive for first-neighbours and compet ing as well as anisotropic for second-neighbours, for certain choices of the spring constants (K, K') and the distortion parameter (alpha). The model thus provides a mechanism for the structural transition from the tetragonal phase to the orthorhombic phase. Also, the coupling be tween the spin and lattice degrees of freedom allows for a systematic study of ferroelasticity and phonon properties of this system.