LOW-TEMPERATURE OXYGEN ORDERING IN YBA2CU3O6+X

We consider a model for O ordering in which any two O ions of the CuO( x) planes repel each other with a screened Coulomb repulsion, but the repulsion energy between two second-neighbor O ions with a Cu ion in b etween is reduced by DELTAE, to account for the stabilization energy o f perfect chains due to covalency and charge transfer. We calculate th e screening length lambda using Thomas-Fermi theory, a slave-boson tre atment of correlations and experimental data. For reasonable values of DELTAE, the ground state of the model is consistent with most of the observed diffraction data, with a possible explanation of photoinduced effects for x approximately 0.4, and with recent calculations relatin g T(c) with x.