Magnetic and electronic correlations in YBa sub 2 Cu sub 3 O sub 6+x

YBa{sub 2}Cu{sub 3}O{sub 6+x} is an antiferromagnetic insulator in its tetragonal phase at small x, and it becomes a superconducting metal in the large x orthorhombic phase. The transition between the two phases is controlled by the orientational ordering of CuO{sub x} chain segments and dielectric screening by the BaO layers. In the orthorhombic phase, holes from the chains are transferred to the CuO{sub 2} planes, and the superconducting transition temperature scales with the density of O 2p holes. The antiferromagnetic ordering in the tetragonal phase is dominated by localized Cu moments in the CuO{sub 2} planes. The two-dimensional (2D) coupling between those spins is unusually strong. The interaction between nearest-neighbor planes is sufficiently strong that spins in bilayers remain highly correlated well above the Neel temperature. Long-range order is destroyed when a significant density of O 2p holes is present in the planes; however, there is evidence that Cu moments survive and interact in the metallic phase. 55 refs., 13 figs