Neutron scattering results on the RBa2Cu3O6+x(R=rare earth) class of systems are reviewed. For the oxygenhyphen;deficient systems (x0.5), which are semiconducting, the Cu ions are observed to order with Neacute;el temperatures as high as 500 K. The magnetic structure consists of strong nearesthyphen;neighbor antiferromagnetic couplings of the Cu spins within the Cuhyphen;O2layers, while nearesthyphen;neighbor spins in adjacent layers are also aligned antiferromagnetically. The moments are constrained in the tetragonal plane. At lower temperatures the Cu moments in the oxygenhyphen;deficient lsquo;lsquo;chainrsquo;rsquo; layers also order antiferromagnetically, with a moment that can be quite substantial (sim; 1/2 thinsp;mgr;B). Both transition temperatures are very sensitive to the oxygen concentration. The magnetic heavy rarehyphen;earth compounds, on the other hand, order antiferromagnetically at very low temperatures, analogous to the lsquo;lsquo;conventionalrsquo;rsquo; magnetichyphen;superconductor systems such as RMo6S8and RRh4B4. The ordering is not very sensitive to the oxygen concentration, and the antiferromagnetic state coexists with superconductivity. Differences in the ordering behavior are ascribed to crystalhyphen;field effects, which are briefly discussed. Finally, smallhyphen;angle neutron scattering results in the vicinity of the superconducting transition temperature are also discussed.
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