Neutron-scattering measurements have been performed to study the magnetic ordering of Pr and Cu in semiconducting TlBa2PrCu2O7-y. This compound has a nuclear structure which is similar to that of YBa2Cu3O7 but with the CuO-chain layers fully replaced by TlO layers. At low temperatures the magnetic diffraction pattern reveals magnetic Bragg peaks of the {1/2 1/2 1/2} type, which are associated with the antiferromagnetic ordering of the Pr moments. The magnetic structure consists of nearest-neighbor spins along all three crystallographic directions being antiparallel. The spin direction is along the c axis, with a saturated moment of 1.05 mu(B). The temperature dependence of the intensity of these reflections indicates a Neel temperature T(N) almost-equal-to 8 K, which is about half that observed in the related PrBa2Cu3O7 system. A second set of magnetic reflections, of the {1/2 1/2 1} type, are also observed, and these are associated with the Cu magnetic sublattice. This magnetic structure is identical to the ''plane ordering'' observed in the related 1:2:3 system, where the nearest-neighbor Cu spins are again antiparallel. The spin direction is in the tetragonal ab plane, with a saturated ordered moment of 0.59 mu(B). The temperature dependence indicates a Neel temperature of almost-equal-to 370 K for the Cu spins.
