Crystal structure and magnetic ordering of the rare-earth and Cu moments in R${\mathrm{Ba}}_2$${\mathrm{Cu}}_2$${\mathrm{NbO}}_8$ (R=Nd,Pr)

Using neutron diffraction, we have studied the crystal structure and magnetic ordering of the rare-earth and Cu moments in the layered perovskite systems ${\mathrm{NdBa}}_2$ ${\mathrm{Cu}}_2$ ${\mathrm{NbO}}_8$ (Nd 2:2:1:8) and ${\mathrm{PrBa}}_2$ ${\mathrm{Cu}}_2$ ${\mathrm{NbO}}_8$ (Pr 2:2:1:8). These are systems similar to R${\mathrm{Ba}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{6+\mathit{x}}$ [R 1:2:3:(6+x)] for which the chain Cu-O layers have been fully replaced by layers of Nb-O octahedra. Powder profile refinements below 20 K show that both Nd 2:2:1:8 and Pr 2:2:1:8 are in space group I4/mcm, because the near-neighbor Nb-O octahedra are rotated about the c axis in opposite senses. The Cu moments are ordered below 375(10) K for Nd 2:2:1:8 and 340(15) K for Pr 2:2:1:8, with saturation moments in both cases of 0.5(1)${\mathrm{\mu }}_{\mathit{B}}$. The near-neighbor Cu spins are aligned antiparallel, just as for the Cu ‘‘plane’’ ordering in R 1:2:3:(6+x). The rare-earth moments order at 1.69(5) and 12.6(1) K for Nd 2:2:1:8 and Pr 2:2:1:8, respectively, with saturated values of 〈${\mathrm{\mu }}_{\mathit{z}}$〉=0.74(5)${\mathrm{\mu }}_{\mathit{B}}$ and 1.2(1)${\mathrm{\mu }}_{\mathit{B}}$. The near-neighbor R spins, both in the ab plane and along the c axis, are also aligned antiparallel. As in the case of Pr 1:2:3:(6+x), the Pr sublattice of Pr 2:2:1:8 has an ordering temperature that is much higher than other rare-earth moments in these layered perovskites, suggesting that the Pr in Pr 2:2:1:8 experiences the same f-electron hybridization as has been observed in Pr 1:2:3:(6+x). The rare-earth ordering of both Nd 2:2:1:8 and Pr 2:2:1:8 is not completely three-dimensional in nature: rather, the ordering within the a-b plane is two-dimensional with only short-range order along the c axis. The presence of oxygen defects appears to affect the c-axis correlation length.