CRYSTAL-STRUCTURE AND MAGNETIC-ORDERING OF THE RARE-EARTH AND CU MOMENTS IN RBA2CU2NBO8 (R = ND,PR)

Using neutron diffraction, we have studied the crystal structure and m agnetic ordering of the rare-earth and Cu moments in the layered perov skite systems NdBa2Cu2NbO8 (Nd 2:2:1:8) and PrBa2Cu2NbO8 (Pr 2:2:1:8). These are systems similar to RBa2Cu3O6+, [R 1:2:3:(6+x)] for which th e chain Cu-O layers have been fully replaced by layers of Nb-O octahed ra. Powder profile refinements below 20 K show that both Nd 2:2:1:8 an d 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 mom ents 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)mu(B). The near -neighbor Cu spins are aligned antiparallel, just as for the Cu ''plan e'' 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 satur ated values of [mu(z)] =0.74(5)mu(B) and 1.2(1)mu(B). The near-neighbo r 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 P r in Pr 2:2:1:8 experiences the same f-electron hybridization as has b een 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: rat her, the ordering within the a-b plane is two-dimensional with only sh ort-range order along the c axis. The presence of oxygen defects appea rs to affect the c-axis correlation length.