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 magnetic ordering of the rare-earth and Cu moments in the layered perovskite systems NdBa[sub 2]Cu[sub 2]NbO[sub 8] (Nd 2:2:1:8) and PrBa[sub 2]Cu[sub 2]NbO[sub 8] (Pr 2:2:1:8). These are systems similar to [ital R]Ba[sub 2]Cu[sub 3]O[sub 6+[ital x]] [[ital R] 1:2:3:(6+[ital 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 [ital I]4/[ital mcm], because the near-neighbor Nb-O octahedra are rotated about the [ital 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)[mu][sub [ital B]]. The near-neighbor Cu spins are aligned antiparallel, just as for the Cu plane'' ordering in [ital R] 1:2:3:(6+[ital 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 [l angle][mu][sub [ital z]][r angle]=0.74(5)[mu][sub [ital B]] and 1.2(1)[mu][sub [ital B]]. The near-neighbor [ital R] spins, both in the [ital ab] plane and along the [ital c] axis, are also aligned antiparallel.more » As in the case of Pr 1:2:3:(6+[ital 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 [ital f]-electron hybridization as has been observed in Pr 1:2:3:(6+[ital 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 [ital a]-[ital b] plane is two-dimensional with only short-range order along the [ital c] axis. The presence of oxygen defects appears to affect the [ital c]-axis correlation length.« less