Studies of rare earth cuprates with structures related to NaCuO2

Solid solutions with structures based on an orthorhombic “NaCuO2-type” subcell were formed over a wide range of composition in the CaOM2O3CuO systems, where M = Y, Nd, Gd. These structures contain infinite one-dimensional chains of edge-shared square planar cuprate groups crosslinked by the larger cations, which occupy sites in the inter-chain tunnels. Samples with CaM ≥ 1 have the stoichiometry Ca(2+x)M(2−x)Cu5O10; the maximum solubility range corresponded to x = 0.8, 0.4, and 0.3 for M = Y, Gd, and Nd, respectively. Single phase samples with CaM < 1 were also formed in the Gd and Nd systems; these have the stoichiometry M(103−y)Ca(3y2)Cu5O10, with 56 ≤ y ≤ 43 for Nd and 1.13 ≤ y ≤ 1.33 for Gd. Long-range ordered arrangements of the inter-chain ions lead to the formation of superstructures whose symmetry and periodicity is a sensitive function of the Ca:M3+ ratio and the size of M. In general the superstructures are monoclinic and incommensurate along, and perpendicular to the cuprate chains. The ordering can be described in terms of a periodic displacive modulation of the inter-chain cations from their “ideal,” equally spaced positions. Models based on a simple sinusoidal modulation give excellent agreement with experimental diffraction profiles collected using X rays and electrons.