Subsolidus phase relations and crystal structures of R–Ca–Cu–O ( R=Nd, Sm, Gd, Tm) systems

Abstract

The subsolidus phase relations of R 2O 3–CaO–CuO ternary systems ( R=Nd, Sm, Gd, Tm) have been investigated by X-ray powder diffraction. All samples were synthesized at about 950° in air. There exists a ternary compound Ca 14− x R x Cu 24O 41 ( x = 4 for R=Nd, Gd and x = 5 for R = Sm) and a ternary solid solution Ca 2+ x R 2− x Cu 5O 10 ( R=Nd, Sm, Gd, Tm) with a wide composition range Δ x of about 0.6. The compound Ca 14− x R x Cu 24O 41 possesses a layered orthorhombic structure and is isostructural to Sr 14− x Ca x Cu 24O 41. The lattice parameters a and c of the compound are basically independent of the ionic radius of R, while the lattice parameter b and unit-cell volume V decrease substantially with the decrease of the ionic radii of R. The Ca 2+ x R 2− x Cu 5O 10 solid solution is isostructural to Ca 2+ x Y 2− x Cu 5O 10, the structure of which is based on an orthorhombic “NaCuO 2-type” subcell containing infinite one-dimensional chains of edge-shared square planar cuprate groups crosslinked by the layered cations Ca and R that locate in the inter-chain tunnels.