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structure structure (solids and liquids) D 2000 -008Cation Disorder in Ga1212.-The compounds LnSr 2−x CaxCu 2 GaO 7 (Ln: La, Pr, Nd, Gd, Ho, Er, Tm, Yb, and Lu, 0 ≤ x ≤ 2) are prepared by solid state reaction of the lanthanide oxides, strontium carbonate, calcium carbonate, copper oxide, and gallium oxide (alumina boat, 975 • C, 10 d). The samples are characterized by XRD and neutron diffraction (space group Ima2). Calcium is completely substituted by strontium in the lanthanum and praseodymium systems and partially substituted in the other lanthanide systems. -(GREENWOOD, KEVIN B.; KO, DONGGEUN; VANDER GRIEND, DOU-GLAS A.; SARJEANT, GREGORY M.; MILGRAM, JOHN W.; GARRITY, ELIZABETH S.; DELOACH, DEBORAH I.; POEPPELMEIER, KENNETH R.; SALVADOR, PAUL A.; MASON, THOMAS O.; Inorg. Chem. 39 (2000) 15, 3386-3391; Dep. Chem., Northwest. Univ., Evanston, IL 60208, USA; EN)

The structure of LaSr2CuTiO6.5, a novel (ABO3) n AO n = 2 Ruddlesden−Popper phase, has been solved by powder X-ray diffraction and electron microscopy. The diffraction patterns are consistent with I4/mmm symmetry, with tetragonal lattice parameters a = 3.8816(1) Å and c = 20.296(2) Å. The structure of LaSr2CuTiO6.5 is similar to Sr3Ti2O7, with copper and titanium disordered over the single B-cation site, and lanthanum and strontium almost perfectly disordered over the two A-cation sites. The oxygen content, determined by thermogravimetric reduction to be 6.5 (±0.1), is consistent with divalent copper with the oxygen vacancies distributed within the perovskite layers. The absence of local order and strategies for inducing a layered arrangement of the B-cations are discussed.

Substitution of calcium for strontium in LnSr2-xCaxCu2GaO7 (Ln = La, Pr, Nd, Gd, Ho, Er, Tm, and Yb) materials at ambient pressure and 975 degrees C results in complete substitution of calcium for strontium in the lanthanum and praseodymium systems and partial substitution in the other lanthanide systems. The calcium saturation level depends on the size of the Ln cation, and in all cases, a decrease in the lattice parameters with calcium concentration was observed until a common, lower bound, average A-cation size is reached. Site occupancies from X-ray and neutron diffraction experiments for LnSr2-xCaxCu2GaO7 (x = 0 and x = 2) confirm that the A-cations distribute between the two blocking-layer sites and the active-layer site based on size. A quantitative link between cation distribution and relative site-specific cation enthalpy for calcium, strontium, and lanthanum within the gallate structure is derived. The cation distribution in other similar materials can potentially be modeled.

1995 structure structure (solids and liquids) D 2000 46 -008 Crystal Chemistry of Ln'Ln"Ba2Cu2Ti2O11 (Ln = Lanthanide, Y) Materials. -Investigations of the stabilization of the quadruple perovskite structure Ln2Ba2Cu2Ti2O11 as a function of the constituent lanthanides lead to novel Ln'Ln"Ba2Cu2Ti2O11 (Ln = Lanthanide, Y) materials. The layered mixed A-site compositions with Ln'Ln": LaY, LaHo, LaEr, NdDy ( tetragonal, space group P4/mmm) exhibiting smaller cell constants and volumes than expected, are composed of closely spaced, facing sheets of Cu-O square pyramids separated by double layers of Ti-O octahedra. The A cations, Ln', Ln", and Ba order on three distinct sites with the smaller Ln" occupying the 8-coordinate site between the Cu-O planes. These materials are promising candidates for high Tc materials. -(GREENWOOD, K. B.; SARJEANT, G. M.; POEPPELMEIER, K. R.; SALVADOR, P. A.; MASON, O.; DABROWSKI, B.; ROGACKI, K.; CHEN, Z.; Chem. Mater. 7 (1995) 7, 1355-1360; Dep.