Joseph T. Lewandowski

Ion Exchange of the Layered Perovskite KCa2Nb3O10 by Protons.

Abstract The alkali cations in the layered perovskites MCa 2 Nb 3 O 10 (M ≡ K, Rb, Cs) are exchanged by protons in aqueous acid in a topochemical reaction to give HCa 2 Nb 3 O 10 . The unit cell of HCa 2 Nb 3 O 10 , as determined from powder X-ray diffraction, is tetragonal with a = 3.851 A and c = 14.38 A and contains one formula unit. The anhydrous hydrogen compound hydrates to form a composition HCa 2 Nb 3 O 10 ·1.5H 2 O which is also tetragonal with a = 3.854 A and c = 16.23 A . The hydration-dehydration behavior at 24 °C as a function of the partial water pressure shows significant hysteresis as a result of the expansion and contraction of the unit cell along the c axis.

Intercalation of the layered solid acid HCa2Nb3O10 by organic amines

Layered compounds of formula C/sub n/H/sub 2n+1/NH/sub 3/Ca/sub 2/Nb/sub 3/O/sub 10/ are formed by reaction of n-alkylamines with the solid acid HCa/sub 2/Nb/sub 3/O/sub 10/. Other organic bases such as pyridine can also be intercalated. The lattice constants of the new compounds have been determined by powder X-ray diffraction. The unit cells are tetragonal with a axes that are unchanged with variation of the intercalated amine. The c axes lengths (layer spacings) vary systematically with the hydrocarbon chain length of the alkylamine.

New oxide pyrochlores: A2[B2−xAx]O7−y (A = Pb, Bi; B = Ru, Ir)

A new series of mixed metal oxides with the pyrochlore structure has been discovered and can be described by the general formula A2[B2−xAx]O7−y where A = Pb or Bi, B = Ru or Ir, 0 < x ≲ 1 and 0 ≤ y ≤ 0.5. Values of x appreciably greater than zero may only be obtained at relatively low temperatures. It has been observed that when A = Pb and B = Ru, y = 0.5 for all values of x. Linear correlations between lattice parameter and x are presented for both Pb2[Ru2−xPbx]O6.5 and Bi2[Ru2−xBix]O7−y.

Hydrothermal synthesis of calcium - niobium and tantalum oxides with the pyrochlore structure

Abstract The mixed metal oxides Ca 2 B 2 O 7 (B = Nb, Ta) have been synthesized with the pyrochlore structure. The oxides were prepared by hydrolysis of Nb and Ta alkoxides and subsequent crystallization in the presence of the divalent cation in alkaline solution at temperatures up to 100°C. The reactions yield crystalline phases directly, though with small particle size and high surface area. The pyrochlore oxide Ca 2 B 2 O 7 was also found to undergo ion-exchange in acid under mild conditions to form the hydrated oxide HNbO 3 ·xH 2 O, also with small particle size and high surface area. Confirmation of the structures of Ca 2 Nb 2 O 7 and HNbO 3 ·xH 2 O was obtained by Rietveld analysis of the powder X-ray diffraction data.

Solution synthesis and characterization of sillenite phases, Bi24M2O40 (M=Si, Ge, V, As, P)

Abstract A low temperature, aqueous solution synthesis route is reported for a number of sillenite phases, Bi 24 M 2 O 40 , where M=Si, Ge, V, As and P. The technique gives highly crystalline products at temperatures ⪕75°C, but the range of compositions is found to be more limited than can be synthesized by high temperature, solid state reaction techniques. The structurally related γ-Bi 2 O 3 could not be prepared in pure form by the solution synthesis route. Phases synthesized in this study were characterized by X-ray and neutron diffraction, infrared spectroscopy and elemental analysis.

Ion exchange reactions of the layered solid acid HCa2Nb3O10 with alkali metal cations

Abstract Ion exchange reactions of the layered solid acid HCa2Nb3O10 with alkali metal cations have been investigated in aqueous solutions at temperatures between ambient and 100°C. The extent of exchange differs for the different alkali metal cations. The exchange reaction, in general, does not alter the stacking of adjacent oxide layers and the structures of the exchange products differ only slightly in their interlayer separations. The results obtained in aqueous solution are compared with previous measurements of the exchange of KCa2Nb3O10 in molten salts at higher temperatures (1).

A neutron powder diffraction study of the ordering in LixNi1−xO

Abstract Lithium nickel oxide phases, Li x Ni 1− x O( x =0.400, 0.347, 0.320) have been synthesized by direct solid state reaction of Li 2 CO 3 and NiO. The structures and cation distribution have been determined at ambient temperature and 60 K from time of flight neutron powder diffraction data collected on the Neutron Powder Diffractometer at LANSCE, Los Alamos National Laboratory. The rhombohedral distortion and the order parameter characterizing the lithium distribution have been determined as x decreases. The partitioning of Ni 2+ and Ni 3+ between the two layers has been inferred from the average metal-oxygen bond lengths for the two inequivalent sites.

Orthorhombic and tetragonal structures in the La(Ba2−yLay)Cu3O7−x system

Abstract The crystal structures present in a series of La(Ba 2− y La y )Cu 3 O 7− x samples with y =0.5, 0.375, 0.25, and 0.125 have been investigated by Rietveld refinements of X-ray and neutron powder diffraction data. For y =0.5 and x =-0.20, the structure is tetragonal, P4/mmm, similar to YBa 2 Cu 3 O 6 , but with 60(2)% occupancy of the O1 (and, equivalently, O5) sites. At all compositions barium populates only the Ba site. For y x x >∼0.0, both O and T phases can occur, although there is apparently a significant barrier to converting such O components to more oxygen rich T materials with x y =0.125 (which significantly improves the superconducting behavior) increases slightly the orthorhombic distortion of the O component, but little affects the relative amount of the T component. The tendency towards O and/or T character is apparently determined primarily by the temperature at which the compound is initially formed, possibly as a result of differences in the La/Ba distribution in the Ba planes. A comparison between the structural results and Meissner effect data from these same samples, suggests that optimal bulk high temperature superconductivity in this system is associated with an orthorhombic distortion. The average structural details of these various materials are discussed and compared with those of the YBa 2 Cu 3 O 7− x system.