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Gonçalves Oliveira Valente da Cruz-Lopes, Luísa Paula
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- Non-stoichiometric mixed lead and tin niobatesPublication . Cruz-Lopes, Luísa; Savariault, J.-M; Morais, C.M; Fernandez, C; Pedrosa de Jesus, J.D; Rocha, JNovel non-stoichiometric mixed lead and tin niobates with the pyrochlore structure have been synthesized via a solid-state route. The materials have been characterized by bulk chemical analysis, single-crystal and powder X-ray diffraction, 207Pb, 119Sn and 93Nb (single and triple quantum) solid-state NMR spectroscopy. The structures of the new non-stoichiometric mixed lead–tin niobate pyrochlore compounds have been Rietveld refined in the cubic system, space group Fd-3m. The steroactivity of the Sn(II) free electron pair leads to the general formula (Sn1.6−xPbx)(Nb2−ySny)O6.6−0.5y, where x (0–1.6) is the Pb(II) and y (0–0.34) is the Sn(IV) contents. 207Pb, 119Sn and (in particular) 93Nb NMR support the isomorphous substitution of tin for lead in the compounds studied. 93Nb triple-quantum magic-angle spinning NMR spectroscopy show that the materials contain, at least, two distinct niobium sites, in a 2:1 population ratio, which may be associated with different sample
- Synthesis and Characterization of Tin NiobatesPublication . Cruz-Lopes, Luísa; Savariault, J.-M.; Rocha, J.; Jumas, J.-C.; Pedrosa de Jesus, J.D.The synthesis and the structural characterization of tin niobates Sn11x Nb2 O61x , x50.0, 0.5, and 1.0, are reported. The materials have been characterized by bulk chemical analysis, single-crystal and powder X-ray di4raction, and 119Sn Mossbauer and 93Nb and 119Sn solid-state NMR spectroscopies. SnNb2 O6 is a synthetic analog of the rare mineral foordite. Red Sn2 Nb2 O7 crystallizes in a pyrochlore structure with a510.5386 A. 119Sn Mossbauer reveals the presence of both Sn(II) and Sn(IV) in pyrochlore samples. 119Sn MAS NMR spectroscopy supports the presence in synthetic foordite of Sn(II) in eight-fold coordination. The six-coordinated Sn(IV) and eight-coordinated Sn(II) 119Sn MASNMRresonances are not resolved in the Sn2 Nb2 O7 pyrochlore spectrum. 93Nb NMR indicates a fairly distorted local environment for niobium in synthetic foordite, much more so than in the Sn2 Nb2 O7 pyrochlore.
- An easy way to Pb(Mg1/3Nb2/3)O3 synthesisPublication . Cruz-Lopes, Luísa; Segadães, Ana M.; Rocha, João; Pedrosa De Jesus, Júlio D.This work describes the very expeditious combustion synthesis of Pb(Mg1/3Nb2/3)O3 perovskite powders from mixtures of Pb(NO3)2, Mg(NO3)2·6H2O and Nb2O5, as cation precursors, and urea as fuel. The powders produced were characterized by XRD, SEM (EDS), and ICP and MAS NMR spectroscopies. Since, with this method, exposure to high temperatures is reduced to a minimum, the as-prepared combustion product shows no PbO loss. The % PMN in the as-prepared combustion powder is sensitive to, and increases with, the rise in the ignition temperature, and gets further improved upon calcination. However, the benefit of calcination, in terms of the reached maximum in the % PMN, becomes more modest when the ignition temperature is increased. Subsequent heat treatments of powder compacts in air (normal processing usually includes some kind of sintering step in order to improve crystallinity and densify the material), showed that over 90% Pb(Mg1/3Nb2/3)O3 can be readily obtained at 870°C.
- Pyrochlore-type tin niobatePublication . Cruz-Lopes, Luísa; Savariault, Jean-Michel; Rocha, JoãoA single crystal of Sn(1.59)Nb(1.84)O(6.35) was grown at 1273 K from a mixture of sodium niobate and tin(II) chloride. The structure is of pyrochlore type A(2)B(2)O(7). The tin is partially oxidized to tin(IV) and competes with niobium for the occupation of site B. The stereoactivity of the Sn(2+) lone pair induces displacement of tin towards the O atoms of the tunnel.