Preparación y caracterización de óxidos de manganeso no estequiométricos

Prieto, O.; Rives, V.

doi:10.3989/cyv.2000.v39.i3.832

Cited by 3 publications

(4 citation statements)

References 15 publications

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“…Some aspects of the order/disorder features of these samples can be deduced from these XRD patterns. Among the LDH samples, the sharpest XRD peaks are observed for the sample ZnAlMo . They are symmetric, suggesting an ordered stacking of the layers.…”

Section: Resultsmentioning

confidence: 96%

“…Among the LDH samples, the sharpest XRD peaks are observed for the sample ZnAlMo. 12 They are symmetric, suggesting an ordered stacking of the layers.…”

Section: Resultsmentioning

confidence: 99%

“…This charge is compensated by protons or interlayer cations: In our case, there are sodium cations and water molecules in the interlayer. The sample (further labeled NaBIR) was prepared by a sol−gel method − with a glucose/NaMnO 4 molar ratio of 1. It was characterized as described above for LDH samples.…”

Section: Methodsmentioning

confidence: 99%

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Rotational Fluctuations of Water Confined to Layered Oxide Materials: Nonmonotonous Temperature Dependence of Relaxation Times

et al. 2007

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The rotational molecular dynamics of water confined to layered oxide materials with brucite structure was studied by dielectric spectroscopy in the frequency range from 10(-2) to 10(7) Hz and in a broad temperature interval. The layered double hydroxide samples show one relaxation process, which was assigned to fluctuations of water molecules forming a layer, strongly adsorbed to the oxide surface. The temperature dependence of the relaxation rates has an unusual saddlelike shape characterized by a maximum. The model of Ryabov et al. (J. Phys. Chem. B 2001, 105, 1845) recently applied to describe the dynamics of water molecules in porous glasses is employed also for the layered materials. This model assumes two competing effects: rotational fluctuations of water molecules that take place simultaneously with defect formation, allowing the creation of free volume necessary for reorientation. The activation energy of rotational fluctuations, the energy of defect formation, a pre-exponential factor, and the defect concentration are obtained as main parameters from a fit of this model to the data. The values of these parameters were compared with those found for water confined to nanoporous molecular sieves, porous glasses, or bulk ice. Several correlations were discussed in detail, such as the lower the value of the energy of defect formation, the higher the number of defects. The pre-exponential factor increases with increasing activation energy, as an expression of the compensation law, and indicates the cooperative nature of the motional process. The involvement of the surface OH groups and of the oxygen atoms of the interlayer anions in the formation of hydrogen bonds was further discussed. For the birnessite sample, the relaxation processes are probably overlaid by a dominating conductivity contribution, which is analyzed in its frequency and temperature dependence. It is found that the conductivity of birnessite obeys the characteristics of semiconducting disordered materials. Especially the Barton/Nakajima/Namikawa relationship is fulfilled. Analyzing the temperature dependence of the direct current (dc) conductivity sigma0 in detail gives some hint that sigma0(T) has also an unusual saddlelike form.

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Section: Resultsmentioning

confidence: 96%

“…Among the LDH samples, the sharpest XRD peaks are observed for the sample ZnAlMo. 12 They are symmetric, suggesting an ordered stacking of the layers.…”

Section: Resultsmentioning

confidence: 99%

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Rotational Fluctuations of Water Confined to Layered Oxide Materials: Nonmonotonous Temperature Dependence of Relaxation Times

et al. 2007

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“…Este procedimiento se utilizó para la incorporación de Cu 2+ , Mg 2+ o Li + a la región interlaminar de birnesitas preparadas por el método solgel [28]. Para ello, se dispersaron 0.5 g de la birnesita en su forma só-dica o potásica en 100 mL de disolución 1 M de M(NO 3 ) 2 .4H 2 O (M=Cu y Mg) o LiCl.…”

Section: Cambio Iónicounclassified

Birnesitas obtenidas mediante cambio iónico. Evolución estructural con la calcinación

Rives¹,

Arco²,

Prieto³

2004

Bol. Soc. Esp. Ceram. Vidr.

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Se han preparado birnesitas con Na + o K + en la interlámina mediante un método sol-gel, con una relación inicial glucosa/catión de 1 y 1.5. Estas muestras se han sometido a cambio iónico con los cationes Li + , Mg 2+ y Cu 2 , caracterizando las muestras intercambiadas por diversas técnicas (análisis químico elemental, difracción de rayos X, espectroscopía FT-IR, análisis térmico y determinación de superficie específica). En ningún caso se alcanza un cambio iónico completo de los iones sodio o potasio iniciales, aunque el sodio se intercambia en mayor medida que el potasio. El intercambio es más efectivo al incorporar el Li + , menor en el caso del Cu 2+ y menos eficiente, todavía, al incorporar Mg 2+. El cambio iónico se produce topotácticamente, manteniéndose en todos los casos la estructura laminar con una distancia interlaminar de, aproximadamente, 7 Å. Las muestras son estables hasta 600 ºC; a partir de esta temperatura se produce la transformación a estructura túnel 2x2 (criptomelano) en el caso de la birnesita de K y a la espinela de Li-Mn (LiMn 2 O 4 ) en el caso de tener Li en la interlámina. Palabras claves: birnesita, óxidos de Mn, no estequiometría. Birnessites prepared by ion exchange. Structural evolution with temperatureBirnessites containing Na + or K + in the interlayer have been prepared following a sol-gel method, with an initial glucose/cation ratio of 1 or 1.5. The solids have been submitted to ion exchange with Li , and all the materials have been characterised by several physicochemical techniques (elemental chemical analysis, X-ray diffraction, FT-IR spectroscopy, thermal analysis and specific surface area assessment). Total cation exchange was not achieved in any case, although sodium is better exchanged than potassium. Exchange is larger for Li + , lower for Cu 2+ and even lower for Mg 2+. Ionic exchange is topotactic, and the layered structure is maintained in all cases, with an interlayer spacing close to 7 Å. The samples are stable up to 600 °C; above this temperature cryptomelane (a 2x2 tunnel structure) is formed in most of the samples containing potassium, while LiMn 2 O 4 spinel is formed is Li was present in the birnessite.

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