Lutetium I 4200Hydrothermal Reactivity of Na-n-Micas (n = 2, 3, 4). -Na-n-micas (n = 2, 3, 4) are synthesized from near-stoichiometric powder mixtures of molar compositions (8-n)SiO2, (n/2)Al2O3, 6MgF2, and (2n)NaCl (Pt crucibles, 900°C, 15 h) followed by hydrothermal treatment with Lu(NO3)3 solutions (stainless steel reactor, 300°C, 48 h). The samples are characterized by powder XRD and MAS NMR spectroscopy. The hydrothermal treatments lead to the formation of Lu 2 Si 2 O 7 with Na-4-mica exhibiting the highest reactivity. Na-4-mica can be considered as an appropriate material for the decontamination of heavy metal cations not only through an ion exchange reaction but also through the formation of new crystalline disilicate phases. It is of special relevance for the storage of high activity radioactive waste. -(ALBA, M. D.; CASTRO*, M. A.; NARANJO, M.; PAVON, E.; Chem.
Reconstructive transformations in layered silicates need a high temperature in order to be observed. However, very recently, some systems have been found where transformation can be studied at temperatures 600 degrees C below the lowest experimental results previously reported, including sol-gel methods. We explore the possible relation with the existence of intrinsic localized modes, known as discrete breathers. We construct a model for nonlinear vibrations within the cation layer, obtain their parameters, and calculate them numerically, obtaining their energies. Their statistics show that, although there are far less breathers than phonons, there are much more above the activation energy, making them good candidates to explain the reconstructive transformations at low temperatures.
High charge mica samples, with layer charge values between 2 and 4, have been synthesized via the
sodium chloride melt procedure, and their hydrothermal reactivities for the formation of lutetium disilicate
have been evaluated. Characterization of the synthesized micas, through a combination of techniques
that inform of both the long and the short range order of the solids, indicates a progressive incorporation
of aluminum in the tetrahedral sheet as the layer charge increases. Hydrothermal treatment of the
synthesized samples causes the reaction between the lanthanide ions in the solution and the tetrahedral
silicate species from the micas to form a new crystalline lanthanide disilicate phase. Those samples with
higher layer charge show the higher reactivity, in accordance with our initial hypothesis. In conclusion,
these samples have a novel and attractive application through a new reaction mechanism, of special
relevance for the storage of high activity radioactive waste.
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