On the thermo-elastic behaviour and phase-stability of Cs-bearing open-framework aluminosilicates
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Data di Pubblicazione:
2012
Citazione:
On the thermo-elastic behaviour and phase-stability of Cs-bearing open-framework aluminosilicates / G.D. Gatta, M. Merlini, M. Fisch, P. Lotti. ((Intervento presentato al 1. convegno European Mineralogical Conference tenutosi a Frankfurt, Germany nel 2012.
Abstract:
In the last decades, synthetic Cs-aluminosilicates have been prepared in search for suitable crystalline phases as
potential solid hosts for 137Cs
-radiation sources to be used in sterilization applications, or for fixation and deposition
of other radioactive isotopes of Cs. We have investigated the thermo-elastic behaviour, the phase-stability and
the main P=T -induced deformation mechanisms of three Cs-bearing open-framework silicates: CsAlSiO4 (ABW
topology, a 9.446, b 5.439, and c 8.927 Å, space group Pc21n), CsAlSi5O12 (CAS topology, a 16.753,
b 13.797 and c 5.023 Å, space group Ama2), and (Cs,Na)AlSi2O6 x nH2O (pollucite, ANA topology, a 13.68
Å, space group Ia-3d).
The high-pressure experiments of the aforementioned materials were performed under hydrostatic conditions with
a diamond anvil cell, by in-situ single-crystal or powder X-ray diffraction. The in-situ high-temperature experiments
were performed by single-crystal and powder X-ray diffraction using heating stages. Axial and volumetric
bulk moduli (along with their P-derivatives) and axial and volumetric thermal expansion coefficients were obtained.
The experiments show that these three compounds remain crystalline at least up to 8-9 GPa (at room-T) and up
to 1000°C (at room-P). This result is surprising if we consider their microporous nature. Pollucite only shows
a P-induced phase-transition at a modest pressure ( 0.7 GPa). Its high-pressure polymorph shows an almost
isotropic elastic behaviour (i:e: KT0(a):KT0(b):KT0(c) = 1.10 : 1 : 1.09). The elastic response of Cs-CAS is
more anisotropic (i:e: KT0(a):KT0(b):KT0(c) = 1 : 1.50 : 2.36). In contrast, Cs-ABW appears to be one of the
most anisotropic crystalline materials, with KT0(a) : KT0(b) : KT0(c) = 19 : 12 : 1. The elastic anisotropy of these
materials at high-pressure reflects the configuration of the channels, and follows a general principle concerning the
HP-behavior of microporous materials: the open-framework structures tend to accommodate the effect of pressure,
by cooperative rotation of the tetrahedra, usually increasing the ellipticity of the channel systems and maintaining
the original elliptical configuration, without any “inversion” in ellipticity.
At high-temperature, only Cs-CAS undergoes a phase-transition: at 773 K, a displacive transition from the acentric
low-T space group Ama2 to the high-T centrosymmetric Amam occurs. The anisotropic P-behaviour of Cs-ABW
is observed even at high-T, with a negative thermal expansion coefficient along [100] and almost no expansion
along [010].
Moreover, the chemical stability of the three Cs-bearing compounds is surprising. Pollucite, for example, retains
Cs better than several other Cs-bearing materials when immersed into a fluid phase, even under hydrothermal
conditions. This behavior can be ascribed to the topological configuration of the Cs-polyhedron and its bonding
environment, to the small dimension of the sub-nanopores, where the Cs-sites lie, and to the high flexibility of the
ANA framework type.
On the basis of their high thermo-elastic and chemical stability, the three aforementioned Cs-bearing materials,
especially Cs-ANA, may be considered as functional materials usable for fixation and deposition of radioactive isotopes of Cs, or as solid hosts for 137Cs
-radiation source to be used in sterilization applications.
Tipologia IRIS:
14 - Intervento a convegno non pubblicato
Elenco autori:
G.D. Gatta, M. Merlini, M. Fisch, P. Lotti
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