Looking for potential nuclear waste disposal materials: Thermo-elastic behaviour of CsAlSiO4 (ABW)

The thermo-elastic behaviour of CsAlSiO4 have been investigated up to 1000 °C. The microporous structure shows a remarkably anisotropic expansion and remains stable.

  G.D. Gatta et al., 
Microporous Mesoporous Mater., 163, 147 (2012).

Over the last years, an increasing number of studies has been devoted on Cs-bearing crystalline compounds suitable as potential nuclear waste disposal materials. The fission products are, in fact, usually grouped into four classes of materials: “volatile elements”, metallic alloys, oxide precipitates, and radionuclides incorporated into the UO2 structure. Cs (as 137Cs and 135Cs) is one of the principal “volatile elements”. Short-lived radionuclides, such as 137Cs and 90Sr, which have half-lives of about 30 years, cause a strong radiation field (mainly gamma and beta radiation). We have performed a series of experiments aimed to optimize the synthesis protocol and to describe the phase stability and the thermal-elastic behaviour of silicates/borates as potential host for Cs, among those: CsAlSiO4, CsAlSi5O12, (Cs,Na)AlSi2O6·nH2O, CsAl4Be5B11O28, and Cs(Be2Li)Al2Si6O18. CsAlSiO4, in particular, is an open-framework silicate with ABW framework topology. Its structure is orthorhombic, with Pc21n space group and lattice parameters: a~9.414, b~5.435, c~8.875 Å. The framework consists of tetrahedral sheets parallel to (001), in which 6-membered rings of corner-sharing tetrahedra lie. Apical oxygens of three neighboring tetrahedra in a ring point upward, whereas apical oxygens of the other three tetrahedra point down (Fig. 1), with distorted eight-membered rings channels running along [010], where the extra-framework sites (populated by Cs) lie. The Si/Al-distribution in the tetrahedral framework of CsAlSiO4 was found to be fully ordered.

As no experiments were performed on the thermal stability of this compound, the aim of our study was the investigation of the phase stability and of the thermo-elastic behavior of CsAlSiO4 by in situ high temperature (HT) synchrotron powder diffraction. In situ HT data were collected at the MCX beamline at Elettra. For this study, powder sample of CsAlSiO4 was contained in a quartz-glass capillary. The use of a gas blower allowed controlling the sample temperature up to 1000 °C.
The structure reacts, in response to the applied T, by a negative thermal expansion along [100], almost no expansion along [010] and a pronounced positive expansion along [001]. The diffraction data collected at room-T after the HT experiment show that any T-induced deformation mechanism in CsAlSiO4 up to 1000°C is completely reversible. The Rietveld structure refinements performed at high-T allowed explaining the anisotropic thermo- elastic behaviour of CsAlSiO4. The framework deformation of CsAlSiO4 in response to the non-ambient conditions occurs through tetrahedral tilting, as usually observed in this class of materials. The main effect of the inter-tetrahedral rotation on (001) is the pronounced di-trigonalization of the 6mR. The general bonding configuration of the Cs-polyhedron appears to be preserved within the T-range investigated, and with no Cs-migration through the channels running along [010]. The HT experiment confirms that CsAlSiO4 maintains its crystallinity within the T-range here investigated under elastic regime, and this is surprising if we consider the microporous nature of this material. On the basis of the topological configuration of the Cs-polyhedron (and its bonding environment), the small dimension of the sub-nanopores and the high flexibility of the ABW framework type, we can consider CsAlSiO4 as a functional material potentially usable for fixation and deposition of radioactive isotopes of Cs, and as a potential solid host for a 137Cs γ-radiation source to be used insterilization application.

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Phase stability and thermo-elastic behavior of CsAlSiO4 (ABW): A potential nuclear waste disposal material;
G. Diego Gatta, Marco Merlini, Paolo Lotti, Andrea Lausi and Milan Rieder,
Microporous and Mesoporous Materials 163, 147 (2012)
Ultima modifica il Giovedì, 13 Giugno 2019 11:21