Cs-zeolites under extreme conditions: comparative thermoelastic behaviour of Cs-ABW,Cs-CAS and Cs-ANA
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Data di Pubblicazione:
2011
Citazione:
Cs-zeolites under extreme conditions: comparative thermoelastic behaviour of Cs-ABW,Cs-CAS and Cs-ANA / G.D. Gatta, M. Merlini, M. Fisch. ((Intervento presentato al 1. convegno Advances in Zeolite Science and Technology tenutosi a Napoli nel 2011.
Abstract:
1. Introduction
Cs-bearing zeolites are extremely rare in nature. Pollucite, the Cs-counterpart of analcime, is the only natural Cs-rich zeolite. In the last decades, synthetic Cs-aluminosilicates have been prepared in search for suitable crystalline phases potentially usable as solid hosts for 137Cs γ-radiation source to be used in sterilization applications, or for fixation and deposition of radioactive isotopes of Cs. The thermo-elastic behaviour, the phase-stability and the main P-induced deformation mechanisms of two synthetic Cs-bearing zeolites, Cs-ABW (CsAlSiO4) and Cs-CAS (CsAlSi5O12), and one natural zeolite, pollucite Cs-ANA [(Cs,Na)AlSi2O6 x nH2O] have been recently investigated. Here we provide a comparative analysis of the response at high-pressure and high-temperature of the three mentioned Cs-rich zeolites.
2. Experimental Methods and Results
2.1. Cs-ABW
The elastic behavior of the synthetic zeolite CsAlSiO4 (a~9.446, b~5.439, and c~ 8.927 Å, space group Pc21n)[1] is under investigation by in-situ powder synchrotron X-ray diffraction with a diamond anvil cell under hydrostatic conditions. Data are currently available up to ~7 GPa. The material preserves its crystallinity and no phase transition appears to occur within the P-range investigated. Fitting the P-V data with a third-order Birch-Murnaghan equation-of-state (BM-EoS), we obtained: V0 = 457.9(4) Å3, KT0 = 42(1) GPa and K’ = 3.9(3). The “axial moduli” were calculated with a third-order “linearized” BM-EoS, substituting the cube of the individual lattice parameter (a3, b3, c3) for the volume. The refined axial-EoS parameters are: KT0a = 271(9) GPa (b_a = 0.00123(4) GPa-1), K’a = 4 (fixed) for the a-axis; KT0b = 181(3) GPa (b_b = 0.00184(3) GPa-1), K’b = 4 (fixed) for the b-axis; KT0c =14.5(5) GPa (b_c = 0.0230(8) GPa-1), K’c = 2.6(1) for the c-axis (KT0a : KT0b : KT0c = 19 : 12 : 1).
Previous high-temperature experiments showed that Cs-ABW transforms irreversibly to Cs-ANA framework-type zeolite at 1423 K [2].
2.2. Cs-CAS
The elastic and structural behavior of the synthetic zeolite CsAlSi5O12 (a~16.753, b~13.797 and c~5.023 Å, space group Ama2) were investigated up to ~8.5 GPa by in-situ single-crystal X-ray diffraction with a diamond anvil cell under hydrostatic conditions [3]. No phase-transition occurs within the P-range investigated. Fitting the P-V data with a third-order BM-EoS gives: V0 = 1155(4) Å3, KT0 = 20(1) GPa and K’ = 6.5(7). The “axial moduli”, calculated with a third-order “linearized” BM-EoS, are: KT0a = 14(2) GPa (b_a = 0.024(3) GPa-1), K’a = 6.2(8) for the a-axis; KT0b = 21(3) GPa (b_b = 0.016(2) GPa-1), K’b = 10(2) for the b-axis; KT0c = 33(3) GPa (b_c = 0.010(1) GPa-1), K’c = 3.2(8) for the c-axis (KT0a : KT0b : KT0c = 1 : 1.50 : 2.36). The HP-crystal structure evolution was studied on the basis of several structural refinements at different pressures. The main deformation mechanisms at high-pressure are governed by tetrahedral tilting, the tetrahedra behaving as rigid-units. A change in the compressional mechanisms was observed at P ≤ 2 GPa. The P-induced structural rearrangement from 0.0001 up to 8.5 GPa is completely reversible.
Further experiments have been devoted to the high-temperature behavior of CsAlSi5O12. In-situ high-temperature single-crystal and powder X-ray diffraction experiments were performed to describe its anisotropic thermal expansion [4]. The evolution of the unit-cell constants show a significant decrease in expansion above 773 K. At 773 K, a displacive phase transition from the acentric low-T space group Ama2 to the high-T centrosymmetric Amam was found. Thermal expansion below the phase-transition is governed by rigid-body tetrahedra rotations, accomp
Tipologia IRIS:
14 - Intervento a convegno non pubblicato
Elenco autori:
G.D. Gatta, M. Merlini, M. Fisch
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