Cs-zeolites under extreme conditions: comparative thermoelastic behaviour of Cs-ABW,Cs-CAS and Cs-ANA

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, accompanied by stretching of T–O–T angles. Temperature-dependent unpolarized Raman spectra between room temperature and 1270 K confirm the nature of the phase-transition (i.e. disordered static–disordered dynamic type) and the crystallinity of CsAlSi5O12 at least up to 1270 K. 2.3. Cs-ANA The elastic behavior and the phase-stability of a natural pollucite, (Cs,Na)AlSi2O6 x nH2O, were investigated at hydrostatic pressure by in-situ single-crystal X-ray diffraction with a diamond anvil cell [5]. Pollucite experiences a P-induced phase transition at P=0.66 +/- 0.12 GPa from cubic (Ia d) to triclinic symmetry (P ). The phase transition is completely reversible and without any appreciable hysteresis effect. No further phase transition has been observed up to ~9 GPa. Fitting the P-V data of the low-P cubic polymorph with a BM-EoS, we obtained: V0=2558.3(4)Å3, KT0= 41(2)GPa and K’T = 4 (fixed). For the high-P triclinic polymorph, a third-order BM-EoS fit gives: V0=2577.5(40)Å3, KT0=25.1(9)GPa and K’T = 6.5(4). The axial bulk moduli of the high-pressure triclinic polymorph were calculated with a third-order “linearized” BM-EoS. The EoS parameters are: KT0(a)= 25.5(17)GPa (b_a = 0.0131(11) GPa-1) and K’T(a)= 6.8(6) for the a-axis; KT0(b)= 23.2(15)GPa (b_b = 0.0145(10) GPa-1) and K’T(b)= 7.7(7) for the b-axis; KT0(c) = 25.2(10)GPa (b_c = 0.0132(6) GPa-1) and K’T(c) = 6.8(4) for the c-axis, resulting in a modest elastic anisotropy (KT0(a):KT0(b):KT0(c) = 1.10:1:1.09). A previous experiment based on in-situ high-temperature powder X-ray diffraction up to 1470 K (at room P) showed that synthetic cubic pollucite (CsAlSi2O6) preserves its crystallinity within the T-range investigated, without any evidence of phase transition between 290 and 1470 K [6]. 3. Discussion The experiments aimed to describe the phase-stability fields, the thermo-elastic behavior and the P/T-induced structure evolutions of Cs-ABW, Cs-CAS and Cs-ANA show that these three compounds are crystalline at least up to 8-9 GPa. This results is surprising if we consider their microporous nature. Pollucite only shows a P-induced phase-transition, at a modest pressure (~0.7 GPa). The high-pressure polymorph of Cs-ANA shows an almost isotropic elastic behaviour (i.e. KT0(a):KT0(b):KT0(c) = 1.10 : 1 : 1.09). More anisotropic is the elastic response of Cs-CAS (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 in Cs-CAS and Cs-ABW reflects the configuration of the channels, and follow a general principle concerning the HP-behavior of microporous materials [7]: 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. An in-situ high-pressure single-crystal diffraction experiment on Cs-ABW is in progress, aimed to describe the main deformation mechanisms responsible for the so drastic anisotropic behavior of this zeolites. The thermal stability of the three mentioned microporous materials is also surprising. Despite its microporous structure, pollucite is currently considered as a “ceramic” material [6], with potential technological applications due to its modest thermal expansion. Previous experiments suggest that, among the crystalline phases of the Cs2O-Al2O3-SiO2 system, Cs-ANA shows the highest T-stability [2]. Even the chemical stability of the Cs-bearing zeolites is someway surprising. Pollucite, for example, is able to retain Cs when immersed into a fluid phase, even under hydrothermal conditions, better than several other Cs-bearing materials [5]. This behavior is ascribable 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. The Cs-retention of CsAlSi5O12 was recently examined by treating the powdered compound in boiling 1M NaCl solution [8]. After more than one month of exposure at extreme exchange conditions, only small amounts of Cs occupying the eight-membered ring channels are extracted. Results show that Cs is strongly bonded to the tetrahedral framework. The key to explain the thermal and chemical stability of the mentioned zeolites is in: 1) the significantly modest, or absent, amount of H2O as extra-framework molecules, 2) the (long) ionic radius of Cs+, which allows high coordination-numbers with the framework anions, 3) the modest free-diameters of the channel systems, which hinder the cation migration. Only natural Cs-ANA contains a low amount of H2O (<2wt%), whereas Cs-ABW and Cs-CAS are anhydrous materials. On the basis of their high thermo-elastic and chemical stability, the three aforementioned Cs-bearing zeolites, 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 γ-radiation source to be used in sterilization applications. However, further experiments are needed to investigated potential self-radiation damage of the crystal structures due to 137Cs. References [1] GATTA G.D., ROTIROTI N., ZANAZZI P.F., RIEDER M., DRABEK M., WEISS Z., KLASKA R., Am. Mineral. 93 (2008), 988-995. [2] DIMITRIJEVIC R., DONDUR V., PETRANOVIC N., J. Solid State Chem. 95 (1991), 335–345. [3] GATTA G.D., ROTIROTI N., FISCH M., KADIYSKI M., ARMBRUSTER T., Phys. Chem. Miner. 35 (2008), 521-533. [4] FISCH M., ARMBRUSTER T., KOLESOV B., J. Solid State Chem. 181 (2008), 423-431. [5] GATTA G.D., ROTIROTI N., BOFFA BALLARAN T., SANCHEZ-VALLE C., PAVESE A., Am. Mineral., 94, (2009), 1137-1143. [6] KOBAYASHI H., YANASE I., MITAMURA T., J. Am. Ceramic Soc. 80 (1997), 2161–2164. [7] GATTA G.D., LEE Y., Phys. Chem. Minerals 32 (2006), 726 – 732. [8] FISCH M., ARMBRUSTER T., LIBOWITZKY E., Topics in Chemistry and Materials Science, vol 4 (2010) 61-70. Advanced Micro- and Mesoporous Materials – 08, Eds. K. Hadjiivanov, V. Valtchev, S. Mintova, G. Vayssilov, Sofia, Heron Press Ltd, ISSN 1314-0795.