High-Pressure Spectroscopic Study of Hydrous and Anhydrous Cs

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High-Pressure Spectroscopic Study of Hydrous and Anhydrous Cs-Exchanged Natrolites Dan Liu,† Donghoon Seoung,† Yongmoon Lee,† Zhenxian Liu,‡ Jong-Won Lee,§ Ji-Ho Yoon,|| and Yongjae Lee†,* †

Department of Earth System Sciences, Yonsei University, Seoul 120-749, Korea Geophysical Laboratory, Carnegie Institution of Washington, Washington D.C. 20015, United States § Department of Environmental Engineering, Kongju National University, Chungnam 330-717, Korea Department of Energy and Resources Engineering, Korea Maritime University, Busan 606-791, Korea

)

‡

ABSTRACT: Structural phase transitions in hydrous Cs-exchanged natrolite (Cs-NAT-hyd) and anhydrous Cs-exchanged natrolite (Cs-NAT-anh) have been investigated as a function of pressure and temperature using micro-Raman scattering and synchrotron infrared (IR) spectroscopy with pure water as the penetrating pressure medium. The spectroscopic results indicate that Cs-NAT-hyd undergoes a reversible phase transition around 4.72 GPa accompanied by the discontinuous frequency shifts of the breathing vibrational modes of the four-ring and helical eight-ring units of the natrolite framework. On the other hand, we observe that Cs-NAT-anh becomes rehydrated at 0.76 GPa after heating to 100 °C and then transforms into two distinctive phases at 2.24 and 3.41 GPa after temperature treatments at 165 and 180 °C, respectively. Both of these high-pressure phases are characterized by the absence of the helical eight-ring breathing modes, which suggests the collapse of the natrolite channel and formation of dense high-pressure polymorphs. Together with the fact that these high-pressure phases are recoverable to ambient conditions, our results imply a novel means for radionuclide storage utilizing pressure and a porous material.

’ INTRODUCTION The typical structure of a zeolite is composed of a corner connected network of (Al,Si)O4 tetrahedra, yielding cavities and channels where nonframework charge-balancing cations and water molecules are located.1 Such distinctive crystal-chemical characteristics endow them with diverse applications in catalysis, wastewater treatment, ion exchange, and gas absorption, etc.2 5 For example, the selective ion-exchange capabilities of zeolites have been utilized to remove pollutant-heavy metal cations and radionuclides from the aqueous environment.6,7 Cesium (137Cs) is one of the most pollutant radionuclides with a half-life of about 34 years and is present in the form of aqueous radioactive wastes from a nuclear fission reactor.8 Recently, Lee et al. have successfully shown that Cs exchange can be induced in a small-pore zeolite natrolite under mild aqueous conditions if the ordered distribution of the nonframework cation and water molecules in the parent sodium-natrolite is modified into a disordered fashion via potassium exchange.9 In ion-exchange applications using zeolites, however, one has to consider the possibility of back-exchange under changing environments as long as the access to the pores and channels remains unaltered. Natrolite (Na16Al16Si24O80 3 16H2O) belongs to the group of “fibrous zeolites”. The framework flexibility of natrolite-group zeolites has recently been shown to be greatly altered by pressure (and the choice of pressure-transmitting medium).10 17 For r 2011 American Chemical Society

example, when a nonpenetrating pressure transmitting medium is used, a crystal-to-crystal transition is induced at 3.7 GPa, followed by a complete amorphization above 7 GPa.10 On the other hand, when penetrating pressure transmitting media are used, anomalous structural changes are observed in natrolite and its compositional variants.14 17 Using Raman spectroscopy, Belitsky et al. first reported that two anomalous phase transitions involving volume expansion and contraction occur in natrolite under a pure water pressure medium at 0.75 and 1.25 GPa, respectively.17 Subsequently, Lee et al. used synchrotron X-ray powder diffraction and the Rietveld method to establish that these anomalous changes are due to the successive pressureinduced hydration to form the ordered paranatrolite and superhydrated natrolite with 24 and 32 water molecules per unit cell, respectively.18,19 Using micro-Raman and synchrotron infrared spectroscopy, we have recently confirmed that the water content in the pressure media plays a crucial role in triggering the pressure-induced hydration in natrolite.20 Special Issue: Chemistry and Materials Science at High Pressures Symposium Received: July 13, 2011 Revised: August 22, 2011 Published: September 16, 2011 2159

dx.doi.org/10.1021/jp206678e | J. Phys. Chem. C 2012, 116, 2159–2164

The Journal of Physical Chemistry C

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Figure 1. Polyhedral representations of (a) Cs-NAT-hyd and (b) Cs-NAT-anh, shown along the c-axis. The bridging oxygen atoms are shown as blue circles. The four-ring and helical eight-ring units are emphasized with bold outlines. Cesium cations and water molecular oxygen atoms are shown in pink and two-tone colored balls, respectively.

On the other hand, natrolite and its compositional variants exhibit dehydration and associated structural changes at elevated temperatures. The mineral sodium-natrolite becomes fully dehydrated at 548 K to form “metanatrolite” (Na16Al16Si24O80, F112, V = 1785 Å3), which subsequently transforms to “high natrolite” (Na16Al16Si24O80, Fdd2, V = 1960 Å3) at 823 K. When exposed to ambient conditions, rehydration takes place, and “post natrolite” (Na16Al16Si24O80 3 16H2O, Fdd2, V = 2183 Å3) is formed.21 When the sodium cations are replaced by K, Rb, and Cs metals, the dehydration temperature is systematically lowered to 175, 150, and 100 °C, respectively.9 Intriguingly, we found that the dehydrated phases of Rb- and Cs-exchanged natrolites remain anhydrous at ambient conditions.22 To the best of our knowledge, there has been no report on high-pressure (and temperature) behavior on anhydrous forms of natrolite or other zeolites. Here we present pressure- (and temperature-) induced structural changes of hydrous Cs-exchanged natrolite (Cs-NAT-hyd) and anhydrous Cs-exchanged natrolite (Cs-NAT-anh) under pure water pressure medium using the combination of in situ micro-Raman scattering and synchrotron infrared spectroscopy.

’ EXPERIMENTAL METHODS The preparation and structural characterization of Cs-NAThyd and Cs-NAT-anh were described in detail by Lee et al.9,22 Each sample was loaded into a modified Mao-Bell-type diamond anvil cell (DAC)23 with a small ruby ball as a pressure marker and filled with pure water as a penetrating pressure-transmitting medium. Pressure was determined from the frequency shift of the ruby R1 fluorescence line.24 High-pressure synchrotron IR spectra were collected at the U2A beamline of the National Synchrotron Light Source (NSLS), Brookhaven National Laboratory (BNL). The mid-IR spectra were collected in a transmission mode using a Bruker Vertex80v Fourier transform infrared (FTIR) spectrometer

at the U2A side station with a nitrogen-cooled MCT detector. The far-IR measurements were performed using a Bruker IFS 66v/S FTIR spectrometer and a custom-made vacuum microscope system equipped with a bolometer (Infrared Laboratories) and a 3.5 μm thick mylar beamsplitter. The spectral resolution of both systems was set to 4 cm 1. The detailed optical layout of the beamline has been described elsewhere.25 High-pressure Raman spectra were recorded using a customized micro-Raman system with a backscattering geometry at the beamline U2A. The excitation source used for all Raman measurements was a Newport Excelsior-532 nm laser with maximum power at 150 mW. The laser spot was intentionally focused to ∼20 μm on the sample via a combination of the 10 beam expander and 20 Mitutoyo objective to avoid any laser-induced decomposition of the sample. The spectral resolution was about 1.5 cm 1 with a SP-2556 spectrograph and a Spec-10:100BR/LN CCD detector made by Princeton Instruments. The average acquisition time used for a single spectrum was about 120 s. A conventional oven was used to heat the DAC ex situ at desired high temperatures.

’ RESULTS AND DISCUSSION The crystal structure models of hydrous Cs-exchanged natrolite (Cs-NAT-hyd) and anhydrous Cs-exchanged natrolite (Cs-NAT-anh) at ambient conditions are shown in Figure 1.9,22 The framework of both compounds is built from the T5O10 (T = Al and Si) tetrahedra chain units, linked along the c-axis and interconnected to yield respective elliptical channels. For CsNAT-hyd (Cs14.6Al16Si24O80 3 14.7H2O, Fdd2, V = 2658.84 Å3), the cesium cations and water molecules are distributed along the channel walls and the channel center, respectively. Upon dehydration above 100 °C, the channel becomes far more elliptical, and the cesium cations migrate to the walls of the channel’s minor axis. Once dehydrated, this phase becomes stable at ambient conditions 2160

dx.doi.org/10.1021/jp206678e |J. Phys. Chem. C 2012, 116, 2159–2164


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Figure 2. (a) Raman and (b) synchrotron infrared spectra of Cs-NAT-hyd and Cs-NAT-anh at ambient conditions.

and hence labeled to be Cs-NAT-anh (Cs14.6Al16Si24O80, Fdd2, V = 2235.9 Å3). The structural changes of Cs-NAT-hyd were studied using in situ Raman scattering at high pressures. According to the group theory, 99 optical modes have been predicted from the natrolite framework and nonframework species.26 They are assigned as 24A1, 25A2, 25B1, and 25B2 with Raman and IR active modes. As for the original sodium-natrolite, alkali-metal (Li+, K+, Rb+, and Cs+) exchanged natrolites both in the hydrated and dehydrated states maintain the same space group Fdd2.9 We expect overall similar Raman and IR spectra to be observed for the alkalication-exchanged natrolites, and the previous band assignments for the original natrolite would hold for these compositional variants.27 The measured Raman and IR spectra of Cs-NAT-hyd and CsNAT-anh at ambient conditions are shown in Figure 2.28,29 As expected, the spectra of Cs-NAT-hyd are distinguished from those of Cs-NAT-anh by showing the bands of O H stretching and H O H bending modes of the water molecules in the region of 3300 3600 and 1620 1665 cm 1, respectively. The O H stretching vibrations are observed as three weak bands (3408, 3424, and 3440 cm 1) in the Raman spectrum and three strong bands (3286, 3424, and 3463 cm 1) in the mid-IR spectrum. The frequency of the H O H bending band is clearly seen at 1661 cm 1 in the mid-IR spectrum, whereas it is weak in the Raman spectrum. None of the O H vibrational bands are observed in the spectra of Cs-NAT-anh, which further proves that the dehydration in Cs-exchanged natrolite is irreversible, and Cs-NAT-anh is indeed anhydrous. It should be, however, pointed out that there is a very weak and broad band around 3400 cm 1, which is most likely due to the adsorbed water on the sample surface of Cs-NAT-anh. The Raman bands in the range of 900 1100 cm 1 and the mid-IR active bands in the range of 900 1320 cm 1 are assigned to the antisymmetric T O stretching vibrational modes (Figure 2). The bending vibrational modes of O T O correspond to the bands in the range of 340 420 cm 1 in the far-IR spectra and

400 750 cm 1 in the Raman spectra. Here, the strongest Raman band is identified as the breathing mode of the four-ring unit ( Al O3 Si2 O4 Al O3 Si2 O4 ), which is doubly capped by the Si1O4 tetrahedra.30 The second strongest Raman band at 443 cm 1 (and corresponding IR band at 350 cm 1) is identified as the breathing vibrational mode of the helical eightring unit forming the “elliptical pore opening” perpendicular to the c-axis.28,31 The Raman bands observed below the frequency of 420 cm 1 have been assigned as libration modes of water and other optical modes of the lattice vibrations. Pressure-dependent changes in the Raman spectra of Cs-NAThyd are shown in Figure 3 in the spectral region of 120 1200 cm 1. First of all, both the lattice vibrational modes and the T O stretching vibrational modes exhibit substantial blue shift with increasing pressure. In addition, these Raman bands exhibit progressive broadening and decrease in intensity with increasing pressure. The frequencies related to the breathing vibrational modes of the eight-ring and four-ring units, however, display discontinuous changes at 4.72 GPa (Figure 4). In our previous study on Na-NAT-hyd, the frequency of the eight-ring breathing mode showed a large red shift at 0.86 GPa followed by a blue shift at 1.53 GPa, resulting from an expansion and subsequent contraction of the elliptical channels associated with the formation of the ordered paranatrolite and superhydrated natrolite, respectively.20 For Cs-NAT-hyd, the frequency of the eight-ring breathing mode does not show any abrupt shifts in the corresponding pressure region. This demonstrates that pressure-induced hydration does not occur in Cs-NAT-hyd under the same pure water pressure medium. Instead, the frequency of the four-ring breathing mode exhibits a slight red shift by 2 cm 1 at 0.17 GPa. This might be related to the initial distortion of the T5O10 building unit upon applying hydrostatic pressure. Overall, Cs-NAT-hyd is stable up to 3.53 GPa. The discontinuous changes observed at 4.72 GPa are the emergence of a new four-ring breathing Raman band at higher frequency and a large blue shift of the frequency of the eightring breathing mode (Figure 4). The original four-ring band from 2161



Figure 3. Raman spectra of Cs-NAT-hyd at different pressures under a pure water pressure medium. The top spectra were measured after decompression from 7.5 GPa.

Figure 4. Pressure dependence of the frequencies of (a) the eight-ring and (b) the four-ring breathing vibrational modes of Cs-NAT-hyd and Cs-NAT-anh.

Cs-NAT-hyd becomes weaker with further increasing pressure and disappears at the final pressure of 7.5 GPa. This indicates the gradual transformation of Cs-NAT-hyd into a new contracted

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Figure 5. Raman spectra of Cs-NAT-anh at different pressures under pure water pressure medium. The top spectra were measured after decompression from 4.21 GPa.

phase Cs-NAT-hyd-I in the pressure range between 4.72 and 7.5 GPa. Upon release of pressure, the Raman spectra are restored to those of the initial pattern, indicating that the pressure-induced transition from Cs-NAT-hyd to Cs-NAT-hyd-I is fully reversible. Pressure- (and temperature-) induced changes in the Raman spectra of Cs-NAT-anh are shown in Figure 5 in the frequency ranges of 100 1200 cm 1. Abrupt changes are induced at 0.76 GPa after heating at 100 °C for 2 h, and the breathing vibrational frequencies of the four-ring and eight-ring units both exhibit large red shifts by 12 and 27 cm 1, respectively (Figures 4 and 5). This is the characteristic signature from the expansion of the unit cell volume and, accordingly, the elliptical channels. Interestingly, the shifted frequencies of the four-ring and eight-ring units are very close to those of the four-ring and eight-ring units in the Cs-NAT-hyd phase (Figure 4). These results imply that rehydration of Cs-NATanh takes place near 0.76 GPa and after heating under water pressure medium. This newly hydrated phase is labeled as Cs-NATanh-I, and future structural characterizations will reveal its comparative crystal chemistry with respect to the as-prepared Cs-NAT-hyd. Upon further increasing pressure to 2.24 GPa and after heating at 165 °C for 1 h, the Raman band related to the eight-ring breathing mode disappears, and a new Raman peak appears in the lowerfrequency side to the original four-ring vibrational mode (Figure 5). The original four-ring band then disappears completely, and the new four-ring band persists, showing a red shift at 2.48 GPa after heating at 170 °C for 1 h (Figure 5). These observations suggest that Cs-NAT-anh-I transforms into a new high-pressure phase, post-Cs-NAT-anh-I, which might not bear a topological relationship to the parent natrolite framework in the absence of the characteristic eight-ring channels. With further increasing pressure to 3.41 GPa and after heating the sample at 180 °C for 1 h, the Raman spectra undergo complete reorganization, signaling the formation of yet another high-pressure phase, post-Cs-NAT-anh-II (Figures 4 and 5). The Raman spectra at 3.41 GPa are characterized with eleven new bands in the frequency range below 600 cm 1. Studies are underway to detail the structures of the post-Cs-NATanh phases. 2162



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Figure 6. (a) Synchrotron infrared and (b) Raman spectra of the respective quenched samples, compared to those of Cs-NAT-hyd and Cs-NAT-anh at ambient conditions.

A series of IR and Raman spectra were measured on the respective quenched samples and compared to the starting phases, Cs-NAT-anh and Cs-NAT-hyd (Figure 6). First, in the frequency range below 1500 cm 1, the IR spectra measured after 30 min from quenching the Cs-NAT-anh-I phase bear more similarity to those from Cs-NAT-hyd than those from Cs-NATanh, the starting anhydrous phase (Figure 6a). In the mid-IR region, on the other hand, the O H stretching band is not seen clearly, whereas the O H bending band is present as a weak peak near 1663.5 cm 1. When the quenched sample is measured again after one month, however, the Raman data show close resemblance to that of Cs-NAT-anh. This is an indication that the rehydrated CsNAT-anh-I phase under pressure becomes slowly dehydrated and transforms back to Cs-NAT-anh at ambient conditions. The Raman and IR spectra of the quenched post-Cs-NAT-anh-I and post-Cs-NAT-anh-II are in clear contrast to the spectra of the starting Cs-NAT-anh phase and any other phases maintaining the natrolite framework topology. This reveals that the transitions to the post-Cs-NAT-anh phases might be reconstructive in

nature and are irreversible. For the post-Cs-NAT-anh-II phase, two broad and one shape O H stretching bands are observed at 3397, 3448, and 3590 cm 1, respectively, but no O H bending band is detected around 1660 cm 1. This suggests that the (quenched) post-Cs-NAT-anh-II phase might possess hydroxyls species instead of H2O molecules.

’ CONCLUSION In summary, we presented here the pressure- (and temperature-) induced structural changes of hydrous and anhydrous Csexchanged natrolites under pure water pressure medium. The evolution of Raman spectra from the Cs-NAT-hyd sample revealed a formation of a new phase Cs-NAT-hyd-I starting at 4.72 GPa. This transition is completed at 7.5 GPa and found reversible. On the basis of the similarities and differences of the measured Raman spectra, we conjecture that this new phase is possibly a channelcontracted Cs-NAT-hyd and might show different distribution of the cesium cations and water molecules along the channel. 2163


The Journal of Physical Chemistry C On the other hand, the Cs-NAT-anh sample showed a series of transitions with increasing pressure. First, the starting anhydrous phase becomes hydrated at 0.76 GPa and after heating to 100 °C. This rehydrated Cs-NAT-anh-I phase under pressure then undergoes irreversible transformations into new phases of post-CsNAT-anh-I and post-Cs-NAT-anh-II at 2.24 GPa after heating to 165 °C and at 3.41 GPa after heating to 180 °C, respectively. These post-Cs-NAT-anh phases do not show the characteristic eight-ring breathing vibrational mode, and hence we propose that their structural topologies would differ from the parent natrolite framework. We conjecture that the cesium cations would have been trapped inside the dense networks of the recoverable postCs-NAT-anh phases. Our results thus shed light on a possibly novel means of radionuclide storage utilizing moderate pressure temperature conditions. Studies are underway to detail the structures of these high-pressure phases.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT This work was supported by the National Research Foundation through the Nuclear R&D Program (Grant No. M2AM062008-03931). J.-W. Lee and J.-H. Yoon thank the support by the Midcareer Researcher Program through NRF (No. 2008-0061974). The use of the U2A beamline at the NSLS is supported by COMPRES, the Consortium for Materials Properties Research in Earth Sciences under NSF Cooperative Agreement EAR 0649658, and by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC0298CH10886. Research carried out in part at the NSLS at BNL is supported by the U.S. Department of Energy, Office of Basic Energy Sciences.

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High-Pressure Spectroscopic Study of Hydrous and Anhydrous Cs

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