Structure and Phase Transformations in the Titanosilicate, Sitinakite

Jul 6, 2010 - Sitinakite has been suggested for the removal of 137Cs+ from liquid radioactive waste. Additionally, it has been shown to intercalate li...
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4222 Chem. Mater. 2010, 22, 4222–4231 DOI:10.1021/cm100727h

Structure and Phase Transformations in the Titanosilicate, Sitinakite. The Importance of Water )

Gordon J. Thorogood,*,† Brendan J. Kennedy,^ Christopher S. Griffith,† Maragaret M. Elcombe,§ Maxim Avdeev,§ John V. Hanna, Samantha K. Thorogood,‡ and Vittorio Luca#,z Institute of Materials Engineering, ‡Government and Public Affairs, and §Bragg Institute, ANSTO, Locked Bag 2001, Kirrawee DC New South Wales, Australia 2232, ^The University of Sydney, School of Chemistry, NSW 2006 Australia, Department of Physics, University of Warwick, Gibbet Hill Road, Coventry CV4 7AL, UK, and #Comision Nacional de Energia Atomica, Gerencia Quimica. z Current Address: Comisi on Nacional de Energia At omica, Programa Nacional de Gesti on de Residuos Radioactivos, Gerencia de Quı´mica, Centro At omico Constituyentes, Av. General Paz 1499, 1650 San Martı´n, Argentina. )

†

Received March 12, 2010. Revised Manuscript Received June 3, 2010

Synchrotron X-ray diffraction and neutron diffraction have been used to investigate the phase changes that the titanosilicate mineral sitinakite undergoes when dehydrated. Refinements of the powder diffraction data of the material before and after heating to 573K indicate a phase change from space group P42/mcm to P42/mbc. Upon exposure to normal atmospheric conditions for an extended period the material transforms back to P42/mcm with a reduced lattice parameter. If the material is heated in a sealed capillary, it is possible to get the two phases coexisting. The coexistence of these two phases in sealed system suggests that it is only the loss of H2O that is driving the reversible phase transformation. Introduction The safe long-term storage of legacy wastes from the nuclear weapons programs is a pressing issue that could be simplified by the removal of 137Csþ and 90Sr2þ from the currently stored liquid waste since these two isotopes account for a large amount of the radioactivity. Crystalline titanosilicates are one group of inorganic ion exchangers that because of their high selectivity and radiation stability have been suggested as possible materials for the removal of 137Csþ and 90Sr2þ.1-5 This synthetic analogue of the titanosilicate isostructural with the mineral sitinakite,6 was originally described by Sokolova7 as Na2(H2O)2[(Ti4O5(1) Anthony, R. G.; Zheng, Z.; Gu, D.; Philip, C. V. R. Soc. Chem. 1997, 196, 267–274. (2) Anthony, R.G.; Dosch, R.G.; Philip, C.V. (Sandia National Laboratories). Novel silicotitanates and their methods of synthesis and use. Patent WO9419277, 1994. (3) Xia, Q.; Zheng, L.; Wang, G.; Ying, M. Shiyou Huagong 1994, 23, 337–41. (4) Xia, Q.; Wang, G.; Ying, M.; Cao, G.; Zheng, L. Cuihua Xuebao 1994, 15, 109–14. (5) Poojary, D. M.; Cahill, R. A.; Clearfield, A. Chem. Mater. 1994, 6, 2364–8. (6) Menshikov, Y. P.; Sokolova, E. V.; Yegorov-Tismenko, Y. K.; Khomjakov, A. P.; Polezhaeva, L. I. Zap. Vseross. Mineral. O-va. 1992, 121, 94–9. (7) Sokolova, E. V.; Rastsvetaeva, R. K.; Andrianov, V. I.; EgorovTismenko, Y. K.; Men’shikov, Y. P. Dokl. Akad. Nauk SSSR 1989, 307, 114–17. (8) Anthony, R.G.; Dosch, R.G.; Philip, C.V. (Sandia National Laboratories). Patent US6110378, 1995. (9) Anthony, R. G.; Philip, C. V.; Dosch, R. G. Waste Manage. 1993, 503–512. (10) Gu, D. Ph.D. Thesis. Kinetics Catalysis and Reaction Engineering Laboratory, Department of Chemical Engineering, Texas A&M University, College Station, TX, 1995.

pubs.acs.org/cm

(OH)(SiO4)2]K(H2O)1.7, and further developed at Sandia National Laboratories.8-10 The composition of sitinakite can be idealized as Na2Ti2O3(SiO4) 3 2H2O, and synthetic samples were first reported by Anthony and Poojary et al.5,9 It is also possible, as shown by various authors,10-13 to substitute other elements for Si or Ti in the structure, which enhances the material’s selectivity for certain cations.10-13 The substitution of Nb5þ for Ti 4þ greatly enhanced the material’s selectivity for Csþ as revealed in a patent held by Sandia National Laboratories;8 this material is available as a commercial product (IONSIV IE 910/1) from UOP, Inc., Des Plaines, IL. In addition to its promise as a host for 137Csþ and 90Sr2þ, Milne14 has recently investigated the use of sitinakite to intercalate lithium. Milne found that the material’s performance improves if heated to 493 K prior to use. Thermal gravimetric analysis indicates a mass loss and possible phase change at this temperature.14 This altered behavior due to heating is of great interest as the radiogenic heat produced by the decay of radioactive Cs can be quite significant. For example, Carter et al15 stated that high (11) Zheng, Z.; Philip, C. V.; Anthony, R. G.; Krumhansl, J. L.; Trudell, D. E.; Miller, J. E. Ind. Eng. Chem. Res. 1996, 35, 4246– 4256. (12) Anthony, R. G.; Dosch, R. G. (Sandia National Laboratories, Alburquerque, NM). U.S. Patent 5 177 045, 1993. (13) Luca, V.; Hanna, J. V.; Smith, M. E.; James, M.; Mitchell, D. R. G.; Bartlett, J. R. Microporous Mesoporous Mater. 2002, 55, 1–13. (14) Milne, N. A.; Griffith, C. S.; Hanna, J. V.; Skyllas-Kazacos, M.; Luca, V. Chem. Mater. 2006, 18, 3192–3202. (15) Carter, M. L.; Gilen, A. L.; Olufson, K.; Vance, E. R. J. Am. Ceram. Soc. 2009, 92, 1112–1117.

Published on Web 07/06/2010

r 2010 American Chemical Society

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Figure 2. Representation of the structure of synthetic sitinakite viewed down the a-axes. The format is the same as Figure 1. Figure 1. Representation of the structure of synthetic sitinakite viewed down the c-axes. The Ti atoms are at the center of the larger TiO6 polyhedra (light blue), and the Si atoms are at the center of the smaller SiO4 polyhedra (dark blue). The Na atoms are the smaller spheres (yellow) shown at half of their ionic radius to distinguish them from the O atoms that are shown as the larger spheres (red) associated with the H2O. The O atoms associated with the vertices of the polyhedra along with the H-atoms are not illustrated. This size and color scheme is used for the remainder of the paper.

waste loadings of Cs in hollandite could increase the temperature of the ceramic to 1073 K. This is far in excess of the temperatures Milne employed, and so, if sitinakite is to be used for the removal of radioactive Cs from waste streams, its characteristics at elevated temperatures need to be fully examined. The ion-exchange ability of sitinakite can be understood by examination of the Na2Ti2O3(SiO4) 3 2H2O structure which consists of filaments of edge-sharing TiO6 octahedra connected to SiO4 tetrahedra to form one-dimensional tunnels as viewed down the c-axis, (Figure 1). The associated Naþ cations and H2O molecules then occupy positions at the center of these tunnels. Perpendicular to the c-axis are cavities in the faces of the four sides of the tunnels, Figure 2. It is also possible that the Naþ cations can sit atop the SiO4 tetrahedra in the cavities. The removal of some or all of the H2O from the tunnel positions is likely to influence the precise positions of the Naþ cations, and it is possible that these will migrate to a new position and hence a phase transformation can occur. The structure of sitinakite and the Nb-substituted variant have been examined extensively both before and after ion exchange via X-ray diffraction and NMR.5,8-12,16-21 Pertierra22 reported a neutron diffraction study of the material that had been converted to the hydrogen form (16) Tripathi, A.; Medvedev, D. G.; Nyman, M.; Clearfield, A. J. Solid State Chem. 2003, 175, 72–83. (17) Poojary, D. M.; Bortun, A. I.; Bortun, L. N.; Clearfield, A. Inorg. Chem. 1996, 35, 6131–6139. (18) Bortun, A. I.; Bortun, L. N.; Poojary, D. M.; Xiang, O.; Clearfield, A. Chem. Mater. 2000, 12, 294–305. (19) Clearfield, A. Solid State Sci. 2001, 3, 103–112. (20) Nyman, M.; Nenoff, T.M.; Headley, T.J. SAND2001-0999, Sandia National Laboratories, Albuquerque, NM, 2001. (21) Cherry, B. R.; Nyman, M.; Alam, T. M. J. Solid State Chem. 2004, 177, 2079–2093. (22) Pertierra, P.; Salvado, M. A.; Garcia-Granda, S.; Bortun, A. I.; Clearfield, A. Inorg. Chem. 1999, 38, 2563–2566.

H2Ti2O3-SiO4 3 1.5H2O via repeated treatments with 0.05-0.1 M HCl to remove all the Na from the material. In comparison with the relatively large body of work now available investigating the influence of dehydration on the structure and ion exchange properties of defect pyrochlores such as NaW2O6þδ 3 nH2-zO23,24 very little is known about sitinakite, and the present work seeks to address some of the outstanding questions regarding the dehydration dynamics of sitinakite. The aim of the present work is to fully describe the sitinakite structure with emphasis on establishing the positions and behavior of H2O and Na within the structure and to investigate the structural changes that occur when the H2O is removed. Experimental Section Sitinakite was synthesized using the method of Poojary et al5 with minor variations. A Ti-peroxo solution was made by adding 47 mL of TiCl4 to 235 mL of doubly deionized water and 285.6 mL of 30% H2O2. After reaction, the Ti concentration of the solution was 0.000730 mol/g. A Si/Na loaded solution was made by dissolution of 1.876 g of Si-gel (SiO2.0.69H2O) produced by hydrolysis of tetramethyl orthosilicate in 60 mL of water containing 14.8 g of dissolved NaOH. Sitinakite was then synthesized by combining this solution with 70 g of the Ti-peroxo solution while stirring vigorously. The solution was left to stand overnight, and the resultant gel was loaded into a Teflon-lined autoclave and heated at 473 K for 5 days. The white precipitate was filtered and washed with doubly deionized water to remove any excess sodium. Simultaneous thermogravimetric analysis (TGA) and differential thermal analysis (DTA) were performed on a Setaram TAG 24 (Setaram, France). Approximately 10 mg of powder was placed on a platinum crucible and heated to 1100 K at a rate of 5 K min-1 in flowing air. High-resolution solid-state 23Na and 29Si magic-angle-spinning (MAS) nuclear magnetic resonance (NMR) spectra were acquired at ambient temperatures using an MSL-400 NMR spectrometer (Bo = 9.4 T) operating at 23Na and 29Si frequencies of 105.20 and 79.48 MHz, respectively. 23Na MAS NMR (23) Thorogood, G. J.; Kennedy, B. J.; Luca, V. Phys. B 2006, 385-386, 91–93. (24) Thorogood, G. J.; Kennedy, B. J.; Luca, V.; Blackford, M.; van de Geest, S. K.; Finnie, K. S.; Hanna, J. V.; Pike, K. J. J. Phys. Chem. Solids 2008, 69, 1632–1640.

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Table 1. Conditions Employed in the Variable-Temperature Synchrotron Diffraction Measurements environmenta

rampb

soakc

coolb

unsealed sealed

373-453 (20) K 453-493(10) K 493-573(20) K

573 K 30 min

573-373 (20) K

a Describes if the capillary was sealed or open to atmosphere. b The number in parentheses is the temperature interval of the ramp. c Time at the highest temperature before cooling the sample.

Figure 4. TGA and DTA of the sample showing an endothermic event occurring during mass loss of the sample, with an exothermic event occurring at high temperatures. See text for discussion.

Figure 3. Mass loss (TGA) and temperature dependence of the appropriately scaled unit cell volume variation of sitinakite determined using synchrotron XRD, illustrating the significant change in cell volume associated with the loss of water. See text for discussion.

data were acquired using a Bruker 4 mm double-air-bearing probe with single-pulse (Bloch decay) methods and MAS frequencies of ∼15-16 kHz. “Nonselective” (solution) π/2 pulse times of 3 μs were calibrated on a 1 M NaCl solution, from which “selective” pulse times of 0.6 μs were implemented for data acquisition. A preacquisition delay of 3 μs and a relaxation delay of 8 s were employed, with checks for abnormally long T1s also being undertaken (recycle delays up to 60 s). The 1 M NaCl solution also served as a 23Na primary chemical shift reference at 0.0 ppm. 29Si MAS NMR data were acquired using a Bruker 7 mm double-air-bearing probe with single-pulse (Bloch decay) methods that used high-power 1H decoupling during data acquisition. The MAS frequencies implemented for these measurements were ∼5 kHz. For these 29Si MAS single-pulse/high-power 1 H decoupling measurements, a single 29Si π/4 pulse width of 2.5 μs and a preacquisition delay of 10 μs were used in conjunction with recycle delays of 30-60 s for quantitative 29Si speciation analysis. All 29Si MAS chemical shifts were externally referenced to tetramethylsilane (TMS) at δ = 0 ppm via a secondary reference sample of high-purity kaolinite (δ = -91.2 ppm). Confirmation of phase purity and initial high-temperature studies were performed on a Panalytical X’pert pro X-ray diffractometer. The instrument was equipped with a Cu long fine focus tube, a programmable incident beam divergence slit, programmable diffracted beam scatter slit, an X’celerator high speed detector and an Anton Paar HTK 2000 sample cell. Synchrotron X-ray diffraction patterns were recorded on the large Debye-Scherrer diffractometer at the Australian National Beamline Facility, Beamline 20B at the Photon Factory, (25) Sabine, T. M.; Kennedy, B. J.; Garrett, R. F.; Foran, G. J.; Cookson, D. J. J. Appl. Crystallogr. 1995, 28, 513.

Figure 5. In-situ X-ray diffraction patterns for synthetic sitinakite measured using Cu KR radiation at 393 (bottom), 473, 593, and 723 (top) K. The bottom pattern at 393 K corresponds to the untransformed material, upon heating this to 473 K peaks corresponding to the dehydrated sample appear. By 593 K, the sample is essentially fully dehydrated, and no further changes are evident in the pattern upon heating to 723 K. Each pattern took ∼60 min to collect. The 473 K pattern has been offset by 3 degrees 2θ with each successive pattern offset by a further 3 degrees 2θ.

Japan, using 0.79628 A˚ X-rays.25 This wavelength was chosen to avoid the absorption edges of the elements within the samples. The interior of the large Debye-Scherrer diffractometer was evacuated to 10-1 mbar to reduce air scatter of the X-rays. All data were collected on Image Plates and then processed to extract diffraction patterns. Variable-temperature data were recorded using a custom built furnace under various conditions, Table 1. Powder neutron diffraction data were collected either on the medium resolution powder diffractometer (MRPD)26 instrument (λ = 1.6649 A˚) at the HIFAR facility or on the Echidna diffractometer (λ = 1.8850 A˚) at the OPAL facility of the Australian Nuclear Science and Technology Organisation (ANSTO).27 Samples were exchanged with D2O to remove the H2O. The sample was sealed into a vanadium sample can and placed into a closed cycle cryostat. Powder neutron diffraction profiles were collected at a series of temperatures ranging (26) Kennedy, S. J. Adv. X-Ray Anal. 1995, 38, 35. (27) Liss, K.-D.; Hunter, B.; Hagen, M. E.; Noakes, T. J.; Kennedy, S. J. Phys. B 2006, 385-386, 1010–1012.

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Figure 6. (a) TEM image of the, untransformed material with inset SAD pattern down the [021] zone axis and (b) transformed material with inset SAD pattern down the [221] zone axis.

between 10 and 675 K and over an angular range of 6-132 in increments of 0.1. Structure refinements were carried out by the Rietveld method using the RIETICA program28 with pseudo-Voigt peak shapes and refined backgrounds for all temperatures. Transmission electron microscopy was performed using a JEOL JEM 2010F (JEOL, Japan) equipped with a field emission gun electron source operated at 200 kV. The TEM was equipped with a Noran System Six energy dispersive X-ray spectrometer and microanalysis system (Thermo Electron Corporation, U.S.A.) and GIF 2001 electron energy filter (Gatan, U.S.A.). Bright field images and selected area diffraction patterns were recorded using the 1k  1k CCD camera in the GIF 2001. The sample was prepared by lightly crushing under ethanol then pipetting onto a holey carbon coated copper grid.

Results and Discussion TGA measurements of sitinakite (Figure 3) revealed a mass loss of 10% by 700 K. The change in slope of the TGA at ∼475 K infers the beginning of a possible phase transformation caused by loss of H2O, which is supported by the endotherm visible in the DTA (Figure 4). Complete loss of two water molecules from Na2Ti2O3(SiO4) 3 2H2O corresponds to a 11.32% mass loss. A continuous mass loss from room temperature, corresponding to a weakly bonded (and presumably nonstructural) component, is observed during heating to around 475 K, above this there is a rapid mass loss corresponding to the removal of the more tightly bound structural water. This latter process is essentially complete by 700 K. We saw no evidence for loss of organic template residues, and it should be noted that we did not observe any exothermic events in the DTA around 475 K in contrast to the reports of Nyman and Cherry.20,21 Variable-temperature X-ray diffraction measurements confirmed the onset of a phase transformation near 493 K (Figure 5), with this transformation being complete by 573 K. No further phase transformations were observed upon heating to 723 K suggesting that the phase transformation is driven by H2O loss. The X-ray diffraction (28) Howard, C.J.; Hunter, B.A. A Computer Program for Rietveld Analysis of X-ray and Neutron Powder Diffraction Patterns; Lucas Heights Research Laboratories: New South Wales, Australia, 1998; pp 1-27.

pattern of the as-prepared sample could be indexed to a tetragonal cell in P42/mcm with a = 7.8225(9) A˚ and c = 11.986 (1) A˚ in good agreement with the previous study of Poojary et al. However the X-ray diffraction pattern of the transformed material could only be indexed to a larger cell with a = 10.672(2) A˚ and c = 11.870(2) A˚. The space group of this larger cell was identified as P42/mbc. The background in the diffraction patterns are unaltered by heating and, thus, provide no evidence for the loss of any organic material (such as organic template), which is consistent with the lack of exothermic event in this temperature region seen in the DTA. Single-crystal selected-area electron diffraction patterns were recorded from three individual grains and showed the presence of the two tetragonal phases, in P42/mcm and P42/mbc, Figure 6. It was not possible to observe a single-phase P42/mcm sample by electron diffraction, despite the phase purity apparent in the X-ray diffraction measurements. It is believed that the high vacuum environment of the 2010F TEM results in partial loss of water from the sample with a concurrent phase transition to P42/mbc. It should be noted that there was no difference in grain morphology between the transformed and untransformed material, indicating that the material remained intact during H2O loss. Multinuclear Solid-State MAS NMR. The 23Na MAS NMR data of the as-synthesized parent material, shown in Figure 7a, indicates that the Na position within the sitinakite channel structure is represented by a single featureless asymmetric resonance with an apparent center of gravity shift at δ ≈ -9.0 ppm. Pronounced tailing on the upfield side of this quadrupole-dominated resonance is indicative of some local disorder that influences the Na position, with significant distributions of the isotropic hemical shift (δiso) and quadrupolar parameters (quadrupole coupling constant (CQ) and asymmetry parameter (η)) being observed.29-32 (29) Kunath-Fandrei, G.; Bastow, T. J.; Hall, J. S.; J€ager, C.; Smith, M. E. J. Phys. Chem. 1995, 99, 15138–15141. (30) Kunath-Fandrei, G.; Losso, P.; Schneider, H.; Steuernagel, S.; J€ager, C. Solid State Nucl. Magn. Reson. 1992, 1, 262–266. (31) Kohn, S. C.; Dupree, R.; Mortuza, M. G.; Henderson, C. M. B. Am. Mineral. 1991, 76, 309–312. (32) Toplis, M. J.; Kohn, S. C.; Smith, M. E.; Poplett, I. J. F. Am. Mineral. 2000, 85, 1556–1560.

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Figure 7. (a) 23Na MAS NMR spectra of as-synthesized sitinakite. (b) 29Si MAS NMR spectra of as-synthesized sitinakite. The insert demonstrates that it is comprised of a resonance associated with a single site. (c) 23Na MAS NMR spectra of sitinakite heated to 573 K for one hour. (d) 29Si MAS NMR spectra of sitinakite heated to 573 K for one hour. The insert demonstrates that it is comprised of a multitude of contributions from closely related structural moieties.

The corresponding 29Si MAS spectrum of this sample (see Figure 7(b)) exhibits a very narrow resonance at δiso = -81.7 ppm, suggesting that the silico-titanate framework is not influenced by the local disorder of the Naþ cations (and presumably the water molecules) that are located within the tunnels that run along the c-axis of sitinakite. After the sample was heated to 573 K for 1 h, significant changes to the 23Na and 29Si MAS spectra are observed, as evidenced by Figure 7c and d. The 23Na resonance in Figure 7c experiences a large upfield shift to an apparent position of δ ≈ -22.1 ppm, suggestive of a reduced oxygen influence on the immediate coordination sphere of the Naþ channel species as expected by dehydration. The fullwidth-at-half-height has increased from ∼2 to ∼4.5 kHz, and the asymmetric tailing on the upfield side of the resonance is dramatically enhanced, suggesting that the magnitude of CQ for the Naþ channel species has greatly increased and that the structural disorder influencing these species has also increased producing larger distributions of the 23Na isotropic chemical shift and quadrupolar parameters. Unlike the case for the parent material, the increased channel disorder experienced in the dehydrated sitinakite is correlated with some structural disorder in the silico-titanate framework, as evidenced by the 29Si MAS spectrum of Figure 7d. The expansion of this 29Si resonance shown in the inset of Figure 7d demonstrates that it

is comprised of a multitude of contributions from closely related structural moieties which are associated with loss of symmetry of the SiO4 unit and with the increased Naþ disorder; the downfield shift to δiso =-77.2 ppm corroborates the unit cell contraction and transformation from P42/mcm to P42/mbc The synchrotron X-ray diffraction pattern of a sample dried at 403 K could be indexed to cell in P42/mcm as originally described by Sokolova7 and an acceptable fit was obtained using the model described by Poojary et al,5 although it should be noted that, since Naþ and H2O/ D2O have a similar number of electrons (ten for both Naþ and H2O/D2O), it is not possible to unequivocally establish the positions of these. As evident from the atomic displacement parameters (ADP’s), see Supporting Information, the majority of the atoms appear to be welldefined; however, the positions of other atoms, such as the O associated with the D2O were less well located because of the lack of scattering contrast. The average Ti-O and Si-O distances obtained via Rietveld refinement are reasonable though the derived polyhedra are not regular. Even in simple titanites, such as rutile, the TiO6 octahedra are not strictly regular.33 There was no (33) Bolzan, A. A.; Fong, C.; Kennedy, B. J.; Howard, C. J. Acta Crystallogr. B 1997, 53, 373–380.

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Table 2. Refined Lattice Parameters (A˚) and Selected Bond Distances (A˚) for Sitinakite bond

Poojary et al5

synchrotron 300 K

neutron 20 K

neutron 298 K

a c volume (A˚3) Ti-O1 Ti-O1a Ti-O2 Ti-O2a Ti-O20 Ti-O4 Ti-Oavg Si-O1 Si-O10 Si-O100 Si-O1000 Si-Oav OX-OXa

7.8082(2) 11.9735(4) 730 2.000(4) 2x

7.8159(4) 11.9681(6) 731.11(6) 1.8838(1) 2x

7.880(8) 11.953(2) 725.0(1) 1.764(9) 2x

7.8099(7) 11.956(1) 729.2(1) 1.87(1) 2x

2.032(6) 2x

2.0205(1) 2x

2.159(7) 2x

2.02(2) 2x

2.112(5) 1.878(3) 2.00(1) 1.631(4) 4x

2.1172(1) 1.897(4) 1.9705(2) 1.67559(9) 4x

2.12(1) 1.836(6) 1.967(8) 1.649(8) 4x

2.10(3) 1.92(2) 1.97(1) 1.637(7) 4x

1.631(4) 2.66(1)

1.67559(9) 2.5228(1)

1.649(8) 2.66(2)

1.637(7) 2.72(2)

a

dehydrated combined synchrotron and neutron 573 K 10.6772(3) 11.8850(4) 1354.93(6) 1.913(5) 1.930(6) 1.982(6) 1.995(6) 2.110(7) 1.864(1) 1.966(6) 1.55(2) 1.65(2) 1.57(1) 1.63(1) 1.60(2) 2.74(2)

Average O-O distance within the SiO4 tetrahedra. Table 3. Refined Positional and Atomic Displacement Parameters for As Prepared Sitinakite Obtained from Rietveld Refinement against Neutron Diffraction Data Recorded at 20 K atom

site

x

y

z

Uiso (A˚3)

Ti Si Na1 O1 O2 O4 O(W1) O(W2) D(W1) D(W2)

8o 4e 4f 16p 8o 4i 4j 8o 8o 8o

0.156 (2) 0 0 0.121 (1) 0.116 (1) 0.151 (2) 0.283 (2) 0.5 0.211 (2) 0.548 (3)

0.156 (2) 0.5 0.5 0.379 (1) 0.116 (1) 0.151 (2) 0.283 (2) 0.5 0.211 (2) 0.548 (3)

0.153 (2) 0.25 0.5 0.1686 (6) 0.327 (1) 0 0.5 0.192 (9) 0.434 (2) 0.950 (3)

0.3(4) 1.4(5) 1.1(6) 1.01(4) 0.4(2) 2.3(4) 2.4(5) 12.1(9) 6.2(6) 2.8(9)

Figure 8. Rietveld refinement profiles of the neutron diffraction data obtained at 20 K. The red crosses are the data; the black line through the data is the calculated pattern, and the bottom black trace is the difference between the calculated pattern and acquired data. The black vertical markers are the Bragg peaks. The higher than usual background is a consequence of incomplete exchange for D.

evidence of scattering from a second Naþ cation, suggesting that some of the cation exchange capacity is a consequence of the H2O molecules located within the tunnels. It is most likely that these H2O molecules are in fact H3Oþ; using NMR, Cherry and Nyman20,21 provided evidence of such species in sitinakite. Although these molecules are different in size to Naþ, their charge may enable them to act in the same manner as the Naþ. To establish a more precise description of the structure, neutron diffraction data were collected at 20 K, see Figure 8 and Table 2. The motion of the Na and H2O/D2O species was greatly reduced at this temperature. These data proved accurate atomic coordinates for all of the oxygen atoms present in the system, as evident from Table 3. In part, this is a consequence of the strong contrast between Ti that has a negative scattering length of -3.438 fm and the Si and O that have positive scattering lengths of 4.1491 and 5.803 fm, respectively. At 20 K the Ti-O distances ranged from 1.764 to 2.159 A˚, while the Si-O distance was 1.649 A˚ (Table 2). Note also that the only

Figure 9. Structure of sitinakite at 20 K. There are no shared positions of oxygen associated with H2O (red) and Na (yellow). Only one Na position is visible, above the Si tetrahedra. The format is the same as Figure 1.

species present in the channel running down the z axis is H2O/D2O (Figure 9), which is contrary to the structural model proposed by Poojary et al5 who suggested the tunnel was occupied by both H2O/D2O and Na cations. This highlights the advantage of neutron diffraction over X-ray diffraction in distinguishing the Naþ cations and H2O/D2O molecules. A second advantage of neutron diffraction is its sensitivity to D atoms and a feature of the structure at 20 K is

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site

x

y

z

Uiso (A˚3)

Ti Si Na1 Na2 Na3 O1 O2 O4 O(W1) O(W2) D(W1) D(W2)

8o 4e 4f 4i 4h 16p 8o 4i 4j 8o 8o 8o

0.147 (2) 0 0 0.27 (1) 0.5 0.127 (1) 0.109 (1) 0.147 (1) 0.294 (2) 0.439(4) 0.217 (2) 0.439 (3)

0.147 (2) 0.5 0.5 0.27 (1) 0.5 0.384 (1) 0.109 (1) 0.147 (1) 0.294 (2) 0.439(4) 0.217 (2) 0.439 (3)

0.160 (2) 0.25 0.5 0 0.38(1) 0.1710 (6) 0.327 (1) 0 0.5 0.333 (5) 0.445 (2) 0.124 (4)

1.7(4) 2.6(6) 7(1) 3(3) 3(3) 0.9(8) 0.9(8) 0.9(8) 9(2) 3(2) 6.2(6) 6(1)

Figure 10. Rietveld refinement profiles from neutron diffraction data obtained at 298 K. The format is the same as in Figure 8.

that the D-O distances of the D2O molecule offset from the center of channels have a bond distance of 1.12 A˚, while the bond distance of the D-O in the center of the channel is 1.27 A˚. Both of these distances are longer than the expected 0.97 A˚ and may be the result of attraction of the O or D in the material to the outer part of the tunnel given that the D2O closest to the edge has a smaller bond distance than the D2O in the center. Having established the structure of sitinakite at 20 K, we sought to use neutron diffraction data to refine the structure at room temperature. A striking feature of the neutron diffraction pattern recorded at 298 K, Figure 10, is the large diffuse feature evident near 2θ = 30. That this was absent in the neutron diffraction pattern obtained at 20 K demonstrates that this is the result of thermally induced disorder within the structure. Further, that this feature is considerably stronger in the neutron, compared with, the X-ray data shows that this is disorder of a light atom. The most likely candidates are the Naþ cations or water molecules located within the channels running along the c-axis. Rietveld refinement against the 298 K data rapidly confirmed the major features of the structure; however, a satisfactory fit could only be obtained if disorder of the Naþ cations from the 4f site at 0 1/2 1/2 was included. The final refined parameters are collected in Table 4 and selected bond distances are given in Table 2. Four Ti atoms are clustered together and are bridged by the O2 oxygen which lies on the mirror plane and the O4 oxygen which lies on the c-axis. These clusters are then connected in the a and b directions by Ti-O1-Si-O1-Ti linkages, which form the outside edges of the channels. The position of the O4 at z = 0 places it halfway between the Ti clusters along the z-axis and so connects two Ti atoms of one cluster above or below via corner sharing. The Ti coordination sphere is completed via bonding of the O1 associated with the Si. The Na and Si atoms alternate in the ac and bc faces to form a linear arrangement of atoms parallel to the c axis, Figure 11. The Si atoms are tetrahedrally coordinated to the O1 atoms. The Na atoms are, in turn, bonded to O1 atoms associated with four different Ti tetrahedra. The D2O molecules lie

Figure 11. Structure of sitinakite that best fits the neutron diffraction data obtained at room temperature. Note there are no shared positions of oxygen associated with H2O (red) and Na (yellow), rather they alternate throughout the structure. There are three Na positions visible, one above the Si tetrahedral and two running down the channels. The format is the same as Figure 1.

in the channel and appear to only weakly interact with the framework. The thermal transformation of sitinakite was also examined using synchrotron X-ray diffraction. Initial examination of the data, recorded with increasing temperature, showed there to be a rapid decrease in the cell size near 475 K. This discontinuity was not evident as the material was recooled to room temperature; rather, there was an approximately linear decrease in lattice parameters as the material cooled, Figure 12. The discontinuous reduction in the cell parameters occurs at approximately the same temperature, where the water was lost from the system as identified in the thermal analysis. This correlation is highlighted in Figure 3. As noted above the diffraction data from the transformed material could be indexed to the tetragonal space group P42/mbc that involves a 21/2 increase in the a lattice dimension. The structure of the dehydrated material was refined using the model developed by Poojary et al5 to describe the structure of sitinakite after exposure to 0.5 M HCl. This model provides an acceptable fit to the synchrotron diffraction data (Supporting Information, Figures S-1a

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Table 5. Positional and Thermal Parameters for Dehydrated Sitinakite Estimated from Synchrotron and Neutron Diffraction Data

Figure 12. Temperature dependence of the lattice parameters a A˚ (circles) and c A˚ (squares) of sitinakite showing a sudden reduction in both on heating and then constant contraction in the lattice upon cooling at 463 K.

Figure 13. Rietveld refinement profiles for neutron diffraction data recorded for dehydrated sitinakite. The format is the same as in Figure 7.

and b and Table S-1) and adequately describes the structure of the dehydrated material; however, to obtain an optimal model, a neutron diffraction data set was also collected at 573 K. As apparent from comparison of the neutron diffraction patterns recorded at 20 (Figure 8), 298 (Figure 10), and 573 K (Figure 13), heating the sample reduces, but does not eliminate, the disorder, as shown from the structured background. To establish a suitable model for the structure of the dehydrated material structure factors were extracted from the neutron data using a LeBail fit in space group P42/mbc. After several refinement cycles of the structure based on the 573 K neutron data five partially occupied sites were revealed in a difference Fourier map. Initially the neutron diffraction data and finally joint neutron/ X-ray refinements of the structure were accomplished. In the final refinement, the occupancies of the five Na sites were allowed to vary freely, and partial validation of the refinement is obtained by noting the sum of the refined occupancies for the Na cations is in excellent agreement

atom

site

x

y

z

Uiso (A˚3)

Ti Si O1 O2 O3 O4 Na1 Na2 Na3 Na4 Na5

16i 16i 8h 16i 16i 16i 8h 8h 16i 8h 4d

0.8599(3) 0.311(1) 0.853(1) 0.7127(9) 0.8859(7) 0.7870(7) 0.717(1) 0.648 (1) 0.585(2) 0.475 (3) 0.5

0.0329(2) 0.198(2) 0.0162(9) 0.9350(9) 0.0276(8) 0.1983(7) -0.199(1) 0.258 (1) 0.121(2) 0.917 (3) 0

0.1560(2) 0.25 0 0.1828(7) 0.3320(6) 0.1582(9) 0 0 0.189 (1) 0 0.25

0.71(6) 1.5(2) 0.6(3) 1.8(2) 1.6(2) 1.9(2) 2.6(3) 1.3(4) 1.5(5) 3(1) 6(2)

with the bulk composition. The refined structural parameters are given in Table 5, and the refined neutron data is shown in Figure 13. When this structure is compared with the reported room-temperature sitinakite structure, the connectivity of the framework remains the same with rotations of the Ti2O7 and SiO4 units about the z-axis in opposite directions with the major distortion occurring in the Si tetrahedra, Figure 14. Similar distortions of the Si tetrahedra can be seen in CaTiSiO5,34 (Y,REE)2Ti2SiO9,35 and Ca1-xNax/2Smx/2TiSiO5.36 The size of the tunnel contracts as a result and hence the lattice parameters decrease. The advantage in dealing with material that had been transformed by the loss of H2O/D2O was that the confusion between Naþ positions and H2O/D2O was now removed. Dehydration results in a collapse of the tunnel structure and introduces distortion of both the Si tetrahedra and the Ti octahedra, Table 2. The collapse in the tunnel structure results in the view down the z-axis resembling a Chinese finger puzzle, Figure 14a. The loss of the water results in a movement of the Na cations to several positions in the channels, allowing greater access to the Na cations from three directions. In addition the H2O is no longer occupying space in the channels adjacent to the Na, Figure 14 b. Interestingly, Cherry21 concluded that the Na sites located on the ac planes are held by the four O1 atoms that are coordinated to the Si, and hence, they would not be available for exchange. Our results contradict this in that removal of H2O is sufficient to move these particular Naþ ions into the tunnels thus making them available for exchange. Our results agree with the findings of Pertierra22 who removed all the Naþ from the structure by exposure to dilute HCl. The structure reported by Pertierra is very similar to that of the present dehydrated structure having a tetragonal space group P42/mbc with an a value of 11.0343(5) A˚ and a c value of 11.8797(7) A˚. The reversibility of the transformation that occurs upon dehydration was noted by Cherry et al,21 although that work did not establish the nature of the structural changes. Mechanism of Transformation. To better understand the importance of the water molecules in controlling the (34) Speer, J. A.; Gibbs, G. V. Am. Mineral. 1976, 61, 238–247. (35) Kolitsch, U. Eur. J. Mineral. 2001, 13, 761–768. (36) Liferovich, R. P.; Mitchell, R. H. Mineral. Petrol. 2005, 83, 271–282.

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Figure 14. (a) Final structure of sitinakite at 573 K that best fits the synchrotron and ND data viewed down the z direction and (b) viewed down the x direction. The format is the same as Figure 1.

Conclusions

Figure 15. Synchrotron diffraction patterns for sitinakite (a) heated in a sealed capillary at 573 (green) and heated at 573 K in an unsealed capillary (red).

phase transformation two additional series of measurements were performed. The first simply involved remeasurement of diffraction data of the heated sample after re-exposure to the atmosphere, to establish the original P42/mcm structure can be re-established. Allowing the dehydrated sample to stand overnight resulted in the appearance of a number of peaks in the diffraction pattern that are indicative of the initial P42/ mcm phase demonstrating facile interconversion of the two phases, see Supporting Information, Figure S-2. Prolonged standing yielded a pure P42/mcm sample. Likewise rerecording an X-ray diffraction pattern of the material heated to 570 K during the neutron diffraction measurements demonstrated rehydration had occurred. The second study was to establish the importance of water molecules on the stability of the original material (P42/mcm) through a series of measurements performed on samples in sealed capillaries. As can be seen in Figure 15 the presence of water clearly plays an important role in the transformation. If water loss from the system is retarded the transformation can be prevented from reaching completion and so the main mechanism driving the phase transformation is H2O loss.

The structure of sitinakite has been investigated using a combination of NMR, synchrotron XRD, and neutron diffraction. It is evident from the results in this work that the structure is more complex than has been previously reported by Poojary et al.5 In particular there is a large amount of disorder of the Naþ cations and H2O that occupy the tunnels that run parallel to the c-axis of sitinakite. Establishing precise details of this was only possible through the use of variable-temperature synchrotron XRD and neutron diffraction. Neutron diffraction was also critical in differentiating between the Na cations and D2O molecules, which have the same number electrons and therefore the same scattering contrast when examined using X-ray diffraction. The disorder of the Naþ cations and H2O molecules is immediately evident by the observation of diffuse features of the neutron diffraction profiles. This is diminished upon cooling the sample to 20 K, illustrating this disorder is thermally induced. It is reasonable to propose that the mobility of the Naþ cations is fundamental to the ion exchange properties of this material. Removal of the water by heating the sample to above 475 K resulted in a change in space group from P42/mcm to P42/mbc, though we found that the H2O can be easily removed by placing the material in the high vacuum of a TEM. Both structures retain the same essential arrangement of edgesharing TiO6 connected by SiO4 tetrahedra, but removal of the water from the tunnels results in movement of the Naþ cations and distortions of the SiO4 tetrahedra. The sensitivity of the Naþ cation distribution to the presence of water is correlated with changes in ion exchange properties observed upon heating sitinakite.14 The fact that the material did not change morphology upon transformation is of importance for the future application of this material, where swelling of the ion exchanger may cause complications. The importance of the water on the stability of the two structures was evident from both in situ and ex situ diffraction studies where heating sitinakite in an unsealed environment results in a phase transformation because of the loss of H2O from the tunnels. Upon transformation all of the Na migrates into the center of the channels and

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partially occupies a number of disordered sites allowing the Na to be easily removed from the structure. This has not been previously reported and may explain the improved performance of the material seen by Milne.14 The transformed cell is very distorted with a large variation in bond lengths for the Si tetrahedra, which makes the material unstable in the transformed state. Upon standing for extended periods sitinakite will reabsorb H2O and transform back to the original phase with a reduced lattice parameter and in addition improved crystallinity. Our studies demonstrate that the Naþ cations are located inside the framework, presumably as hydrated or partially hydrated cations. It is likely that these interact with the framework via weak electrostatic charges. This explains both the labile nature of the cations and sensitivity of exchange to the precise water content of sitinakite. By containment of sitinakite in a sealed capillary, the transformation to the final phase can be retarded, resulting in a mixture of the original and final phases with

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altered lattice parameters because of the retention of H2O. These altered phases also persist over short periods. If exposed to elevated temperatures for a long enough period sitinakite can be made to lose water and transform regardless of its environment, for example, sealed or unsealed. Acknowledgment. The work performed at the Australian National Beamline Facility was supported by the Australian Synchrotron Research Program, which is funded by the Commonwealth of Australia under the Major National Research Facilities program. We thank Dr James Hester for assistance at the ANBF, as well as the anonymous reviewers whose comments have helped to improve this paper. Supporting Information Available: Structure of sitinakite, synchrotron X-ray diffraction profiles, and refined positional and atomic displacement parameters. This material is available free of charge via the Internet at http://pubs.acs.org.