Chem. Mater. 1994,6, 2364-2368
2364
Synthesis, Crystal Structures, and Ion-Exchange Properties of a Novel Porous Titanosilicate Damodara M. Poojary, Roy A. Cahill, and Abraham Clearfield* Department of Chemistry, Texas A&M University, College Station, Texas 77843 Received June 2, 1994. Revised Manuscript Received October 3, 1994@
A sodium titanosilicate of ideal composition Na2Ti203Si04*2H20was synthesized hydrothermally in highly alkaline media. Its crystal structure was solved from X-ray powder data by ab initio methods. The compound is tetragonal, a = 7.8082(2), c = 11.9735(4) A, space group P4dmcm, 2 = 4. The titanium atoms occur in clusters of four grouped about the 42 axis and are octahedrally coordinated by oxygen atoms. The silicate groups serve t o link the titanium clusters into groups of four arranged in a square of about 7.8 in length. These squares are linked to similar ones in the c direction by sharing corners to form a framework which encloses a tunnel. Half the Na+ ions are situated in the framework coordinated by silicate oxygen atoms and water molecules. The remaining sodiums are present in the cavity, but evidence indicates that some of them are replaced by protons. The composition is then closer to Na1.64H0.36Ti203Si04°1.8H20.The sodium ions within the tunnels are exchangeable. Exhaustive exchange with CsN03 yielded a Cs+ phase of whose structure was revealed from application composition Na1.49Cs0.2H0.31Ti203Si04~H20 of Rietveld methods to X-ray powder data.
Introduction Over the past 50 years, nuclear defense activities have produced large quantities of nuclear waste that now require safe and permanent disposal. The general procedure to be implemented is the removal of 137Csand from the waste solutions for disposal in permanently vitrified media. These ions are present in the waste solutions a t concentrations of 10-3-10-5 M admixed with many other species and at high salt concentrations. Therefore, highly selective sorbents or ion exchangers are required. Further, at the high radiation doses present in the solution, organic exchangers or sequestrants are likely t o decompose over time. Inorganic ion exchangers are resistant to radiation damage and can exhibit remarkably high se1ectivities.l A recent report2 indicated that a titanosilicate was selective for both Cs+ and Sr2+in 5.7 M sodium ion and 0.6 M hydroxide ion solutions. These properties are remarkable and indicated that a rather unique structure must be involved to produce such high selectivities. The published X-ray powder pattern2 indicated that a poorly crystalline material had been made. Our own work with other inorganic ion exchangers3t4has shown that the key to understanding their behavior is a detailed knowledge of their structure. Therefore, we decided to prepare better crystallized samples and to attempt structure solutions. We were encouraged by the recent results obtained with other titanium sili-
* To whom correspondence should be addressed.
Abstract published in Advance ACS Abstracts, November 1,1994. (1) Clearfield, A,; Nancollas, G. H.; Blessing, R. H. In Ion Exchange and Solvent Extraction; Marinsky, J. A., Marcus, Y., Eds.; M. Dekker: New York, 1973; Vol. 5, Chapter 1. (2) Anthony, R. G.; Philip, C. V.; Dosch, R. G. Waste Manage. 1993, 13, 503. (3) (a) Clearfield, A.; Smith, G. D. Inorg. Chem. 1969, 8, 431. (b) Troup, J. M.; Clearfield, A. Inorg. Chem. 1977, 16, 3311. (4) Inorganic Ion Exchange Materials; Clearfield, A,, Ed.; CRC Press: Boca Raton, FL, 1982. @
0897-475619412806-2364$04.50/0
~ a t e s . These ~ , ~ compounds have zeolite-like pores and also behave as ion exchangers.
Experimental Section Synthesis of the Crystalline Titanosilicate. Titanium isopropoxide (4.56 g, 97%, Aldrich Chemical Co.) was mixed with 3.33 g of tetraethylorthosilicate (99.8%, Alfa Products). The mole ratio of Ti:Si was 1:l. A 6.32 M NaOH solution (26 mL) was added to the stirring mixture, and a white precipitate formed. The new mixture was then transferred into a 100 mL Teflon-linedpressure vessel using 15 mL of distilled-deionized water. The Teflon bomb was then placed in a stainless steel vessel, sealed, and then heated in a 170 "C oven for 8 days. Then the solid was collected by filtration using a millipore filtration apparatus. The material was washed once with a 10% water 90% ethanol solution and then several times with pure ethanol. The product was dried a t 65 "C. The X-ray powder pattern showed that a highly crystalline product was prepared by this procedure. Thermogravimetric analysis was carried out with a DuPont thermal analysis unit, Model No. 951, at a heating rate of 10 W m i n . The IR spectrum was recorded on a Digilab FTS-40 FTIR unit by the KBr disk method. Cs Exchange for Na in the Titanosilicate. Na2TizSiO.l (0.562 g) was placed in a column that contained a glass frit platform. A 0.2 M CsN03 (200 mL, 99%,Aldrich) solution was then passed through the solid dropwise, in a columnlike fashion. The duration of this process took 2 days for the entire 200 mL of Cs solution to drip through the titanosilicate powder. The product of this reaction was then washed with acetone and ethanol after which the solid was dried in a 60 "C oven for 5 h. Cesium analysis was carried out by dissolving the titanosilicate in HF, diluting t o 250 mL and determining the concentration of Cs+ by AA on a Varian Model A-250 plus spectrometer. X-ray Data Collection, Structure Solution and Rietveld Refinement. Initially, the X-ray powder data for ( 5 ) Kuznicki, S. M.; Thrush, K. A,; Allen, F. M.; Levine, S. M.; Hamel, M. M.; Hayhurst, D. T.; Maknoud, M. In Synthesis oflwicroporous Materials; Ocelli, M., Robson, H., Eds.; Van Nostrand Reinhold: New York, 1992; Vol. I, p 427. (6) Kuznicki, S. M. U.S. Patent 4,938,989, 1990.
0 1994 American Chemical Society
A Novel Porous Titanosilicate
Chem. Mater., Vol. 6, No. 12, 1994 2365
Table 1. Crystallographic Data for Sodium and Cesium Titanosilicatesa Na pattern range (28), deg step scan increment (at?),deg step scan time, s radiation source wavelength, A empirical formula space group a, A C, A
z
no. of contributing reflections no. of geometric observations Ti-0 distances and tolerance (A) Si-0 distances and tolerance (A) 0-0 distances for si04 (A) no. of structural parameters no. of profile parameters expected R,, RWP RP RF a
cs
9.5-70 0.02 15 rotating anode 1.5406, 1.5444 Na1,fi4&.3fiTizSiOg P4dmcm (No. 132) 7.8082(2) 11.9735(4) 4 205 12 2.00(1) 1.63(1) 2.66(1) 27 11 0.035 0.142 0.110 0.055
9.5-70 0.02 15 rotating anode 1.5406, 1.5444 N~~.~~C~O.ZHZ.~~T~ZS~OE P4dmcm (No. 132) 7.8258(2) 11.9815(4) 4 207 12 2.00(1) 1.63(1) 2.66(1) 23 11
0.038 0.154 0.12 0.082
R, = (ZwU, - Ic)z~,[w~021)1/2. R, = (ClZ, - zcILUc).RF = (lFoI - FcI)/)Fol). Expected R,
the titanosilicate were collected on a Scintag PAD-V automated difiactometer using Ni-filtered Cu Ka radiation from a sealedtube X-ray generator operating at 30 kV and 40 mA. Data were mathematically stripped of the Kaz contribution, and peak picking was conducted by a modification of the doublederivative method.' The powder pattern was indexed by Ito and trial-and-error methods8 Systematic absences: 001; 1 = 2n 1 and h01; 1 = 212 1. These absences indicated space group P4dmcm. However, because of a spurious peak which could be indexed as 201, space group P4dm was initially chosen. X-ray powder data used for the Rietveld refinement (sample packed into a flat aluminum sample holder) were collected by means of a Rigaku computer automated diffractometer. The X-ray source was a rotating anode operating a t 50 kV and 180 mA with a copper target and graphitemonochromated radiation. Data were collected between 5 and 70"in 28 with a step size of 0.02" and a count time of 15 sfstep. Refinement of 25 reflections up to a 20 limit of 50" yielded preliminary unit-cell dimensions of a = 7.814(1) A and c = 11.98(1) A. Integrated intensities were extracted from the profile over the range 9.5 < 28 < 51 by decomposition (MLE) method^.^ This procedure produced 27 nonoverlapping reflections. Due to this limited number of reflections the structure solution was attempted through heavy-atom methods.1° A Patterson map was computed using this data set in the space group P4dm. The position of the Ti atom (0.14, 0.14, 0.165) was identified from this vector map. A difference Fourier map computed using the Ti position yielded the position of the Si and one of the oxygen atoms (01). Subsequent difference Fourier maps revealed the positions of 0 2 , 03, 04. At this stage it was realized that Ti, 0 2 , and 0 4 all had the same x and y coordinates and that 0 3 is related to 01 (y, x , 2). The space group was then switched to P4dmcm which is consistent with the data obtained from the rotating anode source. These positions were used as a starting model for Rietveld refinement in the GSAS program package.ll In the early stages of refinement, the atomic positions were refined with soft constraints consisting of both Ti-0 and Si-0 bond distances and 0-0 nonbonded distances. As the refinement progressed, the weight of these constraints was reduced, but they could not be removed completely without some structural distortions. The position of N a l and O(W1) could
+
+
(7) Mellory, C. L.; Snyder, R. L. Adu. X-ray Anal. 1979,23,121. (8) Visser, J. W. Appl. Crystallogr. 1969,2,89. Werner, P.-E. 2. Kristallogr. 1969.2,89. (9) Rudolf, P. R.; Clearfield, A. Inorg. Chem. 1989,28,1706. (10)TEXSmTEXRAY Structure Analysis Program, Molecular Structure Corp., The Woodlands, TX,1987, revised. (11) Larson, A.; von Dreele, R. B. GSAS: Generalized Structure Analysis System, LANSCE, Los Alamos National Laboratory, Copyright 1985-88 by the Regents of the University of California.
= Rw&2)1/2. x2 = Cw(Zo- Ic)'/(Nobs
- Nvar).
Table 2. Positional and Thermal Parameters for the Sodium Titanosilicatea Ti Si1 Nal Na2b 01 02 04 O(W1) O(W2)C
X
Y
2
vi,,, A2
0.1382(2)
0.1382(2) 0.5 0.5 0.4351(19) 0.3934(5) 0.1210(8) 0.1377(11) 0.2643(14) 0.4314(25)
0.1568(2) 0.25 0.5 0.0598(19) 0.1692(5) 0.3325(4) 0 0.5 0.3101(20)
0.011(2) 0.021(3) 0.072(5) 0.079(7) 0.012(3) O.OlO(4) 0.012(5) 0.035(6) 0.054(6)
0 0
0.4351(19) 0.1301(5) 0.1210(8) 0.1377(11) 0.2643(14) 0.4314(25)
Ui,,= Bi,,48n2. Occupancy refined to 0.32 (expected 0.5). Occupancy refined to 0.42 (expected 0.5). be easily identified from the difference Fourier maps. These positions were included in the refinement. Additional Fourier maps, calculated at this stage, revealed the positions of Na2 and O(W2). Since these positions are close to themselves and to each other, they should have partial occupancy. All the atoms were refined isotropically. The final difference Fourier map contained some residual density peaks (-0.5 e/A3)a t the center of the pore, but these peaks could not be refined. The Na and lattice water molecules were refined without any geometrical constraints. In the final cycles of refinement the shifts in all the parameters were less than their estimated standard deviations.loJ1 Neutral atomic scattering factors were used for all atoms.12 No corrections were made for anomalous dispersion, absorption, or preferred orientation. The refined positions of the framework atoms of the sodium phase were used as a starting model for the structure solution of the Cs-exchange phase. The positions of the sodium atom N a l and water molecule O(W1) were located from the very first difference Fourier map. After these atoms were refined, an additional Fourier map was computed which allowed the positioning of the cesium atoms in the pore. Attempts t o identify any residual density due to water and unexchanged sodium ions in the pore were not successful.
Results Crystallographic and experimental parameters are given in Table 1, final positional and thermal parameters in Tables 2 and 3 for the sodium and cesium derivatives, respectively, and bond lengths and angles in Tables 4 and 5. The final Rietveld refinement (12) Cromer, D. T.; Waber, J. T. International Tables for X-ray Crystallography; Kynoch Press: Birmingham, U.K., 1974; Vol. IV, Table 2.2A (distributed through Nuwer Academic Publishers, Dor-
drecht).
Poojary et al.
2366 Chem. Mater., Vol. 6, No. 12, 1994 Table 3. Positional and Thermal Parameters for the Cesium Titanosilicatea Ti Si1 Nal Csl' C S ~ 01 02 04 O(W1)
X
Y
2
0.1401(2) 0 0 0.5 ~0.5 0.1287(5) 0.1228(7) 0.1386(11) 0.2738(12)
0.1401(2) 0.5 0.5 0.5 0.5 0.3925(5) 0.1228(7) 0.1386(11) 0.2738(12)
0.1570(2) 0.25 0.5 0.25 0.131(2) 0.1673(5) 0.3329(4) 0 0.5
Ul,,, A2 0.012(3) 0.009(3) 0.069(5) 0.042(6) 0.085(8) O.OlO(3) 0.015(4) 0.023(6) 0.024(6)
U,,,= B,,d8x2. Occupancy refined to about 20% of the expected 2-fold position. Occupancy refined to about 10% of the expected 4-fold position.
1
i
I
J3 .
I
1
Table 4. Bond Lengths (A)and Bond Angles (deg) for the Sodium Titanosilicate atoms distance Til-01 2.000(4) 2x Til-02 2.032(6) 2x Til-02 2.112(5) Til-04 1.878(3) Sil-01 1.631(4) 4x atoms 0-Til-0 0-Til-0 0-Sil-0
(cis) (trans)
atoms Ti- 01-Si Ti- 02-Ti Ti- 04-Ti atoms Nal-01 Nal-O(W1) Nal-04 Na2-O(W1) Na2-O(W2) Na2-O(W2) Na2-01
angle range
average
82.2(3)-94.35(3) 170.4(3)-174.7(3) 103.0(3)-118.7(3)
90 172 110
angle 124.9(3) 97(4)-97.3(3) 179.7(4) distance 2.414(5) 2.765(1) 3.026(5) 2.79(2) 3.0 3.17(4) 2.74(2)
4x 2x 2x 2x
Nal-O(Wl)-Nal
average 97
173.4(7)
2x 2x
Table 5. Bond Lengths (A)and Bond Angles (deg) for the Cesium Silicotitanate atoms distance Til-01 1.981(4) 2x Til-02 2.065(5) 2x Til-02 2.116(5) Til-04 1.881(3) Sil-01 1.645(4) 4x atoms 0-Til-0 (cis) 0-Til-0 (trans) 0-Sil-0 atoms Ti-01 -Si Ti-02-Ti Ti- 0 4-Ti atoms Nal-01 Nal-O(W1) Nal-04 Csl-01 cs2-01 Cs2-O(W1)
angle range 82.3(4)-94.9(3) 170.6(3)-174.3(3) 104.5(3)-118.5(4) angle 125.1(3) 96.8(2)-97.3(3) 179.0(8)
distance 2.395(6) 2.779(1) 3.029(5) 3.183(5) 3.057(6) 2.95(2)
4x 2x 2x 8x 4x 2~
Nal-O(Wl)-Nal
average 90
172 110 average 97
169.1(5)
difference plots are shown in Figures 1 and 2. Views ofthe sodium and cesium structures down the c axis are shown in Figures 5 and 7, respectively.
Figure 2. Same as Figure 1 but for the cesium-exchanged phase.
The titanium atoms are octahedrally coordinated and are located on a diagonal mirror plane and close to the 42 axis in the crystal. Thus, the symmetry generates a cluster of four titanium atoms bridged together by 0 2 atoms (average Ti-Ti = 3.1 A) that also lie on the mirror plane. Ti clusters are bridged by 0 4 atoms along the c axis while along the a and b directions they are connected by Ti-01-Si-01-Ti linkages. 0 4 also resides on the mirror plane but a t z = 0 and V 2 . This positioning places these oxygen atoms half way between the Ti4 clusters along the z axis and connects two Ti atoms of one cluster to two in a cluster above or below by corner sharing. The coordination sphere of each Ti is completed through bonding t o 01 of the silicate groups. A single cluster of the four titanium octahedra is shown in Figure 3. The arrangement of silicate tetrahedra and titanium octahedra create tunnels parallel to the c axis. The Si atoms alternate with Nal in the ac and bc faces which produces a linear arrangement of these atoms parallel to the c axis (Figure 4). The silicon atoms are tetrahedrally coordinated by symmetry related 01 atoms which also bond to the Nal atoms in the tunnel framework. The water molecules are also bonded to these sodium ions completing their octahedral coordination. The ideal formula for the sodium titanosilicate is NazTizSi07-2HzO. There are two different sodium ions in the structure. The Nal position is fully occupied, and this Na+ is rigidly held in the lattice by four silicate 01 atoms and two water molecules [O(Wl)l. Nal is found in both the sodium and cesium ion structures, indicating that this ion is not exchanged for the cesium ions as
A Novel Porous Titanosilicate
Chem. Mater., Vol. 6, No. 12, 1994 2367
Figure 3. Section of the titanosilicate structure showing the cluster of four titanium octahedra. 01 atoms connect this cluster to silicon tetrahedra along the a and b axis while 0 4 links these clusters along the c axis.
Q
Figure 5. Structure of sodium titanosilicate as viewed the c axis. The disordered sodium ions in the pore are represented by filled circles.
Y
-33%4003
1
3502
1
I
3300
2500
2000
,
is00
1000
500
Uavenurbers
Figure 6. FTIR spectrum of the sodium titanosilicate.
Figure 4. Plot of the titanosilicate structure down the b axis showing the arrangement of silicon tetrahedra and sodium (Nal) octahedra in the ac and bc faces.
the tunnels are fully one dimensional. The other sodium ion, Na2, is located in the tunnels (Figure 5). From our refinement it is found that Na2 has only 64% of its full occupancy, the deficiency being compensated by protons. During occupancy refinement, the isotropic thermal parameters of the atoms were fixed at a reasonable value. It is possible that there are other disordered positions for this sodium ion in the pore or for some of the oxygen atoms that are protonated. These disordered Na+ and possibly protons are exchangeable for cesium ions. The structure also consists of two types of water molecules. O(W1)is fully occupied in both the Na+ and Cs+ structures and is involved in binding to Nal. O(W2)is disordered in the tunnel closer to Na2 ions (Figure 5). Its population parameter refined to about 84% of its full occupancy. This water is not present in the cesium-exchanged phase suggesting that its role in the structure is only to complete the coordination of the disordered sodium ions located in the tunnels. An infrared spectrum of the sodium titanosilicate is reproduced in Figure 6. This spectrum clearly shows
the presence of an 0-H stretching band at 3529 cm-l and hydronium ion like bands a t 3283 and 3071 cm-l superimposed on a broad water band. Thus, a more realistic formula would incorporate the presence of these protons and based on the X-ray refinement would be close to Na1,64H0.36Tiz03(Si04)'1.84H20. This formula requires a total weight loss of 11.85% as compared to the observed value of 14%. The compound dehydrates to yield a phase with a smaller unit cell. The dehydration occurs slowly at 285 "C requiring several hours for completion. The reaction is reversible as exposure t o atmospheric conditions results in water uptake with reversion t o the original unit cell. Figure 7 provides a view of the Cs+-exchangedphase observed down the tunnels. Only the sodium ions in the tunnels are exchanged, those held in the framework (Nal) retain their positions in the framework. Replacement of the Na+ by Cs+ requires only a very small enlargement of the tunnel as shown by the small increases in the unit cell dimensions (Table 1). The cesium ions (Csl) are located exactly at the center of the tunnel wherein they can bind to eight 01 atoms with very normal Cs-0 distances. These structural parameters clearly show that the pore size is ideally suited for the cesium ions. The other site (Cs2) observed for cesium ions, is also located at the center of the pore, but it binds to four 01 atoms and two water molecules [O(Wl)l. Some residual density for a third cesium position was seen at 0.5, 0.5, 0.0 in the difference
Poojary et al.
2368 Chem. Mater., Vol. 6, No. 12, 1994
dilute HF and analyzed for Cs+ by A4 to yield a value of 4.57%. This value compares t o a calculated one of 8.4% and indicates that the Cs+ occupancy factor was overdetermined. Comparison of the two powder patterns (Figures 1and 2) shows that the intensities of the reflections changed slightly in going from sodium ion to the cesium ion exchanged phase confirming the low level of uptake. This low Cs+ loading is surprising. However, the tunnels are one dimensional so that crowding in the tunnels may prevent the free flow of ions leading to low exchange levels. Furthermore, once a Cs+ is situated in the stable l/z l/2 '14, l/z l/2 3/4positions they may be difficult to dislodge without the expenditure of sufficient energy. For example, if Cs+ enters both ends of the tunnel further exchange in that tunnel is blocked. Further exploration of the ion exchange behavior of the sodium titanosilicate is underway. Near the completion of this study, we became aware of a Russian paper13 describing a mineral sodium Figure 7. Structure of the cesium exchanged phase down the titanosilicate. This single crystal study yielded a strucc axis. Cesium ions are represented by filled circles. ture very similar to that described here with unit cell dimensions a = 7.819(2),c = 12.099(4)A. The difference Fourier map but could not be refined as its occupancy in cell dimensions and other observed differences in the was very low. In contrast the sodium ions inside the two studies stems from the composition of the mineral tunnel do not fit neatly into the center but are shifted which was found t o contain a considerable amount of along the diagonal mirror plane away from the center potassium and smaller amounts of Ca, Sr, Ba, and Ce in order to bind to the 01 atoms. Thus, the Na2 ions substituting for sodium and about 5 mol % of Nb and are disordered over four sites near the center of the of Zr and Fe substituting for Ti in the smaller amounts tunnel, and each sodium ion then interacts with two 01 framework. Even this mineral sample contained about atoms and oxygens from two water molecules. 2 mol of HzO per formula. There are several related compounds with structures Discussion similar to that described here. Pharmacosiderite, a ~-~HZO, mineral of composition K F ~ ~ ( O H ) ~ ( A S O ~ ) ~ . has It is evident from the structural model revealed in the four iron atoms arranged in a fashion similar to that the present study why the titanosilicate has such a high of Ti in the titano~i1icate.l~ The unit cell is cubic, a = affinity for Cs+. The sodium ions are loosely held and 7.98 A, but the tunnels are about the same size as those disordered over several sites. In contrast the Cs+ fits of the titanosilicate because Cs+ fits into the tunnels. neatly into the tunnel in an ideal coordination site. The tunnels in pharmacosiderite and the subject comHowever, as we have indicated, there are two sites for pound are almost identical in size and shape. However, Cs+ and we need to inquire why the Cs+ distributes it should be noted that because of the cubic symmetry, itself over two sites since the six-coordinate site is much pharmacosiderite has similar tunnels about all three less favorable than the eight-coordinate one. A closer axes. Similar germanium-based compounds are also consideration of the stoichiometry involved may provide known.14-18 Interestingly Chapman and Roelg synthea clue as to the observed siting of the ions. In the ideal sized a number of titanosilicates differing slightly in Ti empirical formula, NazTizOs(SiO4).2HzO, Nal is in a 4-fold special position, while Na2 is in an 8-fold special and Si content that are related in structure to pharmacosiderite. By varying the framework compositions of position within the tunnels. However, only half the these compounds,it may be possible to change the cavity tunnel sites can be occupied due to spacial requiresize and therefore influence the selectivity toward ions ments. In actual fact, the occupancy refined to 0.32 with of different size and charge. We have initiated a protons satisfying the remaining positive charge. Thus the nonideal formula is Nal.64Ho.36Ti203(Si04).l.84HzO. program in this direction. The unit cell would then contain four such units. In Acknowledgment. We acknowledge with thanks the cesium ion phase Cs+ is situated at l/z l/2 '14, l/2 l/2 the financial support of this study by the State of Texas 3/4 which is a 2-fold position. Complete filling of these through its Advanced Technology Program, Grant No. positions displaces only half the sodium in the tunnel, 010366-205, and the R. A. Welch Foundation through assuming that ideally the sodium sites are filled and Grant No. A-673. the Na+ in the framework is unaffected. The second cesium site is close to the first one, so it is impossible (13) Sokolova, E. V.; Ratsvetaeva, R. K.; Andrianov, V. I.; EgorovTismenko, Yu. K.; Men'shikov, Yu. P. Dokl. Akad. Nauk SSSR 1989, for both sites t o be occupied in the same unit cell. This 307, 114. means that the maximum number of Cs+ ions that can (14)Buerger, M. J.; Dollase, W. A,; Garaycoehea-Wittke, I. Z. be accommodated in the unit cell is two and that some Kristallogr. 1967,125, 92. (15) Wittman, A.Fortschr. Miner. 1966,43, 230. Na+ in the tunnels must not be exchanged. However, (16) Sturua, G. I.; Belokoneva, E. L.; Simonov, M. A,; Belov, N. V. the Cs+ positions did not refine to full occupancy but Sou. Phys. Dokl. 1978,23, 703. rather to a relatively low occupancy corresponding t o a (17) Mutter, G.; Eysel, W.; Greis, 0.;Schmetzer, K. N. Jb. Miner. M h . 1984,4,183. total of 0.8 atom in the unit cell. The refined formula Nenoff, T. M.; Harrison, T. A,; Stucky, G. D. C h e n . Mater. ~ check ~O~( this S ~ O ~1994, )(18) .H O. is then N ~ ~ , ~ ~ C ~ O . Z H O . ~ ~ TTo 6, ~ 525. (19)Chapman, D. M.; Roe, A. L. Zeolites 1990,10, 730. value for cesium, a portion of the solid was dissolved in ~~~~~
