J. Phys. Chem. 1994, 98, 5768-5772
5768
A Cationic Cesium Continuum in Zeolite X Tao Sun' and Karl Seff Department of Chemistry, University of Hawaii, Honolulu, Hawaii 96822
Nam Ho Heo Department of Industrial Chemistry, Kyungpook National University, Taegu 702- 701, Korea
Vitalii P. Petranovskii A . F. Iofle Physical Technical Institute, Academy of Sciences of Russia, St. Petersburg 194021, Russia Received: July I , 1993; In Final Form: March 9, 1994"
A single crystal of fully dehydrated sodium zeolite X, Nag2Alg&-,&84 per unit cell, reacted with cesium vapor a t 450 OC. Its structure was determined by X-ray diffraction methods a t 24 'C in the cubic space group F d 3 ( a = 25.155(3) A). Not only have all 92 N a + ions been replaced with Cs+ by redox reaction, but a n additional 36 atoms of C s were sorbed to give Cs128A192Si1000384. Most cesiums participate in a cationic continuum with a formula of (Cs12#6+ per unit cell. An additional six Cs+ ions are found a t the centers of double six-oxygen rings. The continuum, per unit cell, consists of eight prismatic clusters, six of which are icosahedral (12 cesiums) and two of which are expanded to contain an additional cesium atom. Each cluster contains a cesium atom a t its center (at the center of zeolite X's supercage). These clusters, arranged as the carbon atoms in diamond, are tetrahedrally connected to give the continuum, filling all supercages. Each sodalite cavity contains two cesiums (at sites I' and 11') which attach to the continuum, one per cluster, via a large-cavity cesium at site 11. 1', 11', and I1 are linear with intercesium distances of 3.89(2) and 3.92(2) A. Each cluster plus 1'41' appendage contains 4.5 electrons on average, one of which should reside principally on the cesium a t the cluster center and another principally in the sodalite unit. The remaining electrons appear to be widely delocalized as in a metal.
Introduction Zeolites can absorb alkali metal atoms to give cationic clusters. The smaller of these have been identified by ESR methods, but the larger are described only as "metallic". Kasai and Rabol-4 first found both the small cationic clusters, Na43+,and the larger sodium metallic clusters in dehydrated zeolite Y which had been exposed to sodium vapor. In zeolite X, they identified Na6s+ and the larger metallic clusters. Subsequently, a series of ESR experiments weredone by Edwards,%*Martens,"' and Kevan12J3 on samples prepared by various synthetic methods. They confirmed the formation of Na43+and Na65+ and found the new cationic alkali metal clusters Nas4+, Ks2+,and K43+ in sodalite cavities, as well as Na, K, Rb, and Cs metal particles, in zeolites A, X, and Y. Smeuldersl4J5found Nad3+clusters in sodalite. In each of these small clusters, the ESR method indicated that the atoms were equivalent, so, considering the symmetry of available sites within the zeolite, it was reasonable to speculate that they have high symmetry, e.g., are tetrahedral, octahedral, and triangular. However, this is not possible with Nas4+, so in this case at least the equivalence of the ions seen by ESR must be attributed to rapid exchange and not to structure. The triangular clusters Rb3n+ (n = 1 and n = 217),synthesized by exposing dehydrated zeolite A to R b vapor, were found crystallographically in the sodalite units of zeolite A; Rb3"+ is stabilized by coordination to one to three Rb+ cations. The linear Cq3+cluster, which extends from a largecavity through a sodalite unit into another largecavity, was foundin zeoliteA.18-22 Optical and magnetic susceptibility measurements indicated the presence of (Kn+4)"+ in the large cavities of zeolite A.23 Recently, the K32+,27and Nas4+,28have been structures of Cs64+,24K43+,25,26 determined in zeolites A and X. f Current address: Chemical Engineering Department, M.I.T., Cambridge, MA 02139. Abstract published in Advance ACS Absfrocfs,May 1, 1994.
Harrison et a1.6 found by ESR that the Nad3+ionic cluster can be formed by reacting Na-Y with Na, K, or R b vapor. They generalized that the ionic clusters are composed of newly reduced metal atoms, not metal atoms newly introduced to the zeolite. Kevan12J3 did a complete matrix of experiments involving the reaction of all five alkali metal vapors with all five alkali cation exchanged forms of zeoltie X and also concluded that the ionic cluster atoms must originate from cations originally within the zeolite rather than from alkali metal vapor atoms. However, crystallographic results on samples in which sorption reactions went stoichiometrically to completion showed that Rbf+ I 6 + l 7 or C S ~19-22 ~ + clusters, rather than Na43+, had formed in zeolite A upon prolonged exposure of Na-A to Rb or Cs vapor. This study was initiated with the hope that fully Cs+ ion exchanged zeolite X could be prepared without the use of solvent. This had never been achieved by conventional aqueous methods of e ~ c h a n g e . 2 ~It- ~was ~ also hoped that excess Cs atoms would be sorbed to form Cs clusters as had occurred in zeolite A.lg-22 A preliminary account of these results has been published.32
Experimental Section A colorless crystal of Na-X (stoichiometry Na92A192Si1~0384) prepared in St. Petersburg, Russia,33an octahedron ca. 0.25 mm on an edge, was lodged in a fine quartz capillary. It was washed with 0.001 M NaOH solution for 6 h, after which the capillary was connected to a vacuum line. After complete dehydration at 450OCand 1 X lO"Torrfor48 h,cesium(99.98%purity, Johnson Matthey Inc.) was introduced by distillation from a side-arm break-seal ampule to the quartz-tube extension of the crystalcontaining capillary. This quartz reaction vessel was then sealed off under vacuum and placed in a long horizontal cylindrical oven. The reaction was carried out at 450 OC for 36 h (vapor pressure of Cs(1) at 450 OC = 44 Torr). At this temperature, the vapor pressure of sodium (an expected product from the reduction)
0022-365419412098-5768%04.50/0 0 1994 American Chemical Society
A Cationic Cesium Continuum in Zeolite X
The Journal of Physical Chemistry, Vol. 98, No. 22, 1994 5769
TABLE 1: Positional, Thermal. and Occupancy Parameters’ cation Wyckoff fixed varied u23 ~CUp’ OCCu~ site position X Y Uiso/ullb u22 u33 92 4 3 Si 96(g) -0.0545(3) 0.0348(3) 0.1240(3) 169(31) 167(33) 133(30) -59(30) 16(27) -9(34) 96 A1 96(g) -0.0551(3) 0.1233(3) 0.0352(3) 109(34) 155(32) 159(33) -8(26) -2(29) -86(29) 96 O(1) 96(g) -0.1084(6) -0.0001(7) 0.1096(6) 168(87) 195(88) 249(103) -20(71) 83(62) -97(72) 96 96(g) -0.0044(6) -0.0052(6) 0.1411(5) 217(91) 161(85) 202(88) 9(68) 41(68) -99(69) 96 O(2) o(3) 96(g) -0.0377(6) 0.0681(7) 0.0711(8) 212(91) 241(103) 350(116) -40(83) -50(87) -12(78) 96 o(4) 96(g) -0.0687(6) 0.0745(7) 0.1753(7) 224(98) 319(121) 338(124) 12(91) -79(89) -229(80) 96 16(c) 0.0000 0.0000 0.0000 395(49) 395(49) 395(49) 60(32) 60(32) 60(32) 5.7(2) Cs(1) I Cs(2) I’ 32(e) 0.0809(3) 0.0809(3) 0.0809(3) 435(34) 435(34) 435(34) -54(36) -54(36) -54(36) 8 8.3(4) Cs(3) 11’ 32(e) 0.1702(4) 0.1702(4) 0.1702(4) 944(70) 944(70) 944(70) -263(60) -263(60) -263(60) 8 6.2(3) Cs(4) I1 32(e) 0.2603(1) 0.2603(1) 0.2603(1) 325(8) 325(8) 325(8) 18(8) 18(8) 18(8) 32 31.0(5) Cs(5) 111 48(f) 0.4206(2) 0.1250 0.1250 263(28) 908(51) 754(47) 0 0 -96(35) 27.9(5) Cs(6) 111’ 96(g) 0.4329(2) 0.0573(4) 0.0578(3) 289(35) 1063(65) 1093(67) 62(41) 95(84) 393(44) 38.1(6) 8(b) 0.3750 0.3750 0.3750 904(33) 8 7.6(2) Cs(7) IV 0 u = 25.155(3) A, space group Fd3, origin at center, at 1/8, 1/8, 1/8 from 43m. Thermal parameters are given in A2 X 104. b The anisotropic + k2U22 + PU33 + 2hkU12 + 2hlUl3 + 2klU23)). Occupancy factors are given as the number of ions per temperature factor = exp{(-2~~/u~)(h~U11 unit cell. is sufficiently high (1.4 Torr) for it to have distilled from the surface of the crystal to dissolve in the liquid cesium source. Then the extension of the crystal-containing capillary was cooled to room temperature to allow excess cesium to condense away from the crystal. The resulting black lustrous crystal was sealed off from the reaction vessel by torch after cooling to room temperature. This is expected to be the true color of this crystal: no powder X-ray lines attributable to any bulk metal were seen in subsequent examination. A Siemens four-circle computer-controlled P3 diffractometer with a graphite monochromator and a pulse-height analyzer was used for preliminary experiments and for the subsequent collection of diffraction intensities a t 24 OC. Molybdenum radiation (Kal = 0.70930 A; Ka2 = 0.713 59 A) was used throughout. Thecell constant, u = 25.155(3) A, was determined by a leastsquares treatment of 35 intense reflections for which 18O C 28 C 30’. The space group Fd5 was used for this work. The 8-28 scan technique was used for data collection. Each reflection for which 5O C 28 C 65O was scanned at a constant rate of O S 0 min-I in o from 0.6’ below the calculated Kal peak to 0.6O above the Kaz maximum. Background intensity was counted a t each end of a scan range for a time equal to one-half the scan time. The intensities of three reflections in diverse regions of reciprocal space were recorded every 97 reflections to monitor crystal and instrument stability. Only small random fluctuations of these check reflections were observed. Standard deviations were assigned to individual reflections by
04
Figure 1. A linear tricesium subcluster, Cs(2)-Cs(3)-Cs(4), occupying
each sodalite cavity and extends into a supercage. A cesium cation at
Cs(1) is shown in a double six-oxygen ring cavity. Bold lines indicate
bonds from cesiums to oxygens of the zeolite framework and to each other. Ellipsoids of 20% probability are shown.
+ + B2) + (pZ)2]’/2
r(Z) = [w2(CT B ,
where C T is the total integrated count, B I and B2 are the background counts, and I is the intensity. The value o f p refined to 0.004. The intensities were corrected for Lorentz and polarization effects; the contribution of the monochromator crystal was calculated assuming it to be half-perfect and half-mosaic in character. An absorption correction (pR 0.45) was made empirically using a scan. Of the 1425 unique reflections examined (a single data set), only the 43 1 for which the net count exceeded 3 times its standard deviation were used in subsequent structure determination and refinement.
-
Structure Determination Full-matrix least-squares refinement was intiated with fixed positional and thermal parameters for the framework atoms [Si, Al, 0(1), 0(2), 0(3), and 0 ( 4 ) ] and variable positional and anisotropic thermal parameters for Cs(2) a t (0.073,0.073,0.073), Cs(4) at (0.260,0.260,0.260), and Cs(5) a t (0.420,0.125,0.125) taken from the structure of Na,Cs-X.34 This model converged to
A difference Fourier function disclosed four strong peaks at (0.375,0.375,0.375), (O,O,O), (0.173,0.173,0.173) and (0.430,0.055,0.056). Because the first peak was not close to any framework oxygens, it was not included in the following refinement. Inclusion of the remaining three as Cs(l), Cs(3), and Cs(6), respectively, led to R, = 0.16 and R2 = 0.18 in further refinement. The second difference Fourier showed only one very strong peak a t (0.375,0.375,0.375), the positionofsymmetry23 (Td)at thecenter of the large cavity. Including this position in refinement as Cs(7) led to convergence with R1 = 0.0616 and R2 = 0.0643, with occupancies given in the last column of Table 1. All correlations among cesium parameters, including inter-cesium occupancies, were small. The occupancy a t Cs(4) (3 1.O per unit cell) was very nearly 4 times that at Cs(7) (7.6 per unit cell), about 8, and those a t Cs(2) and Cs(3), both in the sodalite cavity, were also nearly equal to 8. Imposing these chemically acceptable results as constraints in least-squares yielded R1= 0.0622, and R2 = 0.0660. The Cs(1)-0(3) distance, 2.65(2) A, is much shorter than the sum of the radii35 2.99 A, of Cs+ and 0 2 - . This suggested that
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The Journal of Physical Chemistry, Vol. 98, No. 22, 1994
Si
02
Sun et al.
AI
Figure 2. A stereoview of the arrangement of cesiums which minimizes repulsive interactions in 75% of the supercages in Cs-X. Ellipsoids of 20% probability are shown.
Selected Bond Distances (A) and Angles (deg) 1.656(17) Cs(6)-0(1) 3.147(18) si-o(zj i.668(17j cs(zj-cS(j) 3.887(23j Si-0(3) 1.628(20) CS(~)-CS(~) 3.923( 19) Si-0(4) 1.672(19) CS(~)-CS(~) 4.664(8) AI-O(1) 1.675(18) CS(~)-CS(~) 4.968(6) AI-O(2) 1.664(17) Cs(4)-Cs(6)' 5.111(8) 1.713(20) CS(~)-CS(~)" 5.126(8) AI-O( 3) AI-O(4) 1.681(1 9) Cs(4)-Ca(7) 5 .OOO(3) CS(1)-0(3) 2.652(19) CS(~)-CS(~) 4.843(8) Cs(2)-0(3) 3.011(18) CS(~)-CS(~) 5.141(5) CS(3)-0(2) 3.095(20) C S ( ~ ) - C S ( ~ ) 4.436(11) Cs(4)-0(2) 3.005(13) CS(~)-CS(~)' 4.796( 16) Cs(5)-0(4) 3.127(17) Cs(6)-Cs(6)' 5.309( 14) Cs(5)-0( 1) 3.521( 17) CS(~)-CS(~) 5.395(6) Cs(6)-0(4) 2.936(17) 0(1)-Si-0(2) 110.9(9) Si-O(1)-A1 129.8(10) O( l)-Si-0(3) 107.9(9) Si-O(2)-AI 146.9(9) O(l)-Si-0(4) 108.0(9) Si-O(3)-AI 141.4( 12) O(2)-Si-O( 3) 108.9(8) Si-O( 4)-AI 151.6(11) 0(2)-Si-0(4) 108.9(8) O(3)-Cs( 1)-0(3) 92.1(5) 0(3)-Si-0(4) 112.2(10) 0(3)-Cs(2)-0(3) 78.7(6) O(l)-A1-0(2) 111.2(9) 0(2)-Cs(3)-0(2) 81.6(5) 0(1)-A1-0(3) 107.9(9) 0(2)-Cs(4)-0(2) 84.6(4) 0(1)-AI-0(4) 108.6(9) 0(4)-Cs(5)-0(4) 69.9(6) 0(2)-A1-0(3) 109.9(8) CS(~)-CS(~)-CS(~) 180' 0(2)-A1-0(4) 107.3(8) CS(~)-CS(~)-CS(~) 109.48' 0(3)-A1-0(4) 112.0(10) CS(~)-CS(~)-CS(~)90' Inter-cesiumcontacts unique to the Cs(Cs13)~+ subcluster shown in Figure 3b. Straight,tetrahedral, and right angles, required by symmetry.
TABLE 2 Si-O( 1)
Cs( 1) should be refined as Na+. The refinement of N a at Cs( 1) at (O,O,O)with fixed full occupancy caused R1 and R2 to increase sharply to 0.083 and 0.087, respectively, and a strong residual peak near (O,O,O)appeared in the resulting difference Fourier. When the occupancy was allowed to vary also, it converged as 25( 1) Na+ ions (more than the limit of 16 which fills this equipoint) with a small fixed isotropic thermal parameter. All this indicates that the Cs( 1) position is not occupied by Na+ ions. The possibility of a mixture of Cs and N a a t site I was eliminated because it will lead to a short 3.54 8, distance to the Cs at site 1', which is unacceptable for a Cs-Cs or Cs-Na contact. When Cs(7) was refined at (0.370,0.370,0.370) to see if it preferred a less special position, it moved back to (0.375,0.375, 0.375). A refinement of the scale factor using low angle data (28 < 15') caused R1 to decrease from 0.062 to 0.048, R2 from 0.074 to0.064: this probably indicates a problem with the data, probably in the absorptioncorrection. The goodness-of-fit, ( c w ( F , - lFc1)2/ (m - s))l/*, is 1.40; the number of observations, m, is 431, and the number of parameters, s, is 84. The largest maximum/ minimum in the final difference function is 0.98/-0.88 e-/8,3. The final structural parameters and selected interatomic distances and angles are presented in Tables 1 and 2, respectively.
Discussion In this crystal structure, cesium is found at seven different crystallographic sites. Cs( 1) at (O,O,O), at the center of the double six-oxygen ring (D6R) (Figure l ) , is only partially occupied (6 ions per 16 sites per unit cell). This site is apparently not suited to large alkalimetal cations because D6R occupancies identical to this were seen for K+ ions in K142-X36and Rb+ ions in Rb28Na,~-X,~~ compositions which were achieved by treating Na-X single crystals with K(g) and Rb(g), respectively. A very short Cs( 1)-Cs(2) contact, 3.54 A, can readily be avoided by placing Cs(1) ions in D6R's far from Cs(2) ions (Figure 1). The unacceptably short Cs(1)-0(3) distance, 2.65 A, is likely to be virtual: the O(3) coordinates are an average over 6 filled and 10 empty D6R's. However, this distance, because it is significantly longer than Na-O(3) in Na-X, 2.55 A, supports the proposition that 6 of the 16 D 6 R s per unit cell have expanded to hold Cs+ cations. (This suggests that the thermal parameters at O(3) should be enlarged, but this is not seen with significance.) The very poor refinement of sodiumat the Cs( 1) position, as discussed in the previous section, confirms that Cs+ cations occupy (are not too big to occupy) D6R's; this had not be observed before. It remains possible crystallographically, despite the arguments presented above, that some Na+ ions remain at site I. Because the scattering power of Cs+ is approximately 5 times that of Na+, site I could contain, instead of six Cs+ ions per unit cell, compositions like 5 Cs+ and 5 Na+, or 4 Cs+ and 10 Na+, or even 3 Cs+ and 13 Na+ to fill site I, Wyckoff 16(c). A chemical basis for the failure of these Na+ ions to react can be found in the tightness-of-fit of Cs+ at this position. Nonetheless, Cs+ ions are found at site I, and a compelling reason for the reaction of Na+ + Cso to have stopped so near to completion is not seen. This possibility is therefore not discussed further. This issue is unimportant with regard to the principal results of this work. The Cs(2) and Cs(3) positions are both in the sodalite cavity, and their occupancies are one-quarter of their available sites. It is unlikely that more than one Cs(2) or more than one Cs(3) would be present in any sodalite cavity because the Cs(2)-Cs(2) = 3.13 8, and Cs(3)-Cs(3) = 3.22 8, distances are even shorter than the sum of the radii of two cesium ions. The only plausible arrangement for the eight Cs(2) and eight Cs(3) ions per unit cell is one of each trans across each sodalite unit to give a maximized Cs(2)-Cs(3) distance of 3.89 8,. Because of the full occupancy at Cs(4), a Cs(3)4s(4) interaction cannot beavoided. Therefore, the linear Cs3subcluster, Cs(2)-Cs(3)-Cs(4), must exist (Figure 1). The bond distances of Cs(2)-Cs(3) = 3.89 8, and Cs(3)Cs(4) = 3.92 8, are very similar to those, 3.92 and 3.87 8,,'*J9 of the linear Cs43+cluster in Cs-A. All approach distances from Cs(4), Cs(5), and Cs(6) to framework oxygens are approximately the sum of radii35 of Cs+
The Journal of Physical Chemistry, Vol. 98, No. 22, 1994 5771
A Cationic Cesium Continuum in Zeolite X
Cs6
Cs6
cs4
cs4
cs5
cs5
Figure 3. (a, left) Distorted icosahedral C s ( C s # + ( n ca. 10) subcluster found in about 75% of the supercages of Cs-X. The shortest contacts, those from 4.66 to 5.40 A, are indicated by solid lines. (The interatomic distance in cesium metal is 5.31 A.) Longer contacts up to 7.27 A appear as broken lines. Cs(4) appears in this figure and in Figure 1, establishingthe connectivity of the linear tricesium subcluster with the supercage subcluster (and, in turn, with the continuum). Ellipsoids of 20% probability are shown. (b, right) The Cs(Csl3)”+ ( n ca. 9) subcluster is found in about 25% of the supercages of Cs-X. The remaining comments in part a of this caption apply here also.
Cs6
cs2
cs3
d
x
Figure 4. (a, left) Bonding between two icosahedral subclusters. On1 the shorter (solid) bonds from Figure 3a are reproduced here. Each such subcluster bonds to another with six contacts ranging from 4.84 to 4.97 in length. Also shown are the Cs(2)-Cs(3) appendixes which attach linearly to one of four Cs(4)’s and which extend into sodalite cavities. Ellipsoids of 20%probability are used. (b, right) A portion of the cesium continuum in the supercages of zeolite X. The central icosahedral unit shows its full tetrahedral bonding to four other units. In this figure, the clusters shown in Figure 3 are shown to be arranged in space like the carbon atoms in diamond. The cesiums in the sodalite cavity, which form appendixes to the
continuum, are omitted for clarity. Ellipsoids of 40% probability are used. and 02-, 2.99 A, or little longer, affirming that they are cations. TheCs(7) position (Figure 2) isvery far (7.25 A) from its nearest framework oxygens (0(4)), indicating that it is occupied not by ions but by atoms. The occupancies at Cs(5) and Cs(6), because they are not simple fractions of the order of their equipoints, do not allow the contents of all supercages to be the same. In six of the eight supercages per unit cell, the atom at Cs(7) is coordinated by 12 cesium ions: four at Cs(4), four at Cs(5), and four at Cs(6) to form a distorted icosahedron (Figures 2 and 3a). In the remaining two, Cs(7) is coordinated by 13 cesium ions: four at Cs(4), two at Cs(5) and seven a t Cs(6), to give a more crowded (and presumably more reduced) arrangement (Figure 3b). Each supercage cluster, whether icosahedral (Figure 3a) or expanded icosahedral (Figure 3b), bonds to four others in a tetrahedral manner. For example, each icosahedral cluster (Figure 3a) bonds to another with six short interactions (Cs(4)-Cs(6) = 4.97 8,twice and Cs(5)-Cs(6) = 4.84 A four times) as shown in Figure 4a. This generates a continuum of supercage
clusters, a portion of which is shown in Figure 4b, filling the crystal. The clusters are arranged and bound together in the same geometry as the carbon atoms in diamond. This is also the way in which the sodalite units are arranged in the structure of zeolite X, so the cesium continuum (cationic) and the aluminosilicate continuum (anionic) form two interpenetrating tetrahedrally connected networks. These have approximately equal volume and charge density. Altogether about 128 cesiums are found per unit cell in this structure. The framework requires only 92 Cs+ ions for charge balance, so the difference, about 36 cesiums per unit cell, must have been sorbed into this structure as neutral atoms. Located crystallographically are 8 Cs(7) atoms, and the following cations or partially reduced cations (judged to be cations by their close approaches to frameworkoxides): 6 Cs( 1) in D6R’s, 8 Cs(2) and 8 Cs(3) in sodalite cavities, and 32 Cs(4), 28 Cs(5), and 38 Cs(6) in the supercages. (In fact, there must be two types of Cs(4): one of four interacts with Cs(2) and Cs(3) to form the linear tricesium subcluster and the remainder do not (Figure 4a).
5772 The Journal of Physical Chemistry, Vol. 98, No. 22, 1994
Additional nonequivalencies must arise as a result of the two types of large-cavity clusters.) The six ions at Cs( 1) are octahedrally (trigonally distorted) coordinated by framework oxygens and are not close to other cesiums, so they may be considered to be simple (not partially reduced) Cs+ cations. There are, then, a total of 128-92 = 36 cesium atoms and 92-6 = 86 Cs+ ions per unit cell at the remaining 128-6 = 122 cesium positions. However, the structure does not indicate this sharp distinction among the remaining positions, except at Cs(7). This situation may be moreconstructively viewed as 36 electrons distributed among 122 Cs+ ions. Each supercage cluster including its single I’-II’ appendage therefore contains (36/8) = 4.5 electrons on average. One of these should reside principally on the cesium at Cs(7), at the cluster center, and approximately another should be in the sodalite cavity. The remainder (ca. 2.5 electrons per supercage cluster surface or 20 electrons per unit cell) appear to be widely delocalized throughout the continuum. This work shows that zeolite X can sorb a large fraction of cesium atoms ((no. of CsO/no. of Cs+) = (36/92) = 0.39) to form a cationic continuum of semireduced metal ions. Compared to zeolite A, which can only accept 0.5-1 .O cesium atoms per 12 Cs+ per unit cell ((0.5 to 1.0)/12 < 0.083),1s-22zeoliteX has a much greater capacity for cesium atoms, presumably because its supercages are big enough to accept large cesium subclusters. To summarize, it can be seen that all sodalite and supercage cavities are “full” of cesiums in this structure. Each sodalite cavity contains two cesiums which attach as a linear appendix (Figure 4a), one per supercage subcluster. There are eight such subclusters per unit cell, six of which areicosahedral(l2 cesiums), and two of which are expanded to contain an additional cesium atom. In addition, each such subcluster contains a cesium atom at its center (at the center of zeolite X’s supercage). These subclusters, arranged as the carbon atoms in diamond, are tetrahedrally connected to give a three-dimensional cationic continuum of partially reduced cesium ions. Of the 128 cesiums per unit cell, 122 participate in this cationic continuum, whose formula is ( C S & ~ + per unit cell. The remaining six Cs+ ions are found at the centers of D6R’s.
Supplementary Material Available: Table of observed and calculated structure factors for the crystal structure of a cesium continuum in zeolite X (6 pages). Ordering information is given on any current masthead page.
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