Preparation, Crystal Structure, and Luminescence Properties of Zeolite

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Preparation, Crystal Structure, and Luminescence Properties of Zeolite LTA Containing Extraframework Tantalum(V), Tantalum(II), Thallium(I), and Chloride Hyeon Seung Lim, Joon Young Kim, Nam Ho Heo, and Karl Seff J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b03943 • Publication Date (Web): 18 May 2016 Downloaded from http://pubs.acs.org on May 27, 2016

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The Journal of Physical Chemistry

Preparation, Crystal Structure, and Luminescence Properties of Zeolite LTA Containing Extraframework Tantalum(V), Tantalum(II), Thallium(I), and Chloride

Hyeon Seung Lim,† Joon Young Kim,† Nam Ho Heo,*,† and Karl Seff§

†

Laboratory of Structural Chemistry,

Department of Applied Chemistry, School of Applied Chemical Engineering, Kyungpook National University, Daegu 41566, Korea

§

Department of Chemistry, University of Hawaii,

2545 The Mall, Honolulu, Hawaii 96822, United States

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(abstract) For the first time pentapositive cations have been introduced into a zeolite at extraframework positions.

Their presence was confirmed by single-crystal crystallography

using synchrotron X-radiation and energy dispersive X-ray analysis.

It was prepared by

the thallous ion exchange (TIE) method: fully dehydrated Tl12-LTA was treated with 3.0 x 104

Pa

of

TaCl5(g)

at

473

K.

The

crystal

structure

of

the

product,

|Ta2+0.123(TaCl6Tl3)2+0.246Tl+9.84|[Si13.4Al10.6O48]-LTA, was refined in the space group Pm3m (a = 12.099(1) Å) with all unique data to the final error index R1 = 0.044 for the 639 reflections for which Fo > 4σ(Fo). TIE did not occur. Instead some TaCl5 molecules decomposed to TaCl2 and Cl2(g), and TaCl2 reacted with additional TaCl5 molecules to form Ta2+ and TaCl6-.

Octahedral TaCl6- ions center about 24.6% of the large cavities.

Each

Cl- ion bonds to a Tl+ ion in the plane of an 8-ring to form a TaCl6Tl32+ continuum in the near-surface volume of the crystal. rings in the large cavities. and 8-rings.

The Ta2+ ions, 0.123(3) per unit cell, lie opposite 6-

All Tl+ ions occupy well established cation positions near 6-

The photoluminescence spectrum of this material is a broad emission band

between 350 and 500 nm, peaking at 410 nm.

Note to Editor: The subscript o above in Fo (two times) is a lower-case letter o. Throughout this ms, the subscripts o and c stand for "observed" and "calculated". They appear often in the footnotes to Table 1.

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1. INTRODUCTION 1.1. Tantalum Containing Compounds. Tantalum compounds have been used in a number of applications. They are photocatalysts for the decomposition of water to the elements under UV irradiation,1-6 storage capacitors with high dielectric constants in dynamic random access memory (DRAM) devices,7-9 antireflectors in solar cells,10 and photoluminescent (PL) materials. irradiation. NiO/Ta2O5.15

These materials luminesce at ca. 400 nm upon UV

Examples are Ba3NaTaO6,11 (Li,K,Na)TaO3,12 Ba5Ta4O15,13 Sr3NaTaO6,14 and Tantalum-doped silicate glass photoluminesces at ca. 420 nm.16

The

incorporation of lanthanide ions as activators into Ta-based compounds results in materials that (1) can be excited by less energetic photons17-19 and (2) show high photoluminosity at longer wavelengths.17-22

Examples are Eu3+-doped LaTaO4,17 Eu3+-doped KLnTa2O7 (Ln =

Gd, Lu, Y),18 EuKNaTaO5,19 RbLa0.7Tb0.3Ta2O7,20 Eu4Na2Ta2O13,21 and Er3+, Yb3+-doped LnTaO4 (Ln = Y, Gd, or La).22

1.2. Ta5+ Ions in Zeolites. There have been no reports of extraframework Ta5+ ions in zeolites.

They cannot be exchanged into zeolites from aqueous solution because simple

soluble salts of Ta5+ from which to make aqueous exchange solutions do not exist.

Even if

they did, 5+ cations, because of their high charge and relatively small size,23 would hydrolyze very strongly in solution, causing their oxides or hydroxides to precipitate. The resulting H+ ions, produced at high concentration, would exchange into the zeolite and, if



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the zeolite has a high aluminum content, destroy it.23,

24

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Altogether, the solution

chemistry of tantalum is sparse compared to those of the elements in adjacent groups.25 Ta5+ ions have, however, been incorporated into zeolite frameworks such as MFI,26 Beta,27 SiBEA,28 and FAU29-31 for catalytic purposes. outset26,

27, 31

or later27-29,

31

Tantalum was introduced at the

during the synthesis process. For example, Tielens et al.

studied tantalum framework sites in SiBEA and found that tantalum incorporation was followed by dealumination, leaving framework Ta5+-OH groups.28

Upon calcination, these

groups formed both Lewis acid and Lewis base sites: 2Ta5+-OH → H2O + Ta5+ + Ta5+-O-.28

1.3. Objectives and Methodology. The objectives of this work were (1) to achieve Ta5+ exchange into a high alumina zeolite by the thallous ion exchange (TIE) method,23 thus bypassing the difficulties associated with aqueous ion-exchange, (2) to learn the positions and coordination environments of the Ta5+ ions within the zeolite, and (3) to determine the luminescence properties of the Ta5+-containing zeolite.

It was hoped the following TIE

reaction would occur per unit cell: Tl12–LTA + 2.4TaCl5(g) → Ta2.4–LTA + 12TlCl(g).

(1)

2. EXPERIMENTAL SECTION 2.1. Ion Exchange. Single crystals of sodium zeolite A (|Na12(H2O)x|[Si12Al12O48]– LTA, Na12–A·xH2O, Na12–A, or Na–A) were synthesized by J. F. Charnell32 in G. T.


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Kokotailo's laboratory.

A single crystal of Na-A, a colorless cube, was lodged in a fine

Pyrex capillary. Fully Tl+-exchanged zeolite A (|Tl12(H2O)y|[Si12Al12O48]–LTA, Tl12–A, or Tl–A) was prepared by the flow method using 0.10 M aqueous TlC2H3O2 (Strem Chemicals, 99.999%) at 294 K for 24 h.

It was then dehydrated at 673 K and 1 x 10-4 Pa for 48 h.23

This or similar procedures had produced the fully dehydrated, fully Tl+-exchanged zeolites Tl-A,33, 34 Tl-X,35, 36 Tl-Y,37, 38 and ZSM-5.39 A sample of Tl-A powder was similarly prepared by the batch method. Na-A powder (1.0 g, Aldrich, < 5 µm ) was stirred in 100 mL of 0.1 M TlC2H3O2, a 1.4-fold excess, at 294 K for 24 h.

This was repeated twice with fresh solution. After filtering, the resulting

Tl-A powder was dried at 378 K.

About 70 mg was placed in a thin walled Pyrex tube (d

= 2 mm) and dehydrated as above. The dehydrated Tl-A crystal was exposed to TaCl5(g) (Aldrich, ampule, 99.999%) at 473 K (3.0 x 104 Pa in equilibrium with TaCl5(s))40 for 48 h under anhydrous conditions in the reaction vessel (the capillary containing the crystal was an extension of a Pyrex tube containing the TaCl5(s)). The crystal was heated further at 473 K for another 24 h to distill away any excess TaCl5 that might be adjacent to or loosely held within it.

After cooling to

294 K, the resulting crystal, Ta,Tl,Cl-A, was sealed off under vacuum in its capillary by torch. Under the microscope it was seen to be colorless with white powder on its surface and white sparkling crystallites within. The dehydrated Tl-A powder sample was also treated with TaCl5(g) as above. After it 5 ACS Paragon Plus Environment


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had cooled to 294 K, the white product, Ta,Tl,Cl-A powder, was sealed off under vacuum.

2.2. X-Ray Diffraction. Diffraction intensities for the single crystal were gathered using synchrotron X-radiation with a silicon(111) double crystal monochromator. were collected by the omega scan method with the BL2D-SMDC program.41

Data About

40,500 reflections were collected from 72 sets of frames with a 5o scan and an exposure time of 1 s per frame.

The basic data files were prepared using the programs HKL3000.42

The reflections were indexed by the automated indexing routine of the DENZO program.42 These were corrected for Lorentz and polarization effects; negligible corrections for crystal decay were also applied. The space group Pm3m, standard for zeolite A unless high precision is achievable, was determined by the program XPREP.43

Additional

experimental data are presented in Table 1.

2.3. SEM-EDX Analysis. The single crystal was removed from its capillary (exposed to the atmosphere) after the diffraction data were collected.

Scanning electron microscopy

energy dispersive X-ray (SEM-EDX) analysis was done using a Horiba X-MAX N50 EDX spectrometer and a Hitachi SU8220 field emission scanning electron microscope at 294 K and 9.0 x 10-4 Pa with a beam energy of 25 keV and current of 1 nA. tabulated in Table 2.

The results are

Subsequent to SEM-EDX analysis, a compositional map was

prepared using Trumap, a feature of the EDX software.

The SEM-EDX and mapping

results show that tantalum, thallium, and chlorine are all present (Figure 1 and Table 2).


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2.4. UV Induced Luminescence. The UV photoluminescence of the dehydrated Ta,Tl,Cl-A powder was measured with a fluorescence spectrometer (SCINCO FluoroMate FS-2) with a xenon flash lamp (Figure 2).

A slit width of 2.5 nm was used to measure the

excitation and emission spectra using photomultiplier tubes (PMT) at 600 V and a 20 ms integration time.

A UV laser induced image of the hydrated Ta,Tl,Cl-A crystal was

obtained using a 266 nm pulsed laser (MPL-F-266 nm-20 mW-11031584) and a Canon EOS 650D camera (5 seconds of exposure) under a microscope (Figure 2).

3. STRUCTURE DETERMINATION Full-matrix least-squares refinements (SHELXL2014)44 using the space group Pm3m were carried out on F2 using all 726 unique reflections measured.

The atomic parameters

of the framework atoms [(Si,Al), O1, O2, and O3] in dehydrated |Tl12|[Si12Al12O48]-LTA were used initially.45

The initial refinements with isotropic thermal parameters and fixed

weights converged to the high error indices (defined in footnotes to Table 1) R1 = 0.62 and R2 = 0.90 (step 1 in Table 3). The steps of structure determination as additional atomic positions were found in difference Fourier functions and included in refinement are given in Table 3.

The final weights were assigned using the formula w = 1/[σ2(Fo2) + (aP)2 + bP]

where P = [max(Fo2,0) + 2Fc2]/3; a and b are refined parameters. Their final values are given in Table 1.



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During structure refinement, the following four observations indicated the presence of TaCl6 groups at the centers of the large cavities: the occupancy ratio Cl1/Ta2 was 1.75/0.256 = 6.8(7), the Cl1 ions were arranged octahedrally about Ta2, Ta2 was at the center of the large cavity, and the Ta2-Cl1 bond length was 2.90(6) Å. Ta2:Cl1 was imposed.

Accordingly, a 1:6 constraint for

Considering the geometry of Tl22 about Ta2 and Cl1, and

observing that the occupancy at Tl22 was reasonably close to three (half of six, see next paragraph) times that at Ta2 (Tl22/Ta2 = 3.7(5)), the constraint 1:3 was imposed for Ta2:Tl22.

In the same way, the occupancy at Tl13 was close to eight times that found at

Ta2 (Tl13/Ta2 = 7.9(5)), so the constraint 1:8 was imposed for Ta2:Tl13. Altogether, the occupancies at these four positions were constrained to be Ta2:Cl1:Tl22:Tl13 = 1:6:3:8. The Cl1-Tl22 bond length, 3.26(6) Å, indicated that TaCl6 centered a larger group, Ta(ClTl)6. The Ta2:Tl13 occupancy ratio and the Cl1-Tl13 bond length, 3.668(18) Å, although long, indicated that eight additional Tl atoms at Tl13 associate with each Ta(ClTl)6 unit.

Subsequent refinement converged quickly with slight increases in the error indices

(step 10 in Table 3). Physical reasons for Tl22 being exactly at x = 0, and Cl1 exactly at y = z = ½, were not apparent at this stage.

Several positions of lower symmetry were carefully examined at

both positions, but none refined successfully.

Therefore, Tl22 at x = 0 must be bridging

between two chloride ions at Cl1 in adjacent large cavities, and the Ta(ClTl)6 clusters must be members of a cationic continuum with a unit cell formula of TaCl6Tl3 (section 4.2.4.2). 8 ACS Paragon Plus Environment

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This sharing of the Tl atoms at Tl22 is responsible for the occupancy ratio Ta2:Tl22 = 1:3, rather than 1:6.

The Cl1 ions are at y = z = ½ because each has four equivalent long

(3.677(11) Å) electrostatic interactions with a square of four Tl+ ions at Tl13 (section 4.2.4.2). The ratio of the occupancies at the two Ta positions, Ta2/Ta1 = 0.258(7)/0.136(25) = 1.9(4), indicated stoichiometry, so the constraint 1:2 was imposed for Ta1:Ta2 (step 11 in Table 3).

The occupancies at these five positions were subsequently constrained to be

Ta1:Ta2:Cl1:Tl22:Tl13 = 1:2:12:6:16. At this point the sum of the occupancies of the 6-ring cations, Ta1, Tl11, Tl12, and Tl13, insignificantly exceeded the capacity of the 6-rings, indicating that these rings are fully occupied.

A constraint for full occupancy was imposed.

Subsequent refinement

converged with slight increases in the error indices (step 12 in Table 3). The final structural parameters are presented in Table 4 and selected interatomic distances and angles are given in Table 5.

Atomic scattering factors for neutral atoms were

used and all were modified to account for anomalous dispersion.46,

47

Additional

crystallographic details are given in Table 1.

4. DESCRIPTION OF THE STRUCTURE 4.1. Framework Geometry. The mean T-Oi (i = 1-3) bond length, 1.668 Å, is



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almost the same as the mean (1.675 Å) of the Si4+-O (1.61 Å) and Al3+-O (1.74 Å) bonds reported in dehydrated Ca-LSX48 and hydrated Na-A.49

As is often seen, T-O3, 1.6883(19)

Å, is noticeably longer than T-O1, 1.6378(18) Å, and T-O2, 1.6580(19) Å (Table 5).

This

is because most of the cations in this structure coordinate to O3 atoms (Tables 5 and 6).33, 50, 51

For the same reason, the T-O3-T angle is the smallest (Table 5).

4.2. Extraframework Ions. 4.2.1. Assignment of Oxidation States. 4.2.1.1. Tl+, Ta2+, and Cl-. The oxidation states of the thallium ions, the tantalum ions at Ta1, and the chloride ions could be assigned on the basis of their bond lengths or ionic radii. Their coordination numbers and environments were also considered (cations with smaller coordination numbers generally bond more closely to their ligands52, 53).

The

results are tabulated in Table 6. The shortest Tl-O bond lengths at the five Tl positions ranged widely from 2.604(13) to 3.010(7) Å.

Those associated with 6-rings (Tl11, Tl12, and Tl13) range from 2.615(5) to

2.977(10) Å, and those with 8-rings (Tl21 and Tl22) are 2.604(13) and 3.010(7) Å.

These

distances are all in general agreement with the sum of the corresponding ionic radii, 2.79 Å.54

Most of them lie within the range seen in dehydrated Tl-A;45 from 2.64(2) to 2.82(3)

Å for 6-ring Tl+ ions and from 2.60(3) to 3.11(5) Å for 8-ring Tl+ ions. Furthermore, no other possible cations (Tan+) can have such long approaches to framework oxygen atoms.


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Thus the ions at these five positions are all considered to be Tl+. The observed ionic radius of the tantalum ion at Ta1, 2.184(12) - 1.32 = 0.86 Å, is considerably longer than those of Ta5+, 0.64 Å,54 Ta4+, 0.68 Å,54 and Ta3+, 0.72 Å,54 indicating that the ion at Ta1 is in an oxidation state lower than 3+.

Because the 1+

oxidation state of Ta has not been seen in oxide structures,55 the ions at Ta1 are judged to be Ta2+.

See also the first paragraph of section 5.1.

From the Tl22-Cl1 bond length, the ionic radius at Cl1 can be calculated: 3.26(5) - 1.47 (ionic radius of Tl+)54 = 1.79 Å.

It is very similar to that given for Cl-, 1.81 Å,54 so the ions

at Cl1 are accepted as Cl-.

4.2.1.2. Ta5+ Ions in TaCl6-.

The oxidation state of the Ta ion at Ta2 cannot be

assigned using the ionic radius or bond length arguments used above.

The Ta2-Cl1 bond

length, 2.89(5) Å, is substantially longer than any of the following sums of radii: Ta5+-Cl- = 0.6452, 54(0.68)56 + 1.81 = 2.45 (2.49) Å, Ta4+-Cl- = 0.68 + 1.81 = 2.49 Å, and Ta3+-Cl- = 0.72 + 1.81 = 2.53 Å.52,

54, 56

Lower oxidation states for Ta are not considered because

octahedral hexachlorotantalum has been reported only for Ta(V) and Ta(IV);55 to date there have been 11 crystallographic reports of TaCl6-, 5 of TaCl62-, and no others. The Ta5+-Cl- bond lengths in various compounds of TaCl6- lie in a narrow range, 2.317(3)

to 2.362(3) Å.

They range from 2.344(1) to 2.354(1) Å in tetraethylammonium

hexachlorotantalate(V),57 from 2.342 to 2.344 Å in [Na(NCCH3)6][TaCl6],58 from 2.317(3)



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to 2.362(3) Å in [C3H5N2][TaCl6] (imidazolium hexachlorotantalate(V)),59 from 2.303(5) to 2.345(4) Å in [Ta(tmhd)4][TaCl6],60 and from 2.3344(10) to 2.3586(11) Å in [(MePhCO)2H][TaCl6].61 These distances are all much shorter than the 2.89(5)-Å distance seen in this structure. The Ta4+-Cl- bond lengths in the hexachlorotantalate(IV) ion were only about 0.05 Å longer.

They were 2.397(3) Å in K2TaCl662 and 2.394(4) Å in Cs2TaCl6,63 again much

shorter than 2.89(5) Å. The difference, 0.40 Å, between the Ta2-Cl1 distance, 2.89(5) Å, and the sum of the Ta5+ and Cl-ionic radii, 0.68 + 1.81 = 2.49 Å,57 is therefore too large to be explained by decreasing the oxidation state at Ta2 from 5+ to a lower value.

Shannon estimates that the

difference in octahedral radius between Ta5+ and Ta3+ is 0.08 Å.52 This difference between the averaged Ta-Cl- bond lengths of Ta4+, (2.394(4)64 + 2.397(3)63)/2 = 2.396 Å, and Ta5+, 2.34 Å (average of all Ta5+-Cl- bond lengths mentioned above), is 0.056 Å.

For other

metallic elements, the difference in ionic radius between the 5+ and 3+ states is 0.15 Å for V, 0.22 Å for Bi, and 0.24 Å for Pa, all much less than 0.40 Å.56 Much longer Ta5+-Cl- bond lengths, however, were seen when the chloride ion bridges to another Ta5+ cation: 2.547(3) Å in (TaCl5)2,64 2.696(2) Å in dimeric TaCl4(N=PCl3),65 2.431(2) Å in TaCl3(NC5H4NPh-2)2,66 from 2.396(8) to 2.781(7) Å in Ta2Cl(µ-Cl)2(2,6-diiso-propylphenoxide)5(µ-O),67

and

2.66(1)

and

2.686(9)

Cl)(NBut)(NHBut)(NH2But)]2.68 12 ACS Paragon Plus Environment

Å

in

[TaCl(µ-

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In this structure, each chloride ion bridges between Ta2 and Tl22 (Figures 3 and 4, Cl1Tl22 = 3.26(5) Å). At a substantially longer distance, 3.678(11) Å, each chloride ion approaches four additional Tl+ ions at Tl13.

It appears that this bridging to inadequately

coordinated Tl+ ions, especially Tl22, is responsible for the long Ta2-Cl1 2.89(5)-Å bond length.

The Tl+-Cl- bond is not only ionic; the volatility of TlCl(s) indicates that it has

appreciable covalent character.69

Thus, the oxidation state of the Ta ion at Ta2 appears to

be 5+. In support of this tentative conclusion are the colors of hexachlorotantalate(V) and hexachlorotantalate(IV) in their compounds with colorless cations. The former are both colorless,57, 58 while the latter are both black.62, 63 Because the crystal studied here was colorless, we conclude that the tantalum ions at Ta2 are in the 5+ oxidation state, unmodified from the TaCl5(g) reagent used.

4.2.2. Ta2+ Ions.

A small number of Ta2+ ions, 0.123(3) per unit cell, lie on 3-fold

axes at Ta1 opposite 6-rings in the large cavities (Figure 5).

Each is 3-coordinate, bonding

to the three O3 framework oxygens of the 6-ring at 2.184(12) Å.

Each extends 0.69 Å into

the large cavity from the (111) plane of its O3 oxygen atoms

4.2.3. Tl+ Ions. Many Tl+ ions, about 10.59, only somewhat fewer than the number in the initial Tl12-A crystal, were found per unit cell at five well established positions (Table 4). Three positions are on 3-fold axes opposite 6-rings: Tl11 is in the sodalite cavity and Tl12



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and Tl13 are in the large cavity (Figures 3, 5, and 6). atoms at 2.765(5), 2.615(5), and 2.977(10) Å, respectively.

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Each bonds to three O3 oxygen The remaining Tl+ ions at Tl21

and Tl22 lie on 8-rings in the large cavities. They bond to 8-ring oxygen atoms: Tl21 bonds to O1 and O2 at 3.090(11) and 3.010(7) Å, and Tl22 to O1 and O2 at 2.826(16) Å and 2.604(13) Å, respectively (Table 5 and Figures 3 and 5). lengths are similar to those in Tl-A, Tl-X and Tl-Y.38, 45, 70

All of these Tl+-O bond

The Tl+ ions at Tl22 are viewed

as members of the Ta(ClTl)65+ cluster (Figures 3 and 4b).

4.2.4. TaCl6Tl32+ Cationic Continuum. 4.2.4.1. Ta5+ Ions in the TaCl6- Unit.

The 0.246(6) Ta5+ ions per unit cell at Ta2 lie at

the very centers of the large cavities, far from the zeolite framework, filling ca. 24.6% of them.

Each is 6-coordinate, bonding octahedrally to six Cl- ions at Cl1 with Ta5+-Cl-

distances of 2.89(5) Å (Figures 3 and 4a).

The anionic TaCl6- unit makes no close

approaches, of course, to the anionic zeolite framework.

4.2.4.2. TaCl6- Unit in the TaCl6Tl32+ Continuum. Each of the Cl- ions in the TaCl6unit bridges to a Tl+ ion (Tl22) located on an 8-ring (Cl1-Tl22 = 3.26(5) Å) to form Ta(ClTl)65+ (Figures 3 and 4b).

The long Ta5+-Cl- bond distances (section 4.2.1.2) indicate

that the six Tl+ ions at Tl22 have withdrawn appreciable bonding electron density from the Ta5+-Cl- bonds in TaCl6-. In turn each ion at Tl22 bridges to a Cl- ion in an adjacent unit cell to form a 3-


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dimensional continuum.

Because the ions at Tl22 are shared, the unit cell formula of the

continuum is TaCl6Tl32+. The six Cl- ions at Cl1 interact further with eight T1+ ions at Tl13 (Figures 3 and 4c); the octahedron of six Cl- ions (alternatively the TaCl6- ion) is symmetrically contained within a cube of eight Tl+ ions.

Each chloride ion is 3.678(11) Å from four T1+ ions at

Tl13, much longer distances than the Cl1-Tl22 bonds, 3.26(5) Å.

These interactions serve

to support the Ta(ClTl)65+ cluster at its position of high symmetry, centered in the large cavity, and to position Cl1 at y = z = ½ where each is equidistant from the four ions at Tl13. The eight ions per unit cell at Tl13 could have been included in the formula of the continuum, which would then have been TaCl6Tl1110+ per unit cell.

It is clear from the long

Tl13-O3 bond length, which is about 0.3 Å longer than the other two 6-ring Tl-O bond lengths, Tl11-O3 and Tl12-O3 (Table 5), that the ions at Tl13 are being drawn away from the zeolite framework to be involved with the continuum.

4.2.4.3. Placement of the TaCl6Tl32+ Continuum.

The crystallographic results

indicate that this continuum occupies only 24.6% of the volume of the crystal.

SEM-EDX,

a surface analysis method, indicates that the surface of the crystal studied is four to five times richer in Ta and Cl (Table 2) than the average composition of the entire crystal (the crystallographic result). When the two kinds of unit cells, one with the continuum and the other without it, were considered, the composition of the unit cells with the continuum is



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close to that obtained from SEM-EDX analysis (Tables 2 and 7).

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Thus the cationic

TaCl6Tl32+ continuum (Figure 4d) is in the near surface volume (24.6% of the total volume) of the crystal.

It is likely to have formed there when the crystal was treated with TaCl5(g)

and, by blocking the large cavities, prevented TaCl5(g) from reaching the central volume of the crystal.

5. DISCUSSION 5.1. Extraframework Tantalum Species: TaCl6- and Ta2+ Ions. The Ta2+ ions must have been produced by the decomposition of TaCl5 during the sorption reaction.

The ratio

of the occupancies at the two Ta positions, Ta2/Ta1 = 2, indicates that the following more comprehensive reaction has taken place. 6TaCl5(g) → 4TaCl6- + 2Ta2+ + 3Cl2(g)

(2)

A reaction like reaction 2, and balanced with respect to charge, cannot be written for Ta2/Ta1 = 1, 3, and 4.

It would require that the ions at Ta1 be Ta+, Ta3+, and Ta4+,

respectively, oxidation states that were dismissed in section 4.2.1.1. Reaction 2 can be envisioned to be a two step reaction. First some TaCl5(g) would decompose TaCl5(g) → TaCl2 + 3/2Cl2(g) and then the TaCl2 produced would react with additional TaCl5 molecules.


(3)

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TaCl2 + 2TaCl5(g) → Ta2+ + 2TaCl6-

(4)

When fully dehydrated zeolite Tl-A was treated with ZrCl4(g) at 623 K, a similar small number of reduced cations per unit cell were seen, 0.28(1) Zr2+ ions as compared to the 0.123(3) Ta2+ ions found here.71

Like here, a small amount of ZrCl4(g) had decomposed.

The Ta2+ ions occupy a conventional cation site in the zeolite. The Ta5+ ions, however, do not; they coordinate to chloride ions rather than to oxide ions of the zeolite framework, as though, except for the addition of a chloride ion, the reagent TaCl5(g) had simply been sorbed.

5.2. Net Reactions.

Treatment of Tl12-A with TaCl5(g) caused the sum of

extraframework charges to decrease from 12.0+ to 10.59(9)+ (Table 6). This diminished charge may to be due to the dealumination of the zeolite framework, as had been reported for dehydrated zeolites treated with SiCl4(g).72 Also indicative of dealumination is the decreased unit cell parameter, 12.099 Å.

(The crystal reconstructs itself as Al is lost to

give a crystal without T site (Si or Al) vacancies and more Si-O than Al-O bonds; Si-O bonds are shorter than Al-O bonds.52, 54) It is somewhat less than the value of ca. 12.2 Å, commonly seen for anhydrous zeolite A with only monopositive or predominantly monopositive cations coordinating to the zeolite framework.45, 73, 74 Dealumination is also and often seen when ion exchange is done using acidic solutions such as those of Eu(NO3)375 and Ni(NO3)2.76

The zeolite framework appears to have suffered a small



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degree of dealumination, 1.41(9) Al3+ ions per unit cell; the final average framework composition is about Si13.4Al10.6O48. Let us now consider how the Al3+ ions left the zeolite structure.

Reaction 2 shows that

all of the chlorine atoms that took part in the reaction are accounted for, and that no chloride ions remain to react with Al3+ to allow it to leave the zeolite as Al2Cl6(g), so it appears that it was lost (stoichiometrically as "TlAlO2") as Tl2O(g) + Al2O3(s).

The white sparkling

crystallites within the zeolite crystal and the white powder on its surface appear to be Al2O3. Although the vapor pressure of Tl2O(s) (black) is only 1.0 x 10-12 Pa at 473 K,77 it appears to have been high enough for it to have vaporized away, leaving behind a transparent colorless crystal.

Assuming the latter, the net reaction per unit cell can be written.

|Tl+12|[Si12Al12O48]-LTA + 0.37TaCl5(g) → |Ta2+0.123(TaCl6Tl3)2+0.246Tl+9.84|[Si13.4Al10.6O48]LTA + 0.18Cl2(g) + 0.71Tl2O(g) + 0.70Al2O3(s)

(5)

This reaction does not account for the decreased number of unit cells due to the loss of Al3+ and the reorganization of the structure, and is thus unbalanced with respect to oxygen. Another possible explanation for the reduction in zeolite framework charge could be the replacement of Al3+ in the zeolite framework by Ta5+; Al3+ is replaced by Si4+ when zeolites are treated with SiCl4(g).72 However, peaks corresponding to framework tantalum ions were not seen on the difference Fourier functions.

Also, the incorporation of Ta5+ (r =

0.64 Å52, 54 or 0.68 Å)56 at framework T sites, replacing Al3+ (r = 0.51 Å)56 or Si4+ (r = 0.42 Å),56 would have increased the unit cell size a little, contrary to the decrease observed.


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5.3. Luminescence Properties. The UV induced photoluminescence (PL) behavior of the dehydrated powder is shown in Figure 2.

Upon UV excitation (maximum intensity

at 270 nm), a broad emission band between 350 and 500 nm, peaking at 410 nm, was seen. These emission bands were not observed for Tl-A, so it is clear that the tantalum ions participate in this PL.

Similar PL properties were reported for thin films of Ta2Zn3O878 and

tantalum-doped silicate glass.16 explain this emission.

A metal-to-ligand radiative transition was proposed to

In tantalum-doped silicate glass, the excited state is the 5d0 state of

tantalum and the ground state is the 2p state of oxygen.16 The PL properties of Ta,Tl,Cl-A seem to be very similar to those in the reports mentioned above, suggesting that their mechanisms are similar. For Ta,Tl,Cl-A, it appears that an electron in a Cl 3p orbital is excited to a Ta 5d0 orbital upon UV irradiation.

The system would then thermally relax to the lowest

vibrational state associated with Ta 5d0 energy level.

Finally, the blue emission appears

when the excited electron relaxes to the valence band. Upon exposure to the atmosphere, the anhydrous Ta,Tl,Cl-A crystal slowly lost its ability to photoluminesce; it was gone after about a week.

6. CONCLUSIONS Tantalum ions, Ta5+ and Ta2+, were introduced into zeolite LTA at extraframework



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positions by the thallous ion exchange (TIE) method.

TIE, however, did not occur.

Instead, TaCl5(g) decomposed to TaCl2 and Cl2(g), and the Cl- ions in TaCl2 moved to additional TaCl5 molecules to form TaCl6-.

These octahedral TaCl6- anions were found at

the centers of 24.6% of the large cavities. Each was surrounded by six bridging 8-ring Tl+ ions, forming a cationic continuum with a unit cell composition of TaCl6Tl32+ in the nearsurface volume of the zeolite crystal studied. Eight additional 6-ring Tl+ ions per unit cell approach the six Cl- ions at longer distances. The Ta2+ and remaining Tl+ ions occupy conventional cation sites near 6- and 8-rings.

Upon UV excitation (270 nm), the Ta,Tl,Cl-

A crystal luminesced blue; the emission spectrum was a broad band from 350 and 500 nm, peaking at 410 nm.

■ ASSOCIATED CONTENT Supporting Information. Observed and calculated structure factors squared with esds. This material is available free of charge via the Internet at http://pubs.acs.org. ■ AUTHOR INFORMATION Corresponding Author * Tel.: +82 53 950 5589; Fax: +82 53 950 6594; E-mail address: [email protected] ■ ACKNOWLEDGEMENT We gratefully acknowledge the Pohang Light Source, Pohang, Korea, for the use of their


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diffractometer and computing facilities.

This work was supported by a National Research

Foundation of Korea (NRF) grant (No. NRF-2014R1A2A1A11054075) funded by the Korean government (MSIP).



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61. Marchetti, F.; Pampaloni, G.; Zacchini, S., Reactivity of Niobium(V) and Tantalum(V) Halides with Carbonyl Compounds: Synthesis of Simple Coordination Adducts, C–H Bond Activation, C=O Protonation, and Halide Transfer. Dalton Trans. 2007, 4343-4351. 62. Jongen, L.; Meyer, G., Dipotassium Hexachlorotantalate(IV), K2TaCl6. Acta Crystallogr. Sect. E.-Struct Rep. Online 2004, 60, i91-i92. 63. Yun, H.; Jang, G.-J., Dicaesium Hexachlorotantalate(IV), Cs2TaCl6. Acta Crystallogr. Sect. E.-Struct Rep. Online 2007, 63, i22-i23. 64. Rabe, S.; Müller, U., Crystal Structure of Tantalum Pentachloride,(TaCl5)2. Z. Kristallogr. NCS 2000, 215, 1-2. 65. Rivard, E.; Honeyman, C. H.; McWilliams, A. R.; Lough, A. J.; Manners, I., Synthesis and Reactivity of Perhalogenated Acyclic and Metallacyclic Tantalum(V) Phosphoraniminato Complexes: Discovery of an Unexpected Ligand Coupling Reaction To Form the Novel Phosphazenium Salt [N (PCl2NH2)2][TaCl6]. Inorg. Chem. 2001, 40, 1489-1495. 66. Polamo, M.; Leskelä, M., Syntheses and Crystal Structures of Phenyl (2-pyridyl) Amido Complexes of Zirconium(IV), Niobium(V) and Tantalum(V). Dalton Trans. 1996, 43454349. 67. Clark, G. R.; Nielson, A. J.; Rickard, C. E., Sterically Congested Tris Phenoxide Complexes of Tantalum(V). The X-ray Crystal Structures of [TaCl2(2,6-Di-t-Butylphenoxide)3] and [Ta2Cl(µ-Cl)2(2, 6-Di-Isopropylphenoxide)5(µ-O)]. Polyhedron 1987, 6, 1765-1774. 68. Jones, T. C.; Nielson, A. J.; Rickard, C. E., [TaCl(µ-Cl)(NBut)(NHBUt)(NH2But)]2; a Tantalum(V) Complex Containing Terminal Imido, Amido, and Amino Ligands. J. Chem. Soc., Chem. Commun. 1984, 205-206. 69. International Series on Materials Science and Technology; Pergamon International Library, Elmsford, NY, 1979; Vol. 24, p 374. 70. Kim, Y.; Han, Y. W.; Seff, K., Crystal Structure of Fully Dehydrated Fully TI+Exchanged Zeolite X. Zeolites 1997, 18, 325-333. 71. Kim, J. Y.; Park, J. M.; Kim, H. J.; Heo, N. H.; Seff, K., Exchange of a Tetrapositive Cation into a Zeolite and a New Inorganic Scintillator. I. Crystal Structures and Scintillation Properties of Anhydrous Zr1.7Tl5.4Cl1.7–LTA and Zr2.1Tl1.6Cl3.0–LTA. J. Phys. Chem. C 2015, 119, 18326-18339. 72. Anderson, M. W.; Klinowski, J., Zeolites Treated with Silicon Tetrachloride Vapour. Part 1.-Preparation and Characterisation. J. Chem. Soc., Faraday Trans. 1 1986, 82, 1449-1469. 73. Subramanian, V.; Seff, K., Crystal Structure of Dehydrated Cesium- and ThalliumExchanged Zeolite A. J. Phys. Chem. 1979, 83, 2166-2169. 74. Kim, Y.; Seff, K., The Octahedral Hexasilver Molecule. Seven Crystal Structures of Variously Vacuum-Dehydrated Fully Ag+-Exchanged Zeolite A. J. Am. Chem. Soc. 1978, 100, 6989-6997. 75. Kim, C. W.; Kang, H. C.; Heo, N. H.; Seff, K., Encapsulating Photoluminescent Materials in Zeolites. Crystal Structure of Fully Dehydrated Zeolite Y (Si/Al= 1.69) Containing Eu3+. J. Phys. Chem. C 2014, 118, 11014-11025. 76. Kim, C. W.; Jung, K. J.; Heo, N. H.; Kim, S. H.; Hong, S. B.; Seff, K., Crystal Structures of Vacuum-Dehydrated Ni2+-Exchanged Zeolite Y (FAU, Si/Al= 1.69) Containing Three-Coordinate Ni2+, Ni8O4·xH2O8+, x≤4, Clusters with Near Cubic Ni4O4 Cores, and H+. J. Phys. Chem. C 2009, 113, 5164-5181. 77. Cubicciotti, D., Thermodynamics of Vaporization of Thallous Oxide. High Temp. Sci. 1970, 2, 213-220. 78. Rack, P.; Potter, M.; Kurinec, S.; Park, W.; Penczek, J.; Wagner, B.; Summers, C., Luminescence properties of thin film Ta2Zn3O8 and Mn doped Ta2Zn3O8. J. Appl. Phys. 1998, 84, 4466-4470.



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TABLE 1. Experimental Conditions and Crystallographic Data

crystal (a cube) edge length (mm) Tl+ ion exchange (T (K), t (h), V (mL)) dehydration of Tl-A (T (K), t (h), P (Pa)) reaction with TaCl5 (T (K), t (h), P (Pa)) X-ray source wavelength (Å) detector crystal-to-detector distance (mm) crystal color data collection temperature (T (K)) space group, No. unit cell constant, a (Å) maximum 2θ for data collection (deg) no. of reflections measured no. of unique reflections measured, m no. of reflections with Fo > 4σ(Fo) no. of variables, s data/parameter ratio, m/s weighting parameters: a, b Final error indices: R1b, R1*c Final error indices: R2d Goodness of fite

0.080 294, 24, 5 673, 72, 1.5 x 10-4 473, 72, 3.0 x 104 PLS(2D-SMC)a 0.7000 ADSC Quantum-210 63 colorless 294(1) Pm3m, 221 12.099(1) 66.79 40,488 726 639 51 14.2 0.0836, 3.14 0.044, 0.050 0.132 1.13

a

Beamline 2D-SMC at the Pohang Light Source, Pohang, Korea. bR1 = Σ|Fo |Fc||/ΣFo; R1 is calculated using only those reflections for which Fo > 4σ(Fo). c * R1 is calculated using all unique reflections. dR2 = [Σw(Fo2-Fc2)2/Σw(Fo2)2]1/2 is calculated using all unique reflections measured. eGoodness of fit = (Σw(Fo2-Fc2)2/(m-s))1/2.


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TABLE 2. Crystal Composition (Atomic %) by SEM-EDX and Crystallographic Analyses Ta0.37Tl10.59Cl1.48-A SXRDb

SEM-EDXa element

average

unit cell 1 (24.6%) (with continuum)

unit cell 2 (75.4%) (without continuum)

Si

13.9

15.88

15.56

15.96

Al

12.94

12.54

11.11

13.08

O

51.34

56.85

53.33

58.08

Ta

1.61c

0.44

1.11d

0.20

Tl

11.7

12.54

12.22

12.68

Cl

8.52c

1.75

6.66d

-

a

The zeolite crystal can be expected to have suffered some decomposition upon exposure to the atmosphere due to H+ generated by the hydrolysis of Ta5+, and by the action of the electron beam. This can be a significant source of error. bSingle crystal X-ray diffraction. cSEM-EDX is a surface analysis technique, so these values, high as compared to those in the following column, indicate that the surface regions of the crystal studied are enriched with Ta and Cl (section 4.2.4.3). Thus the TaCl6Tl32+ continuum observed crystallographically is present in the outer regions of the zeolite crystal rather than within. dNote the reasonable agreement between the SEM-EDX results for Ta and Cl and the composition of unit cell 1.



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TABLE 3. Steps of Structure Determination as Non-framework Atomic Positions Were Found number of ions or atoms per unit cella step error indicesb R1 R2 Ta1 Ta2 Tl11 Tl12 Tl3 Tl21 Tl22 Cl1 1 0.62 0.90 2 4.9(3) 0.31 0.73 3 4.78(24) 1.91(12) 0.25 0.66 1.33(7) 4 5.11(17) 2.71(13) 0.16 0.52 1.51(5) 5 5.17(16) 1.33(10) 2.73(8) 0.12 0.42 1.55(4) 6 0.276(15) 5.17(9) 1.72(7) 2.93(6) 0.074 0.23 7c,d 0.243(10) 1.54(2) 4.72(12) 2.07(12) 1.82(15) 0.98(15) 0.0505 0.1493 8e 0.217(38) 0.258(10) 1.51(2) 4.74(12) 1.93(12) 1.89(13) 0.96(13) 0.0488 0.1471 f 9 0.136(26) 0.256(9) 1.52(2) 4.71(10) 2.01(10) 1.86(13) 0.96(12) 1.75(18) 0.0428 0.1252 10g 0.136(26) 0.258(7) 1.52(2) 4.67(7) 2.07(6) 2.02(4) 0.77(4) 1.55(4) 0.0434 0.1266 11h 0.129(3) 0.258(7) 1.52(2) 4.66(7) 2.07(6) 2.02(4) 0.78(2) 1.55(4) 0.0433 0.1264 12i 0.123(3) 0.246(6) 1.45(2) 4.46(5) 1.97(5) 1.97(4) 0.74(2) 1.48(4) 0.0438 0.1317 a Numbers in parentheses are the estimated standard deviations (esds) in the units of the least significant figure given for the corresponding parameter. bDefined in footnotes to Table 1. cAll atoms were refined anisotropically. dAn extinction parameter (EXTI) was introduced and refined. eTa1 was introduced and refined isotropically. fCl1 was introduced and refined anisotropically. gThe occupancies at Ta2, Cl1, Tl22, and Tl13 were constrained to be 1:6:3:8. hTa2:Ta1 was constrained to be 2:1. iThe sum of occupancies at Ta1,Tl11, Tl12, and Tl13 were constrained to be eight per unit cell.

28


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TABLE 4. Positional, Thermal, and Occupancy Parametersa atomic Wyckoff occupancyc x y z U11 or Uisob U22 U33 U23 U13 U12 position position varied constrained fixed T 24(k) 0d 18237(10) 36634(10) 250(6) 231(6) 184(5) 18(4) 0 0 24 O1 12(h) 0d 20377(71) 50000d 596(39) 673(47) 214(24) 0(0) 0 0 12 O2 12(i) 0d 30482(31) 30482(31) 447(28) 268(14) 268(14) 91(17) 0 0 12 O3 24(m) 11388(27) 11388(27) 32375(37) 382(12) 382(12) 430(21) 15(12) 15(12) 79(16) 24 0 0 0 0 0 0.135(26) 0.123(3)g,h Ta1 8(g) 21662(164) 21662(164) 21662(164) 431(57) d d d e e e Ta2 1(b) 50000 50000 50000 601(22) 601(22) 601(22) 0 0 0 0.256(9) 0.246(6)f,g Tl11 8(g) 9654(9) 9654(9) 9654(9) 397(7) 397(7) 397(7) -57(4) -57(4) -57(4) 1.521(19) 1.450(16)h Tl12 8(g) 25991(5) 25991(5) 25991(5) 317(4) 317(4) 317(4) -12(1) -12(1) -12(1) 4.71(10) 4.46(5)g,h Tl13 8(g) 28578(61) 28578(61) 28578(61) 916(18) 916(18) 916(18) 119(21) 119(21) 119(21) 2.01(10) 1.97(5)f,h d d Tl21 12(h) 0 45913(56) 50000 662(17) 559(39) 856(55) 0 0 0 1.86(13) 1.97(4) Tl22 24(k) 0d 43601(110) 47546(113) 2304(243) 119(30) 343(65) -167(34) 0 0 0.96(12) 0.739(18) f d d 50000 2534(546) 1026(158) 1026(158) 0 0 0 1.75(18) 1.48(4) f Cl1 6(f) 26088(442) 50000 a 5 4 Positional parameters x 10 and thermal parameters x 10 are given. Numbers in parentheses are the estimated standard deviations (esds) in the units of the least significant figure given for the corresponding parameter. bThe anisotropic temperature factor is exp[-2π2a-2(U11h2 + U22k2 + U33l2 + 2U12hk + 2U13hl + 2U23kl)]. c Occupancy factors are given as the number of atoms or ions per unit cell. dExactly, by symmetry. espherical by symmetry. fThe occupancies of Ta2, Cl1, Tl22, and Tl13 were constrained to be 1:6:3:8. gTa2:Ta1 was constrained to be 2:1. hThe sum of occupancies at Ta1,Tl11, Tl12, and Tl13 were constrained to be eight per unit cell.

29



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TABLE 5. Selected Interatomic Distances (Å) and Angles (deg)a

distances

angles

T-O1 T-O2 T-O3 mean

1.6378(18) 1.6580(19) 1.6883(19) 1.668

O1-T-O2 O1-T-O3 O2-T-O3 O3-T-O3

107.6(3) 112.27(20) 107.55(18) 109.4(3)

Ta1-O3 Ta2-Cl1

2.184(12) 2.89(5)

T-O1-T T-O2-T T-O3-T

161.8(6) 143.3(4) 135.1(3)

Tl11-O3 Tl12-O3 Tl13-O3

2.765(5) 2.615(5) 2.977(10)

O3-Ta1-O3

110.6(8)

Cl1-Ta2-Cl1

90, 180b

O3-Tl11-O3 O3-Tl12-O3 O3-Tl13-O3

80.99(15) 86.71(14) 74.2(3)

Tl21-O1 Tl21-O2

3.090(11) 3.010(7)

Tl22-O1 Tl22-O2 Tl22-Cl1

2.826(16) 2.604(13) 3.26(5)

O1-Tl21-O2 O1-Tl22-O2

51.67(10) 53.1(3)

Tl13···Cl1

3.678(11)

Ta2-Cl1-Tl22

165.3(3)

a

The numbers in parentheses are the estimated standard deviations (esds) in the units of the least significant digit given for the corresponding value. bExact values by symmetry.


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TABLE 6. Unit Cell Charge Budget occ.b atom position ionsa

M-O,c Å

r,d Å

CNe

charge x occ.

Ta0.37Tl10.59Cl1.48-A Ta1 Ta2 Tl11 Tl12 Tl13 Tl21 Tl22 Cl1

2+

Ta 0.123(3) 2.184(12) 0.86 3 0.246(6)+ 5+ Ta 0.246(6) 6 1.23(3)+ + Tl 1.450(16) 2.765(5) 1.45 3 1.450(16)+ Tl+ 4.46(5) 2.615(5) 1.30 3 4.46(5)+ Tl+ 1.97(5) 2.977(10) 1.66 3 1.97(5)+ + 1.97(4) 3.010(7) 1.69 3 1.97(4)+ Tl + Tl 0.739(18) 2.604(13) 1.28 4 0.739(18)+ Cl 1.48(4) 5 1.48(4)ΣTa = 0.369(7) ΣTl = 10.59(8) ΣCl = 1.48(4) Σcharges = 10.59(9)+ a Extraframework ions. bOccupancy, ions per unit cell. cShortest M-O (metal ion to framework oxygen) bond lengths. dRadii of M ions obtained by subtracting 1.32 Å (the conventional radius of the oxide ion56) from the shortest M-O bond lengths. eCoordination numbers.



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TABLE 7. Occupancies of the Two Kinds of Unit Cellsa atom observed unit cell 1 (24.6%) unit cell 2 (75.4%) position occupancies (with continuum) (without continuum) Ta1 0.123(3) 0.164(4)b Ta2 0.246(6) 1 Tl11 1.450(16) 1.924(21)b Tl12 4.46(5) 5.93(7)b b,c Tl13 1.97(5) 8 Tl21 1.97(4) 2.63(5) Tl22 0.739(18) 3c Cl1 1.48(4) 6c a This table is based on the occupancy observed at Ta2. Occupancies are given as the number of ions per unit cell. bThe 8-fold 6-ring position is fully occupied in both kinds of unit cells. cThese are integers due to constraints.


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(figure captions)

Figure 1.

(a) EDX spectrum (counts vs. photon energy in keV) (b) the mappings of constituents for hydrated Ta0.37Tl10.59Cl1.48-A.

Figure 2.

(a) Excitation (pink) and emission (blue) spectra of dehydrated Ta,Tl,Cl-A powder.

Images of the Ta,Tl,Cl-A crystal (b) before and (c) upon being

irradiated with a UV 266 nm pulsed laser.

Figure 3. A stereoview of a large cavity containing a Ta(ClTl)65+ cluster. The zeolite A framework is drawn with open bonds; solid bonds are used to show the cluster. T represents the tetrahedral framework atoms Si and Al.

Ellipsoids of 35%

probability are shown.

Figure 4.

Components of the TaCl6Tl32+ continuum: (a) TaCl6-, (b) Ta(ClTl)65+, (c) the Ta(ClTl)65+ unit associating further with eight surrounding Tl+ ions at Tl13, and (d) eight TaCl6- units connected by 12 bridging Tl22 ions to show a portion of the cationic TaCl6Tl32+ continuum in the near-surface volume of the crystal. The ions at Tl13 have been omitted for clarity.

Figure 5.

A stereoview of a large cavity that does not contain a Ta(ClTl)65+ cluster. The Ta2+ ion at Ta1 and the Tl+ ions at Tl11, Tl12 and Tl21 are shown.

See the

caption to Figure 3 for other details.

Figure 6.

Stereoviews of representative sodalite cavities: (a) in the TaCl6Tl32+ continuum; (b) otherwise.

See the caption to Figure 3 for other details.



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Figure 1. Lim, Kim, Heo, and Seff


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Note to editor: This stereoview is the correct width, ca. 4 1/4 inches black to black, for clearest viewing. This width is dictated by the average distance between adult human eyes. enlarged it will not be viewable in stereo.

If it is

If necessary to save journal space, it may be

reduced to as little as one full column width, ca. 3 1/8 inches black to black.


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Note to editor: This stereoview is the correct width, ca. 4 1/4 inches black to black, for clearest viewing. This width is dictated by the average distance between adult human eyes. enlarged it will not be viewable in stereo.

If it is

If necessary to save journal space, it may be

reduced to as little as one full column width, ca. 3 1/8 inches black to black.


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(a)

(b)


Note to editor: These stereoviews are the correct width, ca. 4 1/4 inches black to black, for clearest viewing.

This width is dictated by the average distance between adult human eyes.

they are enlarged they will not be viewable in stereo.

If

If necessary to save journal

space, they may be reduced to as little as one full column width, ca. 3 1/8 inches black to black.



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TOC Art


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Preparation, Crystal Structure, and Luminescence Properties of Zeolite

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