Encapsulating Luminescent Materials in Zeolites. III. Crystal Structure

Article pubs.acs.org/JPCC

Encapsulating Luminescent Materials in Zeolites. III. Crystal Structure and Scintillation Properties of Cs,Na-LTA Treated with Zirconium Chloride Vapor Chung Woo Lee,† Joon Young Kim,† Hong Joo Kim,‡ Nam Ho Heo,*,† and Karl Seff§ †

Department of Applied Chemistry, School of Applied Chemical Engineering, College of Engineering, and ‡Department of Physics, College of Natural Science, Kyungpook National University, Daegu 41566, Korea § Department of Chemistry, University of Hawaii, 2545 The Mall, Honolulu, Hawaii 96822, United States S Supporting Information *

ABSTRACT: With the purpose of developing a new zeolite scintillator, fully dehydrated Cs,Na-A was exposed to ZrCl4 (g, 3.7 × 103 Pa) at 523 K. The crystal structures of Cs,Na-A and of the product, |Zr0.14Cs5.69Na4.13Cl0.85|[Si12Al12O48]LTA (Zr,Cs,Na,Cl-A), were determined by single-crystal crystallography using synchrotron X-radiation. The structure of Zr,Cs,Na,Cl-A was refined in the space group Pm3̅m (a = 12.230(1) Å) with all unique data to the final error index R1 = 0.069 for the 639 reflections for which Fo > 4σ(Fo). Its composition was confirmed by scanning electron microscopy energy-dispersive X-ray analysis (SEM-EDX). Octahedral ZrCl62− ions center 14% of the large cavities; each Cl− ion bonds to an 8-ring Cs+ ion. These Cs+ ions bridge between ZrCl62− ions to form a continuum with unit cell formula Cs3ZrCl6+ in the near-surface volume of the crystal. Other Cs+ ions lie opposite 6-rings in the sodalite and large cavities; Na+ ions occupy the remaining 6-rings. Upon X-irradiation, Zr,Cs,Na,Cl-A luminesces bright sky blue; the emission band is broad, ranging from 370 to 750 nm, peaking at 480 nm. The integrated intensity of the emission is ca. 16 times greater than that of anthracene, a well documented scintillator. This intense luminescence appears to result from charge transfer within the ZrCl62− ions. Tb,11 Ce/Tl,12 Eu/Tb,13 Eu,14,15 Ag,16−18 and Ln.19 Recently, Kim et al. reported a zeolite LTA scintillator containing Zr, Tl, and Cl ions.20 A broader range of compositions can be assembled within zeolites than those of pure and doped compounds, and this could lead to better scintillators. 1.3. Objectives and Methodology. The objectives of this work were (1) to sorb ZrCl4 into the cavities of partially Cs+exchanged zeolite LTA (Cs,Na-A),21 (2) to determine the structure of the product, and (3) to investigate its scintillation properties upon X-irradiation. We expected that ZrCl4 would be readily sorbed at a moderate temperature (523 K), where the vapor pressure of ZrCl4 is reasonably high (3.7 × 103 Pa)22,23 and dehydrated Cs,Na-A is stable.21 The potential metathesis products, CsCl and NaCl, might remain in the zeolite because they are not volatile at that temperature. Zr4+ would be readily identifiable crystallographically because its ionic radius and scattering power are very different from those of any of the other ions in the expected product, Zr,Cs,Na,Cl-A.

1. INTRODUCTION 1.1. Alkali Halide Scintillators. Scintillators convert highenergy photons into more easily detectable (UV−visible) lowenergy photons. As such, they have applications in medical imaging, high-energy physics, γ-ray detection, and homeland security.1−3 Among the many scintillators, alkali halides such as NaI and CsI and doped alkali halides such as NaI(Tl), CsI(Tl), and CsI(Na) are widely used because of their high light output and fast decay times, despite their hygroscopicity.4,5 Flint et al. reported the highly efficient Cs-based X-ray phosphor family Cs2NaLnCl6, where Ln is a trivalent lanthanide or Y3+.6 Bryan et al. reported that Cs+ ions assembled with X-ray absorbers like Zr or Hf, e.g., Cs2ZrCl6 and Cs2HfCl6, had scintillation properties.7 Burger et al. studied Cs2HfCl6 further; unique among scintillators, it is not hygroscopic.8 Recently, Saeki et al. completed a more extensive and comparative study of the scintillation properties of crystals of Cs2ZrCl6 and Cs2HfCl6.9 More recently, Kang and Biswas completed a theoretical study of the electronic and optical properties of Cs2HfCl6 using firstprincipal calculations.10 1.2. Zeolites with Optical Properties. Zeolites, because of their molecular dimensioned channels and cavities, can host relatively small molecules, relatively large cations, and continua. They have been used to encapsulate photoluminescent materials containing the following ions or pairs of ions; Ce/ © XXXX American Chemical Society

Received: July 6, 2016 Revised: July 29, 2016

A

DOI: 10.1021/acs.jpcc.6b06744 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C

2. EXPERIMENTAL SECTION 2.1. Synthesis. Large colorless transparent single crystals of zeolite 4A (|Na12(H2O)x|[Si12Al12O48]-LTA, Na12-A·xH2O, Na12-A, or Na-A) were synthesized by J. Charnell in G. T. Kokotailo’s laboratory.24 2.1.1. Cs,Na-A (Crystal 1). A single crystal of Na-A, a cube 80 μm on an edge, was lodged in a thin Pyrex capillary. Aqueous CsC2H3O2 (Sigma-Aldrich, 99.99%+) (pH = 6.2) was allowed to flow past it (dynamic ion exchange) (Table 1). A

the reaction vessel was kept at 523 K for 24 h. Finally, after it had cooled to room temperature (−50 K/h), the capillary containing the product Zr,Cs,Na,Cl-A was sealed off under vacuum. The resulting crystal was milky white and translucent with a white powder on its surface. 2.1.3. Zr,Cs,Na,Cl-A Powder. About 3 g of Na-A powder (Aldrich, 4σ(Fo) no. of variables, s data/parameter ratio, m/s weighting parameters: a, b final error indices: R1,b R2c goodness of fitd

Zr,Cs,Na,Cl-A, crystal 2

80 294, 24, 10

80 294, 24, 10

673, 48, 1 × 10−4

673, 48, 1 × 10−4 523, 48, 3.7 × 103

PLS(2D-SMC)a 0.7000 ADSC Quantum-210 63 transparent colorless 294(1) Pm3̅m, 221 12.263(1) 66.90 42 011 747 684 41 18.2 0.123, 3.891 0.0655, 0.206 1.20

PLS(2D-SMC)a 0.7000 ADSC Quantum-210 63 opaque white 294(1) Pm3̅m, 221 12.230(1) 66.70 41 879 755 639 43 17.6 0.105, 5.980 0.0690, 0.221 1.17

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). cR2 = [Σw(Fo2 − Fc2)2/Σw(Fo2)2]1/2 is calculated using all unique reflections measured. dGoodness of fit = (Σw(Fo2 − Fc2)2/(m − s))1/2.

similar procedure had been used to prepare partially Cs+exchanged zeolite A, |Cs7Na5|[Si12Al12O48]-A.21 The capillary containing the crystal was then connected to a high-vacuum line and the crystal was dehydrated (Table 1). After it had cooled to room temperature, it was sealed off from the vacuum line for X-ray experiments. It was colorless and transparent. 2.1.2. Zr,Cs,Na,Cl-A (Crystal 2). A break-off sealed tube with anhydrous ZrCl4 (Sigma-Aldrich, 99.99%) was attached as a side arm to the reaction vessel above a capillary containing a hydrated Cs,Na-A crystal prepared as above. After this crystal was fully dehydrated as above, the reaction vessel (the capillary containing the crystal, the tube above it, and the side arm containing the anhydrous ZrCl4) was sealed off from the vacuum line. The internal seal was then broken and the ZrCl4 was transferred to the tube. Finally, the side arm was sealed off and discarded. The resulting linear reaction vessel was heated to allow ZrCl4(g) to react with the zeolite (Table 1). Then, to eliminate any unreacted ZrCl4 in or near the crystal, the capillary end of B


Article


Figure 1. (a) EDX spectrum (counts versus photon energy in keV) for the hydrated surface of the Zr,Cs,Na,Cl-A crystal. (b) That crystal broken to expose an inner “surface”. Here are mappings of the atomic constituents of that hydrated surface.

Table 2. Crystal Composition (Atomic %) by Crystallographic and SEM-EDX Analyses Zr0.14Cs5.69Na4.13Cl0.85-A, crystal 2 SXRDa

Cs6.5Na5.5-A, crystal 1 a

b

element

SXRD

SEM-EDX

unit cell 1 (14%) (with continuum)

unit cell 2 (86%) (without continuum)

average

SEM-EDXb

Si Al O Zr Cl Cs Na

14.23 14.23 57.11

15.85 14.06 55.1

14.70 14.70 58.80

7.67 6.66

10.5 4.5

13.33 13.33 53.33 1.11c 6.67c 12.22

14.49 14.49 57.96 0.17 1.03 6.87 4.99

12.70 13.16 55.15 1.09d 5.40d 8.55 3.94

5.90 5.89

a Single crystal X-ray diffraction. bThe zeolite crystal can be expected to have suffered some decomposition upon exposure to the atmosphere due to H+ generated by the hydrolysis of Zr4+, and by the action of the electron beam. This can be a significant source of error. cNote the reasonable agreement between the SEM-EDX results for Zr and Cl and the composition of unit cell 1. dSEM-EDX is a surface analysis technique, so these values, high compared to those in the previous column, indicate that the surface regions of the crystal studied are enriched with Zr and Cl (section 4.2.2.3). Thus, the ZrCl6Cs3+ continuum observed crystallographically is present in the outer regions of the zeolite crystal rather than within.

The initial refinements with anisotropic thermal parameters for all framework atoms converged to the high-error indices (defined in footnotes of Table 1) R1/R2 = 0.54/0.86 and 0.49/ 0.84 for crystals 1 and 2, respectively (step 1 of Table 3). The detailed progress of structure determination as subsequent peaks were found on difference Fourier functions and identified as extraframework atoms are presented 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 whose final values are given in Table 1.

(WaveRunner 610zi), and the decay time was calculated from that.28

3. STRUCTURE DETERMINATION 3.1. Procedures Common to Both Crystals. Full-matrix least-squares refinements (SHELXL2014)29 were done on F2 using all unique reflections measured for the two single crystals. They were initiated with the atomic parameters of the framework atoms [(Si,Al), O1, O2, and O3] in dehydrated |Na12|[Si12Al12O48]-LTA.30 Fixed weights were used initially. C


Article


Figure 2. CCD images of the single crystal of dehydrated Zr,Cs,Na,Cl-A upon X-irradiation at 298 K: (a) PLS, Pohang, 17.8 keV, 400 mA; (b) PF, Tsukuba, Japan, 13.8 keV, 450 mA; (c) A CMOS image of the UV-induced luminescence of crystal 2, obtained with a 266 nm pulsed laser after brief (ca. 2 min) exposure to the atmosphere. No luminescence remains after longer (ca. 1 week) exposure to the atmosphere.

R values decreased a little. Although Na2 was rather far from the zeolite framework, it was retained in the model because it had been seen before in dehydrated Na12-A (see section 4.2.1.) Because of the high uncertainty of the occupancy at Na2 and therefore of the total Na occupancy, Cs,Na-A is assigned the composition Cs6.5Na5.5-A corresponding to its expected Si/Al framework ratio of 1.00.30,31 3.3. Procedures Unique to Crystal 2 (Zr0.14Cs5.69Na4.13Cl0.85-A). The occupancy at Cs1 was fixed to 3.0, its maximum value by symmetry (Table 3, crystal 2, step 12). The Cl−Zr−Cl angles (octahedral by symmetry), the Zr−Cl bond length, and the Cl/Zr occupancy ratio, 0.79(19)/0.19(3) = 4.3(12), indicated that a ZrCl62− ion centered the large cavity, so the occupancy ratio Cl/Zr was constrained to be six. Furthermore, the occupancy ratio of the Cs+ ions at Cs4 (they approach Cl− ions of ZrCl62−), and the Zr4+ ions at Zr became about eight: Cs4/Zr = 1.15(15)/0.157(24) = 7.3(15). The occupancy ratio Zr:Cl:Cs4 was subsequently constrained to be 1:6:8 (Table 3, crystal 2, step 14). At the final stages of refinement, trial refinements of some minor difference Fourier peaks near 4- and 8-rings were unsuccessful. One of these peaks, near 4-rings in the large cavities, might represent Na+ ions like those found at Na2 in crystal 1. 3.4. Other Crystallographic Data. 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; all were modified to account for anomalous dispersion.32,33 Additional crystallographic details are given in Table 1.

Figure 3. Visible spectra of dehydrated Zr,Cs,Na,Cl-A powder and related materials upon irradiation with polychromatic X-rays (Cu target, 100 kV, 2 mA).

4. DESCRIPTION OF STRUCTURES 4.1. Framework Geometry. The mean T−Oi (i = 1−3) bond lengths, 1.669 Å for both crystals 1 and 2, are about the same as the mean (1.675 Å) of Si4+−O (1.62 Å) and Al3+−O (1.72 Å) bonds in dehydrated Ca-LSX34 and hydrated Na-A.35 This indicates that the framework geometry is relatively undistorted in both crystals. 4.2. Extraframework Ions. The oxidation states of the zirconium, cesium, and sodium ions were assigned and confirmed primarily on the basis of their ionic radii, obtained by subtracting the conventional ionic radii of O2− from their shortest approach distances to framework oxygen atoms. Consideration was also given to their coordination numbers and environments (cations with smaller coordination numbers generally bond more closely to their ligands).36,37 The

Figure 4. Optical decay time of dehydrated Zr,Cs,Na,Cl-A powder monitored at 266 nm (UV laser). The decay curve was fit by a twoexponential function, y = A1 exp(−t/τ1) + A2 exp(−t/τ2) + y0 (R2 = 0.985). Arbitrary units are used for the intensities.

3.2. Procedure Unique to Crystal 1 (Cs6.5Na5.5-A). At the last stage of refinement, a minor difference Fourier peak near a 4-ring in the large cavity was included. Refinement with this peak at Na2 (Table 3, crystal 1, step 9) was stable and the D


Article

The Journal of Physical Chemistry C Table 3. Steps of Structure Determination as Nonframework Atomic Positions Were Found no. of ions per unit cella step

Cs1

Cs2

Cs3

error indicesb

Na1

R1

Na2

R2

Cs,Na-A, crystal 1 1c 2 3 4 5d 6 7e 8f 9

2.26(10) 3.27(7) 3.02(5) 3.01(5) 2.94(3) 3.02(4) 2.95(3) 2.91(3) Cs1

2.24(7) 2.27(5) 2.26(5) 2.24(3) 2.45(4) 2.60(3) 2.62(3) no. of ions per

0.96(5) 0.93(5) 0.97(4) 0.92(4) Cs2

Cs3

4.6(3) 4.60(24) 4.59(18) 4.16(19) 4.41(13) unit cella

Na1

0.54 0.86 0.38 0.79 0.17 0.57 0.118 0.455 0.125 0.452 0.0989 0.384 0.0923 0.272 0.0659 0.211 0.0655 0.206 error indicesb

1.2(4) Cs4

Zr

Cl

R1

R2

0.90(20) 0.79(19) 0.94(14) 0.90(9) 0.85(10)

0.49 0.31 0.17 0.16 0.108 0.109 0.112 0.0933 0.0727 0.0684 0.0680 0.0695 0.0696 0.0694 0.0690

0.84 0.73 0.56 0.54 0.44 0.43 0.41 0.34 0.2240 0.2217 0.2082g 0.2295 0.2233 0.2224 0.2209

Zr,Cs,Na,Cl-A, crystal 2 1c 2 3 4 5 6d 7 8e 9f 10 11 12h 13i 14j 15k

2.56(9) 2.48(5) 2.98(6) 2.93(4) 2.94(4) 3.16(4) 3.21(4) 3.04(3) 3.06(3) 3.11(3) 3.0 3.0 3.0 3.0

0.68(7) 0.81(6) 0.88(6) 0.74(7) 0.78(6) 0.50(4) 0.73(4) 0.72(4) 0.74(4) 0.74(4) 0.74(4) 0.74(4)

1.33(6) 1.63(7) 2.11(6) 2.10(6) 2.04(5) 2.06(4) 1.82(3) 1.05(17) 0.90(16) 0.85(16) 0.81(16) 0.75(12) 0.82(14)

4.9(3) 4.7(3) 4.3(3) 4.25(21) 5.06(17) 4.20(16) 4.33(15) 4.15(15) 4.12(15) 4.12(15) 4.13(15)

1.03(16) 1.10(16) 1.12(15) 1.15(15) 1.20(12) 1.13(14)

0.38(5) 0.40(4) 0.093(24) 0.17(4) 0.23(4) 0.19(3) 0.157(24) 0.150(14) 0.142(17)

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. cThe framework atoms were refined anisotropically. dAn extinction parameter (EXTI) was introduced and refined. eCs1, Cs2, Cs3, and Na1 were refined anisotropically. fA two-parameter weighting system (Table 1) was applied. gAnomalous due to a large shift in the refined weighting parameters. hThe occupancy of Cs1 was fixed at 3.0, its maximum value. iThe occupancies at Zr and Cl were constrained to 1:6. jThe occupancies at Zr, Cl, and Cs4 were constrained to 1:6:8. kCs4 was refined anisotropically.

Na1 is 3-coordinate, bonding to framework oxygen atoms of the 6-rings at 2.281(4) Å in crystal 1 and 2.245(5) Å in crystal 2, close to the sum of the Na+ and O2− ionic radii, 0.98 + 1.32 = 2.30 Å.38 The Na2−O1 bond length, 2.89(13) Å, is, however, much longer than this sum. This may be real or it may be virtual, as discussed in the previous paragraph regarding 8-ring Cs+ ions. Na+ ions had been found crystallographically opposite 4-rings in the large cavities of dehydrated Na12-A,30 and this has been confirmed by theoretical simulations.40 4.2.2. ZrCl6Cs3+ Cationic Continuum. 4.2.2.1. Zr4+ Ions in the ZrCl62− Unit. Per unit cell of crystal 2, 0.142(17) Zr4+ ions at Zr lie at the very centers of large cavities (Table 4 and Figure 6), far from zeolite framework, occupying only 14% of them. These Zr4+ ions do not bond to the zeolite framework. Instead, each bonds octahedrally to six Cl− ions (Cl) (Figure 9a) at 2.52(10) Å, close to the sum of Zr4+ and Cl− ionic radii, 0.79 + 1.81 = 2.60 Å.38,41 Similar Zr4+−Cl− bond lengths have been reported: Zr4+−Cl− = 2.434(2), 2.433(2), and 2.4433(7) Å in the ZrCl62− anions in Te6[ZrCl6],42 [(C4H9)HNC(C6H5)NH2]2[ZrCl6]·2CH2Cl2,43 and C22H42N4ZrCl6,44 respectively. 4.2.2.2. ZrCl62− Unit in the ZrCl6Cs3+ Continuum. Each of the Cl− ions in ZrCl62− bonds in turn to a Cs+ ion (Cs1) located at the center of an 8-ring (Cl−Cs1 = 3.59(9) Å) to form Cs6ZrCl64+ (Figure 9b). The Cs1−Cl bond length, 3.59(7) Å, is close to the sum of Cs+ and Cl− ionic radii, 1.67 +

oxidation states and coordination numbers of all extraframework ions are tabulated in Table 6. 4.2.1. Cs+ and Na+ Ions. Most Cs+ and Na+ positions are similar to those previously reported for partially Cs+-exchanged zeolite A.21 About six Cs+ ions were found per unit cell in both crystals, 6.5 at three positions in crystal 1 and 5.7 at four positions in crystal 2 (Tables 4 and 6). All Csi (i = 1−3) positions were nearly the same in both crystals; Cs3 had differentiated itself into Cs3 and Cs4 in crystal 2. Cs+ ions lie at the centers of the 8-rings (Cs1), opposite 6-rings in the sodalite cavity (Cs2), and opposite 6-rings in the large cavities (Cs3 and Cs4) (Figures 5−8 and Table 4). They bond to framework oxygen atoms with approach distances of 3.388(5) and 3.375(9) Å for Cs1, 3.115(7) and 3.110(10) Å for Cs2, and 2.8937(5) and 2.819(12) Å for Cs3 in crystals 1 and 2, respectively, and 3.011(22) Å for Cs4 in crystal 2. These distances are all close to the sum of the Cs+ and O2− ionic radii, 1.67 + 1.32 = 2.99 Å,38 except for those involving the 8-ring Cs+ ions, which are commonly longer (actually or artifactually; only averaged oxygen positions have been determined31) in the structures of Cs+-containing zeolites A.21,39 Most of the Na+ ions (ca. 5.6 per unit cell in crystal 1 and 4.1 in crystal 2) are located near 6-ring planes (Na1) (Figures 5, 6, and 8). In crystal 1, an additional 1.2(4) Na+ ions were found at Na2, near 4-rings in large cavity (Figure 5). Each of the ions at E


F

24(k) 12(h) 12(i) 24(m) 6(e) 8(g) 8(g) 8(g) 24(l)

24(k) 12(h) 12(i) 24(m) 3(c) 8(g) 8(g) 8(g) 8(g) 1(b) 6(f)

T O1 O2 O3 Cs1 Cs2 Cs3 Na1 Na2

T O1 O2 O3 Cs1 Cs2 Cs3 Cs4 Na1 Zr Cl

y 18340(8) 22373(44) 29605(27) 11280(20) 50000d 8696(38) 27317(9) 20332(33) 33819(953) 18329(10) 22404(70) 29543(37) 11356(29) 50000d 8524(62) 26959(87) 28358(149) 20224(47) 50000d 50000d

x

0d 0d 0d 11280(20) 0d 8696(38) 27317(9) 20332(33) 20553(1070)

0d 0d 0d 11356(29) 0d 8524(62) 26959(87) 28358(149) 20224(47) 50000d 29373(774) 37066(10) 50000d 29543(37) 33632(42) 50000d 8524(62) 26959(87) 28358(149) 20224(47) 50000d 50000d

37108(8) 50000d 29605(27) 33832(31) 50000d 8696(38) 27317(9) 20332(33) 50000d

z

2921(68) 6240(397) 5846(349) 4311(137) 9494(142) 10332(685) 2189(298) 5485(480) 4214(266) 13643(2614) 17867(3918)

1632(55) 4594(264) 4421(249) 2866(101) 6297(94) 8302(339) 3485(58) 2715(173) 11555(5348)

U11 or Uisob

236(40) 0d 1585(216) 257(141) 0d −2591(401) 12(173) −415(306) 1735(230)

U23

Zr,Cs,Na,Cl-A, crystal 2 2595(66) 2209(64) 7394(481) 2653(242) 3635(168) 3635(168) 4311(137) 5351(237) 6844(81) 6844(81) 10332(685) 10332(685) 2189(298) 2189(298) 5485(480) 5485(480) 4214(266) 4214(266)

U33 204(28) 0d 1425(146) 232(91) 0d −2050(229) 745(37) 1002(138)

U22 Cs,Na-A, crystal 1 1453(56) 1135(52) 3775(250) 1578(170) 2265(120) 2265(120) 2866(101) 3530(163) 5696(69) 5696(69) 8302(339) 8302(339) 3485(58) 3485(58) 2715(173) 2715(173)

0d 0d 0d 257(141) 0d −2591(401) 12(173) −415(306) 1735(230)

0d 0d 0d 232(91) 0d −2050(229) 745(37) 1002(138)

U13

0d 0d 0d 1329(168) 0d −2591(401) 12(173) −415(306) 1735(230)

0d 0d 0d 789(113) 0d −2050(229) 745(37) 1002(138)

U12

3.11(3) 0.74(4) 0.82(14) 1.20(12) 4.13(15) 0.19(3) 0.79(19)

2.92(3) 0.92(4) 2.62(3) 4.41(13) 1.2(4)

varied

3.0e 0.74(4) 0.82(14) 1.13(14)f 4.13(15) 0.142(17)f 0.94(14)f

constrained

occupancyc

24 12 12 24

24 12 12 24

fixed

a Positional parameters ×105 and thermal parameters ×104 are given. Numbers in parentheses are the estimated standard deviations (esds) in the units of the least significant figure given for the corresponding parameter. T represents the tetrahedral framework atoms, Si and Al. bThe anisotropic temperature factor is exp[−2π2a−2(U11h2 + U22k2 + U33l2 + 2U23kl +2U13hl +2U12hk)]. cOccupancy factors are given as the number of atoms or ions per unit cell. dExactly, by symmetry. eThe occupancy at Cs1 was fixed at 3.0, its maximum value at this position. fThe occupancies at Zr, Cl, and Cs4 were constrained to 1:6:8.

Wyckoff position

atom position

Table 4. Positional, Thermal, and Occupancy Parametersa

The Journal of Physical Chemistry C Article


Article

The Journal of Physical Chemistry C Table 5. Selected Interatomic Distances (Å) and Angles (deg)a distances T−O1 T−O2 T−O3 mean Cs1−O1 Cs2−O3 Cs3−O3 Cs4−O3

angles

Cs,Na-A, crystal 1


1.6565(19) 1.6598(13) 1.6806(12) 1.669 3.388(5) 3.115(7) 2.8937(5)

1.658(3) 1.6515(17) 1.6831(18) 1.669 3.375(9) 3.110(10) 2.819(12) 3.011(22)

Na1−O3

2.281(4)

Na2−O1 Cl−Cs1 Cl···Cs3 Cl···Cs4

2.89(13)

2.245(5)

3.59(9) 3.996(17) 3.75(3)

Cs,Na-A, crystal 1


O1−T−O2 O1−T−O3 O2−T−O3 O3−T−O3 T−O1−T T−O2−T T−O3−T

106.29(24) 112.45(16) 107.23(14) 110.8(3) 145.2(4) 157.3(3) 142.27(23)

106.4(4) 112.97(22) 106.36(20) 111.2(4) 145.0(6) 157.7(4) 140.7(3)

O1−Cs1−O1 O3−Cs2−O3 O3−Cs3−O3 O3−Cs4−O3 O3−Na1−O3 Zr−Cl−Cs1

90b 77.78(19) 85.03(12)

90b 76.6 (3) 86.2(5) 79.6(7) 118.18(13) 180c

Cl−Zr−Cl Zr−Cl

118.00(9)

90, 180d

2.52(10)

a The numbers in parentheses are the estimated standard deviations (esds) in the units of the least significant digit given for the corresponding value. T represents the tetrahedral framework atoms, Si and Al. bExactly, by symmetry. cZr−Cl−Cs1 is linear by symmetry. dZrCl62− is octahedral by symmetry.

positions, however, indicate that this ZrCl6Cs3+ continuum occupies only 14% of the crystal. 4.2.2.3. Placement of the ZrCl6Cs3+ Continuum. SEMEDX, a surface analysis method, indicates that the surface of the crystal studied was 4−5 times richer in Cl and Zr (Table 2) than the average composition of the entire crystal (the crystallographic result). When 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 close to that obtained from SEM-EDX analysis (Tables 2 and 7). Thus, the cationic ZrCl6Cs3+ continuum (Figure 9d) occupies the near surface volume (14% of the total volume) of the crystal. This can be seen directly in Figure 1b: the concentration of Zr and Cl, unlike those of all the other elements present, are high only at the surface; they are essentially absent from the interior of the crystal. The continuum appears to have formed at the surface during the reaction of ZrCl4(g) with Cs,Na-A and, by blocking the large cavities, prevented additional ZrCl4(g) from reaching the central volume of the crystal.

Table 6. Unit Cell Charge Budget atom position Cs1 Cs2 Cs3 Na1 Na2

Cs1 Cs2 Cs3 Cs4 Na1 Zr Cl Σ Cs

ionsa

occ.b

M−O,c Å

r,d Å

CNe

charge x occ

Cs,Na-A, crystal 1 2.92(3) 3.388(5) 2.08 4 2.92(3)+ Cs+ Cs+ 0.92(4) 3.115(7) 1.79 3 0.92(4)+ Cs+ 2.62(3) 2.8937(5) 1.57 3 2.62(3)+ Na+ 4.41(13) 2.281(4) 0.96 3 4.41(13)+ Na+ 1.2(4) 2.89(13) 1.49 1 1.2(4)+ Σ Cs = 6.46(6), Σ Na = 5.6(4), Σ charges = 12.1(4)+ Zr,Cs,Na,Cl-A, crystal 2 Cs+ 3.0 3.375(9) 2.06 4 3.0 Cs+ 0.74(4) 3.110(10) 1.79 3 0.74(4)+ Cs+ 0.82(14) 2.819(12) 1.50 3 0.82(14)+ Cs+ 1.13(14) 3.011(22) 1.69 3 1.13(14)+ Na+ 4.13(15) 2.245(5) 0.93 3 4.13(15)+ Zr4+ 0.142(17) 6 0.59(7)+ Cl− 0.85(10) 2 0.85(10)− = 5.69(20), Σ Na = 4.13(15), Σ Zr = 0.142(17), Σ Cl = 0.85(10), Σ charges = 9.6(3)+

5. DISCUSSION 5.1. ZrCl62− Ions. The ZrCl62− ions may have been produced as follows.

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 ion)38 from the shortest M−O bond lengths. eCoordination numbers.

3ZrCl4(g) + 2O2 − → 2ZrCl 6 2 − + ZrO2 (s)

1.81 = 3.48 Å,38 and to that in the CsKZr6Cl15B clusters in boron-centered zirconium compounds: Cs+−Cl− = 3.530(2) Å.45 The ZrCl62− ions are further stabilized by longer interactions between all six of its Cl− ions with eight Cs+ ions at Cs4 (Cl···Cs4 = 3.75(3) Å) in the large cavities (Figure 9c). The Cl− ions at Cl are at y = z = 1/2 because each has four such long electrostatic interactions with a square of four Cs+ ions at Cs4. Finally, from the occupancy at Cs1 and its position in the 8ring plane, Cs1 must bridge between two Cl− ions in adjacent large cavities. It must therefore bridge between ZrCl62− ions in adjacent unit cells and form a cationic continuum with a unit cell formula of ZrCl6Cs3+ (Figure 9d). The occupancies at these

The translucency and powdery surface observed for the product crystal may be due to ZrO2(s). It is also possible that Zr4+ replaced Al3+ ions in the zeolite framework to produce Al2O3(s). Lacheen and Iglesia, however, reported no such incorporation of Zr into the zeolite framework of H-ZSM-5 (Si/Al = 13.4) after reaction with ZrCl4(g).46 Instead, they suggested that ZrO2 might have formed external to the zeolite structure in some of their samples. 5.2. Charge Balance. As summarized in Table 6, the sum of the charges of the extraframework cations in crystal 1 is 12.1(4)+ per unit cell. This balances the framework charge, 12.0−, nicely. However, the sum of extraframework charges is G


Article


Figure 5. Stereoview of a representative large cavity of Cs6.5Na5.5-A. The zeolite A framework is drawn with open bonds; solid bonds are used to show the bonding of the extraframework ions. T represents the tetrahedral framework atoms, Si and Al. Ellipsoids of 20% probability are shown.

Figure 6. Steroview of a large cavity containing a Cs6ZrCl64+ group. The Zr4+ ions at Zr, Cs+ ions at Cs1 and Cs4, and Cl− ions at Cl are shown. See the caption to Figure 5 for other details.

Figure 7. Stereoview of a representative large cavity that does not contain ZrCl62−. The Cs+ ions at Cs2 and Cs3 and Na+ ions at Na1 are shown. See the caption to Figure 5 for other details.

emission spectrum of Zr,Cs,Na,Cl-A are similar to those of Cs2ZrCl67,9 and Cs2HfCl6.7,8,10 The light yield calculated for Zr,Cs,Na,Cl-A is ca. 16 times greater than that of anthracene, a well documented, often-used organic scintillator20 (Figure 10). The optical decay time of Zr,Cs,Na,Cl-A was determined by fitting the recorded pulse shape information with a twocomponent exponential function, y = A1 exp(−t/τ1) + A2 exp(−t/τ2) + y0, where the y and y0 are luminescence intensities, A1 and A2 are constants, t is time, and τ1 and τ2 are decay times (Figure 4). The observed decay times, τ1 = 0.21 μs and τ2 = 6.08 μs, are similar to the values 1.5 and 7.5 μs reported for Cs2ZrCl6,9 and 2.2 and 8.4 μs for Cs2HfCl6,9 although τ1 is much less in the zeolite. This similarity with

only 9.6(3)+ in crystal 2, in part, perhaps, because not all of the Na+ ions could be found (section 4.2.1). 5.3. Luminescence of Zr,Cs,Na,Cl-LTA. The luminosities of six samples were measured upon X-irradiation (Figure 3). The X-ray-induced emission spectrum of dehydrated Zr,Cs,Na,Cl-A is a broad band between 370 and 750 nm, peaking at 480 nm (Figure 3). Such broad luminescence bands are often seen in inorganic scintillators because the transitions between electronic states are broadened by vibronic coupling.47 The luminescence of anthracene can be seen peaking at a 440 nm. The remaining four samples do not luminescence, showing that Zr and Cl, together within the zeolite, are responsible for the strong luminescence observed. The general features of the H


Article


Figure 8. Stereoviews of representative sodalite cavities: (a) adjacent to a large cavity with ZrCl62− and (b) otherwise. See the caption to Figure 5 for other details.

Table 7. Occupancies of the Two Kinds of Unit Cells in Zr,Cs,Na,Cl-A (Crystal 2)a atom position

observed occupancies

unit cell 1 (14%) (with continuum)

Zr Cl Cs1 Cs2 Cs3 Cs4 Na1

0.142(17) 0.85(10) 3.0 0.74(4) 0.82(14) 1.13(14) 4.13(15)

1b 6b 3b

unit cell 2 (86%) (without continuum)

3 0.862(4) 0.96(16)

8b 4.81(17)

a

This table is based on the occupancy observed at Zr. Occupancies are given as the number of ions per unit cell. bThese are integers due to constraints.

Figure 9. Complexes and clusters in Zr,Cs,Na,Cl-A: (a) ZrCl62−, (b) Zr(ClCs)64+, (c) the Zr(ClCs)64+ unit associated further with eight surrounding Cs+ ions (Cs4), and (d) eight ZrCl62− ions connected by 12 bridging Cs1 ions to show a portion of the cationic ZrCl6Cs3+ surface continuum. The ions at Cs4 have been omitted for clarity.

Figure 10. Relative light yields for Zr,Cs,Na,Cl-A, anthracene, a physical mixture of Cs,Na-A and ZrCl4, Cs,Na-A, ZrO2, and ZrCl4.

Cs2ZrCl6 may indicate that the luminescence observed for Zr,Cs,Na,Cl-A is due to the ZrCl62− ion. I


Article

The Journal of Physical Chemistry C The luminescence of HfCl62− was explained as “carrier selftrapping” in a recent report by Kang and Biswas.10 In their calculated “atom projected density of states”, a valence band maximum (VBM) arises from the Cl− 3p states, and a conduction band minimum (CBM) comes from the t2g levels of the Hf4+ 5d orbitals. Upon X-irradiation, an electron in the VBM is transferred to the CBM, leaving a hole among the ligands (Cl− 3p orbitals) around the luminescence center (the Zr4+ 4d orbitals in this report).10 At the atomic orbital level, we may say that a chloride ion has lost an electron (Cl− → Cl0). The Cl0 atom polarizes its environment. This polarized system exhibits an axial relaxation (Cl0 + Cl−→ Cl2−) and this, which may be viewed as two anions sharing a hole, is a Vk center.10,48 This hole captures an electron from the CBM to form an excited (Cl22−)* molecule which may be viewed as a selftrapped exciton (STE).10,48 Finally, this STE emits a photon, the Vk center vanishes, and the sample regains its initial properties (Figure 11).48 The luminescence, therefore, arises from the interaction of an electron in the conduction band with a Vk center.48

ZnWO452 whose luminescence properties are due to transitions between highly charged (high oxidation state) transition metal ions with empty low-lying md shells and ligands with filled with np6 shells.52 In these compounds, the transition metal ions and their ligands would work interactively to give the luminescence. The blue-emitting complexes of anion core clusters such as WO 4 2− , WO 6 6− , ZrCl 6 2− , and HfCl 6 2− would behave similarly7,9,52 because their heavy metal ions all have empty md0 orbitals. A comparison of the scintillation properties of Zr,Cs,Na,Cl-A with those of ZnWO4,52−54 PbWO4,55,56 CaWO4,51,52,54,57 CaMoO4,51,52,58 SrMoO4,59 Cs2HfCl6,7−10 and Cs2ZrCl69 suggests that they all luminesce by similar mechanisms (Table 8).

6. CONCLUSIONS When Cs,Na-A was treated with ZrCl4(g) at 523 K, octahedral ZrCl62− ions formed at the centers of 14% of the large cavities. Cs+ ions in 8-rings bridge between ZrCl62− ions to form a ZrCl6Cs3+ continuum in the near-surface volume of the crystal studied. Each ZrCl62− ion is further stabilized by interactions with eight 6-ring Cs+ ions in the large cavity. The X-ray-induced luminescence of dehydrated Zr,Cs,Na,Cl-A is a broad band from 370 to 750 nm, peaking at 480 nm. The light output of Zr,Cs,Na,Cl-A is much higher than that of anthracene. It appears that an X-ray photon causes an electron to move (charge transfer) from a Cl− ion to the Zr4+ ion in ZrCl62−; the luminescence results when the electron returns to a delocalized state among the chloride ligands.

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b06744. Observed and calculated structure factors squared with esds for Cs,Na-A and Zr,Cs,Na,Cl-A (PDF)

Figure 11. Proposed energy scheme for the absorption and emission (exciton recombination) processes. The absorption of an X-ray photon causes a Cl− 3p (valence band maximum) electron to be transferred to a t2g level of Zr4+ 4d (conduction band minimum); this is ligand-tometal charge transfer (LMCT). The Cl0 atom left in the valence band combines with a neighboring Cl− ion to form a Vk center, Cl2−. This center captures an electron from the Zr4+ 4d conduction band to form a self-trapped exciton (STE), alternatively a Cl22− molecule in an excited state. Finally, this STE emits a photon (luminescence) and the Vk center vanishes.

■

AUTHOR INFORMATION

Corresponding Author

*N.H.H. Telephone: +82 53 950 5589. Fax: +82 53 950 6594. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

It is thought, however, that the charge transfer transition may not involve the actual transfer of an electron, but instead a considerable reorganization of the charge density distribution around the metal ion.49,50 This is analogous to other intrinsic (undoped) scintillators such as CaWO4,51,52 CaMoO4,51,52 and

ACKNOWLEDGMENTS We gratefully acknowledge the Pohang Light Source (PLS) and the Photon Factory (PF, High Energy Accelerator Research Organization, KEK, Tsukuba, Japan) for the use of their

Table 8. Scintillation Properties of Some Materials with Charge Transfer Luminescence crystal ZnWO4 PbWO4 CaWO4 CaMoO4 SrMoO4 Cs2HfCl6 Cs2ZrCl6 Zr,Tl,Cl-A Zr,Cs,Na,Cl-A

luminescence color

range (nm)

λmax (nm)

τ1 (μs)

blue-green blue blue green green blue sky blue sky blue sky blue

375−650 340−600 380−650 400−750 400−750 300−600 300−750 330−750 370−750

490 420 437 540 515 400 435 490 480

2.1

Encapsulating Luminescent Materials in Zeolites. III. Crystal Structure

Recommend Documents