Progress toward Zeolite-Based Self-Luminous Sensors for

Article pubs.acs.org/JPCC

Progress toward Zeolite-Based Self-Luminous Sensors for Radioactive Isotopes such as 201Tl and 137Cs: Structures and Luminescence of Hf,Cl,Tl‑A and Hf,Cl,Cs,Na‑A Joon Young Kim,† Hong Joo Kim,‡ Nam Ho Heo,*,† and Karl Seff§ †

Department of Applied Chemistry, School of Applied Chemical Engineering, College of Engineering, Kyungpook National University, Daegu 41566, Korea ‡ 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: Smart materials that both sorb and announce the presence of radioactive isotopes such as 201Tl and 137Cs are needed for human safety in environmental catastrophies. This study reports progress toward zeolite-based self-luminous sensors for radioactive isotopes. Hf4+ was introduced into zeolite A by treating Tl-A and Cs,Na-A with HfCl4(g) under anhydrous conditions. The crystal structures of the products, Hf,Cl,Tl-A and Hf,Cl,Cs,Na-A, were determined by single-crystal crystallography using synchrotron Xradiation, with compositional confirmation by SEM-EDX analysis. Their luminescence properties upon X-ray, UV, and proton beam irradiation were studied. In Hf,Cl,Tl-A, some Hf4+ ions are 3coordinate with three framework oxygen atoms of 6-rings; octahedral HfCl62− ions are at the very centers of 7% of the large cavities within Tl14HfCl612+ clusters. HfCl62− ions are also seen in Hf,Cl,Cs,Na-A; they similarly occupy 7% of the large cavities and are members of a Cs11HfCl69+ continuum. The X-ray-induced luminescence spectra of Hf,Cl,Tl-A and Hf,Cl,Cs,Na-A, and that of Hf,Cl,Tl-A induced by UV, are all broad bands from 300 to 720 nm, peaking near 400 nm. By comparisons with the corresponding spectra of Zr,Cl,Tl-A, Zr,Cl,Cs,Na-A, Cs2HfCl6, and Cs2ZrCl6, several luminescence mechanisms are proposed. These zeolite-based materials can be expected to emit light (be self-luminous) as radioactive isotopes exchange into them.

1. INTRODUCTION Radioactive isotopes released from nuclear power plants (NPP) and medical applications are hazardous environmental pollutants.1,2 In the last 30 years, the two large-scale NPP catastrophies have occurred at Fukushima and Chernobyl. After each of these two accidents, large amounts of highly radioactive elements such as 137Cs, a common nuclear fission product, were released which affected not only the immediate environment but also more distant regions of the earth. Because the half-life of 137Cs is 30.17 years,3 effective sorbents are needed for its sequestration and storage. Zeolites, because of their cationexchange capability,4,5 have been widely employed to capture 137 Cs. Furthermore, radioactive isotopes such as 99mTc, 99Mo, 67 Ga, 131I, and, 201Tl are widely used in hospitals.6 To better retrieve these radioactive materials from wastes, smart materials that both sorb and announce would be useful. High-energy ionizing radiation is widely used for various applications, ranging from medical to industrial to scientific.7,8 Due to their high photon energies and penetrating ability, applications and demands are constantly rising. Although Xrays and γ rays are invisible to the human eye, they can be made visible by the use of scintillating materials such as thalliumdoped NaI and CsI.9 NaI(Tl) and CsI(Tl) can efficiently © XXXX American Chemical Society

convert invisible ionizing radiation to visible light, and this signal can then be amplified using electronic light sensors such as photomultiplier tubes (PMT), photodiodes, or silicon photomultiplier (SiPM) tubes. In addition to our recent reports regarding the scintillation properties of the zeolites Zr,Cl,Tl-A10 and Zr,Cl,Cs,Na-A,11 several new organic/inorganic scintillators have been developed.7,12 None of them, however, have utilized the scintillation properties of porous materials such as zeolites or metal-organic frameworks (MOFs). Practically speaking, the efficient detection of ionizing radiation with porous materials is difficult to achieve due to their low density and poor attenuation capability (stopping power). Recently, several MOFs overcame these shortcomings by the synergistic assembly of metal clusters and luminescent organic bridging ligands.13,14 We seek to develop porous zeolitic materials with enhanced scintillation properties such as light yield, decay time, and radiation hardness. Received: June 8, 2017 Revised: August 2, 2017 Published: August 11, 2017 A

DOI: 10.1021/acs.jpcc.7b05641 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

Figure 1. Schematic illustration for the preparation of Hf,Cl,Tl-A and Hf,Cl,Cs,Na-A and the conceptual self-luminous sensor. (a) Zeolite A, Na12-A. (b) Fully Tl+- and partially Cs+-exchanged zeolite A, both fully dehydrated. (c) Tl-A and Cs,Na-A containing Hf4+ as a radiation absorber and charge carrier creator. (d) Conceptual self-luminous zeolites containing radioactive 201Tl or 137Cs.

ZrCl42−, Zr5Cl810+, ZrCl3+, and ZrCl62− formed within zeolite A upon treatment of Tl-A and Cs,Na-A with ZrCl4(g).10,11 Their Zr−Cl bonds could be the source of the strong scintillation observed (section 5.2). Zr4+ had not been exchanged into zeolites before because it hydrolyzes very strongly in aqueous solution. Hf4+ is widely used in scintillators.15,16 Because hafnium has a higher atomic number than zirconium (both are group 4 elements), zeolites containing Hf4+ should have greater stopping power for ionizing radiation and thus may have enhanced scintillation capability. Recent studies have shown that the hafnate group (HfO68−) and the hexachlorohafnium ion (HfCl62−) luminesce. Moreover, Wang et al. have suggested that Hf4+ serves as an effective X-ray antenna by absorbing Xray photons and converting them to fast electrons (the photoelectric effect).14 It is hoped that Hf-containing zeolites will find application as self-luminous sensing materials for the detection of radioactive waste, specifically the environmentally troublesome ions 201Tl+ and 137Cs+ (Figure 1). The intensity of their luminescence would increase with increasing radioactive ion concentration within the zeolite. In this report, we describe the preparation and luminescence properties of two new Hf4+-containing zeolites, Hf,Cl,Tl-A and Hf,Cl,Cs,Na-A. Tl+ and Cs+ ions were initially introduced into the zeolite with the hope that the product would have enhanced scintillation properties compared to those seen in doped alkali halide scintillators, and to be possible sources of ionizing radiation when they were replaced by radioactive 201Tl+ and 137 Cs+, respectively. On the basis of their crystal structures, we propose some new luminescence mechanisms. To prepare such zeolites, Hf4+ ions were introduced, for the first time, directly into a high alumina zeolite using a vapor-phase ion-exchange method. Because of experimental limitations, we have had to use the stable isotopes 204Tl and 133Cs instead of radioactive 201 Tl and 137Cs, so the possibility of self-luminescence was explored by using high-energy ionizing radiations.

had been shown to be suitable for the preparation of the fully Tl+-exchanged zeolites Tl-A,18,19 Tl-X,20,21 and Tl−Y.22,23 A powder sample of Tl-A was 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 2-fold excess) for 24 h. This was repeated two times with fresh solutions. 2.1.2. Hf,Cl,Tl-A. A single crystal of Hf,Cl,Tl-A was prepared by the TIE method.24 A hydrated crystal of Tl-A was fully dehydrated at 673 K at 1 × 10−4 Pa for 48 h. It was then exposed to HfCl4(g) [Aldrich, ampule, 99.9%, 7.9 × 103 Pa in equilibrium with HfCl4(s) at 523 K]25 at 523 K for 48 h. Within the reaction vessel the vapor pressure of the product TlCl(g) in equilibrium with TlCl(s) at 523 K would be 6.1 × 10−2 Pa,26 about 105 less than that of HfCl4(g), even less if insufficient TlCl was produced for TlCl(s) to condense. The part of the reaction vessel that contained the crystal was then heated further under vacuum at 523 K for additional 24 h to distill away any excess HfCl4 and TlCl that might be near the crystal or loosely held within. After being allowed to cool to room temperature, the capillary containing the crystal was sealed off under vacuum from the reaction vessel. The product crystal was seen under the microscope to be pure white, opaque, and not glossy. About 40 mg of Tl-A powder was placed in a thin-walled Pyrex tube 2 mm in diameter and was dehydrated under the same conditions as above. It was then allowed to react with HfCl4(g) as above. After it had cooled to room temperature, it was sealed off under vacuum. It was again white. 2.1.3. Cs,Na-A. A single crystal of partially Cs+-exchanged zeolite A was prepared by allowing 0.10 M aqueous CsC2H3O2 (Sigma-Aldrich, 99.99%+) to flow past a single crystal of Na-A in a Pyrex capillary at 294 K for 24 h. This had been shown to yield |Cs7Na5|[Si12Al12O48]-A,27 an apparent limit to Cs+ exchange from aqueous solution at ambient temperature. A powder sample of Cs7Na5-A was prepared by the batch method. Na-A powder (1.0 g, Aldrich, < 5 μm) was stirred in 100 mL of 0.1 M CsC2H3O2 (a 2-fold excess) as described above for 24 h. This was repeated two times with fresh solution. 2.1.4. Hf,Cl,Cs,Na-A. A single crystal of Hf,Cl,Cs,Na-A was prepared from Cs,Na-A as described in section 2.1.2. A hydrated crystal of Cs,Na-A was fully dehydrated and exposed to HfCl4(g) under anhydrous conditions. The product crystal was again seen to be pure white, opaque, and not glossy. About 40 mg of Cs,Na-A powder was placed in a thin-walled Pyrex tube 2 mm in diameter and was dehydrated under the same conditions used to dehydrate the Cs,Na-A single crystal. It was then allowed to react with HfCl4(g) as described in section

2. EXPERIMENTAL SECTION 2.1. Synthesis. Large colorless transparent single crystals of zeolite LTA (|Na12(H2O)x|[Si12Al12O48]−LTA, Na12−A·xH2O, Na12−A, or Na−A) were synthesized by J. F. Charnell in G. T. Kokotailo’s laboratory.17 2.1.1. Tl-A. A single crystal of fully Tl+-exchanged zeolite A (|Tl12(H2O)y|[Si12Al12O48]−LTA, Tl12−A, or Tl−A) was prepared by allowing 0.10 M aqueous TlC2H3O2 (Strem Chemicals, 99.999%) to flow past a single crystal of Na-A in a Pyrex capillary at 294 K for 24 h. This and similar procedures B


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the fresh surface were prepared using Trumap, a feature of the EDX software. 2.4. X-ray Induced Luminescence (XIL). Upon synchrotron X-irradiation, during the initial step of diffraction data collection, Hf,Cl,Tl-A and Hf,Cl,Cs,Na-A were both seen to luminesce bright sky blue. CCD images of the two crystals are shown in Figure 3. The X-ray induced luminescence of both dehydrated powders were obtained with a Flame-T spectrometer (Mo Kα, 50 kV, 30 mA) at 298 K; for comparison, other samples, similarly prepared, were also measured. The luminescence decay times were measured by directly coupling the vessel containing each powder to the entrance window of the photomultiplier tube (PMT, H6610). Each sample was excited with a XR200 pulsed X-ray beam (cold cathode type Xray tube, 0.026 to 0.040 mSv/pulse, pulse duration 60 ns). The pulse shape of the PMT output was directly registered with a 1 GHz digital oscilloscope (WaveRunner 610zi), and the decay time was calculated from that.31 2.5. UV Induced Luminescence (UVIL). The UV (VL4.LC UV lamp, 254 nm, 4W) induced luminescence spectra of the dehydrated Hf,Cl,Tl-A and Zr,Cl,Tl-A powder samples were measured with a Flame-T spectrometer at 298 K. 2.6. Proton Induced Luminescence (PIL). Finally, the radiation hardness (stability) of the dehydrated Hf,Cl,Tl-A and Hf,Cl,Cs,Na-A powder samples were examined by proton bombardment (30 MeV, 500 nA). Once again, both samples luminesced. Their PIL spectra were recorded at 30 s intervals for 5 min with a Flame-T spectrometer at 298 K.

Table 1. Experimental Conditions and Crystallographic Data Hf,Cl,Tl-A crystal (a cube) edge length (mm) Tl+ ion exchange [T (K), t (h), V (mL)] Cs+ ion exchange [T (K), t (h), V (mL)] dehydration of both [T (K), t (h), P (Pa)] reaction of both with HfCl4 [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 F0 > 4σ(F0) no. of variables, s data/parameter ratio, m/s weighting parameters: a, b final error indices: R1b, R2c goodness of fitd

0.070 294, 12, 5

Hf,Cl,Cs,Na-A 0.070

294, 12, 10 673, 48, 1.5 × 10−4 523, 72, 7.9 × 103 PLS(2D-SMC)a 0.6300 ADSC Quantum-210 63 opaque white 293(1) Pm3̅m, 221 12.065(2) 60.92 51,699 824 542 60 13.7 0.074, 7.90 0.072, 0.208 1.13

673, 48, 1.5 × 10−4 523, 72, 7.9 × 103 0.6200 ADSC Quantum-210 63 opaque white 293(1) Pm3̅m, 221 12.248(2) 66.88 60,775 1063 930 44 24.2 0.062, 3.78 0.049, 0.148 1.13

Beamline 2D-SMC at the Pohang Light Source, 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. a

3. STRUCTURE DETERMINATION 3.1. Procedures Common to Both Crystals. Full-matrix least-squares refinements (SHELXL2016)32 were done on F2 using all unique reflections measured for both crystals. They were initiated with the atomic parameters of the framework atoms [T (an average of the Si and Al positions), O1, O2, and O3] in dehydrated |Na12|[Si12Al12O48]-A.33 Fixed weights were used initially. The initial refinements with anisotropic thermal parameters for all framework atoms converged to the high error indices (defined in footnotes to Table 1) R1/R2 = 0.54/0.88 for Hf,Cl,Tl-A and 0.49/0.83 for Hf,Cl,Cs,Na-A (steps 1 in 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 and a and b are refined parameters. Their final values are given in Table 1. 3.2. Procedures Unique to Hf,Cl,Tl-A. The Cl−Hf−Cl angles (90° and 180° by symmetry), the Hf−Cl bond length, and the Cl/Hf occupancy ratio, 1.0(3)/0.057(12) = 17(6), indicated that the ion at the center of the large cavity was HfCl62−, so the occupancy ratio Cl/Hf was constrained to be six. Furthermore, the occupancy ratio of the Tl+ ions at Tl13 and Tl22 (they approach the Cl− ions of HfCl62−) and the Hf4+ ions at Hf became about eight and six: Tl13/Hf = 0.42(5)/ 0.057(12) = 7.4(18) and Tl22/Hf = 0.33(9)/0.057(12) = 5.8(20). The occupancy ratio Hf:Cl:Tl22:Tl13 was subsequently constrained to be 1:6:6:8 (Table 3, Hf,Cl,Tl-A, step 14). At the last stage of refinement, a difference Fourier peak, Q, at the very center of the sodalite cavity was included. Q was not close enough to the zeolite framework to bond to it. It must bond to Tl11, and, by its position at 0,0,0, it must bond to two

2.1.2. After it had cooled to room temperature, it was sealed off under vacuum. It was again white. 2.2. X-ray Diffraction. Diffraction intensities for the two crystals were measured with synchrotron X-radiation via a silicon(111) double crystal monochromator (Table 1). The BL2D-SMDC program was used for data collection by the omega scan method.28 Highly redundant data sets were harvested by collecting 72 sets of frames for each crystal with a 5° scan and an exposure time of 1 s per frame. The basic data files were prepared using the programs HKL3000 (PLS).29 The reflections were indexed by the automated indexing routine of the DENZO program.29 These were corrected for Lorentz and polarization effects; negligible corrections for crystal decay were also applied. The space group Pm3̅m, standard for zeolite A unless high precision is achievable, was determined by the program XPREP.30 Additional experimental and crystallographic data are presented in Table 1. 2.3. SEM-EDX Analysis. After diffraction data collection, the two crystals were removed from their capillaries (exposed to the atmosphere) for scanning electron microscopy energydispersive X-ray (SEM-EDX) analysis. The compositions of the crystals were determined using a Horiba X-MAX N50 EDX spectrometer within a Hitachi SU8820-SR FE (field emission) scanning electron microscope at 294 K and 9 × 10−4 Pa with a beam energy of 20 keV and current 2 μA. The SEM-EDX results (Figure 2 and Table 2) show that Hf, Cl, and Tl are all present in Hf,Cl,Tl-A and that Hf, Cl, Cs, and Na are all present in Hf,Cl,Cs,Na-A. Subsequent to these SEM-EDX analyses, the crystals were intentionally broken and compositional maps of C


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Figure 2. EDX spectra (counts vs photon energy in keV) and mapping images of single crystals of hydrated (a) Hf,Cl,Tl-A and (b) Hf,Cl,Cs,Na-A.

Table 2. Crystal Composition (Atomic %) by Crystallographic (SXRD) and SEM-EDX Analyses

a The zeolite crystal can be expected to have suffered some decomposition upon exposure to the atmosphere and the action of the electron beam. This can be a significant source of error.

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Tl11 ions, bridging between them to form a linear Tl2Q ion. The Tl11-Q bond length, 2.040(7) Å, is much shorter than the sum of the conventional radii for Tl+ and Cl−, 1.47 + 1.81 = 3.28 Å, and even for Tl+ and F−, 1.47 + 1.33 = 2.80 Å. However, when Q was refined as F, the Tl11/Q occupancy ratio [0.63(3)/0.35(7)] refined to 1.8(4); also Tl−F distances as short as 2.25 Å were seen in orthorhombic TlF(s).34 These indicate that Tl2F+ ions (linear, minimizing Tl+···Tl+ repulsion) center 32% of the sodalite cavities. Accordingly, the occupancy ratio Tl11/F was constrained to be two. Refinement with this peak as fluoride at F was stable, and the R values remained unchanged (Table 3, Hf,Cl,Tl-A, step 17). Subsequent SEMEDX analysis was able to confirm the presence of fluorine. Fluoride appears to have been present in the HfCl4 reagent. 3.3. Procedures Unique to Hf,Cl,Cs,Na-A. As in section 3.2, the Cl−Hf−Cl angles, the Hf−Cl bond length, and the Cl/ Hf occupancy ratio, 0.25(11)/0.043(10) = 5.8(29), indicated

Figure 3. CCD images of single crystals of dehydrated (a) Hf,Cl,Tl-A and (b) Hf,Cl,Cs,Na-A using synchrotron X-rays (λ = 0.7000 Å, 293 K).

Table 3. Steps of Structure Determination as Nonframework Atomic Positions were Found number of ions or atoms per unit cella error indicesb

Hf,Cl,Tl-A step 1 2 3 4 5 6 7 8 9c 10d 11e 12f 13 14g 15h 16 17i

Hf

Hf11

0.059(15) 0.026(9) 0.028(9) 0.019(7) 0.018(7) 0.14(3) 0.094(18) 0.132(21) 0.057(12) 0.074(7) 0.075(7) 0.073(6) 0.073(6) Hf

1 2 3 4 5 6c,e 7 8j 9 10 11k 12l 13m

0.040(10) 0.043(10) 0.043(9) 0.071(8) 0.066(6)

Hf12

0.21(4) 0.22(4) 0.27(4) 0.30(4) 0.21(3) 0.23(3) 0.21(3) 0.23(4) 0.20(3) 0.21(3) 0.22(3) 0.22(3)

0.18(4) 0.20(5) 0.22(4) 0.19(4) 0.18(4) 0.18(4) Cs1

2.47(8) 2.59(5) 3.23(5) 2.91(4) 2.95(3) 2.94(3) 2.957(16) 2.974(17) 2.984(17) 2.985(17) 3.000(17) 2.986(16)

Tl11

0.84(4) 0.72(4) 0.83(4) 0.79(4) 0.70(3) 0.70(3) 0.65(3) 0.61(3) 0.62(3) 0.63(3) 0.62(3) 0.63(3) 0. 63(3) 0.636(24) Cs2

0.81(5) 0.66(4) 0.67(4) 0.80(3) 0.82(3) 0.84(3) 0.84(3) 0.85(3) 0.83(3)

Tl12 2.79(12) 3.20(11) 2.92(7) 3.05(8) 2.93(7) 2.97(8) 3.01(7) 3.05(7) 3.38(7) 3.07(6) 3.03(5) 2.99(5) 3.02(5) 2.93(5) 2.90(5) 2.90(5) Cs3

1.37(15) 1.39(14) 1.52(12) 1.57(11) 1.57(11) 1.76(6) 1.77(5)

Tl13

0.23(5) 0.31(5) 0.29(5) 0.33(6) 0.53(6) 0.58(6) 0.42(5) 0.59(6) 0.60(6) 0.58(5) 0.58(5) Hf,Cl,Cs,Na-A Cs4

1.43(5) 1.96(5) 2.02(4) 2.07(3) 0.72(15) 0.80(14) 0.75(12) 0.72(11) 0.72(11) 0.56(6) 0.53(5)

Tl21

2.79(12) 2.14(7) 2.16(7) 2.07(7) 2.15(7) 0.69(21) 0.78(21) 2.75(8) 2.55(10) 2.64(9) 2.51(8) 2.54(6) 2.40(6) 2.36(5) 2.36(5) Na

6.0(3) 5.2(3) 5.51(17) 5.53(15) 5.17(9) 5.12(9) 5.12(9) 5.12(9) 5.16(9) 4.87(3)

Tl22

1.96(23) 1.95(23) 0.45(9) 0.44(12) 0.37(10) 0.33(9) 0.44(4) 0.45(4) 0.44(4) 0.44(4)

Cl

1.0(3) 0.44(4) 0.45(4) 0.44(4) 0.44(4)

F

R1

R2

0.35(7) 0.318(12)

0.54 0.29 0.22 0.182 0.184 0.180 0.170 0.137 0.115 0.100 0.086 0.080 0.083 0.079 0.076 0.0715 0.0715

0.88 0.66 0.60 0.52 0.509 0.497 0.487 0.441 0.430 0.374 0.286 0.247 0.260 0.247 0.237 0.2077 0.2077

Cl

R1

R2

0.25(11) 0.26(5) 0.42(5) 0.40(4)

0.49 0.34 0.18 0.13 0.117 0.100 0.090 0.0488 0.0480 0.0479 0.0479 0.0486 0.0490

0.83 0.76 0.59 0.50 0.487 0.281 0.255 0.150 0.1462 0.1447 0.1448 0.1470 0.1482

Numbers in parentheses are estimated standard deviations (esds) in the units of the least significant figure given for the corresponding parameter. Defined in footnotes to Table 1. cAn extinction parameter (EXTI) was introduced and refined. dHf12, Tl11, Tl12, Tl13, Tl21, and framework atoms were refined anisotropically. eA two-parameter weighting system (Table 1) was applied. fHf11 was refined anisotropically. gThe occupancies at Hf, Cl, Tl22, and Tl13 were constrained to be 1:6:6:8. hTl22 was refined anisotropically. iThe occupancies at Tl11 and F were constrained to be 2:1. j Cs1, Cs2, Cs3, Cs4, Na, and framework atoms were refined anisotropically. kThe occupancies at Hf and Cl were constrained to 1:6. lThe occupancies at Hf, Cl, and Cs4 were constrained to 1:6:8. mThe sum of the occupancies at Cs2, Cs3, Cs4, and Na was constrained to be eight per unit cell. a b

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The Journal of Physical Chemistry C Table 4. Positional, Thermal, and Occupancy Parametersa

Positional parameters × 105 and thermal parameters × 104 are given. Numbers in parentheses are 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)]. cOccupancy factors are given as the number of atoms or ions per unit cell. dExactly, by symmetry. eThe occupancies at Hf, Cl, Tl22, and Tl13 were constrained to be 1:6:6:8. fThe occupancies at Tl11 and F were constrained to be 2:1. gThe occupancies at Hf, Cl, and Cs4 were constrained to be 1:6:8. a

that the ion at the center of the large cavity was HfCl62−, so Cl/ Hf was constrained to be six. Furthermore, the occupancy ratio of the Cs+ ions at Cs4 (they approach the Cl− ions in HfCl62−) and the Hf4+ ions at Hf became Cs4/Hf = 0.72(11)/0.043(9) = 17(4). The maximum value of this ratio is eight, so the occupancy ratio Hf:Cl:Cs4 was subsequently constrained to be 1:6:8 (Table 3, Hf,Cl,Cs,Na-A, steps 11 and 12). 3.4. Other Crystallographic Details. 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.35,36 Additional crystallographic details are given in Table 1.

They are in the sodalite and large cavities opposite 6-rings and on or near the 8-rings (Figures 4 and 5 and Table 4). These positions are very similar to those in previously reported Tl-A structures.42,43 4.2.1.2. Hf4+ Ions in 6-Rings. Per unit cell, 0.18(4) Hf4+ ions at Hf11 lie on 3-fold axes near 6-rings. Each extends 0.49 Å into sodalite cavity from the (111) plane of its three 6-ring O3 oxygen atoms (Figure 4b). Also on 3-fold axes near 6-rings, 0.22(3) Hf4+ ions per unit cell at Hf12 extend 0.58 Å into large cavity from that plane (Figure 4b). These ions are all 3coordinate, bonding to three O3 framework oxygens. The Hf4+O3 bond lengths, Hf11−O3 = 2.153(21) Å and Hf12−O3 = 2.177(10) Å, are both close to the sum of the Hf4+ and O2− ionic radii,39 0.78 + 1.32 = 2.10 Å. 4.2.1.3. Tl14HfCl612+ Cationic Clusters. 4.2.1.3.1. HfCl62−. Per unit cell, 0.073(6) Hf4+ ions at Hf lie at the very centers of the large cavities (Figures 5a and 6), so only about 7% of them are occupied. These Hf4+ ions are far from the zeolite framework and do not bond to it. Instead, each bonds octahedrally to six Cl− ions (Cl) (Figures 5a and 6) at 2.63(13) Å, in agreement with the sum of the ionic radii of Hf4+ and Cl−,39 0.78 + 1.81 = 2.59 Å. 4.2.1.3.2. HfCl62− in Tl14HfCl612+. Each of the Cl− ions in HfCl62− (Figure 6a) bonds in turn to a Tl+ ion (Tl22) located near an 8-ring [Cl−Tl22 = 2.95(14) Å] to form Tl6HfCl64+ (Figure 6b). The Tl22−Cl bond length, 2.95(14) Å, is close to the sum of Tl+ and Cl− ionic radii,39 1.47 + 1.81 = 3.28 Å. In addition, each of these six Cl− ions bonds to four members (a square) of a cube of eight Tl+ ions at Tl13 (24 Cl−Tl13 bonds, Cl−Tl13 = 3.35(4) Å; sum of radii = 3.28 Å) in the large cavities to give Tl14HfCl612+ (Figure 6c). The Cl− ions at Cl are

4. DESCRIPTION OF THE STRUCTURES 4.1. Framework Geometry. The mean T-Oi (i = 1−3) bond lengths, 1.660 Å for crystal 1 and 1.667 Å for crystal 2, are about the same as the mean (1.675 Å) of the Si4+-O (1.61 Å) and Al3+-O (1.74 Å) bonds in dehydrated Ca-LSX37 and hydrated Na-A.38 This indicates that the framework geometry is relatively undistorted in both crystals. 4.2. Extraframework Ions. The oxidation states of the hafnium, thallium, cesium, and sodium ions were assigned and confirmed primarily on the basis of their ionic radii, obtained by subtracting the conventional ionic radius of O2−39 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).40,41 4.2.1. Extraframework Ions in Hf,Cl,Tl-A. 4.2.1.1. Tl+ Ions. About seven Tl+ ions were found per unit cell at five positions. F


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The Journal of Physical Chemistry C Table 5. Selected Interatomic Distances (Å) and Angles (deg)a

a

The numbers in parentheses are the estimated standard deviations (esds) in the units of the least significant digit given for the corresponding value. Mean T-O was calculated as {(T-O1) + (T-O2) + 2(T-O3)}/4. cMean O-T-O was calculated as {(O3-T-O3) + (O1-T-O2) + 2(O1-T-O3) + 2(O2-T-O3)}/6. dExact value by symmetry. eHf−Cl−Cs1 is linear by symmetry. fHfCl62− is octahedral by symmetry. b

Figure 4. Stereoviews of representative sodalite cavities of Hf,Cl,Tl-A: (a) adjacent to a large cavity with a Tl14HfCl612+ ion, (b) otherwise. 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 15% probability are shown.

Figure 5. Stereoviews of representative large cavities of Hf,Cl,Tl-A: (a) containing a Tl14HfCl612+ ion. The 24 bonds between the six Cl atoms and the cube of eight Tl13 ions have been omitted for clarity; they can be seen in Figure 6c, (b) otherwise. See the caption to Figure 4 for other details.

4.2.2. Extraframework Ions in Hf,Cl,Cs,Na-A. 4.2.2.1. Cs+ and Na+ Ions. Most Cs+ and Na+ positions are similar to those previously reported for Cs,Na-A27 and Zr,Cl,Cs,Na-A.11 About

held at y = z = 1/2 because each has four such bonds with a square of four Tl+ ions at Tl13. G


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all close to the sum of the Cs+ and O2− ionic radii,39 1.67 + 1.32 = 2.99 Å, except for that involving the 8-ring Cs+ ions (Cs1), which are commonly longer in the structures of Cs+-containing zeolites A.27,44 The Na+ ions at Na, about five per unit cell, are located near 6-ring planes (Figure 7b). Each is 3-coordinate, bonding to framework oxygen atoms of the 6-rings at 2.277(3) Å, close to the sum of the Na+ and O2− ionic radii,39 0.97 + 1.32 = 2.29 Å. 4.2.2.2. Cs11HfCl69+ Cationic Continuum. 4.2.2.2.1. HfCl62−. Per unit cell, 0.066(6) Hf4+ ions at Hf lie at the very centers of the large cavities (Figure 8a), far from zeolite framework, so

Figure 6. Components of Tl14HfCl612+: (a) HfCl62−, and (b) a representative Tl6HfCl64+ unit. (c) Stereoview of a representative Tl14HfCl612+ ion.

six Cs+ ions per unit cell (Table 4) were found at positions nearly the same as seen before.11,27 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) (Figure 7, panels a and b). They bond to framework oxygen atoms with approach distances of 3.391(4) Å for Cs1, 3.131(5) for Cs2, 2.888(4) for Cs3, and 3.124(18) for Cs4. These distances are

Figure 8. Stereoviews of representative large cavities of Hf,Cl,Cs,Na-A: (a) containing a HfCl62− ion (b) otherwise. See the caption to Figure 4 for other details.

Figure 7. Stereoviews of representative sodalite cavities of Hf,Cl,Cs,Na-A: (a) adjacent to a large cavity with a HfCl62− ion (b) otherwise. See the caption to Figure 4 for other details.

only about 7% of them are occupied. As in Hf,Cl,Tl-A, each of these Hf4+ ions bonds octahedrally to six Cl− ions (Cl) at 2.25(13) Å, which appears to be substantially shorter than the sum of Hf4+ and Cl− ionic radii,39 0.78 + 1.81 = 2.59 Å (section 4.4). 4.2.2.2.2. HfCl62− in Cs8HfCl66+. The HfCl62− ion is held in place primarily by 24 bonds between all six of its Cl− ions and a cube of eight Cs+ ions at Cs4 [Cl−Cs4 = 3.655(24) Å] in the large cavities (Figure 8a). Such a cube of eight Tl+ ions was seen in Hf,Cl,Tl-A (section 4.2.1.3.2). For the same reason as before, the Cl− ions at Cl are exactly at y = z = 1/2. 4.2.2.2.3. Cs8HfCl66+ in the Cs11HfCl69+ Continuum. Each of the Cl− ions in Cs8HfCl66+ bridges via a Cs+ ion (Cs1) at the center of an 8-ring [Cl−Cs1 = 3.88(13) Å] to a Cl- ion of another Cs8HfCl66+ ion in an adjacent large cavity (adjacent unit cell) (Figure 8a) to form a three-dimensional continuum with unit cell formula Cs11HfCl69+. The Cs1−Cl bond length, 3.88(13) Å, appears to be somewhat longer than the sum of Cs+ and Cl− ionic radii,39 1.67 + 1.81 = 3.48 Å. 4.3. Charge Balance. As presented in Table 6, the sum of the charges of the extraframework ions in Hf,Cl,Tl-A is 8.1+ per unit cell. This diminished charge (reduced from 12.0+ in Na-A) is attributed to the dealumination of the zeolite framework, as had been found in Zr,Cl,Tl-A.10 As discussed in section 4.2.1.3, 7.3(6)% of the unit cells in Hf,Cl,Tl-A contain Tl14HfCl612+ H


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The Journal of Physical Chemistry C Table 6. Unit Cell Charge Budget

a

Extraframework ions. bOccupancy, ions per unit cell. cShortest M−O (metal ion to framework oxygen) bond lengths. dThe radii of the ions M were obtained by subtracting 1.32 Å (the conventional radius of the oxide ion) from the shortest M−O bond lengths. eCoordination numbers.

Figure 9. X-ray luminescence spectra of (a) Hf,Cl,Tl-A, (b) Hf,Cl,Cs,Na-A, (c) Zr,Cl,Tl-A, and (d) Zr,Cl,Cs,Na-A. The arbitrary units are the same for all spectra.

I


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Figure 10. Scintillation decay times for dehydrated (a) Hf,Cl,Tl-A, (b) Hf,Cl,Cs,Na-A, (c) Zr,Cl,Tl-A, and (d) Zr,Cl,Cs,Na-A upon irradiation with pulsed X-rays. Arbitrary units are used for the intensities.

(HfCl62−)3,45 {(CH3)2NCHSH}2[HfCl6],46 and Te6[HfCl6],47 respectively. The Hf4+−Cl− bond length in Hf,Cl,Cs,Na-A appears to be shortened because of the low covalency of the surrounding Cs+−Cl− bonds, whereas that in Hf,Cl,Tl-A is lengthened due to the high covalency of the surrounding Tl+Cl− bonds.

clusters. For the charge to be balanced in these latter unit cells, it is assumed that they have not been dealuminated. On the basis of the above, the compositions of two kinds of unit cells, unit cell 1 with the Tl14HfCl612+ cluster representing 7.3% of the unit cells and unit cell 2 without the cluster representing the remaining 92.7%, were calculated to match the crystal composition (weighted average of those two unit cells) observed crystallographically (Table 2). It can immediately be seen that the composition of unit cell 1, especially with regard to Hf and Cl, is similar to that obtained by SEM-EDX, which is a surface sensitive analysis technique (Table 2). This indicates that the unit cells 1 occupy the near surface volume of the crystal, as would be expected if the incoming HfCl4(g) molecules reacted with the zeolite upon arrival. The sum of extraframework charges in Hf,Cl,Cs,Na-A, 10.9+ (Table 6), could also indicate dealumination. (Alternatively, it remains possible that some Na+ ions, ca. 1.1 per unit cell, were not found crystallographically, as had been suggested for Zr,Cl,Cs,Na-A.11) Again, a comparison of the compositions of the two unit cells, unit cell 1 with the Cs11HfCl69+ cluster and unit cell 2 without it, with that obtained by SEM-EDX analysis (Table 2) indicates that the unit cells hosting the Cs11HfCl69+ continuum occupy the near surface volume of the crystal. 4.4. Comparison of Hf,Cl,Tl-A and Hf,Cl,Cs,Na-A. In both structures, HfCl62− ions center some of the large cavities 4 2 of zeolite A with the full symmetry, m 3̅ m = Oh, of that position. Their Hf−Cl bond lengths, however, appear to be quite different, 2.63(13) Å in Hf,Cl,Tl-A and 2.25(13) Å in Hf,Cl,Cs,Na-A. Hf4+−Cl− bond lengths shorter than the sum of their ionic radii, 2.59 Å,39 such as 2.446(11), 2.44(1), and 2.421(2) Å were reported for the HfCl62− ions of (Bi+)(Bi95+)-

5. LUMINESCENCE RESULTS 5.1. X-ray Induced Luminescence (XIL) Spectra. The XIL spectra of the dehydrated zeolites Hf,Cl,Tl-A and Hf,Cl,Cs,Na-A (the Hf-A’s) are broad bands ranging from 300 to 720 nm, peaking at 390 and 410 nm, respectively (Figure 9, panels a and b). Each spectrum has a secondary maximum at 490 nm, suggesting that either each has two kinds of luminescence centers or two different relaxation processes. When compared to the XIL spectra of Zr,Cl,Tl-A10 and Zr,Cl,Cs,Na-A11 (the Zr-A’s, Figure 9, panels c and d, remeasured for this report with more intense X-rays), their domain peaks have blue-shifted from 500 to 390 nm for Hf,Cl,Tl-A and from 495 to 410 nm for Hf,Cl,Cs,Na-A. This can be attributed to larger band gaps because the 5d energy levels in Hf4+ are higher than the 4d energy levels in Zr4+, although the luminescence in the Zr-A’s may occur by somewhat different mechanisms (section 5.3). The shorter wavelengths of the domain peaks from the Hf-A’s are very well matched to the best working range of an extremely sensitive commercial photomultiplier tube;9 such spectral matching is desirable for luminescent materials in scintillation counters.tment wit 5.2. XIL Decay. The XIL decay pulse shapes for the Hf-A’s and Zr-A’s were fit with a two-component exponential function, J


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5.3. Luminescence Mechanisms. The four zeolites, M,Cl,Tl-A and M,Cl,Cs,Na-A, M = Hf or Zr, appear to luminesce by different mechanisms. Four possible mechanisms are presented here. 5.3.1. Self-Trapped Exciton (STE) Mechanism in Hf,Cl,Tl-A and M,Cl,Cs,Na-A. The following mechanism was proposed for the luminescence of Cs2HfCl6 and Cs2ZrCl6.48,49 Ionizing radiation causes a Cl− 3p (valence band maximum) electron to be transferred to a t2g level of Hf4+ 5d in HfCl62− (Zr4+ 4d in ZrCl62−) (conduction band minimum): this is ligand-to-metal charge transfer. The resulting Cl0 atom in the valence band combines with a neighboring Cl− ion to form Cl2− which recaptures the electron from the Hf4+ 5d (Zr4+ 4d) conduction band to form a self-trapped exciton (STE), alternatively a Cl22− molecule in an excited state. Finally, this STE luminesces leaving behind two Cl− ions in their ground states. X-irradiated Cs2HfCl6 and Cs2ZrCl6 exhibit STE luminescences peaking at 398 and 435 nm, respectively.49 The luminescence peaks for Hf,Cl,Tl-A, Hf,Cl,Cs,Na-A, and Zr,Cl,Cs,Na-A at 390, 410, and 440 nm (Figure 9, panels a, b, and d, respectively) match those of Cs2HfCl6 (398 nm) and Cs2ZrCl6 (435 nm) very well.49 All three of these zeolites have MCl62− (M = Zr4+ or Hf4+) ions at the centers of their large cavities. There are no ZrCl62− ions in Zr,Tl,Cl-A, and it does not have a luminescence peak in that region (Figure 9c).10 Similarly, none of the following contained MCl62− ions nor luminesced upon X-irradiation: Tl-A, Cs,Na-A, ZrCl4(s), HfCl4(s), and the four physical mixtures of ZrCl4(s) or HfCl4(s) with Tl-A or Cs,Na-A.10,11 Thus, it appears that MCl62− is responsible for major peaks in the luminescence spectra of Hf,Cl,Tl-A, Hf,Cl,Cs,Na-A, and Zr,Cl,Cs,Na-A. 5.3.2. Charge Carrier Migration (CCM) to Tl+ in M,Cl,Tl-A. As discussed in section 5.3.1, the peak at 390 nm for Hf,Cl,Tl-A may be due to STE. However, its secondary peak at 490 nm and the primary peak of Zr,Cl,Tl-A at 500 nm were not seen for Cs2HfCl6 and Cs2ZrCl6.49 This indicates that one or more additional luminescence mechanisms may be operating in M,Cl,Tl-A. The Tl+ ion is generally understood to be the activator in alkali halide scintillators such as Tl+ doped CsI and NaI.7,9 This means that the Tl+ ion effectively combines with hot electrons (e−) and holes (h+) produced by the interaction of ionizing radiation with all parts of the target to form Tl0 and Tl2+ which relax via radiative and nonradiative transitions.8,9 The Tl+ ions in Hf,Cl,Tl-A and Zr,Cl,Tl-A appear to be acting as activators. Because the atomic numbers of Zr and Hf are greater than those of the atoms of the zeolite framework, their stopping power and their ability to generate hot electrons and holes should be greater. The zeolite, however, may act as a medium through which these e−’s and h+’s can travel to Tl+ ions to produce the observed luminescences. 5.3.3. Na0-Perturbed Excitons (NPE) in M,Cl,Cs,Na-A. As seen in section 5.3.2, the secondary peak in Hf,Cl,Tl-A (490 nm) and the primary peak in Zr,Cl,Tl-A (500 nm) may be due to activated Tl+ ions. Similar secondary emission peaks (at 490 and 495 nm) were found in Hf,Cl,Cs,Na-A and Zr,Cl,Cs,Na-A, respectively. Like the Tl+ ions in M,Cl,Tl-A (section 5.3.2), Cs+ and Na+ are located at extraframework positions in these two zeolites. The Na+ ion is also generally understood to be the activator in Na+-doped CsI (CsI(Na)).9,51 The luminescence of CsI(Na) was explained as “Na0-perturbed excitons (NPE)”.9 Upon Xirradiation, electrons in CsI(Na) localize mainly in the form of

Figure 11. Environment of the MCl62− ion in M,Cl,Cs,Na-A (M = Hf4+ and Zr4+): (a) MCl62− ions, each within a cube of eight Cs+ ions (Cs4) to form Cs8MCl66+ units, are bridged by Cs+ ions (Cs1) to form a three-dimensional continuum. (b) Fourteen Cs+ ions in and on the surface of the large cavity of M,Cl,Cs,Na-A bond to MCl62−. In addition, the ions at Cs4 bond to three framework oxygen atoms and those at Cs1 to four. Thus, each MCl62− ion is supported at its position of Oh symmetry at the center of the large cavity.

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 10). Although the XIL spectral shapes and peak positions for the Hf-A’s are very similar, their decay patterns are quite different. So, although the τ1 values are similar, 0.34 and 0.24 μs, respectively, the τ2 values are very different, 1.96 and 22.7 μs, respectively (Figure 10, panels a and b). The A2 component predominates for both, 82.4% for Hf,Cl,Tl-A and 97.1% for Hf,Cl,Cs,Na-A. Accordingly, most of the luminescence arises from the slower decay process (Figure 10, panels a and b). The Zr-A’s behaved similarly (Figure 10, panels c and d). Recently, there have been a few remarkable reports of the scintillation properties of the HfCl62− and ZrCl62− ions in Cs 2 HfCl 6 and Cs 2 ZrCl 6 . 48,49 Like Hf,Cl,Cs,Na-A and Zr,Cl,Cs,Na-A,11 their crystal structures are cubic (Fm3̅m, no. 225),50 and their MCl62− ions occupy sites of high symmetry 4 2 ( m 3̅ m = Oh), surrounded by a cube of eight Cs+ ions. (It is a cube of eight Tl+ ions in Hf,Cl,Tl-A.) Furthermore, the τ1 and τ2 components for Hf,Cl,Cs,Na-A (0.24 and 22.7 μs, respectively) and Zr,Cl,Cs,Na-A (0.16 and 22.2 μs) are quite different from those in Cs2HfCl6 (2.22 and 8.4 μs) and Cs2ZrCl6 (1.5 and 7.5 μs). It seems clear that the active centers are the HfCl62− and ZrCl62− ions, so these differences must be due to the different environments around them. Although all four of the MCl62− ions above are centered within a cube of eight Cs+ ions, the intrazeolitic MCl62− ions have additional Cs+ neighbors,49 they are members of their unique continuum (Figure 11a), and all Cs+ ions bond to three or four framework oxygen atoms (Figure 11b). When the XIL and UVIL decay times for Zr,Cl,Tl-A10 are compared, τ1 and A1 are both less for X-rays (0.43 μs, 11%) than for UV (0.92 μs, 47%). This is also true for Zr,Cl,Cs,NaA11: (0.16 μs, 4%) and (0.21 μs, 74%). At the same time, the τ2 and A2 values are both greater for X-rays (5.38 μs, 89%) than for UV (2.24 μs, 53%) for Zr,Cl,Tl-A and, similarly, (22.2 μs, 96%) and (6.08 μs, 26%) for Zr,Cl,Cs,Na-A. This means that the luminescence mechanisms induced by X-ray and UV radiations are at least somewhat different. The higher (than UV) energy X-rays may awaken a different luminescence center: the Tl+ ions in Zr,Cl,Tl-A and Na+ ions in Zr,Cl,Cs,NaA could be interacting with higher energy electrons (e−) and holes (h+). K


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transferred to the surrounding dopant ions, Eu2+, allowing them to be additional luminescence centers. The charge transfer processes from Hf4+-O2− and Zr4+-O2− could be similar, both emitting UV radiation which could be absorbed by surrounding luminescence centers, here Tl+. By this radiative mechanism, contact between the M4+ ions and the surrounding luminescence centers is not required. To test whether this mechanism is operating in M,Cl,Tl-A, those two zeolites were exposed to UV radiation. A UV wavelength, 254 nm, similar in energy to the M4+-O2− charge transfer was used, and the resulting UVIL spectra are shown in Figure 12. The peak positions and spectral shapes were very similar to those in the XIL spectra of M,Cl,Tl-A (Figure 9, panels a and c). In contrast, Hf,Cl,Cs,Na-A did not luminesce and Zr,Cl,Cs,Na-A luminesced only negligibly with a very weak peak at 480 nm. This strongly supports the radiative UV mechanism of internal energy transfer to the Tl+ ions in M,Cl,Tl-A. 5.4. Radiation Hardness and Proton Beam Induced Luminescence (PIL). During the 5 min that Hf,Cl,Tl-A was in the proton beam (section 2.6), the position of the peak near 500 nm remained unchanged, although its intensity decreased (Figure 13a). The peak near 400 nm decreased much more, and it moved to 450 nm. Its intensity became significantly less than that in the XIL spectrum (Figure 9a). For Hf,Cl,Cs,Na-A, the positions of both peaks (near 400 and 500 nm) remained unchanged, but the peak near 400 nm weakened while that near 500 nm strengthened (Figures 9b and 13b). These changes might be due to the loss of Cl− from HfCl62− to become HfCl5− or even HfCl4(g). Also, framework damage could affect the orientation and symmetry of the HfCl62− units. The integrated PIL light yield decreased for both samples during their time in the proton beam (Figure 13c). 5.5. Light Yield Comparison. The integrated light yields observed for the Hf,Cl,Tl-A and Hf,Cl,Cs,Na-A upon Xirradiation are about 93% and 40%, respectively, of those of the commercially available scintillator bismuth germanate (BGO) whose light yield is about 8000 photons/MeV9 (Figure 14). Although the integrated light yields of Hf,Cl,Tl-A, 7440 photons/MeV, and Hf,Cl,Cs,Na-A, 3200 photons/MeV, are lower than that of BGO, the efficiency of the spectral response is much higher (section 5.1). These integrated light yields could readily be increased by introducing more HfCl62− ions into the zeolite. The integrated light yields of Hf,Cl,Tl-A and Hf,Cl,Cs,Na-A are both about twice those seen with Zr,Cl,TlA and Zr,Cl,Cs,Na-A, and the efficiency of the spectral response toward the PMT is also improved.

Figure 12. Luminescence spectra of (a) Hf,Cl,Tl-A and (b) Zr,Cl,Tl-A upon UV excitation.

Vk centers, which are captured by surrounding Na+ ions. These Na+ ions became Na0 centers, and subsequently thermal migration occurred from Vk centers to Na0 centers. Finally, NPE (VkeNa0) emit luminescence. Similarly, NPE could be responsible for the secondary peaks at 490 nm for Hf,Cs,Na,Cl-A and 495 nm for Zr,Cs,Na,Cl-A. The Vk centers of the MCl62− ions, Cl2−, may take part in the process of energy transfer to surrounding Na+ ions in M,Cl,Cs,Na-A. Finally, NPE (Cl2−eNa0) could relax to emit at 490 and 495 nm, respectively. 5.3.4. Charge Transfer (CT) from M4+ to O2− in M,Cl,Tl-A. Unlike Hf,Cl,Cs,Na-A (this work) and Zr,Cl,Cs,Na-A11 (Figure 8), some Hf4+ ions occupy 6-ring sites in Hf,Cl,Tl-A, as do Zr4+ ions in Zr,Cl,Tl-A,10 where they bond to three framework oxygen atoms (Figure 5). Sometimes they bond to Cl− ions also, resulting in the creation of HfO32−, ZrO3Cl3−, or ZrO3Cl24− units (the metal ions and their immediate environments) in the zeolite (Figures 4 and 5).10 Recently, Wang et al. observed charge transfer (CT) luminescence from Eu-doped BaHfSi3O952 and BaZrSi3O9.53 They proposed that it could be due to the ion pairs Hf4+-O2− and Zr4+-O2− and suggested that it could be due to a radiative recombination of an excited electron in a 5d orbital of Hf4+ (4d orbital of Zr4+) with a hole in a 2p orbital of O2−. These radiative recombination energies, in the UV region, could be

6. DISCUSSION The two materials reported in this work, dehydrated Hf,Cl,Tl-A and Hf,Cl,Cs,Na-A, when matched with an appropriate photomultiplier tube could be used directly to detect ionizing radiation (UV, X-ray, etc.) and ionizing particle beams. Furthermore, it is expected that dehydrated Hf,Cl,Cs,Na-A would function as a self-luminous sensor for the detection of radioactive 137Cs atoms which would enter the zeolite and be the source of the ionizing radiation. The anhydrous redox reaction, to the extent that it proceeds, would be44 Hf,Cl,133Cs,Na‐A +

137

Cs → Hf,Cl,137Cs‐A +

133

Cs + Na

Similarly, it is expected that dehydrated Hf,Cl,Tl-A would be self-luminous for atoms of radioactive 201Tl, which would replace 204Tl in that zeolite. More generally, Hf,Cl,Tl-A should L


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Figure 14. Relative light yields for bismuth germanate (BGO) and six zeolite samples upon X-irradiation at 293 K.

allowing them to have greater stopping power and more luminescence centers. Upon hydration, M,Cl,Cs,Na-A and M,Cl,Tl-A all lost their ability to luminesce. It is expected that the hydrolysis of MCl62+ and M4+ in these zeolites, and therefore the erasure of their luminescence centers, is responsible. Also, the acid produced could have destroyed the zeolite framework. Accordingly, the original objective of this work has not been achieved: radioactive ions cannot exchange into the zeolite, not from the environment nor from aqueous solution. (Nor can they exchange into the zeolite from anhydrous solvents because ion exchange has been shown to fail from a series of strictly anhydrous solvents.54) This work will continue to search for luminescent zeolites that are stable in the presence of water, and thus be able to both remove radioactive ions from the environment by ion exchange while announcing their presence through self-luminosity. Such a zeolite should be self-luminous for any radioactive ion that can exchange into it, including 99m Tc, 99Mo, 67Ga, and 90Sr. It is anticipated that many luminescent zeolites can be prepared. They would have a variety of luminescence centers incorporated into a variety of zeolitic cavities and could be tailored to specific applications. Zeolites, because their composition can be easily varied by ion exchange, offer a unique opportunity for determining luminescence mechanisms.

Figure 13. Luminescence spectra of (a) Hf,Cl,Tl-A and (b) Hf,Cl,Cs,Na-A in a proton beam. (c) Decreasing integrated light yields of Hf,Cl,Tl-A and Hf,Cl,Cs,Na-A with dose (2 megagray/30 s). Arbitrary units are used for the intensities.

also be self-luminous for the detection of any radioactive atom capable of reducing intrazeolitic Tl+. Most generally, the dehydrated Hf-A’s should be self-luminous for any radioactive ion capable of exchanging into them. The results presented here may be unique to zeolite A because of the unique size of its large cavity and its capacity to have enough monovalent ions to support HfCl62− and ZrCl62− at positions of high symmetry. However, it can be expected that similar results would be found with some other zeolites. Zeolites with high ion exchange capacities should be better luminescent materials than those with lower ion-exchange capacities because they will be able to host more heavy ions, M


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7. SUMMARY Hf4+ was introduced into zeolite A by treating Tl-A and Cs,NaA with HfCl4(g) under anhydrous conditions. The crystal structures of Hf,Cl,Tl-A and Hf,Cl,Cs,Na-A were determined by single-crystal crystallography with compositional confirmation by SEM-EDX analyses. Their luminescence behavior upon X- and UV-irradiation were examined, as was their stability (and luminescence) in a proton beam. In Hf,Cl,Tl-A, some Hf4+ ions lie on 3-fold axes opposite 6rings and are 3-coordinate with three bonds to framework oxygen atoms. Octahedral HfCl62− ions center about 7% of the large cavities. In Hf,Cl,Cs,Na-A, HfCl62− also centers about 7% of the large cavities and are members of a Cs11HfCl69+ continuum. The remaining Tl+, Cs+, and Na+ ions occupy well-established cation sites. The X-ray luminescence spectra of Hf,Cl,Tl-A and Hf,Cl,Cs,Na-A, and the UV luminescence spectrum of Hf,Cl,Tl-A, all have broad bands between 300 and 720 nm, peaking at 390, 410, and 390 nm, respectively, with somewhat different decay characteristics. Four possible luminescence mechanisms are suggested: (1) self-trapped exciton luminescence of HfCl62−, (2) migration of charge carriers (hot electrons and holes) to activator ions, (3) luminescence through Na0-perturbed excitons, and (4) secondary energy transfer by UV radiation, arising from charge transfer from Hf4+ to O2−, to surrounding luminescence centers.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b05641. Observed and calculated structure factors squared with esds (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +82 53 950 5589. Fax: +82 53 950 6594. ORCID

Hong Joo Kim: 0000-0001-9787-4684 Nam Ho Heo: 0000-0003-4689-0080 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge the Photon Factory, High Energy Accelerator Research Organization, KEK, Tsukuba, Japan, and the Pohang Light Source, Korea, for the use of their diffractometers and computing facilities. We also gratefully acknowledge the Korea Institute of Radiological & Medical Sciences, Seoul, Korea, for the use of their cyclotron proton beam. This work was supported by a National Research Foundation of Korea (NRF) Grant NRF2014R1A2A1A11054075, funded by the Korean government (MSIP).

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