Photon Factory Activity Report 2017 #35 (2018)

BL-5A/2016G677

Exchange of Tetrapositive Cations into Zeolites for Zeolite-Based
Inorganic scintillators. Crystal Structures and Scintillation Properties
of Hf4+- and Zr4+-Containing LTA
Joon Young Kim1, Hyeon Seung Lim1, Gi Beom Park1, So Yeon Yoon1, Nam Ho Heo1,*,
and Karl Seff2
1
Department of Applied Chemistry, School of Applied Chemical Engineering, Kyungpook National
University, Daegu, 41566, Korea
2
Department of Chemistry, University of Hawaii, 2545 The Mall, Honolulu, Hawaii, 96822, U. S. A.
1 Introduction
Numerous new inorganic scintillator materials haveappeared in the last several decades. They have beenstudied for use in precision calorimetry in high-energy
physics, medical imaging, and homeland security.
Recently, Saeki et al. completed an extensive andcomparative study of the scintillation properties of
Cs2ZrCl6 and Cs2HfCl6 crystals.[1] High atomic number
ions such as Zr4+ and Hf4+ are important for inorganic
scintillation because they serve as effective X-ray antenna
by absorbing X-ray photons to produce fast electrons by
the photoelectric effect.[2] Zeolites containing these ionsmight be exceptional and tunable scintillators.
Unfortunately, it is difficult to introduce tetrapositive
cations into zeolites by conventional liquid-phase ionexchange (LPIE). Because the vapor pressures of HfCl4
and ZrCl4 are significant at a moderate temperature, it
was hoped that Hf4+ and Zr4+ could be introduced into
zeolites by the vapor-phase ion-exchange (VPIE) method.
2 Experiment
Fully dehydrated Tl12-A and Cs7Na5-A were treatedeither with HfCl4 (g, 7.9 x 103 Pa) (crystals a and b) orwith ZrCl4 (g, 3.7 x 103 Pa) (crystals c and d) at 523 K
under anhydrous conditions. The product crystals were
all milky white. Their crystal structures were determinedby single-crystal crystallography using synchrotron Xradiation and their compositions were confirmed by
SEM-EDX analyses. Their structures were refined in thespace group Pm 3 m (a = 12.065(2), 12.248(2), 12.215(1),and 12.230(1) Å) with all unique data to the final error
indices R1 = 0.072, 0.049, 0.47, and 0.069, respectively.
Their luminescence properties upon X-irradiation were
explored. CCD images of the four crystals were obtainedusing synchrotron X-radiation at the Pohang Accelerator
Laboratory (Figure 1).
The X-ray induced luminescence spectra of similarly
prepared dehydrated powder samples were recorded at
293 K with a Flame-T spectrometer (Ocean optics, 50 kV,
30 mA). The scintillation decay times were measured bydirectly coupling the vessel containing each powder to theentrance window of the photomultiplier tube (PMT,
H6610). Each sample was excited with a XR200 pulsed
X-ray beam.

Fig. 1: CCD images of Zr4+ and Hf4+ containing LTA
during exposure to synchrotron X-radiation (17.7 keV,
350 mA).
3 Results and Discussion
In M,Cl,Tl-A,[3,4] M = Hf and Zr, some M4+ ions are
3-coordinate with three framework oxygen atoms of 6rings. In Hf,Cl,Tl-A, octahedral HfCl62- ions are at thevery centers of 7% of the large cavities within
Tl14HfCl612+ clusters (Figure 2a). MCl62- ions are also
seen in Hf,Cl,Cs,Na-A[3] Zr,Cl,Cs,Na-A[5]; they occupy
7% and 14% of the large cavities and are members of
Cs11MCl69+ continua, respectively (Figure 2b). The
remaining Tl+, Cs+, and Na+ ions occupy well-establishedcation sites.
The X-ray induced luminescence spectra of thedehydrated zeolites Hf,Cl,Tl-A, Hf,Cl,Cs,Na-A, Zr,Cl,TlA, and Zr,Cl,Cs,Na-A are broad bands ranging from 300
to 720 peaking at 390, 410, 500, and 495 nm, respectively(Figure 2c). Although the X-ray induced luminescence
spectral shapes and peak positions for the M,Cl,Tl-A and
M,Cl,Cs,Na-A are similar, their decay patterns are quite
different. The four zeolites, M,Cl,Tl-A and M,Cl,Cs,NaA, appear to luminesce by different mechanisms. Fourpossible mechanisms are recognized for their scintillation
processes.
They are (1) self-trapped exciton
luminescence of MCl62-, (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

Photon Factory Activity Report 2017 #35 (2018)

charge transfer from M4+ to O2-, to surrounding
luminescence centers.

[5] C. W. Lee, J. Y. Kim, H. J. Kim, N. H. Heo and K.
Seff, J. Phys. Chem. C, 120 (2016) 18682.
[6] P. A. Rodnyi, Physical Processes in Inorganic
Scintillators, M. J. Weber, Ed.; CRC Press: Boca Raton,
FL, 1997; pp 6-7.
* nhheo@knu.ac.kr

Fig. 2: (a) HfCl62- in Hf,Cl,Tl-A, (b) Cs11MCl69+ contina
in M,Cl,Cs,Na-A, and (c) X-ray luminescence spectra andscintillation decay times of Hf4+ and Zr4+-containing LTA.
The integrated light yields observed for M,Cl,Tl-A and
M,Cl,Cs,Na-A powders are about 20% to 93% that ofbismuth germanate, a commercially available scintillator
whose light yield is well documented.[6] Under the samemeasurement conditions, Tl+- and Cs+,Na+-exchanged
zeolite LTA showed no radioluminescence.
Acknowledgement
We gratefully acknowledge the Photon Factory, HighEnergy Accelerator Research Organization, KEK,
Tsukuba, Japan for the use of their synchrotron facilities
including diffractometer and computing systems.
References[1] K. Saeki, Y. Fujimoto, M. Koshimizu, T. Yanagida
and K. Asai, Appl. Phys. Express, 9 (2016) 042602.
[2] C. Wang, O. Volotskova, K. Lu, M. Ahmad, C. Sun, L.
Xing and W. Lin, J. Am. Chem. Soc., 136 (2014) 6171.
[3] J. Y. Kim, H. J. Kim, N. H. Heo and K. Seff, J. Phys.
Chem. C, 121 (2017) 19619.
[4] J. Y. Kim, J. M. Park, H. J. Kim, N. H. Heo and K.
Seff, J. Phys. Chem. C, 119 (2015) 18326.