473

Sensors and Materials, Vol. 36, No. 2 (2024) 473–479
MYU Tokyo
S & M 3534

Photoluminescence and Scintillation Properties
of Eu:CaHfO3 Single Crystals
Yusuke Endo,* Kensei Ichiba, Daisuke Nakauchi,
Takumi Kato, Noriaki Kawaguchi, and Takayuki Yanagida
Division of Materials Science, Nara Institute of Science and Technology,
Takayama, Ikoma, Nara 630-0192, Japan(Received October 20, 2023; accepted January 23, 2024)

Keywords: scintillator, radiation detector, photoluminescence, CaHfO3

Eu:CaHfO3 single crystals with various Eu concentrations were synthesized by the floatingzone method, and their photoluminescence (PL) and scintillation properties were investigated.
All the samples showed PL peaks at 590, 620, and 650 nm, which were due to the 4f-4f transitionsof Eu3+. The PL quantum yield of 1.0% Eu:CaHfO3 was the highest at 42.2% among the samples.
The X-ray-induced scintillation spectra showed several emission peaks in the range of 400–690
nm, and the emissions were attributed to the 4f-4f transitions of Eu3+. Among the samples,
0.05% Eu:CaHfO3 showed the highest intensity. In the afterglow curves, the levels were within
the range of 1,000–10,000 ppm and those of previous studies.

1.

Introduction

Inorganic scintillators can absorb ionizing radiation with high energy to momentarily convert
it into photons with low energy.(1–3) Taking advantage of this feature, the scintillators have been
applied to radiation detectors by combining them with photodetectors that can convert thephotons into electrical signals.(4,5) The scintillation detectors are classified into current andphoton counting mode measurements depending on their application.(6,7) In particular, thedetectors for the current mode type integrate the signal for a period of milliseconds and havebeen used in applications such as X-ray computed tomography (CT) and flat panel detectors for
X-ray radiography.(8) The scintillators for the current mode type require properties such as highemission intensity, a large effective atomic number (Zeff), high density (ρ), and low afterglow
level (AL). However, since no scintillators satisfy all the required properties, new scintillators
have been developed.(9–14)
HfO2-based compounds such as RE2Hf 2O7 (RE = La, Gd, Lu) and AEHfO3 (AE = Ca, Sr, Ba)
have attracted attention as candidates for scintillators owing to their large Zeff and high ρ. In
previous reports on HfO2-based scintillators, only CaHfO3 with Zeff (65.2) and ρ (6.95 g/cm3)
showed a scintillation light yield above 10,000 photons/MeV.(15–21) Moreover, our research group
has investigated the scintillation properties of CaHfO3 doped with Ti, Ce, Pr, Tb, and Tm,(18,21–26)*Corresponding author: e-mail: endo.yusuke.ey5@ms.naist.jp

https://doi.org/10.18494/SAM4758

ISSN 0914-4935 © MYU K.K.
https://myukk.org/

474

Sensors and Materials, Vol. 36, No. 2 (2024)

and CaHfO3 doped with various rare-earth elements was found to show high luminescence
properties.
Eu3+ is used as a luminescence center for red scintillators, and the emission wavelength iswell matched with a Si-based photodetector. A representative Eu3+-doped material, Eu:(Y,Gd)2O3
ceramic, is used in a commercial scintillator for X-ray CT.(4,27–29) Furthermore, many Eu3+doped scintillators have been developed in previous studies.(30–38) Therefore, it is considered that
Eu3+ is useful for scintillators. However, the scintillation properties of Eu:CaHfO3 have not beenreported. In this study, Eu:CaHfO3 single crystals were synthesized by the floating zone (FZ)
method, and their photoluminescence (PL) and scintillation properties were investigated.

2.

Experimental Setup

An FZ furnace with four Xe lamps (Crystal Systems Corporation, FZ-T-12000-X-VPO-PCYH) was used to synthesize Eu:CaHfO3 single crystals.(16) The Eu concentrations were 0.01,
0.05, 0.1, 0.5, and 1.0 at% with respect to Ca. CaO (99.99%, Furuuchi Chemical), HfO2 (99.95%,
Rare Metallic), and Eu2O3 (99.99%, Furuuchi Chemical) were used as starting materials anduniformly blended using a mortar and pestle. To account for Ca evaporation during the crystal
growth, CaO was raised by 10% in relation to the stoichiometric composition.(18) The mixtures
were placed inside balloons to form cylindrical rods by applying hydrostatic pressure. The rods
with different Eu concentrations were subsequently calcined at 1400 °C for 8 h in air using anelectric furnace (Crystal Systems, BF-1800-IV-II), and the acquired rods were used forcrystallization. During the crystallization, the pull-down speed was 10 mm/h, and the upper andlower rotation rates were 23 rpm to the right and 20 rpm to the left, respectively. The synthesized
samples were split, and a grinder polisher (Buehler, MetaServ 250) was used to mechanicallypolish the surfaces.
A diffractometer (Rigaku, MiniFlex600) equipped with a Cu Kα micro-X-ray tube containing
a Be window was used to measure the powder X-ray diffraction (XRD) patterns of the samples
in the range of 2θ = 10–70°. Diffuse transmission spectra were measured using aspectrophotometer (Shimadzu, Solidspec-3700).
Three-dimensional PL excitation and emission spectra and quantum yield (QY) were
measured using Quantaurus-QY (Hamamatsu Photonics, C11347). The measurement ranges ofemission and excitation wavelengths were 300–800 nm and 250–400 nm, respectively.
Quantaurus-τ (Hamamatsu Photonics, C11367) was used to evaluate PL decay time profiles.
X-ray-induced scintillation spectra were measured using our original setup.(39) The X-rayinduced scintillation decay and afterglow curves were evaluated using our original measurement
system.(40)3.

Results and Discussion

Figure 1 shows a photograph of the 0.01, 0.05, 0.1, 0.5, and 1.0% Eu:CaHfO3 single crystals.
All the samples looked colorless and transparent. Additionally, the length, width, and thicknessof the 0.01, 0.05, 0.1, 0.5, and 1.0% Eu:CaHfO3 were approximately 4 × 4 × 0.75 mm3. Figure 2

Sensors and Materials, Vol. 36, No. 2 (2024)

Fig. 1. (Color online) Photograph of Eu:CaHfO3
single crystals.

475

Fig. 2. (Color online) Powder XRD patterns of
Eu:CaHfO3 single crystals and reference data.

shows the powder XRD patterns of the Eu:CaHfO3 single crystals and the reference data fromInternational Crystallographic Diffraction Data (ICDD) No. 36-1473. The diffraction peaks ofthe samples were observed at the same positions as those of the reference data, and no impurity
phases were detected. Hence, the synthesized samples had a single-phase structure of
CaHfO3.(41,42)
Figure 3 shows the diffuse transmission spectra of Eu:CaHfO3 in the wavelength range of
200 to 850 nm. In the 300–850 nm range, Eu:CaHfO3 showed the maximum transmittance atapproximately 70–80%, and the transmittances of the 0.05 and 0.5% doped samples wereslightly lower than those of the other samples. The decrease in the transmittance arose from thepresence of cracks, as shown in Fig. 1. Absorption attributed to the charge transfer of Eu3+–O2−
was observed in all the samples at wavelengths shorter than 300 nm.(43)
Figure 4 shows the three-dimensional PL excitation and emission spectrum of the 1.0%
Eu:CaHfO3 as a typical sample. This Eu:CaHfO3 showed the three emission peaks at 590, 620,
and 690 nm originating from the 4f-4f transitions of Eu3+.(44,45) A similar trend was observed inother samples. The PL QYs in the range of 550–750 nm under the 300 nm excitation wavelengthof 0.01, 0.05, 0.1, 0.5, and 1.0% Eu:CaHfO3 were 1.5, 4.6, 12.2, 32.6, and 42.2%, respectively.
The increase in PL QY is attributed to an increase in Eu concentration, and no concentrationquenching was observed in 0.01, 0.05, 0.1, 0.5, and 1.0% Eu:CaHfO3.
Figure 5 shows the PL decay time profiles of 0.01, 0.05, 0.1, 0.5, and 1.0% Eu:CaHfO3 single
crystals, where the dashed lines indicate the fitted curves. All the decay curves of the samples
were well approximated by a single-exponential function. Each obtained PL decay time constant
was 0.96–0.99 ms, which was the range of typical values of the Eu3+-doped materials.(46,47)
Hence, the emission of the samples originated from the 4f–4f transitions of Eu3+.
Figure 6 shows the X-ray-induced scintillation spectra of Eu:CaHfO3. All the Eu:CaHfO3
samples showed several emission peaks at 420 nm (5D3–7F1), 430 nm (5D3–7F2), 450 nm (5D3–476

Sensors and Materials, Vol. 36, No. 2 (2024)

Fig. 3. (Color online) Diffuse transmission spectraof Eu:CaHfO3.

Fig. 4. (Color online) Th ree-dimensional PL
excitation and emission spectrum of 1.0% Eu:CaHfO3.

Fig. 5. (Color on l i ne) PL de cay cu r ve s of
Eu:C a H f O 3 . T he mon it or e d a nd excit at ion
wavelengths were 620 and 300 nm, respectively.

Fig. 6. (Color online) X-ray-induced scintillation
spectra of Eu:CaHfO3.

7F ), 470 nm (5D –7F ), 490 nm (5D –7F ), 510 nm (5D –7F ), 530 nm (5D –7F ), 580 nm (5D –
3
2
0
2
2
2
3
2
4
0
7F ), 590 nm (5D –7F ), 620 nm (5D –7F ), 650 nm (5D –7F ), and 690 nm (5D –7F ) owing to the
0
0
1
0
2
0
3
0
4
4f-4f transitions of Eu3+.(33,35,36,48) In addition, the broad emission band at around 400 nm in

0.01% Eu:CaHfO3 was derived from the host material.(22–25) The highest intensity was observed
in 0.05% Eu:CaHfO3, and the trends in PL QY and the scintillation intensity were not consistent.
Generally, the scintillation intensity depends on PL QY and the energy migration efficiency fromthe host to emission centers.(49) Therefore, the energy migration efficiency was more dominant
than PL QY in affecting the scintillation properties of Eu:CaHfO3.

Sensors and Materials, Vol. 36, No. 2 (2024)

477

Figure 7 shows the X-ray-induced scintillation decay time profiles of Eu:CaHfO3, and thedashed lines show the fitted curves. The obtained decay curves of Eu:CaHfO3 were approximatedby a sum of two exponential components except for an instrumental response function (IRF).
The first and second components, which were 0.80–1.1 ms and 2.8–3.0 ms, would be attributed
to the host emission and the 4f–4f transitions of Eu3+,(21,43,50,51) respectively.
Figure 8 shows the afterglow curves of Eu:CaHfO3 samples. AL is defined as follows: AL[ppm] = 106 × (I2 – Ibg)/(I1 – Ibg), where Ibg, I1, and I2 are the mean signal intensity in the
background, with X-ray irradiation, and at t = 20 ms after irradiation, respectively. The AL
values of 0.01, 0.05, 0.1, 0.5, and 1.0% Eu:CaHfO3 were 10000, 3800, 4100, 1100, and 1000 ppm
with a typical error of 8%, respectively. The obtained values were comparable to those of
CaHfO3 crystals doped with other dopant ions.(18,22–24,26) Since general commercial scintillatorssuch as CdWO4 and Bi4Ge3O12 have an AL value of around 10 ppm,(40) the AL value of
Eu:CaHfO3 was quite high.

4.

Conclusions

Eu:CaHfO3 single crystals were synthesized by the FZ method, and their PL and scintillation
properties were investigated. PL QY was the highest at 42.2% in 1.0% Eu:CaHfO3 under theexcitation and emission wavelengths of 300 and 550–750 nm, respectively. The value was higher
than those of Ce- and Pr-doped CaHfO3 (32.6% and 6.8%,(18,24) respectively). The scintillation
spectra showed several emission peaks from 400 to 690 nm originating from the 4f-4f transitionsof Eu3+. 0.05% Eu:CaHfO3 showed the highest emission intensity among the present samples.
0.01% Eu:CaHfO3 showed the lowest AL value of 1,000 ppm among the samples, and this value
was higher than those of Ce- and Pr-doped CaHfO3 (666 and 228 ppm,(18,24) respectively). The
results obtained in this study showed that the material properties of the Eu:CaHfO3 single crystal
were inadequate for practical use. On the other hand, previous studies showed that CaHfO3 has
excellent scintillation properties.(21,26) Hence, the scintillation properties when using dopants
other than Eu and the rare-earth elements considered in previous studies should be evaluated.

Fig. 7. (Color online) X-ray-induced scintillation
decay time profiles of Eu:CaHfO3.

Fig. 8. (Color on l i ne) A f te rglow cu r ves of
Eu:CaHfO3 with 2 ms X-ray irradiation.

478

Sensors and Materials, Vol. 36, No. 2 (2024)

Acknowledgments
This work was supported by Grants-in-Aid for Scientific Research A (22H00309), Scientific
Research B (22H03872, 22H02939, 21H03733, and 21H03736), Early-Career Scientists(23K13689), and Challenging Exploratory Research (22K18997) from the Japan Society for the
Promotion of Science. The Cooperative Research Project of the Research Center for Biomedical
Engineering, A-STEP from JST, Kazuchika Okura Memorial Foundation, Asahi GlassFoundation, Nakatani Foundation, and Konica Minolta Science and Technology Foundation are
also acknowledged.

References
1 S. E. Derenzo, M. J. Weber, E. Bourret-Courchesne, and M. K. Klintenberg: Nucl. Instrum. Methods Phys.
Res., Sect. A 505 (2003) 111. https://doi.org/10.1016/S0168-9002(03)01031-3
2 A. Lempicki, A. J. Wojtowicz, and E. Berman: Nucl. Instrum. Methods Phys. Res., Sect.A 333 (1993) 304.
https://doi.org/10.1016/0168-9002(93)91170-R
3 T. Yanagida, T. Kato, D. Nakauchi, and N. Kawaguchi: Jpn. J. Appl. Phys. 62 (2023) 010508. https://doi.
org/10.35848/1347-4065/ac9026
4 C. Greskovich and S. Duclos: Annu. Rev. Mater. Sci. 27 (1997) 69. https://doi.org/10.1146/annurev.
matsci.27.1.69
5 G. Blasse: Chem. Mater. 6 (1994) 1465. https://doi.org/10.1021/cm00045a002
6 T. Hayashi, K. Ichiba, D. Nakauchi, K. Watanabe, T. Kato, N. Kawaguchi, and T. Yanagida: J. Lumin. 255(2023) 119614. https://doi.org/10.1016/j.jlumin.2022.119614
7 M. J. Willemink, M. Persson, A. Pourmorteza, N. J. Pelc, and D. Fleischmann: Radiology 289 (2018) 293.
https://doi.org/10.1148/radiol.2018172656
8 A. Nassalski, M. Kapusta, T. Batsch, D. Wolski, D. Mockel, W. Enghardt, and M. Moszynski: IEEE Nucl. Sci.
Symp. Conf. Record (2005) 2823. https://doi.org/10.1109/NSSMIC.2005.1596921
9 T. Kunikata, T. Kato, D. Shiratori, P. Kantuptim, D. Nakauchi, N. Kawaguchi, and T. Yanagida: Sens. Mater.
35 (2023) 491. https://doi.org/10.18494/SAM4145
10 Y. Takebuchi, D. Shiratori, T. Kato, D. Nakauchi, N. Kawaguchi, and T. Yanagida: Sens. Mater. 35 (2023) 507.
https://doi.org/10.18494/SAM4142
11 P. Kantuptim, T. Kato, D. Nakauchi, N. Kawaguchi, K. Watanabe, and T. Yanagida: Sens. Mater. 35 (2023) 451.
https://doi.org/10.18494/SAM4141
12 D. Shiratori, H. Fukushima, D. Nakauchi, T. Kato, N. Kawaguchi, and T. Yanagida: Sens. Mater 35 (2023) 439.
https://doi.org/10.18494/SAM4140
13 N. Kawaguchi, K. Watanabe, D. Shiratori, T. Kato, D. Nakauchi, and T. Yanagida: Sens. Mater. 35 (2023) 499.
https://doi.org/10.18494/SAM4136
14 K. Yamabayashi, K. Okazaki, D. Nakauchi, T. Kato, N. Kawaguchi, and T. Yanagida: Radiat. Phys. Chem. 214(2024) 111292. https://doi.org/10.1016/j.radphyschem.2023.111292
15 K. Fiaczyk, A. J. Wojtowicz, and E. Zych: J. Phys. Chem. C 119 (2015) 5026. https://doi.org/10.1021/jp512685u
16 D. Nakauchi, G. Okada, N. Kawaguchi, and T. Yanagida: Jpn. J. Appl. Phys. 57 (2018) 100307. https://doi.
org/10.7567/JJAP.57.100307
17 D. Nakauchi, T. Kato, N. Kawaguchi, and T. Yanagida: Sens. Mater. 32 (2020) 1389. https://doi.org/10.18494/
SAM.2020.2751
18 H. Fukushima, D. Nakauchi, T. Kato, N. Kawaguchi, and T. Yanagida: J. Lumin. 250 (2022) 119088. https://doi.
org/10.1016/j.jlumin.2022.119088
19 E. V. van Loef, W. M. Higgins, J. Glodo, C. Brecher, A. Lempicki, V. Venkataramani, W. W. Moses, S. E.
Derenzo, and K. S. Shah: 2006 IEEE Nucl. Sci. Symp. Conf. Record (2006) 1538. https://doi.org/10.1109/
NSSMIC.2006.354191
20 H. Fukushima, D. Nakauchi, G. Okada, T. Kato, N. Kawaguchi, and T. Yanagida: J. Alloys Compd. 934 (2023)
167929. https://doi.org/10.1016/j.jallcom.2022.167929
21 Y. Endo, K. Ichiba, D. Nakauchi, H. Fukushima, K. Watanabe, T. Kato, N. Kawaguchi, and T. Yanagida: Solid
State Sci. 145 (2023) 107333. https://doi.org/10.1016/j.solidstatesciences.2023.107333

Sensors and Materials, Vol. 36, No. 2 (2024)

479

22 H. Fukushima, D. Nakauchi, G. Okada, N. Kawaguchi, and T. Yanagida: J. Mater. Sci. Mater. Electron. 29(2018) 21033. https://doi.org/10.1007/s10854-018-0249-9
23 H. Fukushima, D. Nakauchi, N. Kawaguchi, and T. Yanagida: Jpn. J. Appl. Phys. 58 (2019) 052005. https://doi.
org/10.7567/1347-4065/ab116c
24 H. Fukushima, D. Nakauchi, N. Kawaguchi, and T. Yanagida: J. Ceram. Process. Res. 20 (2019) 211. https://doi.
org/10.36410/jcpr.2019.20.3.211
25 H. Fukushima, D. Nakauchi, T. Kato, N. Kawaguchi, and T. Yanagida: Radiat. Meas. 133 (2020) 106280.
https://doi.org/10.1016/j.radmeas.2020.106280
26 H. Fukushima, D. Nakauchi, T. Kato, N. Kawaguchi, and T. Yanagida: Sens. Mater. 35 (2023) 429. https://doi.
org/10.18494/SAM4139
27 M. Cao, J. Xu, C. Hu, H. Kou, Y. Shi, H. Chen, J. Dai, Y. Pan, and J. Li: Ceram. Int. 43 (2017) 2165. https://doi.
org/10.1016/j.ceramint.2016.10.198
28 P. Lecoq: Nucl. Instrum. Methods Phys. Res., Sect. A 809 (2016) 130. https://doi.org/10.1016/j.nima.2015.08.041
29 D. Cavouras, I. Kandarakis, G. S. Panayiotakis, E. K. Evangelou, and C. D. Nomicos: Med. Phys. 23 (1996)
1965. https://doi.org/10.1118/1.597769
30 X.-Y. Sun, Y. Liu, X.-L. Liu, R.-P. Cao, Y.-N. Li, and L.-W. Lin: Opt. Mater. 36 (2014) 1478. https://doi.
org/10.1016/j.optmat.2014.03.044
31 Y. Shi, Q. W. Chen, and J. L. Shi: Opt. Mater. 31 (2009) 729. https://doi.org/10.1016/j.optmat.2008.04.017
32 J. Fu, M. Kobayashi, S. Sugimoto, and J. M. Parker: Mater. Res. Bull. 43 (2008) 1502. https://doi.org/10.1016/j.
materresbull.2007.06.024
33 W. Huang, Y. Li, J. Chen, Y. Zhao, L. Chen, and H. Guo: Ceram. Int. 49 (2023) 8863. https://doi.org/10.1016/j.
ceramint.2022.11.042
34 S. Matsumoto, T. Watanabe, and A. Ito: Sens. Mater. 34 (2022) 669. https://doi.org/10.18494/SAM3698
35 C. Mansuy, J. M. Nedelec, C. Dujardin, and R. Mahiou: Opt. Mater. 29 (2007) 697. https://doi.org/10.1016/j.
optmat.2005.10.017
36 C. LeLuyer, M. Villanueva-Ibañez, A. Pillonnet, and C. Dujardin: J. Phys. Chem. A 112 (2008) 10152. https://doi.org/10.1021/jp803339n
37 F. Khrongchaiyaphum, N. Wantana, S. Kansirin, P. Pakawanit, N. Vittayakorn, S. Kothan, N. Chanthima, H. J.
Kim, and J. Kaewkhao: Optik 292 (2023) 171355. https://doi.org/10.1016/j.ijleo.2023.171355
38 S. Chen, Z. Wen, X. Peng, G. A. Ashraf, R. Wei, T. Pang, and H. Guo: Ceram. Int. 48 (2022) 947. https://doi.
org/10.1016/j.ceramint.2021.09.179
39 T. Yanagida, K. Kamada, Y. Fujimoto, H. Yagi, and T. Yanagitani: Opt. Mater. 35 (2013) 2480. https://doi.
org/10.1016/j.optmat.2013.07.002
40 T. Yanagida, Y. Fujimoto, T. Ito, K. Uchiyama, and K. Mori: Appl. Phys. Express 7 (2014) 062401. https://doi.
org/10.7567/APEX.7.062401
41 A. Feteira, D. C. Sinclair, K. Z. Rajab, and M. T. Lanagan: J. Am. Ceram. Soc. 91 (2008) 893. https://doi.
org/10.1111/j.1551-2916.2007.02134.x
42 Y. M. Ji, D. Y. Jiang, Z. H. Wu, T. Feng, and J. L. Shi: Mater. Res. Bull. 40 (2005) 1521. https://doi.org/10.1016/j.
materresbull.2005.04.026
43 T. Kunikata, K. Watanabe, P. Kantuptim, K. Ichiba, D. Shiratori, T. Kato, D. Nakauchi, N. Kawaguchi, and T.
Yanagida: Jpn. J. Appl. Phys. (2023). https://doi.org/10.35848/1347-4065/acfb16
44 K. Qin, J. Sun, X. Zhu, F. Cao, W. Liu, X. Shen, X. Wu, and Z. Wu: J. Lumin. 233 (2021) 117920. https://doi.
org/10.1016/j.jlumin.2021.117920
45 H. Fukushima, D. Nakauchi, T. Kato, N. Kawaguchi, and T. Yanagida: J. Lumin. 223 (2020) 117231. https://doi.
org/10.1016/j.jlumin.2020.117231
46 A. Maurya, A. Bahadur, and S. B. Rai: J. Lumin. 203 (2018) 714. https://doi.org/10.1016/j.jlumin.2018.07.025
47 G. A. Gusev, K. N. Orekhova, V. A. Kravets, A. I. Isakov, A. N. Trofimov, and M. V. Zamoryanskaya: J. Lumin.
222 (2020) 117084. https://doi.org/10.1016/j.jlumin.2020.117084
48 T. Yanagida: Proc. Jpn. Acad. Ser. B 94 (2018) 75. https://doi.org/10.2183/pjab.94.007
49 Q. Liu, Y. Yuan, J. Li, J. Liu, C. Hu, M. Chen, L. Lin, H. Kou, Y. Shi, W. Liu, H. Chen, Y. Pan, and J. Guo:
Ceram. Int. 40 (2014) 8539. https://doi.org/10.1016/j.ceramint.2014.01.068
50 A. P. de Moura, L. H. Oliveira, I. C. Nogueira, P. F. S. Pereira, M. S. Li, E. Longo, J. A. Varela, and I. L. V.
Rosa: Adv. Chem. Eng. Sci. 04 (2014) 374. https://doi.org/10.4236/aces.2014.43041
51 G. Gao, N. Da, S. Reibstein, and L. Wondraczek: Opt. Express 18 (2010) A575. https://doi.org/10.1364/
OE.18.00A575