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Sensors and Materials, Vol. 36, No. 2 (2024) 489–497
MYU Tokyo
S & M 3536

Photoluminescence and Scintillation Properties
of Ce-doped BaHfO3 Crystals
Hiroyuki Fukushima,1* Daisuke Nakauchi,2 Takumi Kato,2
Noriaki Kawaguchi,2 and Takayuki Yanagida2
1Department of Electrical and Electronic Engineering, National Institute of Technology, Fukui College,

Geshi, Sabae, Fukui 916-8507, Japan

2Division of Materials Science, Nara Institute of Science and Technology,

8916-5 Takayama-cho, Ikoma, Nara 630-0192, Japan(Received October 31, 2023; accepted January 22, 2024)

Keywords: scintillation, photoluminescence, BaHfO3, floating zone

The photoluminescence (PL) and scintillation properties of Ce-doped BaHfO3 crystals were
investigated. The Ce-doped BaHfO3 crystal is transparent and colorless after annealing
treatment under a reduction condition. The broad luminescence band at ~370 nm was observed
under ~280 nm excitation with the decay time of ~17 ns. The X-ray-induced scintillation spectra
also showed the broad luminescence band at ~380 nm with a fast decay. The origin of the broad
luminescence band at ~380 nm is attributed to the 5d–4f transition of Ce3+. The pulse height
distribution under 137Cs gamma ray irradiation using Ce-doped BaHfO3 exhibited a
distinguishable photoabsorption peak. The light yield of Ce-doped BaHfO3 is ~1600 photons/
MeV.

1.

Introduction

Ionizing radiation detection is conducted by two measurement methods. One is a direct
measurement using, for example, a Ge semiconductor detector. The other is an indirect
measurement using, for example, a scintillation detector. A scintillation detector uses a
scintillator, which generates luminescence under ionizing radiation irradiation. The generated
luminescence is detected by a photodetector such as a photomultiplier tube (PMT) and aphotodiode. For example, X-ray images are taken using a scintillation detector,(1) and positronemission tomography also uses many scintillators.(2) The typically required properties for
scintillators are a high light yield and a fast decay time. Moreover, there are some requirements
for density and effective atomic number for some applications. In particular, a high density andeffective atomic number of materials are important for the detection of X- and gamma rays.(3)Scintillators with a fast decay are realized by luminescence from the 5d–4f transition of Ce3+ or
Pr3+.(4) Thus, scintillation properties have been investigated in Ce- or Pr-doped dense materials
to develop a novel X- or gamma-ray scintillator.(5–12) Moreover, the scintillation properties of*Corresponding author: e-mail: fukushima@fukui-nct.ac.jp

https://doi.org/10.18494/SAM4762

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

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many materials such as single crystals,(13–22) ceramics,(23–27) and glasses(28–37) have been
investigated.
MHfO3 (M = Ca, Sr, or Ba) have high density and effective atomic number. The scintillation
properties of many Ce-doped MHfO3 have been investigated.(38–42) The light yield of a Cedoped CaHfO3 single crystal, which has a density of 6.9 g/cm3, is ~7800 photons/MeV with afast decay of ~22 ns.(43) The light yield of Ce-doped CaHfO3 is increased by Mg substitution.(44)The density of a SrHfO3 single crystal is 7.6 g/cm3, and the light yield is ~400 photons/MeV.(45)BaHfO3 has the highest density of 8.3 g/cm3 among the MHfO3 series. The scintillation
properties of Ce-doped BaHfO3 transparent ceramics have been investigated.(46) In this work,
the integrated light yield was investigated; however, there is no report on the light yield of Cedoped BaHfO3 determined from the pulse height distribution.
In this study, Ce-doped BaHfO3 crystals were synthesized by the floating zone (FZ) method,
and the photoluminescence (PL) and X-ray-induced scintillation spectra and decay curves were
measured. Finally, the pulse height distribution under 137Cs gamma-ray irradiation wasinvestigated to estimate the quantitative light yield of Ce-doped BaHfO3.

2.

Materials and Methods

Ce-doped BaHfO3 crystals were synthesized by the FZ method. Four xenon lamps as aheating source were installed in the FZ instrument. The details of the equipment are found in
Ref. 47. The feed rods were synthesized by a simple sintering method. The starting materials,
namely, BaCO3 (99.99%, Rera Metallic), HfO2 (99.95%, Furuuchi Chemical), and CeO2 (99.99%,
Furuuchi Chemical), were mixed using a mortar and pestle. The mixed powder was sintered at
1200 °C for 8 h. The sintered powder was remixed and further sintered at 1400 °C for 8 h. The
resintered powder was hydrostatically pressed into a cylindrical rod and again sintered at 1500
°C for 8 h. The amount of BaCO3 in this powder was increased by 30% from the stoichiometriccomposition of BaHfO3 to compensate for the volatilization of Ba during the crystal growth. The
pulling-down rate was 50 mm/h, and the rotation rates of the upper and lower shafts were 28 and25 rpm, respectively. The synthesized crystals were treated under a reduction atmosphere to
suppress the formation of Ce4+ ions. The crystals were put into graphite powder under vacuum.
The heating temperature was 800 °C, and the treatment time was 96–144 h. The treatment time
was determined on the basis of the appearance of the samples.
The measurement range of the X-ray diffraction (XRD) patterns was 2θ = 10 to 70 degrees(Rigaku, Miniflex600), and the X-ray source was CuKα. The PL excitation and emission map
and quantum yield (QY) were measured using the Hamamatsu Photonics C11347 spectrometer.
The PL decay curves were also measured using the Hamamatsu Photonics C11367 spectrometer.
The X-ray-induced scintillation spectra, decay curves, and afterglow curves were measured withoriginal setups.(48,49) The voltage and current applied to the X-ray tube for the scintillation
spectra were 40 kV and 1.2 mA, respectively. The pulse height distribution under 137Cs gammaray irradiation was measured using a laboratory-made setup.(48) The PMT used was of the ultra–bi-alkali type (Hamamatsu Photonics, R7600-200).

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Results and Discussion

A photograph of the synthesized samples is shown in Fig. 1. The as-synthesized samples werebrown. The 1 and 5% samples were transparent, and the 0.3 and 3% samples were opaque. The
0.3 and 1% samples were black because some graphite powders were inside the sample. The 3%
sample was also partially black for the same reason. The 5% sample was orange. The assynthesized sample was brown; thus, the treatment time or temperature of the 5% sample wasinsufficient compared with those of the other samples. To investigate the absorption wavelength,
diffuse transmission spectra were measured, and the results are shown in Fig. 2. Absorption
bands were observed at ~250 and 300 nm in all the samples. In contrast, the 5% sample showed a
deep absorption band at ~450–550 nm compared with the 1% sample. The 5% sample wasorange, as shown in Fig. 1, because the absorption band was at ~450–550 nm. The transmittanceof the 0.3% sample was the lowest among the samples, and the black color contributes to the lowtransmittance.
Figure 3 shows the XRD patterns of the synthesized samples and references. All the samples
included BaHfO3 (ICDD. 24-0102) and monoclinic HfO2 (COD. 9013470). The melting point of
BaHfO3 is quite high, and the vapor pressure of Ba is also high; thus, Ba was volatilized during
crystal synthesis, and HfO2 remained in the samples. The XRD peak position did not seem toshift. The ionic radii of Ba2+ (1.61 Å in twelve-coordination), Hf4+ (0.71 Å in six-coordination),
Ce3+ (1.34 Å in twelve-coordination and 1.01 Å in six-coordination), and Ce4+ (1.14 Å in twelvecoordination and 0.87 Å in six-coordination) are not the same in the same coordination
number.(50) The doped Ce ions were considered to be incorporated into the Ba or Hf sites; thus,
the peak shift was quite small.
Figure 4 shows the PL excitation and emission map of the 1% sample as the representative
sample because the other samples also exhibit similar spectra but with a different intensity, andFig. 1. (Color online) Photograph of as-synthesized(top) and treated Ce-doped BaHfO3 samples (bottom).

Fig. 2. (Color online) Diffuse transmission spectraof Ce-doped BaHfO3 samples.

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(a)

(b)

Fig. 3. (Color online) (a) XRD patterns of Ce-doped BaHfO3 samples and references and (b) enlarged XRD
patterns from 2θ = 27 to 33 degrees.

Fig. 4.

(Color online) PL excitation and emission map of 1% Ce-doped BaHfO3 sample.

the as-synthesized samples showed a rather low luminescence intensity. The horizontal andvertical axes are emission and excitation wavelengths, respectively. All the samples exhibited thebroad luminescence band at ~370 nm under the excitation wavelength of ~300 nm. Theseemission and excitation wavelengths were similar to those in previously reported results of Cedoped BaHfO3.(41,51) The PL QYs were 11.6% for the 0.3% sample, 30.5% for the 1% sample,
18.0% for the 3% sample, and 11.8% for the 5% sample. The highest PL QY was observed in the
1% sample, which was colorless compared with the same sample before treatment. The brownish
color is attributed to Ce4+.(52) The sample treatment was conducted to suppress the formation of
Ce4+ ions because Ce4+ ions do not contribute to the luminescence. Thus, the colorless or whitesample involved less Ce4+ ions and emitted luminescence due to the 5d–4f transition of Ce3+,
leading to an increase in the PL QY.
Figure 5 shows the PL decay curves of Ce-doped BaHfO3. The excitation and monitoring
wavelengths were 280 and 370 nm, respectively. The decay curves were fitted by one exponentialfunction. The obtained PL decay time is listed in Table 1. The PL decay time of ~17 ns was fasterSensors and Materials, Vol. 36, No. 2 (2024)

493

than the typical decay time due to the 5d–4f transition of Ce3+. For example, the decay times
were ~60 ns (Ce-doped Gd2SiO5),(53) ~40 ns (Ce-doped Lu2SiO5),(54) and ~100 ns (Ce-doped
Y3Al5O12).(55) In contrast, the decay time of Ce-doped YAlO3 and LuAlO3 was ~17 ns.(4) Shorter
wavelengths and higher refractive indices are associated with faster decay times. The emission
wavelength of Ce-doped BaHfO3 was relatively short, and the refractive index was greater than2.(56) Therefore, the decay time of Ce-doped BaHfO3 was fast.
Figure 6 shows the X-ray-induced scintillation spectra of Ce-doped BaHfO3. The peakwavelength was ~380 nm, which was slightly shifted to a longer wavelength compared with the
PL emission peak. The measurement geometry of X-ray-induced scintillation spectra was of thetransmission type. In contrast, the PL spectrum measurement was conducted in the reflectiontype geometry. Thus, the scintillation light was absorbed by the sample, and the peak wavelength
was shifted to a longer wavelength.
Figure 7 shows the X-ray-induced scintillation decay curves of Ce-doped BaHfO3. The
observed decay curves contained fast and slow decay components. The obtained decay time is

Table 1PL and scintillation decay times of Ce-doped BaHfO3.
PL decay time Scintillation decay time
Sampleτ1 (ns)τ 2 (ns)
τ1 (ns)0.3% Ce17
18 (96.3%) 375 (3.7%)
1% Ce
17
16 (96.3%) 241 (3.7%)
3% Ce
17
14 (98.4%) 352 (1.6%)
5% Ce
17
15 (96.6%) 343 (3.4%)

Fig. 5. (Color online) PL decay curves of Ce-doped
BaHfO3 samples.

Fig. 6. (Color online) X-ray-induced scintillation
spectra of Ce-doped BaHfO3 samples.

Fig. 7. (Color online) X-ray-induced scintillation
decay curves of Ce-doped BaHfO3 samples.

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listed in Table 1. The fast decay time was ~14–18 ns, which was close to those in PL decay. The
origin of the fast decay was attributed to the 5d–4f transition of Ce3+ owing to the decay time,
the broad emission band, and the emission wavelength, as shown in Fig. 5. The slow decay was
typically observed in the Ce-doped MHfO3 (M = Ca and Sr) series,(38,43,44,57) and the origin was
attributed to some types of defect.
Figure 8 shows the afterglow curves of Ce-doped BaHfO3. The afterglow level is defined as
the ratio of the intensity at 20 ms after stopping X-ray exposure to the intensity during X-ray
exposure. The afterglow levels of the 0.3, 1, 3, and 5% samples were 1550, 420, 1040, and 860
ppm, respectively. These afterglow levels were lower than those in the Ce-doped MHfO3
series.(38,43,45)
Figure 9 shows the pulse height distribution of the 137Cs gamma ray using Ce-doped BaHfO3
and Bi4Ge3O12 (BGO). The 1 and 3% samples exhibited a discriminable peak due tophotoabsorption. The light yields of the 1 and 3% samples were 1600 and 1300 photons/MeV,
respectively, compared with the peak position of BGO (6800 photons/MeV) and the quantum
efficiency of PMT. Although the 3% sample had a black region on the sample surface, the peak
was observed. Thus, the black region did not affect the results of the pulse height distribution.
The reported integrated light yield of Ce-doped BaHfO3 was ~40000 photons/MeV.(46) The
integrated light yield is estimated on the basis of the integrated intensity of the scintillation
spectrum compared with a reference and a sample. The integrated light yield depends on not
only the light yield, but also the stopping power of materials.(58) Thus, we must evaluate the lightyield using the pulse height distribution. The light yield of the present Ce-doped BaHfO3 wasmore than 10 times lower than the previous result.(46)Fig. 8. (Color online) Afterglow curves of Ce-doped
BaHfO3 samples.

Fig. 9. (Color online) Pulse height distribution of
137Cs gamma ray using Ce-doped BaHfO samples.
3

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4.

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Conclusions

The PL and scintillation properties of Ce-doped BaHfO3 were investigated. The synthesized
samples contained not only the BaHfO3 phase but also the monoclinic HfO2 phase. The assynthesized BaHfO3 showed low emission intensity. After the annealing treatment under the
reduction condition, the emission intensity and the PL QY increased. The 1% sample exhibitedthe highest PL QY of 30.5% among the samples. The broad luminescence band at ~370–380 nm
was observed in PL and scintillation. The PL and scintillation decay curves showed a fast decayof ~17 ns and a slow decay of ~241–375 ns. Ce-doped BaHfO3 crystals exhibited a
photoabsorption peak under 137Cs gamma-ray irradiation, and the maximum light yield was1600 photons/MeV. The Ce-doped MHfO3 (M = Ca, Sr, and Ba) showed blue luminescence andfast decay under X-ray irradiation. The light yield of Ce-doped BaHfO3 is lower than that of Cedoped CaHfO3,(43) but higher than that of Ce-doped SrHfO3.(45)Acknowledgments
This work was supported by a Grant-in-Aid for Scientific Research A (22H00309), Grants-inAid for 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 (JPMJTM22DN) from JST, Kazuchika Okura Memorial
Foundation, Asahi Glass Foundation, Nakatani Foundation, and Konica Minolta Science andTechnology Foundation are also acknowledged.

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