Temperature dependence of luminescence properties of praseodymium-doped perovskite CaTiO3:Pr3+

Article abstract of DOI:10.1016/j.tca.2011.02.036

A polycrystalline perovskite CaTiO₃:Pr³⁺ was synthesized and the temperature dependence of luminescence properties was investigated in the temperature range of 6-440 K. CaTiO₃:Pr ³⁺ exhibits red emission ascribed to the f-f transition from the excited state ¹D₂ to the ground state ³H ₄ of Pr³⁺ upon the band gap photo-excitation between valence and conduction band, and upon the IVCT (inter valence charge transfer between Pr³⁺ and Ti⁴⁺) photo-excitation. Below 330 K, the emission intensity upon the band gap excitation increased with temperature, while the temperature dependence of emission intensity upon the IVCT excitation is relatively small, indicating that the electron transfer process between Pr³⁺ and the host accompanied by the band gap excitation is thermally activated. Above 330 K, the thermal quenching of emission was observed upon both of the band gap and the IVCT excitation and the energy barrier for thermal quenching was estimated to be ～5500 cm^-1.

Full text of DOI:10.1016/j.tca.2011.02.036

Thermochimica Acta 532 (2012) 168–171
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Thermochimica Acta
journal homepage: www.elsevier.com/locate/tca
Temperature dependence of luminescence properties of praseodymium-doped
perovskite CaTiO3:Pr3+
Yoshiyuki Inaguma∗ , Takeshi Tsuchiya, Yuki Mori, Yuki Imade, Nobuhisa Sato, Tetsuhiro Katsumata,
Daisuke Mori
Department of Chemistry, Faculty of Science, Gakushuin University, 1-5-1 Mejiro, Toshima-ku, Tokyo 171-8588, Japan
a r t i c l e i n f o
a b s t r a c t
A polycrystalline perovskite CaTiO3:Pr3+ was synthesized and the temperature dependence of lumines-
cence properties was investigated in the temperature range of 6–440 K. CaTiO3:Pr3+ exhibits red emission
ascribed to the f–f transition from the excited state 1D2 to the ground state 3H4 of Pr3+ upon the band
gap photo-excitation between valence and conduction band, and upon the IVCT (inter valence charge
transfer between Pr3+ and Ti4+) photo-excitation. Below 330 K, the emission intensity upon the band gap
excitation increased with temperature, while the temperature dependence of emission intensity upon
the IVCT excitation is relatively small, indicating that the electron transfer process between Pr3+ and the
host accompanied by the band gap excitation is thermally activated. Above 330 K, the thermal quenching
of emission was observed upon both of the band gap and the IVCT excitation and the energy barrier for
Article history:
Available online 21 March 2011
Keywords:
Luminescence
Praseodymium
Perovskite
Band-gap
Calcium titanate
Electron transfer
thermal quenching was estimated to be ∼5500 cm−1
.
© 2011 Elsevier B.V. All rights reserved.
1. Introduction
to the previous studies. However, in order to elucidate the more
detailed mechanism of luminescence, especially energy transfer
process, further investigations are needed.
Pr3+ (electron configuration: 4f 2) ion-doped phosphors usually
exhibit the greenish-blue emission ascribed to the transition from
Since radiative and non-radiative transitions in phosphor
materials often occur via thermally activated processes, the
understanding of thermal processes is vital to elucidate the lumi-
nescence mechanism. In addition, the temperature dependence of
emission intensity of phosphor materials is a crucial issue in uti-
lizing as industrial products. We then measured the temperature
dependence of luminescence properties of a Pr-doped perovskite-
type oxide CaTiO3:Pr3+ in the wide temperature range 6–440 K
and discuss the luminescence mechanism in terms of thermal
process.
3
the excited state 3P0 state to the ground state 3H4 (3P0 → H4) of
Pr3+ and/or the red emission ascribed to that from the excited state
3
1D2 to 3H4 (1D2 → H4) of Pr3+ upon photo-excitation and electron-
excitation [1]. Among them, Pr3+-doped perovskite-type titanates
ATiO3 (A = Ca [2–13], Sr [7,8,11,13–17], Ba [7,11,18–20], (La, Li) [21],
Ln1/2Na1/2(Ln = La [21–23], Gd, Lu, Y [22,23]), La1/2K1/2 [21]) have
recently been considered attractive as red phosphor materials for a
potential flat-panel display, field-emission display (FED) [2,14,15]
3
and EL display [24], since intense red emission (1D2 → H4) domi-
3
nates and greenish-blue emission (3P0 → H4) is quenched. Due to
recent systematic studies with regard to Pr3+-doped oxide phos-
phors with closed-shell cation, Mn+ (electronic configuration d0),
such as perovskite titanates ATiO3: Pr3+, it was found that their
luminescence originated from the f–f transition of Pr3+ ion occurs
via the inter-valence charge transfer (IVCT) between Pr and M
cation (Pr3+ + Mn+ ↔ Pr4+ + M(n−1)+) [9,10,23,25,26] and band-gap
excitation processes[22]. The IVCT relaxation model can explain
2. Experimental procedure
Polycrystalline CaTiO3 and its Pr3+-doped compound,
Ca2+1−3x/2Pr3+xTiO3 (x = 0.002) (abbreviated as CTO and CTO:Pr,
respectively) were synthesized by
a conventional solid-state
reaction at elevated temperature. The starting materials were
CaCO3 (Rare metallic Co., >99.9%), TiO2 (rutile-type, Rare metallic
Co., >99.9%), and Pr(NO3)3 solution. Pr nitrate solution was then
prepared by dissolving Pr6O11 (Rare metallic Co., >99.9%) using
3 M nitric acid. The metal contents of Pr6O11 were determined
by chelatometry with ethylenediaminetetraacetic acid (EDTA)
because the commercial reagent of Pr6O11 is non-stoichometric.
The appropriate mixture of powders and solution was dried and
pressed into a pellet and calcined at 1273 K (1000 ◦C) for 12 h in
3
the quenching of the greenish-blue emission ascribed to 3P0 → H4
above mentioned. The understanding with respect to the lumines-
cence properties in Pr-doped titanates has become clearer owing
∗
Corresponding author. Fax: +81 3 5992 1029.
E-mail address: yoshiyuki.inaguma@gakushuin.ac.jp (Y. Inaguma).
0040-6031/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.tca.2011.02.036

Y. Inaguma et al. / Thermochimica Acta 532 (2012) 168–171
169
Fig. 2. Schematics of the relation between the band structure and the energy levels
of 4f state of Pr3+ in CTO:Pr, emission upon (a) the IVCT excitation and (b) the band
gap excitation.
this band is attributed to Pr3+/Ti4+ ↔ Pr4+/Ti3+ inter-valence charge
transfer state (IVCT). From the viewpoint of the relation between
band structure of CTO and the energy levels of f state of Pr3+, the
shoulder absorption corresponds to the transition from the ground
Fig. 1. Diffuse reflection spectra of pristine CaTiO3 and CaTiO3:Pr0.2% at room tem-
perature.
state of Pr3+ 3H4 to the bottom of conduction band (Fig. 2(a)).
,
air. The calcined powder was again pressed into pellets, heated at
1573 K (1300 ◦C) for 12 h in air and then furnace-cooled.
The phase identification for the samples was carried out by the
powder X-ray diffraction method using a Rigaku RINT 2100 diffrac-
tometer (graphite-monochromatized CuK␣).
The diffuse reflectance spectra were recorded using a JASCO
V-550 ultraviolet and visible spectrophotometer. The emission
and excitation spectra were measured using a JASCO FP-6500
spectrofluorometer. Light from a 150 W xenon lamp was used as
an excitation source. The emission and excitation spectra were
corrected with a standard lamp and the excitation spectrum of
Rhodamine B in ethylene glycol (8 g/l) [27]. The temperature depen-
dencies of emission and excitation spectra were measured using a
cryostat equipped with a closed cycle helium refrigerator (SRDK-
205 Cryocooler, Sumitomo Heavy Industries) in the temperature
rang 6–300 K and using a home-made copper block cell equipped
with a heater in the temperature range 300–440 K.
Using the wavelength of shoulder band (∼375 nm) and the band
gap energy, we can estimate that the ground state 3H4 is located
at higher energy level by approximately 2000 cm−1 (∼0.3 eV) than
that of the top of valence band. Since the bottom of conduction band
is primarily originated from the anti-bonding contribution between
Ti 3d t2g and oxygen 2p atomic orbitals [29], the IVCT model of
Pr3+/Ti4+ ↔ Pr4+/Ti3+ is consistent with the scheme derived by the
approach in terms of the electronic structure of CTO (Fig. 2).
Fig. 3 shows the emission, excitation, and diffuse reflectance
spectra of CTO:Pr at room temperature. Upon a photo-excitation
with a wavelength of 333 nm corresponding to the absorption edge
of CTO, i.e. the band gap excitation, CTO:Pr exhibits intense red
emission in the vicinity of 615 nm ascribed to the transition from
a excited state 1D2 to the ground state 3H4 of Pr3+(1D2 → H4) and
3
weak emission at the longer wavelengths in the vicinity of 700 nm
3
ascribed to 1D2 → H5 of Pr3+. The excitation spectrum was mon-
itored at 615 nm corresponding to the red emission ascribed to
3
1D2 → H4 of Pr3+. As shown in Fig. 3, the main excitation peak
3. Results and discussion
corresponds to the absorption edge. Furthermore, as seen in the
3.1. Optical absorption and luminescent properties at room
temperature of CaTiO3:Pr3+
The powder X-ray diffraction experiments revealed that CTO
and CTO:Pr possess orthorhombic perovskite-type structure with
a space group Pnma [28]. The diffuse reflection spectra of CTO and
CTO:Pr at room temperature are shown in Fig. 1. For pristine CTO,
the absorption edge was observed in the vicinity of a wavelength
of 330 nm. This absorption energy corresponds to the band gap
between the valence band and conduction band of CTO. The optical
band gap energy, Eg is estimated to be 29,000 cm−1 (3.6 eV) from
the optical absorption [22]. For CTO:Pr, the absorption peaks in
the vicinity of 400–500 nm, and 610 nm were observed in addi-
tion to the absorption of band gap. These peaks correspond to
the f–f transitions of Pr3+ from 3H4 to 3P2(450–460 nm), 3P1 and
1I6(475–485 nm), 3P0(490–500 nm), and 1D2(610–620 nm) [1]. In
addition, for CTO: Pr the shoulder band beside the absorption edge
was observed in the wavelength range of 350–410 nm. As seen
in Fig. 1, the shoulder band was not observed in CTO, indicating
that the shoulder band is related to Pr, but does not correspond
to the f–f transition of Pr3+. Boutinaud et al. [9,10,13,25,26] have
firstly found this type of absorption in CTO:Pr, and proposed that
Fig. 3. Excitation and emission spectra of CaTiO3:Pr3+ 0.2%. Dotted line indicates
diffuse reflectance spectrum. The inset shows the enlarged excitation and diffuse
reflection spectra.

170
Y. Inaguma et al. / Thermochimica Acta 532 (2012) 168–171
Fig. 5. Excitation spectra monitored at 615 nm at various temperatures, 6 K, 125 K,
150 K, 200 K, 250 K, 300 K, 356 K, and 370 K for CaTiO3:Pr3+ 0.2%. The excitation
spectra are normalized by the peak intensity at 330 nm.
Fig. 4. Emission spectra upon photo-excitation with a wavelength of 333 nm at
various temperatures, 6 K, 100 K, and 300 K for CaTiO3:Pr3+ 0.2%. The inset shows
the emission spectra at 300 K, 356 K, and 370 K.
temperature dependence of emission intensity upon the photo-
excitation with a wavelength of 375 nm is relatively small. This
indicates that the electron transfer process accompanied by band
gap excitation is thermally activated. The possible explanations for
this phenomenon are as follows. As shown in Fig. 2(b), upon the
band gap excitation [22], the electron in the valence band is photo-
excited into the conduction band and then the electron of Pr3+ is
resonantly transferred to the valence band, i.e. the generated hole
inset of Fig. 3, on the longer wavelength side of excitation peak,
the shoulder band was also observed in the wavelength range of
360–380 nm, which is the same wavelength range as the shoul-
der band observed in the diffuse reflectance spectrum (Fig. 1). The
finding supports that the shoulder band is attributed to the IVCT
excitation. Upon the irradiation with a wavelength of 375 nm cor-
responding to the IVCT excitation, the emission profiles were the
same as those upon the band gap excitation, i.e. only the red emis-
sion was observed and the greenish-blue emission was quenched.
The intensity of excitation peak ascribed to the f–f transitions of
Pr3+ from 3H4 to 3P2 3P1, 1I6 and 3P0, are relatively weak. These
behaviors are consistent with the results reported by Jia et al. [8],
Boutinaud et al. [9,10,13] and Kyômen et al. [11]. Consequently, in
CTO:Pr, there are primarily two different excitation processes, the
IVCT excitation and the band gap excitation (Fig. 2). In the following
section, we focus on the temperature dependence of luminescence
properties, especially emissions upon these two excitation process.
accompanied by the excitation of electron is transferred to the Pr3+
The excited electron in the conduction band relaxes to the excited
state of Pr3+ 1D2 (this relaxation process may be via the IVCT state),
.
,
and then returns to the ground state of Pr3+, which is accompa-
nied by the red emission. Therefore, if the electron transfer process
from the ground state of Pr3+ ion, 3H4, to the valence band is ther-
mally activated, the emission upon band gap excitation would be
promoted with increasing temperature.
Above 330 K, apparent thermal quenching of emission was
detected. The emission intensity, I (T) was fitted to the following
3.2. Temperature dependence of luminescence properties of
CaTiO3:Pr3+
Fig. 4 shows the emission spectra upon the photo-excitation
with a wavelength of 333 nm at 6, 100, 300, 356, and 370 K. Though
the stark splitting was clearer with decreasing temperature, the
similar emission profiles were observed. The finding is consistent
with the fact that CTO undergoes no phase transition in the mea-
suring temperature range [30]. As seen in the inset of Fig. 4 above
330 K the emission intensity evidently decreased with increasing
temperature, i.e. a thermal quenching occurred.
Fig. 5 shows the excitation spectra monitored at 615 nm at var-
ious temperatures. Here, the excitation spectra are normalized by
the peak intensity at 330 nm. The relative intensity of the shoulder
band in the vicinity of 375 nm to the excitation peak decreased with
increasing temperature. The similar tendency was also observed
in the comparison between excitation spectra at 77 K and 300 K
reported by Boutinaud et al. [13]. In order to compare the tem-
perature dependences of emission intensity upon the band gap
excitation and the IVCT excitation, the temperature dependence of
3
emission intensities ascribed to 1D2 → H4 of Pr3+ upon the photo-
excitation with wavelengths of 333 nm and 375 nm are shown in
Fig. 6. Here, the emission intensities are the integral ones of red
emission in the wavelength range of 575–675 nm. Below 330 K,
the emission intensity upon the photo-excitation with a wave-
length of 333 nm increased with increasing temperature, while the
Fig. 6. Temperature dependence of red emission intensity upon photo-excitation at
333 nm (solid circle) and 375 nm (open square) for CaTiO3:Pr3+ 0.2%. The solid lines
denote the fitting ones.

Y. Inaguma et al. / Thermochimica Acta 532 (2012) 168–171
171
equation based on the model by Struck and Fonger [31–33],
Acknowledgments
ꢀ
ꢁ
ꢂꢃ−1
I(T)
I0
s
1 + exp
A
E
The authors thank Mr. Y. Shimokawa and Mr. M. Kurihara for his
technical support in making high-temperature cell and are grateful
to the reviewers for their useful advice and comments. This work
was partially supported by a Grant-in-Aid for Scientific Research
(No. 17560598) of JSPS and the “High-Technology Research Center
Project” and the Promotional Project for Development of Strategic
Research base for Private Universities: matching fund subsidy from
Ministry of Education, Culture, Sports, Science and Technology,
Japan.
=
−
(1)
kBT
where A is the radiative decay rate of the 1D2 state of Pr3+, s is
the attempt rate for thermal quenching, kB is Boltzmann’s con-
stant, and E is the energy barrier for thermal quenching. When
using the data in the temperature range of 330–440 K for the
emission upon a photo-excitation of 333 nm, we obtained the
values, s/A = (1.1 0.8) × 109, E = (541 20) × 101 cm−1. Here, E is
considered to correspond to the energy gap from 1D2 state to
the cross-over point with the quenching state. On the other hand,
when we fitted the data in the temperature range of 225–440 K for
the emission upon a photo-excitation of 375 nm, we obtained the
values, s/A = (1.8 0.6) × 109, E = (551 9) × 101 cm−1. These values
are in accord with those upon the excitation of 333 nm within the
errors. This indicates that the relaxation process including the elec-
tron transfer from the conduction band to the excited state 1D2 of
Pr3+ are the same for the band gap excitation and the IVCT excita-
tion (Fig. 2). The estimated energy of 5400–5500 cm−1 (0.7 eV) is
greater than that reported by Boutinaud et al. [13], E = 4000 cm−1
(0.5 eV) for CTO:Pr, which implies that the temperature depen-
dence of luminescence properties depend on samples of CTO:Pr
and there is a possibility to control the temperature dependence
of emission intensity, e.g. by changing the synthetic condition or
ion-doping. Further investigations are necessary to clarify the phys-
ical meaning of the energy barrier estimated from the temperature
dependence of emission intensity.
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The temperature dependence of luminescence properties for
CTO:Pr was examined in the temperature range 6–440 K. CTO: Pr
3
exhibits intense red emission ascribed to 1D2 → H4 of Pr3+ upon
the band gap and the IVCT (inter-valence charge transfer) photo-
excitation. Below 330 K, the emission intensity upon the band gap
excitation increases with increasing temperature. This implies that
the electron transfer process accompanied by the band gap exci-
tation in CTO:Pr is promoted with increasing temperature. Above
330 K, the thermal quenching of red emission was then observed.
The effects of the promotion of the process related to band gap
excitation and the thermal quenching with increasing temperature
on the emission intensity are competitive. At lower temperature
the former effect is dominant, while at higher temperature the lat-
ter effect is dominant. The energy barrier for thermal quenching
estimated from the temperature dependence of emission intensity,
5400–5500 cm−1 is rather larger than that reported by Boutinaud
et al. The finding indicates that the estimated activation energy is
dependent of samples of CTO:Pr. Further investigations are nec-
essary to clarify the physical meaning of the activation energy,
which would provide us the important information to control the
temperature dependence of emission intensity.
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