Journal of Alloys and Compounds 537 (2012) 54–59

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Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jalcom

Preparation and optical characterization of Eu3+-doped CaTiO3 perovskite powders
Duong Thi Mai Huong, Nguyen Hoang Nam, Le Van Vu, Nguyen Ngoc Long ⇑
Faculty of Physics, Hanoi University of Science, 334 Nguyen Trai Road, Thanh Xuan District, Hanoi, Viet Nam

a r t i c l e

i n f o

Article history:
Received 8 March 2012
Received in revised form 13 May 2012
Accepted 20 May 2012
Available online 29 May 2012
Keywords:
CaTiO3:Eu3+ perovskite
Sol–gel method
Absorption
Photoluminescence

a b s t r a c t
CaTiO3 perovskite powders doped with 1.0, 1.5, 2.0, 2.5, 3.0, and 5.0 mol% Eu3+ were prepared by sol–gel
technique followed by annealing at high temperatures. The powders were characterized by X-ray diffraction, scanning electron microscopy, Raman scattering, absorption, and photoluminescence spectroscopy.
The obtained powders possessed orthorhombic crystal structure. Raman spectra of the CaTiO3:Eu3+ powders exhibited seven new peaks at 798, 1048, 1188, 1371, 1441, 1601, and 1644 cmÀ1 which were
assigned to the localized vibrational modes related to the complexes containing Eu3+. It was found that

⇑ Corresponding author.
E-mail address: [email protected] (N.N. Long).
0925-8388/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jallcom.2012.05.087

CaTiO3 was synthesized by various methods: high temperature
solid state reaction [6], co-precipitation [8], spray pyrolysis [9],
sol–gel [10] and microwave assisted hydrothermal method [13].
Among the above mentioned methods, sol–gel is the simple and
widely used one for preparation of CaTiO3.
In the present paper, we report on CaTiO3:Eu3+ powders prepared by sol–gel technique followed by heating at high temperatures. The powders were characterized by X-ray diffraction,
scanning electron microscopy, Raman scattering, absorption, and
photoluminescence spectroscopy. It was found that the photoluminescence (PL) of Eu3+ ions results from the radiative intra-configurational f–f transitions that happen between the 5DJ (J = 0,1–3)
exited states and the 7FJ (J = 0,1–4) ground states; the photoluminescence excitation (PLE) of Eu3+ ions takes place from the 7F0
ground state to the 5DJ (J = 1–4), 5L6, and 5G2,6 exited states. It
was noted that the photoluminescence intensity was strongest in
the samples doped with 3.0 mol% Eu3+.

2. Experimental
Ca1ÀxEuxTiO3 with x = 1.0, 1.5, 2.0, 2.5, 3.0, and 5.0% mol ratio of Eu3+ ions powders were synthesized by sol–gel method using the following precursors: CaCl2,
TiCl4, and Eu(NO3)3. All chemicals are of analytic grade without further purification.
A mixed aqueous solution contained the above chemicals with the appropriate mol
ratio Ca: Eu: Ti = (1Àx): x: 1 was prepared. The mixture was then constantly stirred
to get an opalescent solution. Citric acid (CA) was dissolved in the double distilled
water to form a 50% CA solution. The CA solution was added into the opalescent
mixture under constant magnetic stirring at 90 °C. After 4 h of stirring, the sol changed into a yellow chrome homogeneous gel. The gel was dried at 120 °C for 24 h to
remove the water, and was then annealed at 300 °C for 30 min. After that, an
ash-gray powdered product was obtained. In order to support the crystallization



3.0 mol% Eu3+ annealed at different temperatures ranging from 300
to 1000 °C for 2 h. As can be seen from the figure, the samples annealed at 300 °C exhibited a bad crystallinity: The characteristic

Fig. 1. XRD patterns of (a) the powders CaTiO3 doped with 3.0 mol% Eu3+ annealed
at different temperatures ranging from 300 to 1000 °C for 2 h, (b) the powders
CaTiO3 undoped and doped with 1.5, 2.0, 3.0, and 5.0 mol% Eu3+ annealed at 1000 °C
for 2 h in air.

55

peaks of CaTiO3 appeared with very weak intensity. The samples
exhibited better cystallinity with increasing annealing temperature. At calcinating temperatures of 800, 900, and 1000 °C the samples displayed a good crystallization.
XRD patterns of the powders CaTiO3 undoped and doped with
1.5, 2.0, 3.0, and 5.0 mol% Eu3+ annealed at 1000 °C for 2 h in air
are shown in Fig. 1(b). All the peaks in the XRD patterns clearly
indicate that the CaTiO3:Eu3+ samples possess orthorhombic crystal structure. No other diffraction peaks are detected except for
the CaTiO3 related peaks.
The lattice constants determined from the XRD patterns are
a = 5.432 Å, b = 7.643 Å and c = 5.390 Å, which are in good agreement with the standard values (a = 5.440 Å, b = 7.643 Å and
c = 5.381 Å, JCPDS card No. 22-0153). The average size of the crystallites was estimated by Debye–Scherrer’s formula [15]:

D¼

b

0:9k
cos h

ð2Þ


1601, and 1644 cmÀ1 (lines b–e in Fig. 4). These vibrational modes
may be related to LVMs of the Eu3+-containing complexes with different configurations.
3.3. Absorption and photoluminescence spectra

Fig. 3. The EDS spectra of the undoped CaTiO3 and Eu3+-doped CaTiO3 powders
with 5.0 mol% Eu3+.

layer deposited on silicon substrate for enhancement of conductivity in the EDS measurement.
It is known that effective radii of Ca2+, Eu3+, and Ti4+ ions in
octahedral sites are 1.00, 0.947, and 0.605 Å, respectively [16]. It
is expected that the Eu3+ ions can substitute for the Ca2+ ions more
easily than for the Ti4+ ions in CaTiO3:Eu3+ lattice because ionic radii for Ca2+ and Eu3+ are close. In addition, Mazzo et al. [13] showed
a simulated orthorhombic lattice of CaTiO3:Eu3+, which illustrated
the substitution of Eu3+ ions for Ca2+ ions in octahedral sites.

3.2. Raman scattering spectra
Raman spectroscopy is an important and useful tool for obtaining information about the vibrational modes of the materials and it
is well known that a small concentration of impurities introduced
into a perfect crystal will have little effect on vibrational modes, in
some case even there may appear vibrational modes lying outside
of the allowed frequency range of the perfect crystal [17]. These are
called localized vibrational modes (LVMs).
Typical room temperature Raman spectra of the undoped
CaTiO3 and the CaTiO3:Eu3+ powders with various contents of Eu
are shown in Fig. 4. As seen from the figure, the spectra can be

Fig. 4. Typical Raman spectra of the CaTiO3 powders undoped and doped with 1.0,
2.0, 3.0, and 5.0 mol% Eu3+.

Fig. 5 depicts diffuse reflection spectra measured at room temperature of the undoped CaTiO3 and the Eu3+-doped CaTiO3 powders with various dopant contents. Can be seen that in addition




h
ð3p2 nÞ2=3
DEg ¼
2mÃeh

ð4Þ

where 
h is the reduced Planck’s constant, mÃeh is the reduced effective mass of electron and hole (m1Ã ¼ m1Ã þ m1Ã ; mÃe and mÃh are the
eh

e

h

where A is a constant and Eg is the band gap of the material. The
plots of ½FðRÞ Â hmŠ2 versus hm for the undoped and the Eu3+-doped
CaTiO3 powders are represented in Fig. 7. By extrapolating the
straight portion of the graph on hm axis at a = 0, we found the band
gaps of the CaTiO3 powders doped with the concentration of 0, 1.5,
2.0, 3.0, and 5.0 mol% Eu3+ to be 3.670, 3.670, 3.687, 3.695, and
3.719 eV, respectively. Thus, with increasing Eu3+-dopant content
from 0 to 5.0 mol%, the optical band gap is gradually increased from
3.670 to 3.719 eV. The similar phenomenon was also observed for
ZnO doped with any of the group III elements (B, Al, Ga, In) and
for many various semiconductors (see, for example, Ref. [27]).
This phenomenon can be explained as follows. When the Eu

CaTiO3:Eu3+ were 28 mol% of Eu3+ in the samples prepared by
solid-state reaction [11] and 16 mol% of Eu3+ in those prepared
by sol–gel method [12], while Mazzo et al. reported that the optimal concentration of Eu3+ is 1 mol% [13]. When the concentration
of an activator is higher than an appropriate value, the luminescence of the phosphor is usually lowered. This effect is called concentration quenching. The origin of this effect is known to be one of
the following: the cross-relaxation between the activators, excitation energy migration to quenching centers or the surface states
acting as quenching centers, the pairing or coagulation of activator
ions and their change to quenching centers. As mentioned above,
the concentration quenching occurs at different concentrations
maybe because the samples were prepared by different methods.
In fact, under various technological conditions Eu3+ ions were differently incorporated into the samples.
In order to interpret the origin of the emission lines, the room
temperature PL spectrum under 398 nm excitation wavelength of
CaTiO3 powder doped with 3.0 mol% of Eu3+ is illustrated in Fig. 9.
The groups of emission lines located in the range of wavelength
from 590 to 725 nm are attributed to the radiative transitions from

Fig. 7. The plots of ½FðRÞ Â hm2 Š versus photon energy hm for the undoped CaTiO3 and
the CaTiO3 powders doped with 1.5, 2.0, 3.0, and 5.0 mol% Eu3+.

Fig. 8. Room temperature PL spectra under excitation wavelength of 398 nm of
CaTiO3 powders doped with various concentrations of Eu3+.

Fig. 6. Plots of Kubelka–Munk F(R) versus photon energy hm for the undoped CaTiO3
and the Eu3+-doped CaTiO3 powders. The inset shows four absorption peaks related
to the optical transitions within Eu3+ ion in the spectra of the 2.0, 3.0, and 5.0 mol%
Eu3+-doped CaTiO3 samples.

ahm ¼ Aðhm À Eg Þ1=2

ð3Þ

Eu3+ ions.
Finally, it is noted that contrary to Pr-doped CaTiO3 powders,
our CaTiO3:Eu3+ samples do not exhibit a long afterglow luminescence. The afterglow luminescence (phosphorescence) occurs due
to the thermally stimulated recombination of trapped charged carriers. Fig. 12 depicts the decay behavior of the 615 nm (5D0 ? 7F2
transition) emission line for Eu3+ in the CaTiO3:3.0 mol% Eu3+ samples. As seen from the figure that the experimental data were very
well fitted using a double-exponential function:

Fig. 12. Decay curve of the 615 nm (5D0 ? 7F2 transition) emission line for Eu3+ in
the CaTiO3:3.0 mol% Eu3+ samples.

IðtÞ ¼ A1 expðÀt=s1 Þ þ A2 expðÀt=s2 Þ

ð5Þ

where I(t) is the phosphorescence intensity, A1 and A2 are the constants, and s1 and s2 are the decay constants (or lifetimes). The results showed that two lifetimes, a fast one s1 = 0.194 ms, and a slow
one s2 = 0.919 ms have been observed for the 5D0 ? 7F2 emission of
Eu3+. The fact that our CaTiO3:Eu3+ samples do not exhibit a long
afterglow luminescence indicated there are not the metastable
traps in these samples.
4. Conclusion
CaTiO3:Eu3+ perovskite powders were synthesized by sol–gel
method followed by annealing at high temperatures. At calcinating
temperatures higher than 800 °C the samples displayed a good
crystallization. The obtained powders possess orthorhombic crystal structure with lattice constants a = 5.432 Å, b = 7.643 Å and
c = 5.390 Å. The average sizes of the crystallites estimated by Debye–Scherrer’s formula are 24 nm. Raman scattering spectra show
7 new peaks observed at 798, 1048, 1188, 1371, 1441, 1601, and
1644 cmÀ1. These vibrational modes may be related to LVMs of
the complexes containing Eu3+ with different configurations. With
increasing Eu3+-dopant content from 0 to 5.0 mol%, the optical
band gap is gradually increased from 3.670 to 3.719 eV, which is

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