Short-range structure and photoluminescent properties of the CaTiO3:Pr,La phosphor

Article abstract of DOI:10.1016/j.jallcom.2016.07.090

Pr³⁺-doped CaTiO₃perovskite is a promising phosphor compound with the possibility of use in several technological applications. In this paper, we report the substitution of Ca²⁺ions with La³⁺ions in order to pro

Full text of DOI:10.1016/j.jallcom.2016.07.090

Accepted Manuscript
Short-range structure and photoluminescent properties of the CaTiO :Pr,La phosphor
3
Guilherme K. Ribeiro, Fabio S. Vicente, Maria Inês B. Bernardi, Alexandre Mesquita
PII:
S0925-8388(16)32123-5
10.1016/j.jallcom.2016.07.090
JALCOM 38258
DOI:
Reference:
To appear in: Journal of Alloys and Compounds
Received Date: 13 April 2016
Revised Date: 5 July 2016
Accepted Date: 7 July 2016
Please cite this article as: G.K. Ribeiro, F.S. Vicente, M.I.B. Bernardi, A. Mesquita, Short-range structure
and photoluminescent properties of the CaTiO :Pr,La phosphor, Journal of Alloys and Compounds
3
(2016), doi: 10.1016/j.jallcom.2016.07.090.
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ACCEPTED MANUSCRIPT
Short-range structure and photoluminescent properties of the CaTiO :Pr,La
3
phosphor
1
1
2
Guilherme K. Ribeiro , Fabio S. Vicente , Maria Inês B. Bernardi , Alexandre
1
,*
Mesquita
Instituto de Geociências e Ciências Exatas, UNESP – Univ Estadual Paulista,
1
Departamento de Física, Rio Claro, SP, Brazil
2
Instituto de Física de São Carlos, Universidade de São Paulo, São Carlos, SP, Brazil
Abstract
3
+
Pr -dopedCaTiO perovskite is a promising phosphor compound with the
3
possibility of use in several technological applications. In this paper, we report the
2+
3+
substitution of Ca ions with La ions in order to promote Ca vacancies to further
enhance photoluminescence (PL). CaTiO :Pr,La samples were prepared by the
3
polymeric precursor method and their short- and long-range structure and
photoluminescent properties were investigated in this paper. PL measurements show a
1
3
3+
narrow emission centered at 612 nm typical of the D - H transition of Pr ions. As
2
4
the La content increases, the intensity of this peak is increased. This enhancement of PL
curves for CaTiO :Pr,La samples is associated with disorder in the CaTiO lattice
3
3
caused by the La incorporation. This disorder as a function of the La content is shown
by both X-ray absorption near edge structure (XANES) spectra at the Ti K-edge and
Raman spectroscopy measurements. CaTiO :Pr,La samples do not present PL with long
3
term afterglow.
Keywords: CaTiO , Calcium titanate, Praseodymium, Lanthanum, photoluminescence,
3
Red phosphor, XANES, Raman, Ca vacancies, Short term afterglow
1
. Introduction
Luminescent materials, or phosphors, are widely used in several technological
applications, such as cathode-ray tubes, light bulbs, lasers, solar concentrators, vacuum
fluorescent displays, medical radiology equipment (including scintillators), field
emission displays and light-emitting diodes [1-7]. Regarding field emission displays,
which are one of the best candidates for advanced flat-panel applications, calcium
*
Corresponding author at: Departamento de Física, Instituto de Geociências e Ciências Exatas, Universidade
Estadual Paulista, Rio Claro, SP, Brazil. Tel.: +55 193526 9179.
E-mail address: mesquita@rc.unesp.br (A. Mesquita).

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3
+
titanate doped with praseodymium (CaTiO : Pr ) is a promising phosphor material for
3
this application [1, 3].
3
+
Since the publication of results reporting red emission from CaTiO : Pr , the
3
study and development of this material, which aims to find applications such as
phosphor, has received much attention [2, 8-11]. This red emission at around 612 nm is
1
3
3+
ascribed to the 4f–4f transition from the excited state D to the ground state H of Pr
2
4
ions. The excitation of the luminescence is achieved through the conduction band states
of the orthorhombic CaTiO lattice and then transferred to the emitting level [8, 9].
3
3
+
Moreover, compared to conventional sulfide compounds, CaTiO :Pr presents better
3
physical and chemical stabilities due to the fact that it is an oxide phosphor [12].
Trivalent praseodymium is well-known to emit efficiently between the blue and the red
regions, depending on the host material, the concentration and the pumping conditions
3
+
[
(
13]. CaTiO :Pr phosphor exhibits red cathodoluminescence with CIE coordinates
3
chromaticity coordinates) at x = 0:680 and y = 0:311, very close to the coordinates of
the “ideal red”, which is rather attractive for potential applications [13].
Many efforts have been employed in order to improve optical properties of this
material, such as intensity of excitation/emission and phosphorescent decay [9, 10, 12,
1
4-16]. For this purpose, different methods of synthesis, co-doping, controlling of
3+
particle size and annealing have been used [9, 10, 12, 14-17]. Co-doping of CaTiO :Pr
3
3
+
with Bi greatly enhances UV excitation in the spectral range 370–390 nm [16, 18],
3
+
whereas enhancement of the photoluminescence (PL) intensity of CaTiO :Pr can also
3
+
+
be achieved by adding H BO or B O as flux [16, 19, 20]. Substitution of Na or Ag
3
3
2
3
2
+
3+
cations for Ca and substitution of trivalent cations, like Al , or divalent cations, like
2
+
2+
4+
Mg or Zn for Ti cations, were also used as methods that improved the red emission
or afterglow [9, 16]. This improvement is attributed to the formation of trapping centers
3
+
related to the reduction of Pr defects due to the charge compensation which increases
3
+
the energy transfer from CaTiO to Pr [4, 16, 19, 20]. If there is no other charge
3
3
+
compensation, some undesirable defects, such as Ti and oxygen vacancies, may form
3
+
3+
near Pr , which can contribute to quench the Pr emission [4].
2
+
4+
Thus, substitutions of monovalent cations for Ca and trivalent cations for Ti
are used in order to achieve charge compensation. On the other hand, a calcium vacancy
3
+
can be promoted to achieve charge compensation due to fact that Pr is typically
2
+
thought to occupy a Ca site [4]. In this case, the formula Ca
Pr TiO is usually
x 3
1
−
3x/2
adopted and the charge compensation occurs by x/2 formula units of Ca vacancies per

ACCEPTED MANUSCRIPT
3
+
3+
3+
formula units of Pr . As every Ca vacancy is close to the Pr ion in the lattice and Pr
concentration is quite low, statistically, the possibility of the Ca vacancies meeting the
3
+
Pr ions is not favorable [4]. Regrettably, there are so few reports in the literature
2+
concerning this kind of study [4]. Hence, in this paper we report the substitution of Ca
ions by trivalent La ions in order to promote more Ca vacancies than x/2 to compensate
the additional positive charge. In principle, the disorder structure caused by Ca
3
+
vacancies can originate lower symmetry at Pr sites, which can mix opposite-parity
1
3
into 4f configurational levels, subsequently increasing the D - H transition
2
4
3
+
3+
probabilities of Pr ions and to get the further enhanced luminescence of Pr [21].
Calcium titanate doped by praseodymium and lanthanum (CaTiO :Pr,La) samples were
3
prepared and structural characteristics were probed by several techniques, which are
correlated to PL properties.
2
. Experimental
CaTiO :Pr,La samples with a nominal composition of Ca
La Pr TiO
3
3
1-3/2(x+y)
x
y
were prepared by the polymeric precursor method (details of this method can be found
elsewhere [22]) with x = y =0.00 (CT), x = 0.05 and y = 0.00 (CLT5), x = 0.10 and y =
0
.00 (CLT10), x = 0.05 and y = 0.01 (CLPT5) and x = 0.10 and y = 0.01 (CLPT10).
This method has proven to be an effective procedure for the preparation of
.
nanostructured samples [3, 23]. Calcium nitrate tetrahydrate Ca(NO ) 4H O (99%,
3
2
2
Aldrich), titanium isopropoxide [Ti(OC H ) ] (98%, Aldrich), praseodymium oxide
3
7 4
Pr O (99.9% Alfa Aesar), lanthanum nitrate hexahydrate (99%, Aldrich), ethylene
2
3
glycol, C H O (99.5%, Synth) and citric acid C H O (99.5%, Synth) were used as raw
2
6
2
6
8
7
materials. Titanium citrate was formed by titanium isopropoxide dissolution in a citric
acid aqueous solution under constant stirring. The citrate solution was stirred at 70 °C to
obtain a clear homogeneous solution. Stoichiometric quantities of calcium nitrate
tetrahydrate, lanthanum nitrate hexahydrate and praseodymium nitrate (Pr O was
2
3
dissolved with HNO to convert into Pr(NO ) ) were added to the Ti citrate solution.
3
3 3
After the cation homogenization in the solutions, C H O was added to promote a
2
6
2
polyesterification reaction. The citric acid/ethylene glycol mass ratio was fixed at 60/40.
As the preparation involves the polymeric network formation, the samples were heated
at 400 °C for 4 h in order to eliminate the organic precursors. After this heat treatment,
CaTiO :Pr,La samples were annealed at 500, .600 and 700 °C for 2 h.
3

ACCEPTED MANUSCRIPT
In order to confirm the nominal concentration of synthesized samples, a field
emission scanning electron microscope (FE-SEM) Zeiss Sigma with an Oxford system
of qualitative and quantitative chemical analysis was used. Energy dispersive
spectroscopy (EDS) was performed on prepared samples with the following nominal
concentration: Ca0.91La0.05Pr0.01TiO (CLPT5) and Ca0.885La0.10Pr0.01TiO (CLPT10).
3
3
The average effective cation concentrations measured with EDS analysis were: 0.887
for Ca, 0.055 for La, 0.011 for Pr and 1.058 for Ti (CLPT5 sample) and 0.846 for Ca,
0
.099 for La, 0.012 for Pr and1.043 for Ti (CLPT10 sample). For samples without Pr,
the effective cation concentrations are: 0.878 for Ca, 0.049 for La and 1.053 for Ti
CLT5 sample) and 0.857 for Ca, 0.101 for La and 1.041 for Ti (CLT10 sample). These
(
results show that the sample compositions are quite similar to the nominal
compositions. The EDS spectrum for the CLPT10 sample is shown in Fig. 1.
The X-ray powder diffraction (XRD) measurements were performed at room
temperature on a Rigaku Ultima 4 powder diffractometer with geometry θ-2θ, a rotating
anode X-ray source (Cu-K radiation, λ = 1.542 Å) and a scintillation detector. The data
α
were collected with a step size of 0.02° and the count time was 5 s per step.Ti K-edge
X-ray absorption spectra (4966 eV) were collected at the LNLS (National Synchrotron
Light Laboratory, Brazil) facility using the D04B-XAS1 beamline. The LNLS storage
ring was operated at 1.36 GeV and 100-160 mA. The spectra were collected in
transmission mode using a Si(111) channel-cut monochromator at room temperature.
The sample thicknesses were optimized at each edge by the Multi-Platform
Applications for XAFS (MAX) software package Absorbix code [24]. Normalized
XANES (X-ray absorption near edge structure) spectra were extracted with the MAX-
Cherokee code. The theoretical XANES spectra were calculated by the FEFF9 ab initio
code [25] whose input files were issued from MAX-Crystalffrev software, which takes
into account substitution disorder and random vacancies in the structure [24]. Raman
spectra were measured with a MonoVista CRS Raman spectrometer from S&I. The
samples were irradiated with a laser beam focused with an Olympus microscope. For
the excitation in backscattering geometry, the 633 nm line of a He-Ne laser with 35 mW
was used. Photoluminescence spectra were collected with a Thermal Jarrel-Ash
Monospec monochromator and a Hamamatsu R446 photomultiplier. The 350.7 nm
exciting wavelength of a krypton ion laser (Coherent Innova) was used; the output of
the laser was maintained at 200 mW. The phosphorescence decay curve was measured

ACCEPTED MANUSCRIPT
using a Varian Cary Eclipse fluorescence spectrophotometer. All measurements were
taken at room temperature.
3
. Results and discussion
Fig.2 shows the X-ray diffraction (XRD) patterns collected at room temperature
as a function of the annealing temperature. The samples crystallized completely without
the presence of secondary phases and the samples annealed at 500 °C exhibited a
pattern characteristic of an amorphous phase. In good agreement with the literature, the
diffraction planes of CaTiO :Pr,La samples annealed at 600 and 700 °C correspond to
3
an orthorhombic structure with Pbnm space group [3]. The diffraction planes
corresponding to this space group were also indexed in Fig. 2 (JCPDS card N.o 22-
0
153). Table 1 shows the lattice parameters calculated using least squares fitting for
CaTiO :Pr,La samples annealed at 700 °C. The cell parameters, a, b and c, and the
3
volume of unit cell show a slight increase as a function of La and Pr content. This minor
increase is consistent with the presence of cation vacancy which can induce a lattice
expansion in order to minimize anion repulsions in the vicinity of the vacancy.
3
+
2+
3+
In CaTiO , Pr can substitute for Ca because the ionic radius of Pr almost
3
2
+
3+
coincides with that of Ca . The ionic radii of Pr in the twelve and six-coordinated
2
+
state are 1.30 and 0.99 Å, respectively, whereas the ionic radius of Ca in twelve-
4
+
coordinated state is 1.34 Å and the ionic radius of Ti in the six-coordinated state is
0
.605 Å [4]. Thus, since the samples were prepared considering stoichiometric Ca1-
3
+
La Pr TiO formula, Pr ions would preferentially occupy Ca sites instead of Ti
3
/2(x+y)
x
y
3
sites because of the similar ionic radii. Moreover, our X-ray diffraction (XRD) results
3
+
3+
did not show any secondary phase, suggesting that Pr or La incorporates into
CaTiO lattice.
3
In order to investigate the effect of La and Pr incorporation and annealing
temperature on the local order structure around the Ti atom, Ti K-edge XANES spectra
were measured. XANES spectra give information on the coordination symmetry and the
valence of ions incorporated in a solid. The energy of the absorption edge shifts
according to the valence of the absorbing ion, since the binding energy of bound
electrons rises as the valence increases. Also, the shape of the absorption edge depends
on the unfilled local density of states and the coordination symmetry of the absorbing
element. The XANES spectra are shown in Fig 3(a) and (b).

ACCEPTED MANUSCRIPT
The pre-edge region of K-edge spectra of some transition metal oxides is
characterized by some features several electron volts before the absorption edge [26]. In
transition metal oxides that crystallize in centrosymmetric structures, the intensity of
these pre-edge features is very small; however, in non-centrosymmetric structures it can
be relatively large [26]. According to Vedrinskii et al., the physical origin of the pre-
edge feature between 4964 and 4966 eV is related to quadrupolar transitions of t -type
2
g
molecular orbitals located in the absorption atom [26]. The physical origin of the pre-
edge feature between 4966 and 4970 eV (shown in more details in the insets of Fig. 3(a)
and (b)) is the transition of the metallic 1s electron to an unfilled d state. This forbidden
electronic transition dipole is normally allowed by the mixture of p states from
surrounding oxygen atoms into the unfilled d states of titanium atoms [23, 26].The pre-
edge feature between 4970 and 4975 eV in Fig.3 is related to the Ti 1s electron
transition to the unoccupied 3d-originated e -type molecular orbital of TiO polyhedral
g
6
neighboring the absorbing Ti atoms, which are weakly affected by the core hole
potential [26]. The features beyond the absorption edge are related to electronic
transitions and the atomic structure of the second and third neighbors of the Ti atom ata
distance of up to 8 Å from this atom [27, 28].
The XANES spectra for the CT sample as a function of annealing temperature
are shown in Fig. 3(a). As the annealing temperature increases, the intensity of all
features in pre-edge region decreases. Moreover, the intensity of first features beyond
the edge (between 4980 and 5010 eV) increases with the increase of annealing
temperature. This behavior is also observed for CLT5, CLT10, CLPT5 and CLPT10
sample (not shown). The higher intensity of these peaks indicates that the local
environment of the Ti atom is non-centrosymmetric, which distorts the octahedral
configuration [3, 23]. This higher intensity has also been attributed to five-fold
coordination (TiO ) clusters [3, 23]. In order to confirm this hypothesis, calculated
5
XANES spectra at the Ti K-edge for the CaTiO compound using ab initio FEFF code
3
[
25] were obtained and the results are shown in Fig. 4. The input files for FEFF code,
with cluster radius of 6.0 Å, were generated using CRYSTALFFREV software [24] and
crystallographic model according to XRD results. As can be seen in Fig. 4, calculated
XANES spectra satisfactorily reproduce the experimental spectra. The calculated
XANES spectra considering a five-fold coordination of Ti atoms in the CaTiO lattice is
3
also shown. The intensity of the pre-edge increases in the XANES spectrum of TiO5
compared to TiO octahedra, whereas the intensity of features between 4980 and 5010
6

ACCEPTED MANUSCRIPT
eV decreases. As stated earlier, the same behavior is observed with decreasing
annealing temperature for experimental spectra shown in Fig. 3(a).
XANES spectra as a function of La and Pr content for samples annealed at 700
°
C are shown in Fig. 3(b). No changes at the pre-edge region are observed as a function
of La and Pr concentration. As these transitions are also related to distortion of the
+
3
+3
octahedral configuration, the incorporation of La and Pr ions into the CaTiO host
3
matrix does not cause structural disorder around the local environmental of Ti atoms.
Additionally, O vacancies are not expected, which could also originate from the
increasing of the pre-edge features by forming TiO clusters. Moreover, no change in
5
energy was observed for the pre-edge region or the edge for different La and Pr content.
This result is corroborated by the calculated XANES for the CLTP10 sample shown in
Fig. 3. The ab initio calculation was performed considering La atoms and Ca vacancies
located in the A-sites of the perovskite lattice stoichiometrically. All transitions in pre-
edge and post-edge regions of the experimental spectra were replicated quite well, as
can be seen in Fig. 4. No differences in energy or relative intensity of these transitions
are observed in calculated the spectrum for the CLTP10 composition compared to that
of the CT composition, in agreement with the experimental spectra as a function of the
La or Pr content. The displacement of these transitions for lower energies would be an
indicative of the decreasing in Ti valence [29]. Therefore, in principle, La and Pr
3
+
incorporation to the CaTiO host matrix does not cause the formation of Ti ions
3
according to XANES results.
Raman spectroscopy is a well-known and useful technique for estimating the
degree of structural order-disorder at short-range of the materials. Raman spectra of the
CT sample annealed at different temperatures are shown in Fig. 5(a). According to
literature, there are 24 Raman-active modes for the orthorhombic structure with four
B
molecular units in the primitive cell and space group Pbnm (Z = 4),which can be
described by the representation ΓRaman,Pbnm= 7A + 5B + 7B + 5B [3, 30-32].
g
1g
2g
3g
However, most of these modes cannot be detected because of their low polarizabilities
and nine Raman modes (labeled as P , P , P , P , P , P , P , P and P in Fig. 4) can be
1
2
3
4
5
6
7
8
9
detected in the spectra of the pure CT sample, which are in agreement with previous
studies [3, 30-32].
-1
Hence, the Raman mode at 160 cm is related to the CaTiO lattice mode
3
(
vibration of Ca bonded to a TiO group (Ca–TiO )). Raman modes at 184, 227, 248,
3
3
-1
2
90 and 340 cm are related to the bending of the O-Ti-O linkages, caused by the “tilt”

ACCEPTED MANUSCRIPT
phenomenon between the TiO -α-TiO adjacent clusters (α is the rotation angle between
6
6
-1
adjacent TiO ) [30]. The 473 and 499 cm Raman modes are ascribed to the Ti-O
6
6
-1
torsional mode and 636 cm Raman mode is assigned to the Ti-O symmetric stretching
vibration [3, 30-32]. Besides the nine Raman modes, five additional modes, indicated
by asterisk in Fig. 5(a), are observed in spectrum for the CT sample. The soft mode at
-1
1
20 cm is not commonly active from first-order Raman scattering in an orthorhombic
perovskite-type structure [31]. It is attributed to a perturbation of the perfect crystal
symmetry by grain boundaries or second order processes and can be related to the
-1
CaTiO lattice mode [31]. Additionally, the mode at 539 cm can be assigned to Ti-O
3
torsional mode and can also be interpreted as a signal of the disordered structure by an
increase in the tilts of the titanium octahedron cluster [31]. The presence of the modes at
-1
3
93 and 811 cm is observed, as well as in other perovskite compounds, and these two
modes are characteristic of B-site ordering in complex perovskites [32, 33]. The Raman
-1
modes at 539 and 811 cm are related to oxygen motions which can be represented as
asymmetric and symmetric ‘‘breathing’’ of the TiO octahedra, respectively [32, 33].
6
In Fig. 5(a), Raman modes are not observed for the CT sample annealed at 500
°
C. This result is consistent with XRD results which show an amorphous phase for this
sample. As the annealing temperature increases (600 and 700 °C), Raman modes
described above are observed, in agreement with XRD results. Raman spectra of
CaTiO :Pr,La annealed at 700 °C are shown in Fig. 5(b). As the La and Pr content
3
-1
increases, the mode labeled P and the mode positioned at 120 cm in the CT spectrum
1
-1
shift to lower wavenumbers (intensity of mode at 120 cm also increases). Both modes
are attributed to the CaTiO lattice mode, as stated earlier. An increase of intensity of
3
-1
+3
+3
Raman modes at 539 and 811 cm as La and Pr ions are incorporated to the CT
lattice is also observed. These differences in position and intensity is indicative of
symmetry break along TiO and CaO clusters, which is mainly caused by disorder in
6
12
2
+
the lattice modifiers due to distorted CaO12 clusters and/or Ca vacancies [3, 30].
Moreover, no changes are observed in P , P , P , P , P and P modes with increasing La
2
3
4
5
6
9
and Pr content. As stated earlier, these modes are associated with O-Ti-O bending or Ti-
O symmetric stretching. This result is consistent with XANES spectra, which did not
show an increase of disorder around Ti atoms with the addition of La and Pr ions.
Fig. 6 depicts the PL curve of CaTiO :Pr,La samples. PL curves for the CT
3
sample annealed at 500, 600 and 700 °C are shown in Fig. 6(a). A broad peak centered
at around 590 nm is observed for the sample annealed at 500 °C, whose structure is

ACCEPTED MANUSCRIPT
amorphous, as shown in the XRD results. As the annealing temperature increases and
the CT composition crystallizes, the PL intensity decreases significantly (the inset in
Fig. 6 (a) amplifies PL curves for these samples). This behavior was also observed by
Lazaro et al. for CT samples; these authors decomposed the PL curve for completed
distorted samples in three components that represent different types of electronic
transitions which are linked to a specific structural arrangement [23]. Such electronic
transitions would be due to the existence of electronic levels in the band gap of a
material, which are possible due to structural disorder. These authors propose that PL
emission is affected not only by the structural disorder in the network former but also by
structural disorder in the network modifier due to charge exchanges between Ca–O11
andCa–O (or TiO and TiO ) clusters and intermediate levels in the gap state [23]. Our
1
2
5
6
XANES measurements revealed structural disorder as well as the possible presence of
TiO for lower annealing temperatures.
5
Fig. 6(b) shows the PL curves for CaTiO :Pr,La annealed at 700 °C. The curves
3
for CLT5 and CLT10 samples exhibit a broad peak with low intensity, similar to the CT
sample, as expected. On the other hand, CLPT5 and CLPT10 samples reveal a narrow
peak centered at 612 nm and its intensity increases as a function of La content. The
origin of this peak is well known and ascribed to the 4f–4f transition from the excited
1
3
3+
state D to the ground state H of Pr ions [2, 8-10]. Some low-intensity peaks are
2
4
1
also observed in these PL curves. The features at 705 and 820 nm are related to D -
2
3
1
3
H and D - H transitions, respectively [34]. The features that are observed in the
5
2
6
blue-green region around 500 nm (which are not observed for CaTiO :Pr compound)
3
could be ascribed to the presence of a La-rich phase, for instance the perovskite
1
3
La TiO [35]. Fig. 6(c) shows the excitation spectrum for 612 nm emission ( D - H
4
2
/3
3
2
transition) for the CLPT10 sample annealed at 700 °C.
Fig.7 shows the fluorescence decay time of the emission at 612 nm (λexc = 350
nm) for the CLPT10 sample. The decay curve can be well fitted as a function of time by
a double exponential equation, in agreement with previous works reporting the
fluorescence lifetime of CaTiO :Pr [12, 15]. In Fig. 7, the fit was performed using the
3
t/τ1
t/τ2
following double exponential expression: I(t) = A e +A e , where I is the
1
2
fluorescence intensity, A and A are the amplitudes for each component and τ and τ
2
1
2
1
are the decay constants for the two components, respectively [12, 15]. The average
3+
2
1
2
2 2
lifetime for Pr can be determined by the formula τ = (A τ + A τ )/(A τ + A τ )[12].
1
1 1
2 2
According to the results obtained with the fit, whose values of amplitudes and decay

ACCEPTED MANUSCRIPT
constants are shown in Fig. 7, the average lifetime for the CLPT10 sample is 74 ꢀs. It is
important to note that the CaTiO :Pr,La samples do not show PL with long term
3
afterglow. This is an important factor in the synthesis of phosphors for fast response
applications [36].
The mechanism of the UV excitation to red emission has been related to energy
1
3+
transfer and charge transfer models of UV photoelectrons to the D level of Pr ions via
2
3
+
the valence-to-conduction band transition, Pr 4f5d states, self-trapped excitons and
4
+
3+
Pr /Ti states [37]. Several studies have shown an enhancement of red emission from
4
+
the CaTiO :Pr host matrix by adding divalent or trivalent cations replacing Ti or
3
2
+
monovalent cations replacing Ca [4, 9, 16, 19, 20]. In this case, the formation of
3
+
trapping centers is related to the reduction of Pr defects due to the charge
3
+
compensation and increases in the energy transfer from CaTiO to Pr would be
3
3
+
associated to this improvement [4, 9, 16, 19, 20]. However, in our study, La ions are
2
+
+3
incorporated into Ca sites and it is supposed that any additional charge (from La or
3
+
Pr ) was compensated by the formation of a stoichiometric quantity of Ca vacancies.
3
+
Therefore, the charge defects are located in the third nearest neighbors of Pr .
4
+
Moreover, distortions can also be located in the 8 second nearest neighbors of Ti , as
3
+
shown by the Raman results. This situation is expected to induce perturbed Pr centers
that have different optical spectra than those of unperturbed centers, especially
1
1
concerning the position of 4f 5d bands and charge transfer states [34].
It is well known that the PL efficiency of a rare-earth ion is sensitively
influenced by the symmetry and strength of the crystal field and that an increase in the
degree of distortion of the crystal field may enhance the rare-earth ion’s luminescence
efficiency [37]. According to Fujiwara et al., in a study comparing CaTiO , SrTiO and
3
3
3
+
2+
BaTiO host lattices, Pr substituted for the Ca site in CaTiO :Pr can emit intense red
3
3
light without the addition of any enhancers, since CaTiO has a distorted crystal
3
structure due to the crystal framework constructed by tilted Ti octahedrons [37]. The
ions of large ionic radius or vacancies substituted for A-site ions tend to induce the
3
+
structure asymmetry of the host, approaching the lower symmetry around Pr [21]. In
3
+
principle, more uneven crystal fields due to the lower symmetry at Pr sites can mix
1
3
opposite-parity into 4f configurational levels, subsequently increasing the D - H
2
4
3
+
transition probabilities of Pr ions [21]. As shown by Raman measurements, disorder
structure by an augment in the tilts in the titanium octahedron cluster is increased as the

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La content increases. Thus, the enhancement of the PL curves for CaTiO :Pr,La samples
3
is associated with disorder in the CaTiO lattice caused by La incorporation.
3
4
. Conclusions
CaTiO :Pr,La samples were prepared by the polymeric precursor method and
3
theirshort- and long-range structures and photoluminescent properties were investigated.
1
3
PL measurements show a narrow emission centered at 612 nm typical of D - H
2
4
3
+
transition of Pr ions. As the La content increases, the intensity of this peak is
increased. This enhancement of PL curves for CaTiO :Pr,La samples is associated with
3
disorder in the CaTiO lattice caused by La incorporation. In principle, more uneven
3
3
+
crystal fields due to the lower symmetry at Pr sites can mix opposite-parity into 4f
1
3
configurational levels, subsequently increasing the D - H transition probabilities of
2
4
3
+
Pr ions. This disorder, as a function of the La content, was probed by both XANES
and Raman measurements.
Acknowledgements
The authors thank FAPESP (through projects 2013/12993-4 and 2013/07909-4)
and CNPq funding agencies. The research was partially carried out at LNLS National
Laboratory of Synchrotron Light (proposal number XAFS1-17750), Brazil. The authors
also thank Prof. Dr. Ervino C. Ziemath and Leandro X. Moreno for Raman
measurements and Manoel R. Roncon for EDS measurements.
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Table 1 – Lattice parameters determined by XRD results fromCaTiO :Pr,La samples
3
annealed at 700 °C.
3
sample
CT
a (Å)
b (Å)
c (Å)
Volume (Å )
222.72(4)
223.93(4)
224.56(4)
224.15(4)
224.52(4)
5.3966(5)
5.4079(5)
5.4107(5)
5.4108(5)
5.4129(5)
5.4053(5)
5.4134(5)
5.4196(5)
5.4139(5)
5.4193(5)
7.6355(5)
7.6491(5)
7.6579(5)
7.6518(5)
7.6539(5)
CLT5
CLTP5
CLTP5
CLTP10

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Figure captions
Fig. 1 – EDS spectrum for CLPT10 sample annealed at 700 °C.
Fig. 2 – X-ray diffraction for (a) CT, (b) CLT5 and CLT10, (c) CLPT5 and CLPT10
samples annealed at 500, 600 and 700 °C.
Fig. 3 – XANES spectra at the Ti K-edge for (a) CT samples annealed at 500, 600 and
7
00 °C and (b) for CLT5, CLT10, CLPT5 and CLPT10 samples annealed at 700 °C.
The insets magnify the features in the pre-edge region.
Fig. 4 - Calculated XANES spectra for CaTiO for five-fold and six-fold Ti
3
coordination and for CLTP10 composition. The inset magnifies the features in pre-edge
region.
Fig. 5 – Raman spectra for (a) CT samples annealed at 500, 600 and 700 °C and (b)
CLT5, CLT10, CLPT5 and CLPT10 samples annealed at 700 °C.
Fig. 6 – PL curves for (a) CT samples annealed at 500, 600 and 700 °C, (b) CLT5,
CLT10, CLPT5 and CLPT10 samples and (c) the excitation spectrum for 612 nm
1
3
emission ( D - H transition) for CLPT10 sample annealed at 700 °C.
2
4
Fig. 7 – Fluorescence decay profile for the CLPT10 sample.

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