Systematic study of photoluminescence upon band gap excitation in perovskite-type titanates R1/2Na1/2TiO3:Pr (R=La, Gd, Lu, and Y)

Article abstract of DOI:10.1016/j.jssc.2007.03.017

Pr³⁺-doped perovskites R_1/2Na_1/2TiO₃:Pr (R=La, Gd, Lu, and Y) were synthesized, and their structures, optical absorption and luminescent properties were investigated, and the relationship between structures and optical properties are discussed. Optical band gap of R_1/2Na_1/2TiO₃ increases in the order R=La, Gd, Y, and Lu, which is primarily due to a decrease in band width accompanied by a decrease in Ti-O-Ti bond angle. Intense red emission assigned to f-f transition of Pr³⁺ from the excited ¹D₂ level to the ground ³H₄ state upon the band gap photo-excitation (UV) was observed for all compounds. The wavelength of emission peaks was red-shifted in the order R=La, Gd, Y, and Lu, which originates from the increase in crystal field splitting of Pr³⁺. This is attributed to the decrease in inter-atomic distances of Pr-O together with the inter-atomic distances (R, Na)-O, i.e., increase in covalency between Pr and O. The results indicate that the luminescent properties in R_1/2Na_1/2TiO₃:Pr are governed by the relative energy level between the ground and excited state of 4f² for Pr³⁺, and the conduction and valence band, which is primarily dependent on the structure, e.g., the tilt of TiO₆ octahedra and the Pr-Ti inter-atomic distance and the site symmetry of Pr ion.

Full text of DOI:10.1016/j.jssc.2007.03.017

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Journal of Solid State Chemistry 180 (2007) 1678–1685
www.elsevier.com/locate/jssc
Systematic study of photoluminescence upon band gap excitation in
perovskite-type titanates R1/2Na1/2TiO3:Pr (R ¼ La, Gd, Lu, and Y)
Yoshiyuki Inaguma , Takeshi Tsuchiya, Tetsuhiro Katsumata
Ã
Department of Chemistry, Faculty of Science, Gakushuin University, 1-5-1 Mejiro, Toshima-ku, Tokyo 171-8588, Japan
Received 14 December 2006; received in revised form 3 March 2007; accepted 15 March 2007
Available online 23 March 2007
Abstract
Pr3+-doped perovskites R1/2Na1/2TiO3:Pr (R ¼ La, Gd, Lu, and Y) were synthesized, and their structures, optical absorption and
luminescent properties were investigated, and the relationship between structures and optical properties are discussed. Optical band gap
of R1/2Na1/2TiO3 increases in the order R ¼ La, Gd, Y, and Lu, which is primarily due to a decrease in band width accompanied by a
decrease in Ti–O–Ti bond angle. Intense red emission assigned to f–f transition of Pr3+ from the excited 1D2 level to the ground 3H4 state
upon the band gap photo-excitation (UV) was observed for all compounds. The wavelength of emission peaks was red-shifted in the
order R ¼ La, Gd, Y, and Lu, which originates from the increase in crystal field splitting of Pr3+. This is attributed to the decrease in
inter-atomic distances of Pr–O together with the inter-atomic distances (R, Na)-O, i.e., increase in covalency between Pr and O. The
results indicate that the luminescent properties in R1/2Na1/2TiO3:Pr are governed by the relative energy level between the ground and
excited state of 4f2 for Pr3+, and the conduction and valence band, which is primarily dependent on the structure, e.g., the tilt of TiO6
octahedra and the Pr-Ti inter-atomic distance and the site symmetry of Pr ion.
r 2007 Elsevier Inc. All rights reserved.
Keywords: Luminescence; Praseodymium; Perovskite; Band gap; Titanate
1. Introduction
kites is related to the studies on photo-catalyst because
both cases are common in the viewpoint of the usage of
energy by band gap photo-excitation. Photoluminescence
in most of rare-earth doped inorganic phosphor materials
is attributed to the f–f transition or f–d transition of rare-
earth ion as an activator and the intensity of photolumi-
nescence depend on the site symmetry or sorts of ligands,
i.e., the covalency between rare-earth ion and ligands. If
the band gap photo-excitation process of phosphors and
photoluminescence regarding f-transition of rare-earth ion
are closely linked and the band excitation energy can be
effectively converted, intense photoluminescence would be
expected.
Diallio and Boutinaud et al. [4] have first found that
Pr-doped perovskites CaTiO3 shows intense red emission
corresponding to f–f transition [5] of Pr3+ from 1D2 to 3H4
on ultraviolet light excitation. After that, Cho et al. [6]
reported the red cathodoluminescence properties. Then,
Toki et al. [7] and Okamoto et al. [8–10] reported that
SrTiO3:Pr3+ by the addition of Al exhibits an intense red
Energy transfer process is of much importance in photo-
functional materials, and the novel materials presenting
effective energy transfer and the elucidation of mechanism
are strongly desired. Among them, much attention has
been paid to metal oxides with the band gap energy
corresponding to near ultraviolet or visible light, e.g. TiO2
[1] and perovskite-type compounds such as SrTiO3 [2,3]
because of their potential use in photo-catalytic processes.
Its promising applications include water splitting into O2
and H2, and the decomposition of harmful organic
compounds in polluted air and water. Their photo-catalytic
process depends upon the photo-excitation in accordance
to band gap and successive energy transfer to split water
into H2 and O2 or decompose organic compounds. Our
research regarding photoluminescence in Pr-doped perovs-
Ã
Corresponding author. Fax: +81 3 5992 1029.
E-mail address: yoshiyuki.inaguma@gakushuin.ac.jp (Y. Inaguma).
0022-4596/$ - see front matter r 2007 Elsevier Inc. All rights reserved.
doi:10.1016/j.jssc.2007.03.017

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Y. Inaguma et al. / Journal of Solid State Chemistry 180 (2007) 1678–1685
1679
emission approximately 200 times higher than for Al-free
2. Experimental
samples. Since their reports, Pr3+-doped perovskite-type
titanates ATiO3 (A ¼ Ca [10–17], Sr [13,14,17,18], Ba
[9,13,17,19,20], (La, Li) [21], La1/2Na1/2 [21], La1/2Ka1/2
[21]) have been attractive as red phosphor materials for a
potential flat-panel display, field-emission display (FED).
According to the previous reports [7–10,13], the red
emission is attributed to the excitation from the valence
band to the conduction band in host materials, and the
energy transfer from the host to the activator Pr3+. It was
claimed that the energy transfer occurs from the host
to the activator Pr3+ through the recombination of photo-
excited carriers in ATiO3:Pr3+ on the basis of the fact that
the photoluminescence excitation spectra reveal the
absorption edge of the ATiO3 host lattice. It was then
speculated that the overlapping between the absorption
band of ATiO3 host and the band for 4f–5d transition
in Pr3+ play an important role in the emission enhance-
ment. Jia et al. [14] suggested that the energy transfer
from Pr3+ to the charge transfer state of the Ti-complex
is related to the non-radiative transition, which is
ascribed to the much weaker red emission in SrTiO3:Pr
than in CaTiO3:Pr. Boutinaud et al. [15,16,22,23] proposed
that the Pr3+/Ti4+2Pr4+/Ti3+ inter-valence charge
transfer state (IVCT) in Pr-doped titanates such as
CaTiO3:Pr is the final relaxation channel to the red
emitting 1D2 level. Kyomen et al. [17] then reported
that the intense emission was observed when the IVCT
band and the valence-to-conduction band edge are over-
lapped in (Ca1ꢀxSrx)TiO3:Pr. The understanding with
respect to the emission in Pr-doped titanates has become
clearer owing to the previous studies. However, in order to
elucidate the more detailed mechanism of photolumines-
cence, especially energy transfer process, further investiga-
tions are needed.
The entitled perovskite-type oxides R1/2Na1/2TiO3:Pr
(R ¼ La, Gd, Lu, and Y) are appropriate materials to
elucidate the mechanism of photoluminescence system-
atically as well as potential red phosphors for the following
reasons. Shan et al. [24–28] reported that R1/2Na1/2TiO3
(R ¼ La, Pr, Nd, Gd, Eu, Tb, Yb, Y) have perovskite-type
structure and the lattice parameter decreased and the tilting
angle of TiO6 octahedra increases with a decrease in the
ionic radius [29] of R ion. The change in lattice parameter
and tilt angle of TiO6 octahedron brings in the change in
chemical bond between Ti and O, i.e., the change in width
of valence and conduction bands which primarily consist of
Ti 3d and O 2p orbitals [30]. Consequently, the change in
band gap energy and energy transfer to Pr3+ ion can be
expected.
Perovskites R1/2Na1/2TiO3 and R1/2ꢀxPrxNa1/2TiO3
(x ¼ 0.002) (abbreviated as RNTO and RNTO:Pr below,
respectively) (R ¼ La, Gd, Lu, and Y) were synthesized by a
conventional solid-state reaction at elevated temperature [26].
The starting materials were La2O3 (4N), Gd2O3 (4N), Lu2O3
(3N), Y2O3 (3N), Na2CO3 (3N), Pr6O11 (3N) and TiO2 (3N).
The metal contents of La2O3 and Pr6O11 were determined by
a chelatometry with ethylenediaminetetraacetic acid (EDTA),
and R2O3 (R ¼ Gd, and Lu) was fired at 1000 1C over a night
prior to the weighing because the commercial reagents are
non-stoichometric or include carbonates and hydroxides. The
starting materials were mixed in their stoichiometric ratio
with the addition of 2–5 mol% Na2CO3 powder, because
Na2O has a tendency to vaporize at high temperature. The
mixture of powders was pressed into a pellet and calcined at
1100 1C for 6 h in air. After grinding, the calcined powder was
again pressed into pellets, sintered at 1250 1C for 4h in air,
and then furnace-cooled. Na2CO3 powder was added to the
calcined powder in order to minimize impurity phases when
they were observed. As reference compounds, CaTiO3 and
SrTiO3 were also synthesized under the same heating
condition by using the stoichometric mixture of CaCO3
(3N) or SrCO3 (3N) and TiO2 (3N).
The phase identification and determination of lattice
parameter for the sample were carried out by the powder
X-ray diffraction method using a Rigaku RINT 2100
diffractometer (graphite-monochromatized CuKa). The
crystal structures of RNTO (R ¼ La, Gd, Lu, and Y) were
refined using the Rietveld method with the RIETAN 2000
program [31]. The X-ray diffraction data for Rietveld
analysis were collected in the range 2y ¼ 20–1201 at 0.021
intervals at room temperature. The bond valence sum
(BVS) was calculated from inter-atomic distances with the
concept of bond valence [32,33].
The photoluminescence and excitation spectra measure-
ments were carried out using a JASCO FP-6500 and 6600
spectrofluorometer at room temperature. Light from a
150 W xenon lamp was used as an excitation source.
Photoluminescence spectra were recorded from 420
to 1010 nm with an excited wavelength of 345 and
440–460 nm, and excitation spectra were recorded from
250 to 550 nm with respect to the wavelength of lumines-
cence peak. The emission and excitation spectra were
corrected with the excitation spectrum of Rhodamine B in
ethylene glycol (8 g/L) [34] and standard lamp. The diffuse
reflectance spectra measurement was carried out using a
JASCO V-550 ultraviolet and visible spectrophotometer.
In this study, we investigated the structure, luminescent
properties and their relationship of R1/2Na1/2TiO3:Pr(R ¼ La,
Gd, Lu, and Y). Here we chose La3+, Gd3+, Lu3+, and Y3+
as R ion because they exhibit no absorption of near
ultraviolet and visible light due to the f–f transition. The
mechanism of photoluminescence of Pr-doped perovskite
titanates upon band gap photo-excitation is discussed.
3. Results and discussion
3.1. Crystal structure and optical band gap of RNTO
(R ¼ La, Gd, Lu, and Y)
The powder X-ray diffraction experiments revealed that
all the RNTO samples possess perovskite-type structure

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1680
(please see Figs. ES1–ES5 in Electronic Supplementary
Information). A trace of impurity phases was observed
only in LuNTO. The Pr-doped RNTO samples have the
same structure as the pristine RNTO. The structure of
LaNTO was refined with two different space groups,
same BVSs of Ti (see Table 1). This means that the tilt of
TiO6 octahedron primarily compensates the decrease in the
space of A-site ion.
The absorbance spectra of RNTO (R ¼ La, Gd, Lu, and
Y) at room temperature are shown in Fig. 1. Here, the
diffuse reflectance data were transformed into absorbance
with the Kubelka-Munk function. With a decrease in
the ionic radius of R, the wavelength with respect to
absorption edge decreases; i.e., the optical band gap
energy, Eg, increases. Since the Ti–O inter-atomic distances
are almost the same, the difference in Eg cannot be
explained by the difference in Ti–O inter-atomic distances.
In ATiO3, it is known that the orbital character of the
valence band is primarily derived from Oxygen 2p orbitals,
while the conduction band has strong Titanium 3d orbital
character. Therefore, the chemical bond between Ti3d and
O2p orbital become less covalent accompanied by the
decrease of Ti–O–Ti bond angle and consequently the band
width of bonding and anti-bonding orbitals decrease,
resulting in the increase in band gap. Since the tilt angle
of TiO6 octahedron increases; i.e., Ti–O–Ti bond angle
decreases with a decrease in the ionic radius of R for
RNTO, the Eg is expected to increase. Fig. 2 shows the
variation of optical band gap energy, Eg, versus the average
the Ti–O–Ti angles. As seen in Fig. 2, the Eg of RNTO
increases with decreasing the Ti–O–Ti bond angle. The
result therefore implies that the Eg of RNTO is primarily
determined by the Ti–O–Ti angle. The data of CaTiO3 and
SrTiO3 are then also plotted for reference. The estimated
¯
I4/mcm (No. 140) and R3-c (No. 167) [35,36] since the axis
of anti-phase tilt of TiO6 was not discretely determined due
to the low degree of lattice distortion. The Glazer notation
[37,38] of tilt of TiO6 is a0a0cꢀ for I4/mcm, and is aꢀaꢀaꢀ
¯
for R3-c. In the former, the TiO6 octahedron tilts along
the tetragonal c-axis, resulting in the bond angles of
Ti–O–Ti of 1801 along tetragonal c-axis and less than 1801
perpendicular to tetragonal c-axis. In the latter, the TiO6
octahedron tilts along the hexagonal c-axis, resulting in one
type of bond angle of Ti–O–Ti less than 1801. On the other
hand, the structure of RNTO (R ¼ Gd, Lu, and Y) was
refined with the space group Pnma (No. 62) the same as the
previous report [24,26]. The notation is aꢀb+aꢀ for Pnma,
as a result, there are two types of Ti–O–Ti bond whose
angles are less than 1801. The results of Rietveld refinement
are summarized in the Electronic Supplementary Informa-
tion. The selected inter-atomic distances and angles
of RNTO (R ¼ La, Gd, Lu, and Y) were summarized in
Table 1. The same as previous reports [24–28], the lattice
parameters, R, Na–O distances, and Ti–O–Ti bond angles
decrease; i.e., tilt angles increase, with increasing the ionic
radius of R ion in RNTO (R ¼ La, Gd, Lu, and Y). On the
other hand, the Ti–O distances are similar despite of a
decrease in the lattice parameters, resulting in the almost
Table 1
Selected inter-atomic distances (nm), bond angles (deg), and calculated bond valence sum (BVS) of Ti at room temperature for R1/2Na1/2TiO3 (R ¼ La,
Gd, Lu, and Y)
La1/2Na1/2TiO3
Gd1/2Na1/2TiO3
Lu1/2Na1/2TiO3
Y1/2Na1/2TiO3
Model-1 (I4/mcm)
Model-2 (R3-c)
0.19494(4) ꢁ 6
Distance
Ti–O1
Ti–O2
0.19378(6) ꢁ 2
0.19540(9) ꢁ 4
0.1952(2) ꢁ 2
0.1932(7) ꢁ 2
0.1965(7) ꢁ 2
0.19551(12) ꢁ 2
0.1950(4) ꢁ 2
0.1959(4) ꢁ 2
0.19540(10) ꢁ 2
0.1951(3) ꢁ 2
0.1966(3) ꢁ 2
R, Na–O1
R, Na–O2
R, Na–Ti
0.27406(6) ꢁ 4
0.2533(3) ꢁ 3
0.27479(5) ꢁ 6
0.2951(3) ꢁ 3
0.2353(10) ꢁ 1
0.2523(6) ꢁ 1
0.2980(7) ꢁ 1
0.3047(10) ꢁ 1
0.2303(5) ꢁ 1
0.2327(4) ꢁ 1
0.3056(5) ꢁ 1
0.3219(4) ꢁ 1
0.2329(4) ꢁ 1
0.2405(3) ꢁ 1
0.3055(4) ꢁ 1
0.3121(3) ꢁ 1
0.2570(4) ꢁ 4
0.2923(5) ꢁ 4
0.2397(6) ꢁ 2
0.2571(8) ꢁ 2
0.2720(8) ꢁ 2
0.3193(6) ꢁ 2
0.2345(3) ꢁ 2
0.2501(4) ꢁ 2
0.2648(4) ꢁ 2
0.3360(3) ꢁ 2
0.2350(2) ꢁ 2
0.2557(3) ꢁ 2
0.2666(3) ꢁ 2
0.3310(2) ꢁ 2
0.33565(6) ꢁ 8
0.33546(6) ꢁ 2
0.33578(8) ꢁ 6
0.31826(14) ꢁ 2
0.3270(2) ꢁ 2
0.3338(2) ꢁ 2
0.34658(15) ꢁ 2
0.30663(9) ꢁ 2
0.32049(11) ꢁ 2
0.33252(12) ꢁ 2
0.35757(11) ꢁ 2
0.31197(10) ꢁ 2
0.32414(13) ꢁ 2
0.33320(13) ꢁ 2
0.35301(12) ꢁ 2
Bond angle
Ti–O1–Ti
Ti–O2–Ti
180
165.3(4)
167.69(15)
4.17
156.7(5)
157.4(3)
151.77(17)
151.7(2)
154.2(2)
153.12(13)
BVS of Ti
4.18
4.17
4.09
4.09

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1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
Ti–O–Ti angles derived from the average structure, and
other factors, e.g., the local structure originated from the
different A-site ions, R3+ and Na+, must be taken into
account.
La
3.2. Optical absorption and luminescent properties of
Pr-doped RNTO
Gd
The diffuse reflection spectra of RNTO:Pr (R ¼ La, Gd,
Lu, and Y) at room temperature are shown in Fig. 3. The
absorption peaks in the vicinity of 400–500 and 610 nm
were observed in addition 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),
Y
1
3P0(490–500 nm), and D2(610–620 nm) [5]. The shoulder
band beside the absorption edge for RNTO:Pr (R ¼ Gd,
Lu, and Y) was then observed in the long wavelength side,
and on the contrary the shoulder band was not observed in
LaNTO:Pr. Fig. 4 shows the diffuse reflectance spectra of
YNTO:Pr and YNTO. As seen in Fig. 4, the shoulder band
was not observed in pristine YNTO, indicating that this
Lu
340
360
380
Wavelength/nm
400
420
band is related to Pr but is not an f–f transition of Pr3+
.
Fig. 1. The absorbance spectra of RNTO (R ¼ La, Gd, Lu, and Y) at
room temperature.
Boutinaud et al. [15,16,22,23] found this type of absorption
in Pr-doped CaTiO3, and they proposed that this band is
attributed to Pr3+/Ti4+2Pr4+/Ti3+ IVCT as the final
relaxation channel to the emitting 1D2 level. They also
pointed out [22,23] that the IVCT is dependent on the
Pr–Ti shortest distance in Pr-doped d0 transition-metal
oxides and the IVCT can be observed when the distances
are in the 0.315–0.330 nm. Supposing that Pr–Ti distance is
approximately equal to R,Na–O inter-atomic distance, the
shortest inter-atomic distances of Pr–Ti in RNTO:Pr
3.6
CaTiO3
Lu
3.5
Y
Gd
1.0
0.8
0.6
3.4
model-1
model-2
(I4/mcm)
(R3-c)
3.3
R = La
SrTiO3
175
3.2
150
155
160
165
170
180
Average angle of Ti-O-Ti /deg.
0.4
R = La
Fig. 2. Variation of optical band gap energy, Eg, versus average Ti–O–Ti
angles of RNTO (R ¼ La, Gd, Lu, and Y) at room temperature. The data
of CaTiO3 and SrTiO3 are also plotted for reference.
R = Gd
R = Lu
0.2
R = Y
band gap energies, 3.56 and 3.26 eV are in good agreement
with those previously reported, 3.57 eV for CaTiO3 [39] and
3.2 eV for SrTiO3 [40], respectively. In Fig. 2, the structural
data of CaTiO3 after Sasaki et al. [41] is adopted. It seems
that the data of CaTiO3 and SrTiO3 deviate from those of
RNTO. This result implies that the band gap energies of
0.0
300
400
500
600
700
800
Wavelength / nm
Fig. 3. Diffuse reflectance spectra of R1/2Na1/2TiO3:Pr 0.2% (R ¼ La, Gd,
A
2+TiO3 and R31+/2 Na1+/2TiO3 cannot be ruled only by the
Lu, and Y).

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1.0
0.8
0.6
0.4
0.2
0.0
R = La
R = Gd
R = Lu
R = Y
Gd
Y
R = La
Lu
580
600
620
640
660
Wavelength / nm
Pr-doped YNTO
YNTO
500
600
700
800
900
1000
Wavelength / nm
Fig. 5. Emission spectra of R1/2Na1/2TiO3:Pr 0.2 mol% (R ¼ La, Gd, Lu,
1
and Y) under an excitation at 345 nm. Inset shows the profiles of red D2
300
400
500
600
700
800
emission.
Wavelength/nm
Fig. 4. Diffuse reflectance spectra of Y1/2Na1/2TiO3:Pr (solid line) and
Y1/2Na1/2TiO3 (dashed line).
R ¼ La, Gd, Y, and Lu. These results indicate that the
inter-atomic distance of Pr–O in RNTO:Pr also decreases
together with a decrease in inter-atomic distance of
(R, Na)–O, resulting in the increase in covalency between
Pr and oxygen. The weak green-blue emission correspond-
ing to the f–f transition of 3P0 to 3H4 was then observed in
LaNTO though the green-blue emission in other com-
pounds was little observed. This phenomenon will be
discussed later.
(R ¼ La, Gd, Lu, and Y) are 0.336, 0.318, 0.307, and
0.312 nm seen in Table 1, respectively. The obtained result
is therefore in good agreement with the findings by
Boutinaud et al. In addition, they stated that the IVCT is
present at a higher energy level and overlaps the absorption
of host materials even when the inter-atomic distance of
Pr-Ti is beyond 0.330 nm. This idea can explain no
absorption corresponding to the IVCT in LaNTO:Pr. As
another possibility, the finding is also likely to be ascribed
to poor energy transfer from 4f orbital of Pr to 3d orbital of
Ti due to the long inter-atomic distance of Pr–Ti despite
that the energy difference between Pr 4f and Ti 3d being the
same as RNTO:Pr (R ¼ Gd, Lu and Y).
Fig. 6 shows the emission and excitation spectra of
RNTO:Pr (R ¼ La, Gd, Lu, and Y). Here the excitation
spectra were recorded with respect to the red luminescence
peak. Compared with Figs. 4 and 6, the peak of excitation
is in agreement with the optical absorption edge. This
finding indicates that the red intense luminescence is mainly
based upon the band-gap photo-excitation of RNTO:Pr.
The shoulders adjacent to absorption edge seen in diffuse
reflectance spectra of RNTO (R ¼ Gd, Lu, and Y), also
corresponds to the shoulder of excitation spectra. The
IVCT pointed out by Boutinaud et al. [15,16,22,23], is
therefore one of the pathway of energy transfer for red
emission in RNTO:Pr (R ¼ Gd, Lu, and Y).
Fig. 5 shows the emission spectra of RNTO:Pr (R ¼ La,
Gd, Lu, and Y) under an excitation at 345 nm. All the
compounds show intense red emission with a wavelength,
l, of 610–616 nm, corresponding to the f–f transition
of Pr3+(1D2 to 3H4), and several emissions in near
infrared-red region, corresponding to the transitions of
1D2 to 3H5 (lꢂ700 nm), 1D2 to 3H6 (lꢂ820 nm), 1D2 to 3F2
(lꢂ880 nm) and 1D2 to 3F3, 3F4 (lꢂ1000 nm) [5]. As seen in
the inset of Fig. 5, the wavelength of red emission peaks
was red-shifted in the order R ¼ La, Gd, Y, and Lu. Other
peaks in the infrared-red region were red-shifted in the
same way. Furthermore, the absorption peaks correspond-
On the other hand, upon photo-excitation to the excited
3
3
3
states of Pr3+, P2, P1, and P0, poor red emission was
observed. Fig. 7(a) and (b) show the luminescence spectra
upon band gap excitation (l ¼ 345 nm) and the excitation
3
in accordance to f–f transition (3H4 to P2, l ¼ 455 nm) of
RNTO:Pr (R ¼ La and Y), respectively. The red emission
upon band gap excitation is much more intense relative to
that upon the f–f excitation of 3H4 to 3P2 for both
compounds, indicating the effective energy transfer of
band gap excitation energy. The calculated peak intensity
ratio of green-blue emission (ꢂ490 nm) to red emission
(ꢂ610 nm) (G/R ratio) upon band gap excitation and
3
3
1
ing to the f–f transitions of Pr3+ from H4 to P2, I6 and
3P1, P0, and D2 is seen in Fig. 4, also show the same
tendency. These findings originate from the increase in the
crystal field spilitting due to the increase in covalency
between Pr and oxygen, i.e., the decrease in inter-atomic
distance of Pr–O in RNTO:Pr. As seen in Table 1, the inter-
atomic distance of (R, Na)–O decreases in the order
3
1
3
3
upon H4 to P2 excitation using the spectrum data shown

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1683
excitation
emission
R = La
LaNTO:Pr0.2%
R = Gd
R = Lu
R = Y
450 500 550 600 650 700 750
Wavelength/nm
300
400
500
600
700
Wavelength / nm
Fig. 6. Emission (excitation: 345 nm) and excitation (emission:
610–616 nm) spectra of R1/2Na1/2TiO3:Pr 0.2 mol% (R ¼ La, Gd, Lu,
and Y).
450
500
550
600
650
700
750
Wavelength/nm
in Fig. 7 are 5% and 40% for LaNTO:Pr, and o0.1% and
6% for YNTO:Pr, respectively. These results imply that the
red emission occurs via the different route upon two
YNTO:Pr0.2%
3
different excitations. Upon the photo-excitation from H4
3
3
to P2, the route is simply from the P0 excite state to the
3H4 ground state. On the other hand, upon band gap
3
excitation, the main relaxation channel is not that via P0
excite state, but the channel from the conduction band via
an excited state D2 of Pr3+ to finally H4 ground state.
Furthermore, the evident differences in G/R ratio between
LaNTO:Pr and YNTO:Pr were also found. Upon photo-
excitation from 3H4 to 3P2, the difference between two
compounds (G/R ratio: 40% and 6%) is partly ascribed to
the difference in transition probability due to the difference
of site symmetry of Pr between LaNTO (space group:
1
3
450 500 550 600 650 700 750
Wavelength/nm
¯
¯
I4/mcm or R3-c, point group: 42m or 32) and YNTO
(space group: Pnma, point group: m). Upon band gap
excitation, the weak green-blue emission was observed for
LaNTO:Pr, on the contrary, the emission for RNTO:Pr
(R ¼ Gd, Lu, and Y) was almost quenched as mentioned
before. The large difference in R/B ratio (5% and o0.1%)
cannot be explained only by the difference in site
symmetry. Boutinaud et al. [15,16,22,23] claimed that the
450
500
550
600
650
700
750
Wavelength/nm
Fig. 7. Emission spectra upon photo-excitation of 345 nm (solid line) and
of 455 nm(dashed line) for R1/2Na1/2TiO3:Pr 0.2 mol% (R ¼ La and Y).
The enlarged figures are shown in the insets.
3
quenching of green-blue emission from P0 upon band gap
excitation is related to the energy level of IVCT relative to
the energy level of conduction band. Their idea can be
applied to account for our results. Considering another
possibility, it is likely that a part of Pr ions are
accommodated in Ti-site in LaNTO and the green-blue
emission originates from the Pr ion in Ti-site. We therefore
also need to elucidate the relationship between the site
preference of Pr and the luminescent properties.
Ti4+2Pr4+/O2ꢀ/Ti3+proposed by Boutinaud et al.
[15,16,22,23]. The electron of Pr3+ is excited to the IVCT
state and here charge transfer to Ti4+ occurs via O 2p
orbital, and then the electron returns to the Pr ion in the
excited state, D2 [42]. Finally the transition accompanied
3
by red emission to the ground state, H4, occurs. Another
one is upon band gap excitation. The schematic diagram
of the band gap excitation and emission processes is
shown in Fig. 8. The electron in the valence band which
primarily consists of O 2p orbital is photo-excited into the
1
The obtained results indicates that the intense red
emission in RNTO:Pr is based on the two different
routes. One is due to the IVCT via oxygen, Pr3+/O2ꢀ
/

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Y. Inaguma et al. / Journal of Solid State Chemistry 180 (2007) 1678–1685
1684
intense red emission assigned to f–f transition from the
excited 1D2 level to the ground 3H4 state of Pr3+. The tilt of
TiO6 octahedra changes the band width, bringing in the
increase in the optical band gap of R1/2Na1/2TiO3 in the
order R ¼ La, Gd, Y, and Lu. The red-shift in emission
peaks in the order R ¼ La, Gd, Y, and Lu reveals that the
inter-atomic distance between Pr and O decreases with a
decrease in (R, Na)–O average distance. The differences in
luminescent properties for R1/2Na1/2TiO3:Pr are ruled by
the relative energy level between the ground and excited
state of 4f2 for Pr3+, and the conduction and valence band,
which is strongly correlated to the structure, e.g., the tilt of
TiO6 octahedra and the Pr-Ti inter-atomic distance and the
site symmetry of Pr ion.
Conduction band
Ti3d ,O2p*
Pr3+ 3P0
Pr3+ 1D2
excitation
Red emission
Pr3+ 3
H
4
O2p
Valence band
Acknowledgments
Fig. 8. Schematic diagram of the band-gap excitation and emission
processes in R1/2Na1/2TiO3:Pr.
The authors are grateful to the reviewers for their useful
advice and comments. This work was supported by a
Grant-in-Aid for Scientific Research (c) (No. 17560598) of
Japan Society for the Promotion of Science, the Grant-in-
Aid for Scientific Research on Priority Area, ‘‘Nanoionics
(439)’’, and ‘‘High-Technology Research Center Project’’,
of the Ministry of Education, Culture, Sports, Science and
Technology of Japan.
conduction band which primarily consists of Ti 3d and O
2p orbitals, and then the electron of Pr3+ is transferred to
the O 2p orbital in the valence band i.e., the generated hole
accompanied by the excitation of electron is transferred to
the Pr3+. The excited electron in conduction band relaxes
1
to the excited state of Pr3+, D2 (this relaxation process
may be via the IVCT state), and then returns to the ground
state of Pr3+, which is accompanied by the red emission.
The formal charge transfer in these processes is described
as follows:
Appendix A. Supplementary materials
Supplementary data associated with this article can
be found in the online version at doi:10.1016/j.jssc.
2007.03.017.
O2ꢀ þ Pr3þ ! Oꢀð¼ O2ꢀ þ hþÞ þ Pr3þ þ eꢀ
O
!
2ꢀ þ Pr4þ þ eꢀ ! O2ꢀ þ Pr3þꢃ ! O2ꢀ þ Pr3þ
Here, ‘‘Pr3+*’’ denotes the Pr3+ ion which is in the
excited state.
The electron transfers from the conduction band to the
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