Photoluminescence studies on rare earth titanates prepared by self-propagating high temperature synthesis method

Article abstract of DOI:10.1016/j.saa.2008.03.030

The laser-induced luminescence studies of the rare earth titanates (R₂Ti₂O₇) (R = La, Nd and Gd) using 355 nm radiation from an Nd:YAG laser are presented. These samples with submicron or nanometer size are prepared by the

Full text of DOI:10.1016/j.saa.2008.03.030

Spectrochimica Acta Part A 71 (2008) 1281–1285
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Photoluminescence studies on rare earth titanates prepared
by self-propagating high temperature synthesis method
Lyjo K. Josepha,∗, K.R. Dayasb,c, Soniya Damodarb, Bindu Krishnana,b,1
,
K. Krishnankuttyc, V.P.N. Nampooria, P. Radhakrishnana
a International School of Photonics, Cochin University of Science and Technology, Kalamssery, Kochi 682022, Kerala, India
b Centre for Materials for Electronics Technology, Athani PO, Thrissur 680771, Kerala, India
c Department of Chemistry, University of Calicut, Calicut University PO, Tenhipalam, Malappuram 673635, Kerala, India
a r t i c l e i n f o
a b s t r a c t
Article history:
The laser-induced luminescence studies of the rare earth titanates (R2Ti2O7) (R = La, Nd and Gd) using
355 nm radiation from an Nd:YAG laser are presented. These samples with submicron or nanometer size
are prepared by the self-propagating high temperature synthesis (SHS) method and there is no known
fluorescence shown by these rare earths in the visible region. Hence, the luminescence transitions shown
by the La2Ti2O7 near 610 nm and Gd2Ti2O7 near 767 nm are quite interesting. Though La3+ ions with no 4f
electrons have no electronic energy levels that can induce excitation and luminescence processes in the
visible region, the presence of the Ti3+ ions leads to luminescence in this region.
Received 11 January 2008
Received in revised form 29 February 2008
Accepted 19 March 2008
PACS:
78.55.−m
78.55.Mb
© 2008 Elsevier B.V. All rights reserved.
Keywords:
Photoluminescence
Rare earth titanates
Self-propagating high temperature
synthesis
Pyrochlore
1. Introduction
photochemical properties in the photo assisted reactions. It is
closely related to the surface stoichiometry and the nature of sur-
Rare earth titanates (R2Ti2O7) (RET) have interesting dielectric,
piezoelectric and ferroelectric properties [1–3]. These materials
usually possess a cubic pyrochlore structure [4–7] for the ions
face states, which can usually be changed by the annealing process
[14]. Investigations have shown that the luminescence efficiency
of materials can be improved by appropriate impurity addition
[15,16].
with small R3+ (Sm3+–Lu3+), while with the larger R3+ (La3+–Nd3+
)
exhibit a monoclinic structure [2,8]. Pyrochlore RETs and zirconates
find numerous applications such as hosts for fluorescence centres,
high temperature pigments, catalysts, thermal barrier coatings,
ionic/electronic conductors, host phase in nuclear waste control,
etc. [3,9–11]. Some of the pyrochlores exhibit good catalytic activ-
ity, high melting point, low thermal conductivity, high thermal and
phase stability, which make them promising for the catalytic com-
bustion applications and thermal barrier coatings [12,13].
The rare-earth (RE) doped laser crystals, glasses and ceramics,
which possess trivalent RE ions, are popular solid-state gain media.
RE ions are also used as co-dopants for quenching the population in
certain energy levels by the energy transfer processes, or for realiz-
ing saturable absorbers, or as optically passive constituents of laser
crystals [17]. The PL and decay times in nano-structured xerogel and
annealed sol–gel silica glasses doped with RE ions, transition metal
ions, metal complexes and semiconductor nano-crystals are inves-
tigated previously to study the excitation energy transfer processes,
which reflect the optical properties of these confined nano-porous
networks [18–20]. RE metal compounds, especially their oxides,
have become of increasing interest in recent years because of their
special PL and catalytic properties [21,22]. RE-based phosphors play
a critical and indispensable role as luminescent materials in the
display industry. Synthesize of high quality RE-based phosphors
materials in the nano-region is thus important. With the growing
Photoluminescence (PL) is a non-destructive and highly sen-
sitive method commonly used to study the photo-physical and
∗
Corresponding author. Tel.: +91 484 2575848; fax: +91 484 2576714.
E-mail addresses: lyjokjoseph@yahoo.co.in, lyjo@cusat.ac.in (L.K. Joseph).
1
Present address: Indian Institute of Space Science and Technology, VSSC Campus,
Thiruvananthapuram, Kerala, India.
1386-1425/$ – see front matter © 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2008.03.030

1282
L.K. Joseph et al. / Spectrochimica Acta Part A 71 (2008) 1281–1285
interest in nanoscale RE activated phosphors for display applica-
tions, many efforts have been made to develop these nanoscale
phosphor materials because of their good luminescence charac-
teristics, stability in high vacuum, and absence of corrosive gas
emission under electron bombardment [23,24].
The RETs are prepared by self-propagating high temperature
synthesis (SHS) method, which is also known as the self-ignition
synthesis or combustion synthesis. The titanates and oxides
prepared by SHS method are superior to those prepared by con-
ventional methods. The salient feature of SHS method is that it
utilizes reaction heat, instead of electric power. The SHS prod-
ucts are good quality submicron or nanometre sized powders with
non-agglomerated nature. It can be synthesized to phase pure and
sintered to high density at relatively lower temperatures [25–29].
2. Sample preparation
The corresponding RE oxides (0.02 mol) are dissolved in hot
HNO3. This solution is slowly heated on a boiling water bath to
remove the excess acid, if any, and evaporated to get the dry RE
nitrate. To this, TiO2 (0.04 mol) and urea (0.16 mol) are added and
slowly heated on an electric Bunsen and the contents in the beaker
are thoroughly mixed by stirring. Urea acts as the solid fuel in the
reaction and its molar ratio is very crucial. As the temperature
increases to around 300 ◦C, urea melts and starts decomposing with
the evolution of ammonia. When the temperature reaches ∼350 ◦C,
copious brown fumes of oxides of nitrogen start evolving. At this
stage a spontaneous incandescent reaction takes place vigorously
and the mixture self-ignites at about 400 ◦C along with evolution
of large amount of gases. The content of the vessel froths up to a
highly porous network of foam like structure, which almost fills
the reaction vessel and gradually the flame subsides. The highly
porous network of the fine crystallites of the RET formed is cooled to
room temperature. The powder is calcined, pelletised and sintered
at 1350–1550 ◦C.
Fig. 1. Reflectance spectra of the samples.
4. Results and discussion
From the reflectance spectrum, shown in Fig. 1, it is clear that
the samples have absorption at around 355 nm.
The XRD of these SHS powders show the amorphous or micro-
crystalline nature of the products. However, the crystallinity
gradually improves on calcination at different temperatures and
at 1200 ◦C, transforms to 100% phase pure crystalline material. The
peaks in the spectra of R2Ti2O7 synthesized by SHS-urea method
and calcined at 1200 ◦C (Fig. 2), where R = La and Nd match exactly
with the peaks of the standard monoclinic R2Ti2O7; whereas the
spectra of Gd agree well with standard cubic R2Ti2O7. The XRD
and SEM of a cubic and monoclinic RET pyrochlore are shown in
Figs. 2–6 to have an idea about their structure [30].
The SEM photographs show very fine sizes of the order of
nanometer ranges for the powders. The La2Ti2O7 (Fig. 3) and
Nd2Ti2O7 powders appear to be free in nature with fewer inter-
connections of the powder particles.
3. Experimental details
The relative reflectance spectra of the pelletised samples are
taken using a JASCO V-570 UV/VIS/NIR spectrophotometer with the
model SLM-468 single reflection attachment at an angle of inci-
dence approximately 5◦ with aluminium-deposited plane mirror
as reference. The surface area of the samples is measured using the
Quanta chrome Nova model 1200 BET analyzer and from the data
on density of the SHS powders, their average particle size can be
calculated (Table 1). The Bruker AXS 5005 XRD machine with Cu
The SEM microstructure of the sintered (at 1450 ◦C) monoclinic
RETs exhibit a well sintered physical appearance for the broken
cross-sectional structure of the discs. The grains are so smoothly
interconnected with almost zero porosity and appear to be single
domain in structure. The SEM microstructure of the cross-sectional
surfaces of the La2Ti2O7 discs sintered at 1450 ◦C is given in Fig. 4.
˚
K␣ (ꢀ = 1.5418 A) radiation is used for evaluating the products syn-
thesized in this study. The SEM microstructure measurements are
done using A JEOL JSM-840A SEM.
Using the pellets sintered at 1350 ◦C the luminescence stud-
ies are carried out at room temperature with the third harmonic
of an Nd:YAG laser (355 nm, 7 ns Spectra Physics Quanta Ray).
The luminescence spectrum is recorded using 0.5 m triple grating
spectrograph/monochromator assembly (Spectrapro 500i Acton
Research) attached with a CCD camera (Roper Scientific NTE/CCD
1340/100-EM).
Table 1
Details of the RET samples used for the study
R2Ti2O7 (R=) Density (g/cm3) Surface area (m2/g) Average particle size (nm)
Gd
La
Nd
6.56
5.78
6.11
13.67
11.40
16.01
67
91
61
Fig. 2. Typical XRD spectra of Gd2Ti2O7 (match JCPDS-ICDD 23-0259 cubic
Gd2Ti2O7) and La2Ti2O7 (match JCPDS-ICDD 28–0517 monoclinic La2Ti2O7) synthe-
sized by SHS-urea method and calcined at 1200 ◦C.

L.K. Joseph et al. / Spectrochimica Acta Part A 71 (2008) 1281–1285
1283
Fig. 6. SEM of the cross-section of sintered discs (at 1450 ◦C) of cubic Gd2Ti2O7 made
Fig. 3. SEM microstructure of SHS powders of monoclinic La2Ti2O7.
from SHS powders.
The SEM of Gd2Ti2O7 in Fig. 5 shows loosely packed powders
along with some particles in a free state. SEM of cross-section of
the sintered Gd2Ti2O7 (1450 ◦C) discs made from SHS powders of
cubic crystalline RETs is shown in Fig. 6. A comparison with SEM
structure of sintered monoclinic titanates reveals that the cubic
form is not well sintered even at temperatures of 1450–1500 ◦C,
unlike the monoclinic material.
Fluorescence spectra due to the presence of RE ions in solids
give information about the crystal field exerted on the respective
elements in the compounds. The characteristic fluorescence spectra
of different RE ions arise from the electronic transitions in the par-
tially filled 4f orbitals. Electrons present in the occupied 4f shells
can be transferred by light absorption, into unoccupied levels of
higher energies. The 4f orbitals are well shielded by the fully filled or
partially filled 5s and 5p outer shells. As a result, emission lines are
relatively narrow and the energy level structure varies only slightly
from one host to another. The effect of the crystal field is normally
treated as a perturbation on the free-ion levels. The perturbation is
small compared to spin-orbit and electrostatic interactions among
the 4f electrons. Since the splittings are small, the terms and their
levels remain easily identifiable for the REs. The primary change in
the energy levels due to a splitting of the free ion levels is caused
by the Stark effect of the crystal field. In crystals the free-ion lev-
els are then referred to as manifolds. The trivalent lanthanide ions
show efficient extrinsic localized type luminescence of the char-
acteristic line spectra owing to f ←→ f forbidden type transition in
the visible to near-infrared region. When atoms and ions are incor-
porated in crystals, the forbidden character of the electric dipole
transition is altered by the perturbation of the crystal electric field,
so that the forbidden transition becomes allowed to some degree
[31]. Energy transfer from a light gathering meso-structured host
lattice to an appropriate RE ion generates selected PL in the region
600–1540 nm. Exciting the titania in its band gap results in the
energy transfer and it is possible to observe PL from the crystal
field states of the RE ions [32].
The luminescence spectra recorded at a pump power of 100 mW
using the CCD spectrograph assembly are shown in Fig. 7. The
excited ions of the La2Ti2O7 sample are observed to fluoresce with
Fig. 4. SEM of the cross-section of sintered discs (at 1450 ◦C) of monoclinic La2Ti2O7
made from SHS powders.
Fig. 7. Luminescence spectrum of the rare earth titanates when excited using
100 mW laser pulse of 355 nm.
Fig. 5. SEM microstructure of SHS powders of cubic Gd2Ti2O7.

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L.K. Joseph et al. / Spectrochimica Acta Part A 71 (2008) 1281–1285
peak emission within the range 608–612 nm region and also at
1221 nm which is the next harmonic. The spectrum of the TiO2
is also given for comparison. The La2Ti2O7 is reported to have
direct band gap and act as a good photo-catalyst for different light
absorption known for this kind of materials [2,8]. La2Ti2O7 has a
monoclinic structure with a space group of P21 at room tempera-
ture. Compounds with the pyrochlore structure have the formula
A23+B24+O7, where A can be the RE cation and B is Ti or any transi-
tion metal ions [33]. La3+ ion has a [Xe] 4f0 inert gas configuration
leading to a 1S0 ground state. Excited states can arise from a p–d
transition leading to the electronic configuration 5s2 5p5 5d1 which
gives rise to the singlet 1P1, 1D2 and 1F3 and the triplet 3P1, 3D2 and
3F3 states [34]. Gd3+ has a 4f7 configuration and a ground state of
8S7/2 [7,35–37].
The Gd2Ti2O7 has peaks at 767 and 1536 nm. The second peak is
observed to be the multiple of the first and hence can be considered
as the harmonic. The 6IJ to 6GJ transitions give rise to the 767 nm
emission from the Gd3+ ions. A broad spectrum near 1000 nm is
also seen, which is produced by the TiO6 octahedra centre that is
common in all the RETs used for the present study. In the case of
Nd2Ti2O7 the spectrum is more prominent in this region (1000 nm)
and can be associated with the emission due to the Nd3+ ions arising
from 4F3/2 to the 4IJ (4I9/2: 900 nm, 4I11/2: 1060 nm, 4I13/2: 1350 nm)
transitions.
than the absorption edges. There is no contribution from the La to
the valence band [2]. The various Russell–Saunders states arising
from the 4fn configurations are split by the crystal field, but the
splitting is about 100 cm−1. The extremely small splitting makes
the f–f absorption bands very sharp giving rise to line-like spectra.
The R 4f level in R2Ti2O7 is shifted to lower energy as the number
of R 4f electrons increases. This red shift of the R 4f band decreases
the band gap energy of the R2Ti2O7 [2]. Thus a red shift in the lumi-
nescence to Gd3+ ions with respect to La3+ ions is also observed as
the R3+ ion radius is decreased due to lanthanide contraction. This
explains the red shift exhibited by Gd3+ ions which has a smaller
ionic radius with respect to La3+ ions.
The observed luminescence property may be attributed to La3+
centre in the nano-sized La2Ti2O7 whereas for the other titanates
it may be attributed to TiO6 octahedra centres. Thus nano-sized
La2Ti2O7 is a promising red phosphor for display applications.
5. Conclusions
The luminescence shown by the nano-sized La2Ti2O7 in the vis-
ible region is due to the presence of the unfilled 4f orbital of the
La3+ ions in the TiO6 octahedra centres even though ions with no 4f
electrons do not have such emission in this region. The SE of transi-
tion metal activator Ti3+ ions in insulating laser crystals near 611 nm
can be enhanced by the sensitizer La3+ ions and it is sharp due to
the presence of the 4f orbitals. In other RETs, this SE is inhibited
by the presence of R3+ ions in 4f orbital. In the case of Nd2Ti2O7
the fluorescence emission of the Nd3+ ions is overlapped by the
presence of the Ti3+ SE channels. A red shift in the luminescence
of Gd3+ ions with respect to La3+ ions is observed as the R3+ ion
radius is decreased due to lanthanide contraction. The observed
luminescence of nano-sized La2Ti2O7 near 610 nm makes it suitable
for display applications after further characterizations.
It has been reported earlier that the transition metal activator
Ti3+ ions in insulating laser crystals have spectral stimulated emis-
sion (SE) near 611 nm and within the range 660–1180 nm with the
2
SE channel 2E → T2 for operating temperature 300 K and at laser
pumping conditions [38]. Ions with no 4f electrons are reported
to have no electronic energy levels that can induce excitation and
luminescence processes in or near the visible region [31]. However,
in the case of La2Ti2O7 a transition close to 611 nm is observed in
our studies. The luminescent peaks observed at 610 and 1221 nm
are sharp and it can be credited to the unfilled 4f orbital of the
RE (La) added. An enhancement in the emission at around 610 nm
is due to the presence of the unfilled 4f orbital of the La3+ ions
present in the TiO6 octahedra centres. Transition of the La3+ from
the excited 3F4 state to the ground state 1S0 enhances the Ti3+ SE
channel near 611 nm to give rise to the luminescence at 610 nm for
the La2Ti2O7. The La3+ with the unfilled 4f orbital acts as a sen-
sitizer and enhances the SE of the Ti3+. The crystal field splitting
can also be observed in the resolved spectrum which is shown in
Fig. 8. Internal 4f transitions are observed at wavelengths shorter
Acknowledgment
The author LKJ is thankful to UGC (Govt. of India) for financial
support.
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