The role of reaction conditions in the polymorphic control of Eu3+ doped YInO3: Structure and size sensitive luminescence

Article abstract of DOI:10.1039/c4dt02717e

With the aim of exploring the effect of particle size and different polymorphic structures on the luminescence behaviour of Eu³⁺ ions, all in a single compound, the YInO₃:Eu³⁺ system was synthesized. Metastable C-type modification could be obtained in YInO₃ nanopowders synthesized by gel combustion. The broadening in Raman modes has been related to both the particle size and the strain in the metastable C-type modification. The hexagonal polymorph of YInO₃, with a layered structure, quite unlike the C-type modification, could be obtained by heating nanopowders to 1175°C. Such a change in the structure, leading to different site symmetries for Eu³⁺ ions exhibited a tremendous bearing on its luminescence behaviour. Detailed steady state and time resolved luminescence studies revealed a highly distorted local site symmetry for the Y³⁺ ion in hexagonal YInO₃. The nano C-type YInO₃ exhibited greater asymmetry and a shorter lifetime as compared to the bulk C-type YInO₃, attributable to more surface defects in the case of the former. The Judd-Ofelt parameters (Ω₂ and Ω₄) were determined which established a much higher asymmetry in the hexagonal modification. Along with higher lifetime, the hexagonal polymorph also possessed superior quantum yield values. In addition to providing a significant insight into the structure-luminescence correlation in this study, we propose an efficient red phosphor with a high red color purity.

Full text of DOI:10.1039/c4dt02717e

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The role of reaction conditions in the polymorphic
3
+
control of Eu doped YInO : structure and size
3
Cite this: DOI: 10.1039/c4dt02717e
sensitive luminescence†
a
b
a
b
a
R. Shukla, Santosh K. Gupta, V. Grover,* V. Natarajan and A. K. Tyagi*
With the aim of exploring the effect of particle size and different polymorphic structures on the lumines-
3
+
3+
cence behaviour of Eu ions, all in a single compound, the YInO
stable C-type modification could be obtained in YInO nanopowders synthesized by gel combustion. The
broadening in Raman modes has been related to both the particle size and the strain in the metastable
C-type modification. The hexagonal polymorph of YInO , with a layered structure, quite unlike the C-type
modification, could be obtained by heating nanopowders to 1175 °C. Such a change in the structure,
3
:Eu system was synthesized. Meta-
3
3
3
+
leading to different site symmetries for Eu ions exhibited a tremendous bearing on its luminescence be-
haviour. Detailed steady state and time resolved luminescence studies revealed a highly distorted local
3
+
site symmetry for the Y ion in hexagonal YInO
and a shorter lifetime as compared to the bulk C-type YInO
case of the former. The Judd–Ofelt parameters (Ω and Ω ) were determined which established a much
3
. The nano C-type YInO
3
exhibited greater asymmetry
3
, attributable to more surface defects in the
2
4
Received 5th September 2014,
Accepted 5th December 2014
higher asymmetry in the hexagonal modification. Along with higher lifetime, the hexagonal polymorph
also possessed superior quantum yield values. In addition to providing a significant insight into the
structure–luminescence correlation in this study, we propose an efficient red phosphor with a high red
color purity.
DOI: 10.1039/c4dt02717e
www.rsc.org/dalton
3
+
makes the Eu ions the favoured luminescent structural
probe.
1
. Introduction
It is a widely known fact that the control of the polymorph for
YInO belongs to the hexagonal ABO class of compounds
3
3
a particular compound under desired conditions is very vital which despite possessing a similar nominal formula do not
1
,2
for designing materials with favourable properties. Many a crystallize in the perovskite structure. The ABO composition
3
times, the existence of a desired property in a compound can crystallizes in the hexagonal structure when the tolerance
be attributed to its structure, symmetry and more importantly factor given by (r
A
B O
+ rO)/√2 (r + r ) is less than that required
4
to the local arrangements of atoms or ions in the compound. for orthorhombic perovskites. A hexagonal ABO3 possesses
It has been observed that a compound in a particular structure layered structures composed of AO polyhedra and BO trigo-
7
5
5
or in the polymorphic state possesses important functional nal bipyramids. It has been reported that YInO
3
transforms
6
properties while the other polymorphic modifications of the into C-type rare earth oxide structures at higher temperatures.
same compound might not exhibit these properties. In light of They have been investigated for diverse applications like color
7
8
this, it becomes essential to investigate the local structure to pigments and as photocatalysts. These materials have
generate a greater understanding of the structural control of assumed a lot of importance recently because of the possibility
9
properties. Of late, the luminescence behaviour of lanthanide of existence of geometric ferroelectricity. These are called
3
+
ions, in particular, Eu , is being extensively employed as a improper ferroelectrics and their electrical behavior has been
3
structural probe. The reason behind this is that some tran- attributed to the non-centrosymmetric atomic arrangement of
3
+
9
sitions in Eu are extremely sensitive to local symmetry which the crystal. Since these compounds do not owe their ferroelec-
0
tric properties to d ions, as in conventional ferroelectrics, they
offer new templates for designing magnetic-ferroelectrics
a
Chemistry Division, Bhabha Atomic Research Centre, Mumbai-400085, India
that have been historically difficult to achieve, but are appeal-
b
Radiochemistry Division, Bhabha Atomic Research Centre, Mumbai-400085, India.
E-mail: vinita@barc.gov.in, aktyagi@barc.gov.in
10
ing for ‘spintronics’ applications. In our previous work
on rare earth indates, we have demonstrated how the local
structure at A- and B-sites is instrumental in controlling the
†
Electronic supplementary information (ESI) available. See DOI: 10.1039/
c4dt02717e
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electrical behavior of this class of compounds. The importance and analysis were done using F-900 software provided by
of the local environment around A- and B-cations and their Edinburgh Analytical Instruments, UK. A xenon flash lamp
expression in the bulk electrical behavior in perovskites with a frequency range of 10–100 Hz was used as the excitation
1
1
and other complex oxides has been a front area of research.
source. First the excitation spectrum was recorded by fixing
5
7
Herein, due to the properties being controlled primarily by the λem at 610 nm ( D → F of Eu(III)). The λ was selected so as to
0
2
ex
structure adopted by the ABO
an interesting case study for probing the local structure using spectra were recorded in the wavelength range of 350–750 nm
various techniques. with a step of 0.5 nm. The emission spectrum for a particular
3
compositions, this class forms obtain the maximum fluorescence output. All the emission
In view of the emphasis laid on the fine structural features sample was recorded with a lamp frequency of 100 Hz.
and the electrical properties exhibited by the hexagonal Multiple scans (at least five) were taken to minimize the
indates, this study aimed at synthesizing YInO :Eu in two fluctuations in the peak intensity and maximize the S/N ratio.
3
different polymorphic modifications and exploring the local Fluorescence lifetime measurements are based on the well
3
+
structure and its influence on Eu luminescence. This may established time-correlated single-photon counting (TCSPC)
pave the way for obtaining materials with a superior lumine- technique. Lifetime studies were done in a time range of
scence behavior as well by suitable polymorphic control.
10 ms where the frequency of the Xe lamp was fixed at 10 Hz.
ex and λem were kept at 256 and 610 nm, respectively. All the
λ
life-time data obtained were fitted with a single exponential
decay by an equation of the type, I(t) = Aexp(−t/τ), where t is the
2
. Experimental
5
lifetime of the D
0
excited state.
AR (Analytical Reagent) grade powders of Y O , In O , Eu O
2 3
2
3
2
3
and glycine were used as the starting reagents. The synthesis
3
+
of YInO
self-propagating gel combustion method. Stoichiometric
amounts of Y (894.22 mg), Eu (1.11 mg) and In
1110.54 mg) were dissolved in a minimum amount of nitric
3 3
:1 mol Eu (Y0.99Eu0.01InO ) was carried out by the
3
. Results and discussion
3.1 X-ray diffraction studies
2
O
3
2
O
3
2 3
O
YInO :Eu was synthesized by a self-assisted gel combustion
3
(
route followed by calcination at 600 °C. The XRD pattern of
the calcined powder (Fig. 1(a)) reveal the C-type structure
acid and the combustion was carried out in the presence of
glycine as the fuel. The combustion reaction was performed in
the fuel-deficient stoichiometry wherein the oxidant-to-fuel
(
space group: Ia ˉ3 ) for this compound. The X-ray peaks are
relatively broad indicating the nano-nature of the powder. The
nanopowder was heated at 900 °C and its XRD pattern is
depicted in Fig. 1(b). It is obvious that the C-type modification
is retained at 900 °C as well. Both the XRD patterns are quali-
tatively similar except for the narrowing down of the X-ray
peaks indicating an increase in the particle size on heating the
sample at higher temperatures. Interestingly, the XRD pattern
for the sample heated at 1175 °C (Fig. 1(c)) is completely
different from that observed at 600 °C and 900 °C and pertains
(
O/F) ratio was kept at 1 : 1.5 (the stoichiometric O/F ratio for
this reaction is 1 : 3.33). Thus, the amount of glycine taken for
this combustion was 900 mg. No further pH adjustments
were made. The solution was dehydrated to highly viscous
liquids (gels). At this stage, the temperature was raised to
2
50 °C which led to swelling followed by the auto-ignition of
the viscous liquid, with a rapid evolution of a large volume
of gases to produce voluminous powders. The powders were
calcined at 600 °C and partitioned into three parts. Two of
these were then heated at 900 °C and 1175 °C in static air for
3 3
to the hexagonal modification of YInO (space group: P6 cm).
8
h each in order to obtain different polymorphs.
X-Ray diffraction (XRD) studies were performed using a
monochromatized Cu Kα radiation on a PANalytical Xpert Pro.
Silicon was used as an external standard. The patterns were
refined using Rietveld refinement and lattice parameters were
determined. Raman spectroscopic measurements on pelletized
samples were performed on a micro/macro-Raman spectro-
+
meter (make-LABRAM-1, France) using a 488 nm line of an Ar
ion laser for excitation. A single monochromator spectrometer
equipped with a Peltier-cooled CCD detector in the backscat-
tering geometry was used for collecting the scattered Raman
signal. The laser line was focused on a flat surface of the
sample using an optical microscope (Olympus BX-40, 50×
objective lens) connected to the spectrometer. The spectra
recorded were averaged out of 50 scans with a time interval of
−1
2
s and a resolution of 2 cm
.
PL data were recorded on an Edinburgh CD-920 unit
3
Fig. 1 X-ray diffraction patterns for YInO heated at (a) 600 °C, (b)
equipped with M 300 monochromators. The data acquisition 900 °C and (c) 1175 °C.
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This implies that the temperature increment is accompanied
by a polymorphic phase transition.
The X-ray diffraction patterns for YInO
and 1175 °C were refined using Rietveld refinement. It must
3
annealed at 900 °C
12
3
+
be noted that the samples of YInO : 1 mol% Eu heated at
3
6
00 °C, 900 °C and 1175 °C will be referred to as YInO
YInO -900 & YInO -1175 respectively. The Rietveld-fitted XRD
patterns for YInO -900 and YInO -1175 are shown in Fig. S-1
3
-600,
3
3
3
3
(
ESI-I†). Table S-1 (ESI-II†) contains the refined parameters for
YInO -900 and YInO
-1175. The C-type (Sp. Group Ia ˉ3 ) or the
bixbyite structure adopted by YInO -600 and YInO -900 is an
3
3
3
3
ordered structure based on the F-type fluorite lattice. It is
obtained by the removal of a quarter of anions and the sub-
4
sequent rearrangements of cations. The cations possess six-
fold irregular polyhedra in the C-type structure as compared to
the 8-fold regular polyhedra in a fluorite-type structure. On 1175.
3 3 3
Fig. 2 Raman spectra of (a) YInO -600, (b) YInO -900 and (c) YInO -
1
3
the other hand the hexagonal structure (sp. Group: P6 cm) has
3
no crystallographic relationship to the C-type and is a layered
It must be mentioned that the Raman modes observed for
YInO -600 and YInO -900 are tremendously broadened. The
immense broadening in Raman modes has generally been
5
structure wherein the layers of BO trigonal bipyramids are
5
3
3
joined by the YO polyhedra. The appearance of the hexago-
7
nal phase at higher temperatures and C-type at lower tempera-
1
7
1
4
attributed to the smaller particle size and hence this explains
tures is also a significant observation. As has been reported,
the tolerance factor (which predicts the structure adopted by
the ABO compositions) for YInO is higher than that corres-
the broadening observed for YInO -600. Even though the par-
3
ticle size increases on annealing the sample at 900 °C, the
broadening in the Raman modes does not decrease signifi-
cantly. This may be explained on the basis of the prevalence of
cation heterogeneity in the lattice. The cation heterogeneity
leads to a spread in the vibration frequency of a particular
mode which manifests as its broadening. On heating the
sample to a higher temperature, i.e. 1175 °C, it transforms
to the stable hexagonal phase thus showing a completely
different Raman spectrum with much narrower Raman modes.
3
3
ponding to the bixbyite structure and accordingly it should
crystallize in the hexagonal structure at room temperature. The
stabilization of the metastable C-type phase for YInO can be
3
attributed to the kinetic stability brought about by the non-
equilibrium synthesis method adopted here which brings
about the randomization of Y and In ions, despite the ionic
size difference, favoring the C-type configuration. These struc-
tural details have been discussed in detail in our previous
1
4,15
3
The Raman spectrum of hexagonal YInO has been discussed
studies on similar systems.
1
4
in our previous work. It must be noted that to the best of our
knowledge there has been no detailed Raman analysis
3
.2 Vibrational spectroscopic studies on the polymorphs
3
of YInO in the literature and the analysis of the Raman
Raman spectroscopy is an invaluable technique to study
the structural variations at the microstructural level. Raman
spectroscopic studies were performed on all three samples i.e.
spectrum of hexagonal YInO (both in the present work and in
3
ref. 14) has been based on the Raman spectrum exhibited by
YMnO
and has been investigated by dedicated Raman studies. It is
also noticeable that the same composition of YInO , when
3 3
which crystallizes in the same space group as YInO
YInO
3
-600, YInO
3
-900 and YInO
3
-1175 and the Raman spectra
-600 and
1
8
are depicted in Fig. 2. The Raman spectra of YInO
3
3
YInO -900 show the typical modes observed for the C-type rare
3
adopts the thermodynamically stable modification (which is
hexagonal in this case), shows narrower Raman bands which
proves the decrease in the strain prevalent in the lattice. Also,
since in a hexagonal modification both Y and In occupy dis-
tinct sites, there is no cation heterogeneity at any site which
again is manifested in the narrower bands. The increase in the
particle size upon annealing at higher temperatures also con-
tributes to the narrowing down of the Raman modes.
earth oxides. The Raman modes predicted for an ideal C-type
structure are 4A
g g g g
+ 4E + 14F , where A is the Raman active
symmetric stretching vibration, F is the Raman active triply
g
degenerate symmetric stretching vibration, and E
g
is the
Raman active symmetric doubly degenerate bending vibration.
The most intense band occurs at around 380 cm−1 (Fig.
2
g g
(a and b)), which can be attributed to the (F + A ) mode of
1
6
−1
6
the (Y,In)O octahedra. The highest band at 612 cm may be
assigned to the (Y,In)–O stretch. It is obvious from the Raman
spectra that YInO -600 and YInO -900 possess a similar kind
of structure even at the microscopic level. However, it is notice- In order to study the local site symmetry of Y in all the three
3
.3 Photoluminescence (excitation & emission spectroscopy)
3
3
3
+
able that certain C-type features at 300, 483 etc. which were not samples, and its effect on the luminescence behavior of sym-
3
+
visible at 600 °C have started to become visible at 900 °C and metry-sensitive Eu ions, detailed photoluminescence studies
this can be attributed to an increased crystallinity achieved were performed. The excitation spectrum of YInO -600 nano-
powders monitored at an emission wavelength of 610 nm is
3
after annealing at 900 °C.
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3+
3
Fig. 3 (a) Excitation spectrum of YInO :Eu (λem = 610 nm). (b) Emis-
3+
sion intensity of YInO
ex −256 nm).
3
:Eu as a function of annealing temperature
(
λ
shown in Fig. 3(a). It consists of a strong broad band in the
range of 220–280 nm and very weak lines in the region of
3
2
00–550 nm. A broad hump observed in the region of
2
−
20–280 nm peaking at 256 nm can be attributed to the O
→
3
+
Eu charge transfer band (CTB) and the weaker lines in the
higher wavelength regions (350–550 nm) are assigned to the
3
+
interconfigurational f–f transitions of the Eu ions. Fig. 3(b)
depicts the emission spectra of the samples annealed at
Fig. 4 (a) Room temperature emission spectra from the bulk and nano-
particles of 1.0 mol% Eu doped YInO (YInO -900 and YInO -600).
3 3 3
3+
6
00 °C, 900 °C and 1175 °C at 256 nm excitation.
The samples were excited at 256 nm. (b) Decay curve corresponding to
5
3+
3+
A glance at Fig. 3 reveals that the broad spectral features of the D0 level of Eu in a bulk and nanoparticles of 1.0 mol% Eu doped
3
+
the Eu activator remain the same for all the samples. The YInO
3
. The samples were excited at 256 nm and the emissions were
5
7
monitored at 610 nm.
spectra exclusively contain very weak lines of
2 0
D → F
5
7
5
7
19
(
465 nm), D → F (490 nm) and D → F (535 nm), super-
2 2 1 1
posed over a broad host emission, and the strong lines of
peratures were also obtained to gain an in-depth understanding
and are depicted in Fig. 4(b) and 6(b). All the decay profiles
could be fitted using the monoexponential decay:
5
7
5
7
5
7
D
0
→ F
1
(594 nm), D
0
→ F
2
(614 nm), D
0
→ F
3
(653 nm) and
5
7
D → F (704 nm). In all the samples, the broad emission can
0
4
be assigned to the presence of oxygen related defects in
2
0
I ¼ I0 expðꢀt=τÞ
YInO
3
5
.
Upon excitation at 393 nm, corresponding to the
7
3+
F → L band of Eu , emission properties similar to that of
0
6
where t is the times of measurement and τ are the decay time
CTB excitation were observed. It is also noticed that the inten-
sities of the emission bands are relatively lower, when excited
values or the lifetime values. The lifetime values were obtained
to be 1.47, 1.62 and 1.96 ms respectively for the YInO -600,
3
3
+
with the characteristic absorption band of Eu
93 nm, compared to that of the emission intensities obtained
with the LMCT (ligand to metal charge transfer) excitation
ions at
YInO -900 and YInO -1175. Single lifetime values indicate a
3
3
3
+
3
3
similar and homogeneous environment for Eu in YInO .
3
+
band (256 nm). This may be due to the fact that the Eu
3.4 Effect of size and polymorphic modification on
absorption bands corresponding to the f–f transitions are for-
bidden and exhibit poor absorptivities in the UV region and
luminescence behaviour
hence a relatively weaker emission is exhibited. It should be The optical characterisation of the YInO :Eu system was con-
3
3
+
noticed that the emission spectra of the YInO
3
:Eu phosphors sidered worth exploring because it provides a single system,
and wherein using a sensitive luminescence probe like Eu , the
D ) as well. This is interesting because it has been reported effect of size (YInO -600 and YInO -900) and two different
5
3+
consist of emissions from the higher energy levels ( D
1
5
2
3
3
5
7
that the
1, 2, 3 J
3 3
D → F lines usually appear in the emission polymorphs (YInO -900 and YInO -1175) on the local struc-
spectra of compounds with a low-lattice phonon energy. Thus, tural distortions and luminescence can be investigated.
the presence of emission lines from higher excited states of
3.4.1 Effect of size. Fig. 4(a) and (b) show a comparison of
Eu implies the low phonon vibration energy for the host emission spectra and luminescence decay profiles for YInO3-
YInO lattice. This could be significant in the context of the 600 and YInO -900. It is well known that photo-luminescence
development of new host lattices with a high fluorescence due to the D → F transition of Eu is a magnetic dipole
3
+
3
3
5
7
3+
0
1
5
7
yield. Further, the Stark splitting in the emission spectra is a transition while D → F is an electric dipole allowed tran-
0
2
proof of superior crystallinity of the samples.
sition. The latter transition is allowed only for sites with no
inversion center and it is very sensitive to the local environ-
5
3+
The decay curves corresponding to the D0 level of Eu
3
+
ions in all the three YInO
3
samples annealed at different tem- ment around Eu ions. Thus, the intensity ratio of these
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transitions serves as a very effective spectroscopic parameter
3
+
for monitoring the symmetry of the site occupied by Eu .
This ratio, also known as the asymmetric ratio of lumines-
cence (I) (defined as I = I(5D0-7F2)/I(5D0-7F1)) is calculated from
the emission spectra and the higher is this ratio, the lower is
the site symmetry. The values of “I” are found to be 1.89 and
1
.26 for YInO
3 3
-600 and YInO -900 respectively. The signifi-
3
+
cantly higher value of I for YInO -600 suggests that the Eu
3
ions are occupying a more distorted environment in the nano-
particles compared to the bulk. This can be explained on the
basis of a higher surface to volume ratio in the nanopowders
3
+
which leads to a significantly higher fraction of Eu ions on
the surface which are obviously exposed to a more asymmetric
environment and hence the higher value of I. In addition to
the asymmetric ratio, the luminescence lifetime corresponding
to the excited state of the Eu ions and their decay profiles are
also very sensitive to the changes in the environment. The
Fig. 5 Schematics of two types of sites (S
valent cation in the C-type structure.
6
and C
2
) occupied by the tri-
3
+
cationic distribution over both the sites and this will also
cause a decrease in the asymmetry ratio. In fact, a careful
observation of Fig. 4(a) will emphasize this. However, the
effect of this factor on the value of the asymmetry ratio will be
similar to that which brought about an increase in the particle
size (decreased surface defects) as discussed earlier and hence
its contribution to the decrease in the asymmetry ratio cannot
be delineated.
lower emission intensity and luminescence lifetime for YInO
3
-
6
00 (1.47 ms) as compared to YInO -900 (1.62 ms) again indi-
3
cate the presence of a large number of surface defects in the
former which provides additional pathways for the non-radia-
tive decay channels. This is further supported by the lower
quantum efficiency (Table 1) of YInO -600 as compared to
3
3
.4.2 Effect of polymorphic transition. Fig. 6(a) and (b)
show the emission spectra and luminescence decay profiles
for YInO -900 and YInO -1175. Further annealing the sample
YInO
relaxation processes in YInO
As it was discussed earlier, both YInO -600 and YInO -900
3
-900 indicating that non-radiative decay dominates the
3
-600.
3
3
3
3
crystallize in the C-type structure. It is pertinent to mention
here that in the C-type structure (A ), the A-cation enters
into crystallographically non-equivalent sites of six-fold co-
2 3
O
1
3
ordination. One of these sites has the cation on a distorted
cubic lattice with oxygen vacancies on the face diagonal with the
point symmetry of C . The other site, with S symmetry, pos-
2
6
sesses the inversion symmetry and has oxygen vacancies lying
on the body diagonal of the cube. The schematics of S
sites are shown in Fig. 5.
6 2
and C
Considering the fact that the ED transitions (such as
5
7
D
0
– F
with no inversion symmetry, they can be safely assigned to be
emitted by ions occupying the C site. The orange emission at
2
red emission at about 610 nm) originate from sites
2
5
7
3+
5
92 nm belongs to the MD
this transition hardly varies with the crystal field strength and
is thus assigned to both C and S sites. An increase in the
0 1
D – F transitions of Eu , and
2
6
temperature will lead to increased randomisation in the
Table 1 Radiative and non-radiative decay rates, quantum efficiencies
5
5
7
5
7
(
D
0
level), intensity ratios between the
and Judd–Ofelt intensity parameters of the Eu doped YInO
: Judd–Ofelt parameters; ANR and A : non-radiative and radiative
D
0
→ F
2
3
and
D
0
→ F
1
transitions
+
3
. (Ω and
2
Ω
4
R
transition probability respectively; η: quantum efficiency; I02/I01: asym-
metry ratio)
3
+
Eu
doped
Ω
(10
2
Ω
4
Ω
Ω
2
/
4
A
(s
NR
−1
A
R
η
I
I
02
/
01
−
20
2
−20
2
(s−1) (%)
Fig. 6 (a) Room temperature emission spectra of YInO
YInO -1175. The samples were excited at 256 nm. (b) Decay curves
corresponding to the
3
-900 and
cm ) (10
cm )
)
3
5
3+
0 3 3
D level of Eu for YInO -900 and YInO -1175.
YInO
YInO
YInO
3
-600
3
-900
3
-1175
2.96
2.04
2.95
3.29
3.04
2.32
0.89 987 448 31 1.89
0.67 786 412 34 1.26 The samples were excited at 256 nm and the emissions were monitored
1.27 652 369 36 1.96 at 610 nm.
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3 3
at 1175 °C changes the phase and this gives an opportunity to that hexagonal YInO is obtained in the ferroelectric P6 cm
1
4
study the effect of phase change on the luminescence behav- structure.
iour in the same compound.
3
Here again, the emission spectrum of YInO -1175 exhibits
the electric dipole transition to be much more prominent than
3
.5 Judd–Ofelt theory: validation of experimental results
The Judd–Ofelt theory has been successfully applied to the
quantitative determination of the optical properties of trivalent
lanthanides.
trum is a powerful tool for calculating the parity-forbidden
electric-dipole radiative transition rates between the various
levels of a rare earth ion such as Eu . Through these analyses,
the local environment around the metal ion and the bond
covalency of M–L bonds can be interpreted. It is possible to
determine the J–O intensity parameters Ω (where λ = 2, 4 and
λ
6 etc.) from the emission spectral data. Moreover, it is also
possible to determine the radiative transition rate (A ), radia-
tive lifetimes (τ ) and the quantum efficiency for any given
R
the magnetic-dipole transition and the asymmetry ratio (I) for
3
+
Eu is calculated to be 1.96. A significantly higher value of
2
2,23
The Judd–Ofelt analysis of the emission spec-
I for YInO -1175 as compared to YInO -600 suggests that the
3
3
3
+
Eu ions are in a much more asymmetric environment. An
extremely low site symmetry around Eu in hexagonal YInO
is also supported by the appearance of a relatively intense
3
+
3
3
+
5
7
D
0
→ F
transitions from D
both ED and MD forbidden, and that is why only weak tran-
0
line (∼575 nm) transition. It is known that radiative
5
0
to levels with J = 0 or odd J (J = 3, 5) are
5
sitions from
0
D to these levels are observed due to the CF
R
induced J-mixing effect and this is in fact allowed for very few
2
1
site symmetries. Indeed a closer look at the structure of hexa-
5
system from the emission spectrum.
gonal rare earth indates reveals a trigonal anti-prismatic co-
3
+
3+
Hence, in order to further substantiate the observation and
ordination for Y (and consequently Eu ) ions which is very
asymmetric. These results were further validated by life time
measurement studies shown in Fig. 4. The lifetime value for
3
+
gain deeper insight into the local environment around Eu in
the nano and bulk C-type YInO and hexagonal YInO , the
intensity parameters (Ω ) where λ = 2 and 4 along with the
radiative (Arad) and non-radiative (Anrad) rates for Eu ions in
both the samples were determined from the emission spectra.
The application of the JO theory to the quantitative analysis of
3
3
λ
3
YInO3-1175 is 1.96 ms which is much higher than for YInO -
3
+
3
+
9
00. This is because of the asymmetric environment of Eu in
YInO -1175 compared to YInO -900; as a result selection rules
3
3
get relaxed comparatively enhancing the lifetime for the f–f
transition. Also because of the high temperature annealing of
the system, agglomeration or clustering of the particles take
place thereby reducing the surface to volume ratio. This in
turn reduces the number of inherent defect centers which are
sources for non-radiative pathways thus increasing the PL
3
+
Eu emissive properties in the matrix is nicely presented by
Werts et al.
2
4,25
The experimental intensity Ω can be calculated from the
λ
5
7
ratio of the intensity of the D
0
– F
1
(λ = 2, 4, 6) transitions,
Ð
Ð
5
7
I
1
(ν)dν, to the intensity of the D
0
– F
1
transition,
I
1
(ν)dν as
3
+
follows:
intensity and the decay time of Eu in YInO
The observation of a higher asymmetric ratio in hexagonal
YInO has a very important structural implication as well. In a
lucid review by Sleight et al. it is described that YInO
in two types of polymorphic hexagonal modifications. The one
which shows a paraelectric behavior (space group: P6 mmc)
3
-1175.
Ð
DMDν13
e2νλ3
9n3
IλðνÞdν
I1ðνÞdν
Ωλ ¼
Ð
ð1Þ
2
2
3
nðn þ 2Þ jkψJkUðλÞkψ′J′lj
2
5
3
occurs
The probabilities of each spontaneous emission (radiative
rates) can be obtained from eqn (1) using calculated Ω para-
λ
3
meters. The total radiative lifetime can be represented as the
actually has Y atoms at an inversion center, the O to Y poly-
hedra is a trigonal antiprism and two capped oxygens which
are at equal distances and far off are in the first co-ordination
sphere. However, in the functional ferroelectric form (space
reciprocal of the total radiative decay rate, i.e. τ = 1/A . From
R
R
the experimental decay time (τ ) and τ values, the τ i.e. the
f
R
NR
non-radiative life times can be calculated.
In order to calculate J–O parameters using eqn (1) the
group: P6 cm), the capping Y–O distances alternate between
3
average transition energies are obtained from the emission
spectra as an average over the selected transition region:
long and short and the Y–O polyhedra is now an asymmetric
capped trigonal antiprism. Many a times, it is difficult to dis-
5
Ð
tinguish between the P6 cm and P6 mmc modifications by
3
3
νIðνÞdν
λ
λ
νλ ¼ Ð
ðλ ¼ 0; 1; 2; 3 and 4Þ
XRD because of very small differences in their XRD patterns.
Considering, that it is this subtle change that brings about the
ferroelectric behavior in this class of compounds, it would be
IðνÞdν
5
7
Since the emission band for the D – F transition, cen-
0
6
really helpful if this distinction between the two polymorphs tered at around 810 nm, could not be detected due to the
can also be revealed by a complementary technique. In the limitation of the measurement spectral range, we can only
case of hexagonal YInO (unlike the C-type polymorph), Y and determine the values of Ω and Ω parameters. However, it has
3
2
4
In occupy distinct sites and by virtue of the ionic radius as been shown that the influence of Ω
6
can be neglected in the
3
+
26,27
well as the non-preference for a five co-ordinated site occupied determination of radiative properties of Eu ions.
For the
by In , Eu is expected to occupy Y -sites. Assuming that calculation we have adopted a value of 2.04 for the index of
3
+
3+
3+
3
+
3+
majority of Eu lies in the Y site, a strong ED transition refraction which has been calculated from the refractive index
5
7
along with the observation of the D
0
→ F
0
transition, suggests values of its constituents employing the Gladstone Dale
3
+
a highly asymmetric site for Y in YInO . This emphasizes equation. Taking into account that the index of refraction is
3
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wavelength dependent; taking the constant value into calcu-
lation introduces a small error in the obtained values since the
refractive index changes are small over the wavelength region
of interest (570–710 nm). This simplification is justified for
the sake of comparison of JO parameters between the samples
since this cannot change the observed trends. In general, the
parameter Ω
changes in the vicinity of the Eu ion (short range effect) and
and Ω are related to the long-range effects.
The J–O parameters, as determined for the Eu ions YInO
00, YInO -900 and YInO -1175, are mentioned in Table 1. The
2
is related to the covalency and structural
3
+
Ω
4
6
3
+
3
-
6
3
3
3
+
J
J–O intensity parameters (Ω ) of the Eu ion in this matrix
reveal information regarding the covalence and surrounding of Fig. 7 CIE diagram showing the coordinates and representing the color
emitted by YInO
3
-900 ($) and YInO
3
-1175 (*).
the metal ion. These are significant because, Ω is related to
2
3
+
the covalency and structural changes in the vicinity of the Eu
ion (short range effect) and Ω is related to the long-range
4
effects. In general, the higher values of the above parameter
indicate an increase in polarizability and an associated
3
+
increase in the covalent character of the Eu environment.
The higher value of the ratio Ω /Ω , as observed for the nano-
2
4
3
+
3
particle YInO -600, indicates that the distortion around Eu
ions is more in the nano regime than in the bulk sample.
Further, the greater individual values of Ω and Ω also indi-
2
4
cate the greater extent of the covalent character/electron
3
+
density around the Eu ion in nanoparticles. The non-radia-
tive decay rate is also found to be higher for nanoparticles
thus pointing towards the appreciable amount of surface
defects providing alternative pathways and leading to reduced
emission and life times. On comparing the J–O parameters for
3+
Fig. 8 Comparison of the PL data of the Eu doped YInO
a commercial red phosphor.
3
sample with
the two polymorphs, Ω
YInO -1175 which ascertains the presence of Eu in a highly
asymmetric environment whereas reverse trends are observed
in the case of YInO -900.
2 4
is observed to be less than Ω for
3
+
3
3
adopting standard procedures. It is clear from the values that
YInO -900 gives an ‘orangish-red’ emission due to the pres-
ence of relatively intense 592 nm lines ( D
It is noteworthy that all these calculations support the
experimental observation of optical properties like asymmetry
ratios, luminescence intensities and lifetime values.
3
5
7
0
1
→ F ). The color
can be tuned to pure red when annealed at 1175 °C. This is
represented as the point ‘*’ in the CIE diagram shown in
Fig. 7.
3
.6 Potential application of various polymorphs as
luminescent materials
To get an idea about the luminescence efficiency and com-
mercial utility of the gel-combustion derived yttrium indate
sample, its PL intensity was compared with that of a commer-
Other than providing a structural basis for the luminescence
3
+
behaviour of Eu in the YInO lattice, the study brings forth a
3
candidate for phosphor applications which have been hitherto
unexplored. The relatively higher intensities of the red emis-
sion in the nano C-type YInO3 and in hexagonal YInO3
samples make them important candidates for red phosphors.
cial red phosphor sample (Y O :Eu, NP-340, Nichia, Japan)
2
3
under identical conditions. For this, the entire emission spec-
trum was integrated and the value was compared with that of
the commercial phosphor samples (Fig. 8). It was observed
3
+
This is significant in the context of the search for Eu -based
red phosphors with a greater purity of red emission. More
importantly, the hexagonal polymorph which has the highest
asymmetry ratio and shows the best luminescence properties
among these three shows immense promise. It is further
attractive because of the reasonably high fluorescence yield
depicted by this system which can be attributed to lesser non-
3
that the PL intensity of the hexagonal YInO was 35% of the
commercial sample.
4. Conclusions
radiative pathways as is also supported by the calculated non- In summary, this study explores a single compound, YInO for
3
radiative decay constants. To evaluate the material perform- the size and polymorph induced tunability of its luminescence
3
+
ance on color luminescence emission, CIE chromaticity co- behavior by employing Eu as a structure-sensitive lumines-
ordinates were evaluated for YInO -900 and YInO -1175 samples cent probe. The metastable C-type YInO could be obtained by
3
3
3
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a non-equilibrium gel combustion synthesis which on con-
trolled annealing yielded a hexagonal polymorph at 1175 °C.
After extensive characterization by X-ray diffraction and Raman
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