Synthesis of LiNbO3 nanoparticles by citrate gel method

Article abstract of DOI:10.1166/jnn.2015.9769

We present investigations on the preparation of nearly stoichiometric lithium niobate (LiNbO₃) nanoparticles using citrate gel method. Citric acid is used as a chelating agent and ethylene glycol is added for polyestarification between the chelates. In addition to the main lithium niobate phase, the secondary phase of lithium niobate, LiNb₃ O₈ , and an unreacted phase of Nb₂ O₅ were also observed in the resultant product. The appearance of unwanted phases is a serious problem in citrate gel method. We have observed that the synthesis parameters such as molar ratio of citric acid to metal ions (R₁ ), pH, molar ratio of ethylene glycol to citric acid (R₂ ) and calcination temperature strongly influence the presence of the unwanted phases and these parameters are optimized to remove these phases. Evolution of the phase was investigated by powder XRD whereas TG/DTA was done to find out the crystallization temperature. It was observed that nearly stoichiometric and pure LiNbO₃ nanoparticles can be obtained with the optimized parameters, R₁ = 3, pH = 8, R₂ ≥ 2 and calcination temperature = 700 ° C. The stoichiometry of the synthesized LiNbO3 nanoparticles was investigated using Raman spectroscopy.

Full text of DOI:10.1166/jnn.2015.9769

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Nanoscience and Nanotechnology
Vol. 15, 3757–3763, 2015
Copyright © 2015 American Scientific Publishers
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Synthesis of LiNbO3 Nanoparticles by Citrate Gel Method
C. Debnath, S. Kar, S. Verma, and K. S. Bartwal∗
Laser Materials Development and Devices Division Raja Ramanna Centre for
Advanced Technology, Indore 452013, India
We present investigations on the preparation of nearly stoichiometric lithium niobate (LiNbO3)
nanoparticles using citrate gel method. Citric acid is used as a chelating agent and ethylene glycol
is added for polyestarification between the chelates. In addition to the main lithium niobate phase,
the secondary phase of lithium niobate, LiNb3O8, and an unreacted phase of Nb2O5 were also
observed in the resultant product. The appearance of unwanted phases is a serious problem in cit-
rate gel method. We have observed that the synthesis parameters such as molar ratio of citric acid
to metal ions (R1), pH, molar ratio of ethylene glycol to citric acid (R2) and calcination temperature
strongly influence the presence of the unwanted phases and these parameters are optimized to
remove these phases. Evolution of the phase was investigated by powder XRD whereas TG/DTA
was done to find out the crystallization temperature. It was observed that nearly stoichiometric and
pure LiNbO3 nanoparticles can be obtained with the optimized parameters, R1 = 3, pH = 8, R2 ≥ 2
and calcination temperature = 700 ꢀC. The stoichiometry of the synthesized LiNbO3 nanoparticles
was investigated using Raman spectroscopy.
Delivered by Publishing Technology to: Chinese University of Hong Kong
Keywords: Lithium Niobate, Sol–Gel Preparation, X-Ray Techniques, Electron Microscopy,
IP: 177.102.47.9 On: Mon, 21 Dec 2015 03:17:43
Raman Spectroscopy.
Copyright: American Scientific Publishers
ꢀ
1. INTRODUCTION
temperature above 1100 C which leads to inhomogeneity
in composition deviating from stoichiometry because of
serious evaporation of Li2O component resulting in the
niobium ions to change the valence state.7ꢀ8 In recent
years, LN nanocrystalline powders were synthesized using
wet chemical methods such as co-precipitation,9 sol–gel,10
hydrothermal,11 colloid emulsion,12 molten salt,13 from
water soluble maleic acid complex,14 low temperature
solution phase reduction of niobium (V) chloride using
lithium hydride15 etc. These wet chemical methods have
many unique advantages due to lower processing temper-
ature, easier composition control and better homogeneity
of prepared LN nanocrystalline powders. Sol–gel using
alkoxides, hydrothermal and colloid emulsion routes are
time consuming and involve highly unstable metal alkox-
ides which are highly inflammable, relatively high cost,
and high sensitivity to moisture and hence difficult to
maintain reaction conditions. Pechini method15 solves the
problem of niobium alkoxide by using some ꢁ-carboxylic
acid (e.g., citric acid) to coordinate with the niobium
ions. The Pechini method is a wet chemical technique
offering many advantages such as homogeneous mixing
at molecular level, better stoichiometric control, low pro-
cessing temperature and the production of desired powder.
Nanomaterials research is currently an area of intense sci-
entific interest due to potential applications in biomedi-
cal, optical and electronic fields. The basic understanding
of the nanoparticles and their properties have been exten-
sively compiled in the 25 volumes of the “Encyclopedia
of Nanoscience and Nanotechnology” edited by Nalwa.1
Due to quantum-confinement effects nanomaterial’s elec-
tronic and optical properties are drastically different from
its bulk counterpart. Lithium niobate, LiNbO3 (LN) is
one of the important optical and electronic materials hav-
ing photorefractive, acousto-optic, nonlinear optic, electro-
optic, ferroelectric, piezoelectric, pyroelectric properties,
which make it a promising candidate for many appli-
cations such as light modulation, optical wave guides,
second harmonic generation devices, optical parametric
oscillation devices, holographic memory devices, surface
acoustic wave devices etc.2–5 However, its various prop-
erties strongly depend on stoichiometry.6ꢀ7 Many routes
have been developed for synthesis of LN nanoparticles
resulting in different size, shape and crystalline quality.
Traditionally, LN is prepared by solid state reaction at high
∗Author to whom correspondence should be addressed.
J. Nanosci. Nanotechnol. 2015, Vol. 15, No. 5
1533-4880/2015/15/3757/007
doi:10.1166/jnn.2015.9769
3757

Synthesis of LiNbO3 Nanoparticles by Citrate Gel Method
Debnath et al.
Thus Pechini method is a convenient method for synthesis
of oxide nanoparticles. Pechini method using citric acid
as a chelating agent is also termed as citrate gel method.
In recent years a large number of oxide nanoparticles
have been synthesized, which are promising for photonics
applications.16–22
temperatures (550, 600, 650, 700, 750 and 800 ꢀC for
5 h each) which are well above the crystallization temper-
ature as determined from TG/DTA experiment (Setaram
TG-DTA 92B). Finally the powder samples were formed
after calcination.
The powder XRD patterns were recorded for all the
samples with step size of 0.01ꢀ and scanning speed of
In the present work we adopted the citrate gel method
for synthesizing LN powder using niobium oxide, lithium
nitrate, citric acid (CA) and ethylene glycol (EG) as raw
materials. Along with the main LiNbO3 phase the sec-
ondary phase of lithium niobate, LiNb3O8, and an unre-
acted phase of Nb2O5 were also observed in the obtained
powder precursor. Appearance of these two phases is unde-
sirable and required proper optimization of various syn-
thesis parameters to get rid of these phase. The presence
of the unwanted phase was found to depend strongly on
the molar ratio of CA to metal ions (R1), molar ratio of
EG to CA (R2), pH of the reaction mixture and calci-
nation temperature. We have performed parametric inves-
tigations of the above synthesis parameters to remove
these unwanted phases. We have used the X-ray powder
diffraction (XRD) technique to characterize and confirm
the phase, stoichiometry and crystallinity after each syn-
thesis experiment. TG/DTA was done to find the crystal-
lization temperature. The size of the particles was obtained
by powder XRD data, transmission electron microscopy
(TEM) and dynamic light scattering (DLS) experiments.
The stoichiometry of the particles was characterized by
Raman spectroscopy.
ꢀ
5 /min using X-ray diffractometer (Rigaku, Geigerflex).
The particle size was determined by TEM (Philips, Tecnai
G2-20 (FEI)), DLS (Zetasizer Nano ZS90; ZEN-3690) and
XRD peak width. Before performing DLS the LN pow-
ders were suspended in de-ionized water homogeneously
by several stirring followed by ultrasonication. The parti-
cles were also characterized by micro-Raman spectroscopy
(Labram-HR800) in backscattering geometry using an Ar+
excitation source (ꢂ = 488 nm) and concentration of Li in
mol%, cLi = [Li]/([Li]+[Nb]), in LN crystal was obtained.
3. RESULTS AND DISCUSSION
The citrate gel technique was used to prepare the single
phase LN nanoparticles. The outcome of this method
depends on the formation of complexes of alkali metals,
transition metals, or even non-metals with mono-, bi-, and
tridentate organic chelating agent like CA. The polyal-
cohol, EG is added to establish linkages between the
chelates by polyestarification reaction, resulting in gela-
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tion of the reaction mixture. Unlike traditional sol–gel pro-
IP: 177.102.47.9 On: Mon, 21 Dec 2015 03:17:43
cesses, where the metal itself becomes an integral part
Copyright: American Scientific Publishers
of the gel network, in this method the gel network is
formed by the esterification of the chelating agent and
polyalcohol. The metal ions are essentially trapped in the
organic matrix, to which they are weakly bound. Hence
it is also known as modified sol–gel method. The forma-
tion of the complexes of alkali metals (e.g., Li), transition
metals (e.g., Nb) with CA relies on the amount of CA
(i.e., on the value of R1). For higher amount of CA more
2. EXPERIMENTAL PROCEDURE
Niobium (V) oxide (Alfa Aesar, 99.9%), lithium nitrate
(Merck, 99.995%), hydrofluoric acid (Merck, 48% GR),
citric acid anhydrous (Fisher Scientific, SQ) and ethylene
glycol anhydrous (Sigma Aldrich, 99.8%) were used as
the starting materials. A stoichiometric amount of LiNO3
was dissolved in de-ionized water to form lithium hydrox-
ide (LiOH) and Nb2O5 ꢀwas dissolved in HF after heating
in a water bath at 38 C for at least 20 h to form nio-
bium fluoride (NbF5). For the preparation of Li–Nb–CA
precursor solution, the clear solutions of LiOH and NbF5
were mixed in the aqueous solution of CA separately in
two beakers with continuous stirring. Then the solution of
lithium citrate was mixed with the solution of niobium cit-
rate drop by drop with continuous stirring. At this stage
the molar ratio of the CA to metal ions (defined by R1)
was varied from 1 to 4. The pH was varied (1.2, 4.0, 6.1,
8, and 9.7) by adding ammonia solution and nitric acid.
Then EG was added to promote mixed citrate polymer-
ization by polyesterification reaction and the molar ratio
of EG to CA (defined by R2) was varied (0 to 5 with
step of 0.5). The obtained Li–Nb–CA precursor solution
was heated at 80 ꢀC to produce a gelatinous precursor
after evaporation of water. Subsequently, the gel was cal-
cined in air atmosphere by heating successively at various
Figure 1. Simultaneous TG/DTA investigations of preheated Li–Nb–
citrate gel in static argon atmosphere with heating rate 10 ꢀC/min; The
inset graph shows the XRD peaks at different calcination temperatures.
3758
J. Nanosci. Nanotechnol. 15, 3757–3763, 2015

Debnath et al.
Synthesis of LiNbO3 Nanoparticles by Citrate Gel Method
carboxylic groups (COO−) can be available to chelate the
metallic ions in the solution.
phase becomes negligible. After this the exothermic peak
arises due to growth of LiNbO3 particles. There is still
some weight loss in the TG curve even at the higher tem-
perature range as the evaporation rate of the organic com-
pounds in oxide form is much weaker in Ar-atmosphere.
Therefore in citrate gel method Nb2O5 and LiNb3O8
arise as unwanted phase in the resultant product. We have
optimised the experimental parameters to remove these
unwanted phases. The powders prepared from various
molar ratio of CA to metal ions, R1 (1, 2, 3, and 4) with
stoichiometric pH value (∼2.8–3.0), molar ratio of EG
A black–brown powder was obtained by heating the
ꢀ
precursor gel at 200 C for 1 h. The TG/DTA investiga-
tions in static Ar-atmosphere were carried out on this pre-
ꢀ
heated powder with a heating rate 10 C/min as shown in
Figure 1. In the TG graph significant weight loss doesn’t
occur up to 200 ꢀC. But in this range there is a strong
endothermic peak in the DTA graph arising due to melt-
ing of the Li–Nb–CA precursor and rꢀemoval of remain-
ing water from precursor. Above 200 C there is a rapid
weight loss in the TG graph due to decomposition of the
precursor and in the DTA graph there is a strong endother-
ꢀ
to CA, R2 = 1 and calcination temperature 650 C were
examined by XRD, as shown in Figure 2(a). Here all the
XRD patterns are normalized and put in the same scale to
make a comparative study. It was found that the precur-
sor contains a higher amount of LiNbO3 phase and little
amount of un-reacted Nb2O5 and LiNb3O8 phases which
are minimum at R1 = 3. Miller indices corresponding to
the dotted lines are of pure hexagonal phase of stoichio-
metric LiNbO3 structure (JCPDS no. 20-0631). At lower
value of R1, due to lack of sufficient COO− groups the
chelation of the metal ions is not completed. At suffi-
ciently higher value of R1 the concentration of the metal
ions in the complex host matrix becomes very less and
hence some of them are unable to react each other result-
ing in segregation with heating process. Therefore, here the
ꢀ
mic peak at about 300 C. It has been observed that the
descending portion of the decomposition endotherm covers
the temperature interval of the main weight loss step and
this endothermic peak occurs in a multi-step process.23 The
associated exothermic peaks suggest a self-combustion-
like process in which nitrate ions act as an oxidizing
agent and CA as fuel. After this endothermic peak three
broad exothermic peaks arises due to crysꢀtal formation at
the temperatures about 450, 550 and 890 C respectively.
The first exothermic peak arises due to oxidization of the
niobium ions in the form of Nb2O5 confirmed by XRD.
The second exothermic peak arises due to crystal forma-
tion of lithium–niobium–oxygen compound in the form
of LiNb3O8 and LiNbO3ꢀ also confirmed by XRD analy-
sis. Above 550 C, LiNb3O8 decomposes to convert into
ꢀ Delivered by Publishing Technology to: Chinese University of Hong Kong
optimum value of R is 3. With stoichiometric pH value,
R2 = 1 and R1 = 3 the effect of calcination temperature
IP: 177.102ꢀ.47.9 On: Mon, 21 Dec 2015 03:117:43
LiNbO3 and at the temperature about 750 C, LiNb3O8
Copyright: American Scientific Publishers
Figure 2. Normalized XRD spectra of the samples for (a) various R1, R2 = 1, stoichiometric pH value and calcination temperature 650 ꢀC; (b) R1 = 3,
R2 = 1, stoichiometric pH value and various calcination temperature.
J. Nanosci. Nanotechnol. 15, 3757–3763, 2015
3759

Synthesis of LiNbO3 Nanoparticles by Citrate Gel Method
Debnath et al.
on the unwanted phases is shown in Figure 2(b) and it
was observed that at higher temperature (650 ꢀC or above)
the LiNb3O8 phase is greatly reduced. Generally lithium
forms ionic coordination with CA whereas niobium may
form mono-, bi- or tridentate chelating complex depending
upon the availability of COO− groups surrounding it. The
lower decomposition temperature of ionic ligand makes
the chelating of metal ions (e.g., Li+ ions) with COO−
group to collapse easily than bi- or tri-dentate ligands.
Therefore, in lower heating process the Li+ ion coordi-
nated with COO− would be much easier to segregate and
find relatively lesser amount of Nb ions to react and may
form another compounds (not found in XRD patterns).
Hence at moderate temperature (e.g., 600 ꢀC or below)
relatively smaller amounts of Li ions are available to react
with Nb-ions resulting in lithium deficient phase LiNb3O8
(JCPDS no. 75-2154) as a dominating phase. As the cal-
cination temperature increases (650 ꢀC, 700 ꢀC, etc.), a
solid state reaction between LiNb3O8 and Li-compounds
occurs to form LiNbO3 phase resulting in the reduction
of LiNb3O8 phase. In Figure 2(b), it is also observed
that there is an insufficient change in Nb2O5 phase with
calcination temperature due to incomplete chelation as
explained above. The order of chelation increases with pH
because at higher pH conditions, more CA is ionized by
ionizing the COOH group and hence more COO− groups
can be available to chelate the metallic ions in the precur-
of the precursor solution was changed and effect of pH
on the XRD phases of the precursor powder calcined at
650 C is shown in Figure 3(a). It is found that at lower
ꢀ
pH, the un-reacted Nb2O5 phase becomes dominating due
to incomplete chelation for the lack of COO− groups. With
increasing pH, Nb2O5 phase reduces due to increasing
degree of chelation as at higher value of pH more available
COO− groups allow the molecules to behave as bi-, tri,
or tetra-dentate ligands increasing binding ability of CA.
We have observed that the Nb2O5 phase completely dis-
appear when the pH value was increased to 8. Therefore,
at pH = 8 all the Nb-ions are able to complete the chelate
formation with CA. It is also observed that the LiNb3O8
phase is dominating with pH due to higher decomposition
temperature of bi- or tri- or tetradentate ligands associ-
ated with Nb atoms as explained above. Therefore, to react
LiNb3O8 with segregated Li-compounds higher tempera-
tures are required. In Figure 3(b) we have shown the effect
of calcination temperature on the phase of the precursor
powders synthesized with R1 = 3, R2 = 1 and pH = 8. Here
it is observed that at calcination temperature of 650 ꢀC
the LiNb3O8 phase remains in large extent and decreases
adequately when the calcination temperature is increased
ꢀ
to 700 C and no further significant reduction occurs with
higher calcination temperature as in this temperature range
Li-cation diffuses easily; even it can evaporate from the
raw materials.12 Therefore, to remove the LiNb3O8 phase
the segregation of Li-cation should be restricted and it
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sor solution. Hence keeping the value of R1 fixed to 3, pH
IP: 177.102.47.9 On: Mon, 21 Dec 2015 03:17:43
Copyright: American Scientific Publishers
Figure 3. Normalized XRD spectra of the samples for (a) R1 = 3, R2 = 1, various pH value and calcination temperature 650 ꢀC; (b) R1 = 3, R2 = 1,
pH = 8 and various calcination temperature.
3760
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Debnath et al.
Synthesis of LiNbO3 Nanoparticles by Citrate Gel Method
in stoichiometry. The Raman spectrum for the particles
prepared with R1 = 3, R2 = 2, pH = 8 and calcination tem-
perature 700 ꢀC is shown in Figure 6(a). In the present
Raman investigation we have observed 17 peaks for the
corresponding A1(TO), A1(LO), E(TO) and E(LO) modes.
The observed Raman modes listed in the Table I are very
close to the previously reported values.25 Here the dou-
blet modes are at 273 cm−1 (A1(TO)/A1(LO)), 332 cm−1
(A1(TO)/A1(LO)) and 430 cm−1 (A1(LO)/E(LO)). The
exact composition cLi = [Li]/([Li]+[Nb]) of the LN crystal
can be found from the line-width of some Raman lines.26
They have chosen two Raman peaks one at about lower
frequency, 153 cm−1 and the other at about higher fre-
quency, 876 cm−1. It has been reported26ꢀ27 that the line
width of the phonon mode at about 153 cm−1 is accompa-
nied by an additional weak band at about 175 cm−1 which
arises in the stoichiometric composition of LN crystal as
shown (hump like) in Figure 6. The concentration of Li in
LN can be obtained by using the formulae:
Figure 4. Intensity ratio of the highest XRD peaks of LiNb3O8 phase
to the desired LiNbO3 phase with R1 = 3, pH = 8 and calcination tem-
perature 700 ꢀC and various EG to CA molar ratio R2. The inset XRD
patterns are for EG/CA molar ratio of 2, 3 and 4 respectively.
is possible by introducing more complex network to the
gel with the help of the polymerising agent EG which
can make polyestarification reaction between the citrate
chelates. Keeping R1 = 3, pH = 8, the effect of EG to CA
molar ratio (R2) to the LiNb3O8 phase for calcination tem-
cLi[mol%] = 53ꢅ29 −0ꢅ1837ꢆ875ꢇcm−1ꢈꢉ
for the Raman peak at 875 cm−1 and
cLi[mol%] = 53ꢅ03 −0ꢅ4739ꢆ152ꢇcm−1ꢈꢉ
for the Raman peak at 152 cm−1
ꢀ
perature of 700 C is shown in Figure 4. Here we have
shown the variation of the relative intensity of the highest
XRD peak of LiNb3O8 phase with respect to the highest
XRD peak of LiNbO3 phase. It has been observed that for
Here ꢆ represents full width at half maxima of the
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Raman peak. By decoupling Raman peaks with Lorentzian
IP: 177.102.47.9 On: Mon, 21 Dec 2015 03:17:43
R2 ≥ 2 the LiNb3O8 phase vanishes completely.
Copyright: American Scientific Publishers
The distribution of particle sizes for the particles pre-
(a)
pared with R1 = 3, R2 = 2, pH = 8 and calcination temper-
ꢀ
ature 700 C were obtained by TEM and DLS as shown
in Figure 5, and it was found that the size of the particles
is 91.5 nm with a variation of width 43.7 nm. The second
peak at 238.6 nm in DLS spectrum arises due to agglomer-
ated particles. The average crystalline volume can also be
calculated by Scherrer’s formula (t = Kꢂ/ꢃ cosꢄꢃ) from
XRD peak at ꢄꢃ of width ꢃ assuming K = 0ꢅ9 (spherical
particle) where ꢂ is the wavelength of X-ray radiation. We
have found that t ꢁ 87 nm.
LN belongs to the 3 m point group and R3c space
group symmetry at room temperature the primitive cell
of which contains two formula units (10 atoms) giving
30 degrees of vibrational freedom. At zero wave vector
approximation, vibration may be characterized by group
theory as 5A1, 5A2 and doubly degenerate 10E phonon
branches of which one A1 and one E are the three acous-
tic branches and five A2 fundamentals are Raman and
infrared inactive. Therefore, the total Raman effective
modes are found to be (4A1 + 9E). However, long range
electrostatic field in the ionic crystal lifts the degener-
acy between longitudinal (LO) and transverse (TO) opti-
cal phonons and hence doubling the 13 observed phonons.
Thus ideally, total 26 phonon modes are possible to
detect by Raman spectroscopy in LN.24 However, the
total degeneracy is not achieved due to small variation
(b)
Figure 5. (a) TEM image; (b) The DLS spectrum for particle size dis-
tribution of the samples prepared with R1 = 3, R2 = 2, pH = 8 and cal-
cination temperature 700 ꢀC.
J. Nanosci. Nanotechnol. 15, 3757–3763, 2015
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Synthesis of LiNbO3 Nanoparticles by Citrate Gel Method
Debnath et al.
(a)
with their maximum intensity and plotted in the same
scale for comparison. For R2 = 0, the Raman peaks at
136, 542,675 and 701 cm−1 arise due to the presence of
LiNb3O8 phase.28 With increase of R2 these peaks are
tending to disappear for reduction of LiNb3O8 phase as
seen in Figure 4 and become negligible for R2 ≥ 2. We
have also observed that for R2 <2 the Li concentration
in LN is in the range of 48–49 mol%, i.e., congruent
composition.
4. CONCLUSIONS
The amount of CA, EG, pH of the precursor solution and
the calcination temperature play an important role to syn-
thesize single phase LiNbO3 powder samples by citrate gel
method. The unwanted of LiNb3O8 and Nb2O5 phase can
be removed by optimizing the synthesis parameters which
are molar ratio of CA to metal ions (R1), molar ratio of
EG to CA (R2), pH of the reaction mixture and calcina-
tion temperature. We have found that the chelation of Nb
ions with CA is completed when pH of the precursor solu-
tion be 8 with R1 = 3 and unreacted Nb2O5 phase in the
resultant product is disappeared. The Li deficient phase of
LiNb3O8 can be removed by restricting the Li segregation
by introducing sufficient (R2 ≥ 2) polymerising agent EG
for polyestarification between the citrate chelates. Nearly
stoichiometric (∼49.6 mol% Li concentration) LiNbO3
(b)
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nanoparticles (below 100 nm diameter) can be obtained
IP: 177.102.47.9 On: Mon, 21 Dec 2015 03:17:43
by citrate gel method with R1 = 3, R2 ≥ 2, pH = 8 and
(a) Normalized Raman spectrumCofoLpiNyrbiOght: American Scientific Publishers
Figure 6.
powder sample
3
ꢀ
calcination temperature of 700 C.
prepared with R1 = 3, R2 = 2, pH = 8 and calcination temperature 700 ꢀC.
(b) Normalized Raman spectra of LiNbO3 powder samples prepared with
R1 = 3, pH = 8, various R2 (0, 0.5, 1.0, 1.5, 2.0, 3.0, 4.0 and 5.0) and
calcination temperature 700 ꢀC.
References and Notes
1. H. S. Nalwa (Ed.), Encyclopedia of Nanoscience and Nanotechnol-
ogy, American Scientific Publishers, LA (2004/2011), Vols. 1–25.
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Solid State Commun. 129, 747 (2004).
5. M. Niederberger, N. Pinna, J. Polleux, and M. Antonietti, Angew.
Chem. Int. Ed. 43, 2270 (2004).
fitting and from the peak widths ꢆ875 and ꢆ152 we have cal-
culated cLi[mol%] of value 49.50 and 49.61 respectively
by using above empirical formulae. Thus on average cLi
=
49ꢅ56 mol%. We have also observed that for R2 > 2 the
concentration of Li, cLi ꢁ 49ꢅ6 mol%. Therefore our pre-
pared LN nanoparticles are nearly stoichiometric.
6. K. Niwa, Y. Furukawa, S. Takakawa, and K. Kitamura, J. Cryst.
Growth 208, 493 (2000).
In Figure 6(b) we have shown the Raman spec-
tra for different R2 (with R1 = 3, pH = 8, calcination
temperature = 700 ꢀC). All the spectra are normalized
7. M. Liu and D. Xue, Solid State Ionics 177, 275 (2006).
8. V. T. Kalinnikov, O. G. Gromov, G. B. Kunshina, A. P. Kuz’min,
E. P. Lokshin, and V. I. Ivanenko, Inorg. Mater. 40, 411 (2004).
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10. S. D. Chang, C. H. Kam, Y. Z. Zhou, Y. L. Lam, Y. C. Chan, and
S. Buddhudu, Mat. Lett. 45, 19 (2000).
Table I. Observed bands (cm−1) of several fundamental optical modes
of Raman spectrum in LiNbO3 at room temperature and a comparison
with the previous Raman data.25
11. C. An, K. Tang, C. Wang, G. Shen, Y. Jin, and Y. Qian, Mater. Res.
Bull. 37, 1791 (2002).
Observed band (cm−1
)
Reference [25]
12. C. Luo and D. Xue, Langmuir 22, 9914 (2006).
13. P. Afanasiev, Mat. Lett. 34, 253 (1998).
A1 (TO) A1 (LO) E (TO) E (LO) A1 (TO) A1 (LO) E (TO) E (LO)
14. E. R. Camargo and M. Karkihana, Solid State Ionics 151, 413
(2002).
15. M. Aufray, S. Menuel, Y. Fort, J. Eschbach, D. Rouxel, and
B. Vincent, J. Nanosci. Nanotechnol. 9, 4780 (2009).
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104, 3893 (2004).
258
273
332
273
332
430
875
152
238
320
367
582
117
171
302
430
621
253
275
334
273
331
428
874
152
238
322
368
582
117
198
298
428
621
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Received: 4 December 2013. Accepted: 21 March 2014.
Delivered by Publishing Technology to: Chinese University of Hong Kong
IP: 177.102.47.9 On: Mon, 21 Dec 2015 03:17:43
Copyright: American Scientific Publishers
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