Chapter 6
Chapter-6
Synthesis, Characterization and Studies of Zinc
Orthophosphate & Cobalt Orthophosphate Nanoplatelets
 A part of this work has been presented:
1. T. George, S. Joseph, S. Mathew, National Symposium on Current Trends in
Inorganic Chemistry, CTIC-04, CUSAT, Cochin, 15-17 March, 2004
2. T. George, S. Joseph, S. Mathew, Indo–Australian International Symposium
on Nanoscience and Nanotechnology, IISc, Bangalore, 31March–1April,
2006
3. T. George, S. Joseph, S. Mathew, International Symposium on Materials
Chemistry, ISMC-06, BARC, Mumbai, 4 –8 December, 2006
213
Chapter 6
6.1. Introduction
Phosphate ceramics are getting attention due to their variety of
applications in optical, electrical, prosthetics and structural fields as
fluorescent materials, dielectric substances, dental cements, metal surface
coatings, fuel cells, pigments [1-3]. They are also used in catalysis, ion
exchangers and in low thermal expansion ceramic materials [4-6].
Zinc phosphate is a non-toxic white inorganic pigment featuring
corrosion protection and adhesion capability. It is used as a flame retardant
and also as a chemically bonded ceramic material (CBC). Zinc phosphate is
the first bioceramic to be proposed in dental applications [7, 8]. In the color
televisions different phosphors are used, for their emission in frequency ranges
corresponding to each of the primary colors. The decay time of the phosphor is
vital in these applications, with the relevant time scale imposed for the
electron beam to sweep the phase of the tube. Zinc phosphate doped with
manganese is a well-known phosphor used in cathode tubes [9].
Generally violet colored cobalt pigments are called cobalt violet. First
developed in the early 19th century, cobalt violet was the primary permanent
violet pigment available. Cobalt violets range from deep to pale shades with
either a pink or blue hue. The first cobalt violets used were composed of
cobalt arsenate. This highly toxic compound is now rarely used. Instead most
current cobalt violets are non-toxic and are made from either cobalt phosphate,
or cobalt ammonium phosphate. Cobalt violets are used in paints [10].
To prepare metal phosphates, methods like molten salt flux synthesis
[11], sol-gel synthesis [12], thermolysis/flame pyrolysis of polymer matrix
based precursor solution synthesis [13], boron phosphate method [14],
chemical precipitation [15, 16], solid- state reaction [17], had been developed.
214
Chapter 6
One of the oldest techniques for the synthesis of nanoparticles is the
precipitation of products from bulk solutions [18-20]. In this investigation
simple, inexpensive and highly reproducible aqueous precipitation method
was used for the preparation of zinc orthophosphate and cobalt
orthophosphate. This pollution free method has the advantage of good
stoichiometric control and the production of ultra fine particles with high
purity in a relatively short processing time at lower temperatures with
improved compositional homogeneity. The concentration of the metal species
present in the initial reaction mixture has the largest effect on the overall
nanoparticle size.
6.2. Synthesis and characterization of Zn3 (PO4) 2. 4H2O
nanoplatelets
Nanosized powders of Zn3(PO4)2.4H2O were prepared by reacting
stoichiometric amounts of AR grade zinc nitrate (Zn(NO3)2.6H2O, 0.005M)
and disodium hydrogen phosphate (Na2HPO4.2H2O, 0.003M) in distilled
water. The precipitate was filtered, washed a number of times using distilled
water and dried in an oven at 100°C for 3 hours to get fine, soft and white
powders of zinc orthophosphate. Powder X-ray diffraction (XRD) patterns
were recorded on a RINT 2100 X-ray diffractometer with Ni filtered Cu K
radiation in the range 5o-120o in 2θ steps of 0.02o. This recording procedure
enabled us to detect even the weak peaks in our XRD pattern. Phases in the
sample were determined using quantitative phase analysis employing
Powdercell 2.4 program [21].
The specific surface area of zinc orthophosphate nanoparticles was
determined by the BET (Brunauer, Emmett and Teller) method with nitrogen
adsorption using Gemini Micromeritics surface area analyzer. The size and
215
Chapter 6
morphology of the nanoparticles were determined by Transmission Electron
Microscope (TEM) using a JEOL model 1200EX instrument operated at an
accelerating voltage of 120 kV. The sample was ultrasonically dispersed in
ethanol for several minutes prior to depositing onto the grid and allowed the
solvent to evaporate. The surface features were studied using Scanning
electron microscope (SEM Hitachi S -520). The chemical composition of the
prepared sample was studied using energy dispersive X-ray analysis system
attached to the scanning electron microscope. Thermogravimetric analysis
(TGA) and Differential thermal analysis (DTA) of the sample was carried out
in the temperature range from room temperature to 800°C using Schimadzu
DTG 60. The photoluminescence spectrum of pure zinc orthophosphate and
Mn doped zinc orthophosphates were taken using a Perkin Elmer LS-55
luminescence spectrometer (slit width 10 nm).
6.3. Results and discussion
6.3.1. Powder XRD studies of Zn3 (PO4)2. 4H2O
Figure 6.1 shows the powder X-ray diffraction patterns of zinc
orthophosphate nanopowders prepared by aqueous precipitation method. The
pattern reveals the formation of polycrystalline Zn3(PO4)2.4H2O and the phase
confirmed by comparing with JCPDS file [22].
216
Chapter 6
Figure 6.1 XRD pattern of Zn3(PO4)2.4H2O
The structure resembles that of the mineral ‘hopeite’. The unit cell of
the prepared Zn3(PO4)2 4H2O is having orthorhombic crystal structure with
a = 10.5749; b = 18.3768; c = 5.0398 and  =  =  = 90° and the space group
symbol is Pnma (# 62) The average particle size of the nanocrystalline
samples, calculated from the full width at half maximum (FWHM) of major
peaks of the XRD patterns using Scherrer formula [23] is 30 nm.
6.3.2. Electron Microscopic Analysis of Zn3(PO4)2. 4H2O
The SEM micrograph (figure 6.2) and the TEM micrograph in (figure
6.3) shows the morphological features of zinc orthophosphate nanoparticles.
217
Chapter 6
Figure 6.2 SEM images of zinc orthophosphate nanoplatelets
It is clear from the SEM and TEM images that the particles are
aggregated in a peculiar way to form clusters of thin, transparent, platelets.
Figure 6.3. TEM images of zinc orthophosphate nanoplatelets
218
Chapter 6
The energy dispersive X-ray analysis spectrum of Zn3(PO4)2. 4H2O is shown
in figure 6.4.
Figure 6.4 EDX spectrum of Zn3(PO4)2. 4H2O
The EDX analysis (figure 6.4) reveals the presence of Zn, P and O in
appropriate ratios. Trace amount of Na from a secondary phase can also be
seen in the EDX spectrum even after repeated purification of the sample. In
order to quantify the secondary phase, we tried a quantitative phase analysis
by Reitveld refinement.
The Rietveld method is a powerful and relatively new method for
extracting detailed crystal structural information from X-ray and neutron
powder diffraction data since such structural details dictate many of the
physical and chemical attributes of materials. Since most materials of
technological interest are not available as single crystals but often only in
polycrystalline or powder form, the Rietveld method has become very
219
Chapter 6
important and is now widely used in all branches of science that deal with
materials at the atomic level [24]. The Rietveld analysis [25] was carried out
on crystalline (Zn3PO4)2.4H2O heated at 150 °C for 2 h (figure 6.5). The
structure of the mineral hopeite was first described by Liebau in 1965 [26].
This analysis was performed using the fullprof package assuming
Pnma space group for a ‘hopeite’ type orthorhombic structure. The tick marks
indicate the position of the all possible Bragg reflections from the structural
model shown in table 6. 1.
Figure 6. 5. Rietveld plot for crystalline Zn3PO4)2.4H2O powders heat treated
at 150 °C for 2 h in air atmosphere
The experimental and simulated intensity data (Ie and Is) are plotted as
open circles (o) and solid lines, respectively and I = Ie-Is. Zoomed 6-15° 2
region of the Rietveld plot for crystalline Zn3(PO4)2.4H2O powders heat
treated at 150 °C is shown in figure 6.6.
220
Chapter 6
Figure. 6.6. Zoomed 6-15° 2 region of the Rietveld plot for crystalline
Zn3PO4)2.4H2O
Circles are experimental points. Solid lines are sum due to zinc
orthophosphate and the secondary phase. Dotted lines in the zoomed 6-15° 2
region (figure 6.6) of refined plot correspond to the secondary phase identified
as sodium di zinc triphosphate nonahydrate. Here the precursors used were
zinc nitrate and disodium hydrogen phosphate in aqueous medium at room
temperature. The secondary phase sodium di zinc triphosphate nonahydrate
may also be formed along with zinc orthophosphate. The amount of secondary
phase calculated by quantitative phase estimation is 6 %.
221
Chapter 6
Table 6.1. Atomic positions and isotropic thermal parameters of atoms in zinc
orthophosphate.
Atom
X
y
z
Occupancy
Isotropic thermal
parameter (Å2)
O1
0.3046
0.0418
0.1348
1
1.3
O2
0.5307
0.0792
0.1471
1
0.9
O3
0.3678
0.1707
0.2237
1
1.8
O4
0.401
0.0865
0.5795
1
1.9
O5
0.1549
0.1674
0.6558
1
1.7
O6
0.8937
0.25
0.7615
1
0.5
O7
0.6121
0.25
0.3762
1
1
Zn1
0.7651
0.25
0.0681
1
1.7
Zn2
0.1433
0
0.2938
1
2
P
0.3955
0.0929
0.2673
1
1.2
Atomic positions and isotropic thermal parameters of atoms in zinc
orthophosphate obtained by the Rietveld refinement are shown in table 6.1.
The occupancy factor for Zn, P and O atoms obtained by this refinement are
full. Figure 6.7 illustrates the orthorhombic Zn3(PO4)2.4H2O.unit cell with
space group Pnma.
222
Chapter 6
Zn2
O4
O1
Zn2
Zn2
P
O5
O3
O4
Zn2
O2
O6
O2
P
O1
O7
O6
O5
O3
O5
Zn1
O4
O3
P
Zn1
O5
O7
O3
O4
P Zn2
O2
O2
O2
O2
Zn2
O1
O1
Zn2
P
O4
O1
O1
Zn2
P
O7
O3
O5
Zn1
O3
Zn1
O5
O4
O7
O6
O3
O5
P
O2
O6
O3
O1
O2
Zn2
O4
O5
P
Zn2
Zn2
O1
O4
Zn2
b
Po wd erCell 2 .0
a
c
Figure 6.7. Crystal structure of Zn3 (PO4)2. 4H2O
Red circles are oxygen atoms, pink circles are phosphorous atoms, and the
blue circles are Zn. Also note that water basis is replaced by oxygen atoms
because of the poor X-ray scattering power of hydrogen atom. The
phosphorous atoms are coordinated to the four oxygen atoms in a [PO4]3−
tetrahedral configuration (see the polyhedra). The Six-coordinate Zn2+ cations
in this unit cell is cis-bonded to two phosphate groups and to four O atoms of
four water molecules (two of which are located on mirror planes), forming a
framework structure. In addition, hydrogen bonds of the type O—H----O are
present throughout the crystal structure.
223
Chapter 6
6.3.3. Thermal analysis of Zn3 (PO4)2. 4H2O
The TGA and DTA curves are shown in figure 6.8. The TGA cuive
shows two distinct weight loss steps and the DTA curve shows two
Figure 6.8. TGA and DTA curves of the zinc phosphate nanoclusters.
endothermic peaks. The first weight loss step in the temperature range
33–215°C which was accompanied by an endothermic peak at 119.25°C and
the second weight loss step in the temperature range 219–440°C accompanied
by an endothermic peak at 300.81°C. The total weight loss in the two stages
was 14.65 %. Calculations show that the weight loss is equivalent to 4 units of
water of crystallization. The final formula of the prepared sample is assumed
to be Zn3(PO4)2 .4H2O.
224
Chapter 6
6.3.4. Photoluminescent study of Zn3 (PO4)2. 4H2O
Figure 6.9 shows the excitation and emission spectrum of pure zinc
orthophosphate and zinc orthophosphate doped with 3% of Mn2+. Figure 6.9.
(a) is the excitation & emission spectrum of pure zinc orthophosphate.
Figure 6.9 (a) Excitation & Emission spectrum of pure zinc orthophosphate.
(b) Excitation & Emission Spectrum of Mn2+ doped zinc
orthophosphate
225
Chapter 6
Figure 6.9. (b) is the excitation & emission spectrum of Mn2+ doped
zinc orthophosphate. The room temperature photoluminescence studies show a
strong intense emission around 570 nm. It is seen that there is an increase in
the intensity of emission peak (PL) of zinc orthophosphate, when it is doped
with Mn2+.
The experiment is performed with 1, 2 and 3% of Mn2+ doped samples.
But it is noticed that there is no substantial enhancement in the intensity of the
emission peaks when the Mn2+ concentration is increased further. Also there is
no shift in the peak position. The enhancement in the intensity of the emission
peak
may
be
due
to
the 3d-3d transition of Mn2+. The PLE
(photoluminescence excitation) spectra of pure and 3% Mn doped zinc
orthophosphate nanoparticles are also shown in figure 6.9. In doped
semiconducting materials the possible paths for the luminescence excitation of
the impurity ions include indirect excitation of the host lattice and direct
excitation of impurity ions. The maximum excitation peak is centered at 390
nm and it can be attributed to the transitions of the 3d5 multiplet states of
Mn2+. Hence it is evident from the photoluminescence studies that Mn2+ ions
can effectively improve the luminescence of zinc phosphate [27, 28].
6.4. Synthesis and characterization of Co3 (PO4)2. 8 H2O
nanoplatelets
Fine powders of cobalt orthophosphate were synthesized by chemical
reaction between AR grade cobalt nitrate and disodium hydrogen phosphate.
The precipitates were centrifuged, filtered, washed a number of times using
distilled water followed by drying in a vacuum desiccator at room
temperature. The crystal structure and particle size were determined by
226
Chapter 6
powder X-ray diffraction using Cu Kα radiations and the surface morphology
by scanning electron microscope (SEM). The thermal behavior of the sample
was studied by thermogravimetric analysis (TGA) and differential thermal
analysis (DTA). The magnetic behavior of the sample was studied at various
temperatures from 296 K to 573 K using vibration sample magnetometer
(VSM). A typical sample was consolidated into pellet of diameter 10 mm and
thickness 1.9 mm by applying pressure using a hydraulic press. Dielectric
constant (ε') and ac electrical conductivity (σac) were obtained using HIOKI
3532 LCR Hitester, impedance analyzer at different frequencies from 100 Hz
to 1400 KHz over the temperature range from 303 K to 353 K.
6.5. Results and discussion
6.5.1. XRD studies of Co3 (PO4)2. 8 H2O
Figure 6.10 shows the XRD patterns of the as prepared powder
samples of nanocrystalline cobalt orthophosphate with reactant concentrations
227
Chapter 6
Figure 6.10. XRD patterns of cobalt phosphate nanoparticles
(a) 0.0015 M cobalt nitrate and 0.001 M disodium hydrogen phosphate, (b)
0.003 M cobalt nitrate and 0.002M disodium hydrogen phosphate, (c) 0.006 M
cobalt nitrate and 0.004 M disodium hydrogen phosphate and the samples are
named as C1, C2 and C3 respectively. X-raydiffraction pattern reveals that the
powder samples are well crystallized. The pattern agrees well with that of
Co3(PO4)2.8 H2O in the JCPDS card [29]. The crystals are monoclinic in
nature with space group C2/m. The average particle size of the samples
228
Chapter 6
calculated from the full width at half maximum (FWHM) of major peaks using
Scherrer formula is 24 nm for sample C1, 27 nm for sample C2, 30 nm for
sample C3 which shows an increase of particle size with increase of
concentration of the reactants.
6.5.2. Electron microscopic analysis of Co3 (PO4)2. 8 H2O
The SEM images of Co3 (PO4)2. 8 H2O (sample C3) in figure 6.11.
Figure 6.11. SEM image of cobalt orthophosphate nanoparticles
The SEM image shows that the flat crystals are in the form of plate-lets. The
thickness of the plate-lets is estimated to be of the order of 30 nm.
6.5.3. Thermal studies of Co3 (PO4)2. 8 H2O
Figure 6.12 show the TGA and DTA curve of the as synthesized cobalt
phosphate sample C1. The TGA curve shows a major weight loss in the
temperature range 125-180oC followed by a slow weight loss continuously till
229
Chapter 6
up to 500oC. This weight loss is due to the drying of the sample and also due
to the loss of water of crystallization. The DTA curve shows two endothermic
peaks at 162oC and 215oC, which are attributed to the loss of the water of
crystallization in the sample. Calculations using the loss of weight from the
TGA curve agree with the presence of 8 units of water of crystallization in the
basic formula unit.
Figure 6.12. TGA and DTA curve of the as synthesized cobalt orthophosphate
6.5.4. Magnetic studies of Co3 (PO4)2. 8 H2O
Figure 6.13 indicates the variation of the molar magnetic susceptibility
(χ) with temperature of Co3 (PO4)2. 8 H2O (sample C1) from 296 K to 573 K
using VSM. The curve shows the nature of a paramagnetic sample displaying
the Curie-Weiss behavior [30]. At 296 K the value of molar susceptibility is
230
Chapter 6
0.0273 cgs units and at 473 K it is 0.012 cgs units. The literature value of
molar susceptibility of anhydrous Co3 (PO4)2 at 291 K is 0.02811 cgs units
which closely agrees to the above result [31].
0.028
Molar susceptibility (cgs units)
0.026
0.024
0.022
0.020
0.018
0.016
0.014
0.012
0.010
280
300
320
340
360
380
400
420
440
460
480
Temperature K
Figure 6.13 Variation of the molar magnetic susceptibility (χ) with of
Co3 (PO4)2. 8H2O from 296 K to 573 K using VSM
6. 5.5. Dielectric studies of Co3 (PO4)2. 8 H2O
The variation of dielectric constant (ε') with log of frequency of the
applied field for temperatures from 303 K to 353 K of Co3 (PO4)2. 8 H2O
(sample C1) is shown in figure 6.14. It is found that the dielectric constant, for
231
Chapter 6
all temperatures, has high value at low frequencies, which decreases
1800
303 K
323 K
353 K
1600
Dielectric constant '
1400
1200
1000
800
600
400
200
0
-200
2
3
4
5
6
Log f
Figure 6.14 Variation of dielectric constant (ε') with log of frequency
of the applied field for temperatures from 303 K to 353 K of
Co3 (PO4)2. 8 H2O
continuously as frequency increases. For 303 K the value of dielectric
constant, ε' at 100 Hz is 861 which decreases continuously to 0.075 at 1400
KHz. For 323 K the corresponding values of ε' are 1224 and 1.4, and for 353
K the values are 1680 and 2.78 respectively. The variation of tan δ of
Co3 (PO4)2.8 H2O as a function of frequency and temperature is shown in
figure 6.15. At 303 K the value of tan δ is 4.15 at 100 Hz and 49.4 at 1400
KHz.
232
Chapter 6
303 K
323 K
353 K
50
40
tan 
30
20
10
0
2
3
4
5
6
Log f
Figure 6.15 Variation of dielectric loss (tan δ) as a function of frequency of
Co3 (PO4)2. 8 H2O
At 323 K the corresponding values are 2.5 and 4.8 but at 353 K the values are
2 and 3.4 respectively. The dielectric behaviour of nanomaterials is primarily
due to different types of polarizations present in the material. Nanocrystalline
materials possess enormous number of interfaces and large number of defects
such as dangling bonds, vacancies and micropores present in these interfaces
can cause a change of positive or negative space charge distribution [32].
When an electric field is applied these space charges move and are trapped by
these defects resulting in the formation of dipole moments which is called
space charge polarization. Interfaces in nanostructured materials posses many
oxygen or nitrogen ion vacancies, which are equivalent to positive charges
giving dipole moments, which in an electric field will rotate giving a resultant
dipole moment in the direction of the applied field. This is called rotation
direction polarization. The high value of ε' at low frequencies is due to space
233
Chapter 6
charge polarization and rotation direction polarization. [33, 34]. As
temperature increases more and more dipoles will be oriented in the field
direction, which results in the high value of the dielectric constant. When
frequency increases the electron exchange will not follow the external field,
which lowers the values of dielectric constant. In nanophase materials, the
inhomogeneities present at the interfaces produces an absorption current
ac conductivity x 10
-4
Sm
-1
resulting in the dielectric loss [35].
8.0
7.5
7.0
6.5
6.0
5.5
5.0
4.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
-0.5
303 K
323 K
353 K
2
3
4
5
6
Log f
Figure 6.16 Variation of ac electrical conductivity (σac) as a function of
frequency and temperature of a typical sample (b).
The variation of ac electrical conductivity (σac) as a function of
frequency and temperature is shown in figure 6.16. At low frequencies σac
have lower values, which increase continuously up to 1400 KHz. The values
are shifted upwards as temperature increases. The maximum value of σac at
303 K is 2.9 x 10-4 Siemens.m-1, while the corresponding value of σac at 353 K
234
Chapter 6
is 7.3 x 10-4 Siemens.m-1. The low temperature electrical conductivity in
cobalt phosphate can be explained in terms of electron hopping and at high
temperature σac is controlled by thermally activated polaron hopping. Electron
hopping occurs between Co2+ and Co3+ sites. Small polaron formation takes
place in those materials whose conduction band belong to incomplete‘d’ or ‘f’
orbital [33]. In the present case cobalt being a transition metal with
incomplete‘d’ orbital, which is responsible for the formation of small
polarons. The sudden release of space charges accumulated at the grain
boundaries increases the conductivity at higher temperatures.
6.6. Conclusion
Nanoplatelets of zinc orthophosphate and cobalt orthophosphate are
prepared by aqueous precipitation method and characterized. Rietveld analysis
and TGA/DTA studies of zinc orthophosphate confirmed the formula as
Zn3 (PO4)2. 4H2O. The SEM and TEM images show that the particles are thin
transparent platelets with an average particle size of 30 nm. The specific
surface area for the sample calculated by the BET method is  90 m2g-1 and
the pore volume of the sample is  0.026 cm3g-1. Quantitative phase analysis
reveals the presence of 6 secondary phase consisting of sodium
di-zinc-nonahydrate. The photoluminescence spectrum shows an increase in
the intensity of emission peak of zinc phosphate when it is doped with Mn2+
and hence it can be used as a phosphor in cathode tubes.
The average particle size of cobalt orthophosphate estimated from
XRD results ranges from 24-30 nm. It is seen that the particle size increases
with increase of concentration of the reactants. The SEM image shows that the
particles are thin transparent platelets. The VSM study reveals that the molar
235
Chapter 6
magnetic susceptibility of cobalt orthophosphate has a decreasing trend with
increase of temperature. The dielectric constant shows a decreasing trend as
frequency increases whereas the ac conductivity increases with temperature
and frequency.
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