Synthesis and characterization of a novel alkaline earth niobate Ca0.5Sr0.5Nb2O6

Article abstract of DOI:10.1016/j.jallcom.2015.10.220

Ca_0.5Sr_0.5Nb₂O₆ (CSN), a novel compound with crystal structure similar to the well-known orthorhombic columbite compound CaNb₂O₆ (CN) of alkaline earth niobates family was synthesized by a simple co-precipitation technique. The synthesized material was characterized by using X-ray diffraction (XRD) technique, transmission electron microscopy (TEM), UV-Vis diffuse reflectance spectroscopy, FTIR spectroscopy, and photoluminescence emission spectroscopy. It was found that the CSN compound stabilizes in P b c n space group with the improved symmetry as compared to the CN. The lattice parameters of CSN were found to be comparable with those of CN, and they are: a = 15.013 ?, b = 5.757 ? and c = 5.198 ?. The morphology of CSN and CN particles synthesized by a similar method was found to be identical. However, in CSN stronger self activated luminescence than that of CN was observed. The self-activated luminescence property of CSN was assigned to the niobate octahedral group [NbO₆]^7- similar to the CN.

Full text of DOI:10.1016/j.jallcom.2015.10.220

Accepted Manuscript
Synthesis and characterization of a novel alkaline earth niobate Ca Sr Nb O
0.5 0.5 2 6
C.M. Dudhe, S.B. Nagdeote, U.A. Palikundwar
PII:
S0925-8388(15)31476-6
10.1016/j.jallcom.2015.10.220
JALCOM 35775
DOI:
Reference:
To appear in: Journal of Alloys and Compounds
Received Date: 29 July 2015
Revised Date: 13 October 2015
Accepted Date: 23 October 2015
Please cite this article as: C.M. Dudhe, S.B. Nagdeote, U.A. Palikundwar, Synthesis and
characterization of a novel alkaline earth niobate Ca Sr Nb O , Journal of Alloys and Compounds
0.5 0.5 2 6
(2015), doi: 10.1016/j.jallcom.2015.10.220.
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ACCEPTED MANUSCRIPT

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Synthesis and characterization of a novel alkaline earth niobate
Ca0.5Sr0.5Nb2O6
C. M. Dudhe1*, S. B. Nagdeote2, U. A. Palikundwar3
1Department of Physics, Government Institute of Science, Nagpur 440001, India
2Department of Physics, Amolakchand Mahavidhyalaya, Yavatmal 445001, India
3Department of Physics, R. T. M. Nagpur University, Nagpur 440033, India
*e-mail: chandraguptadudhe@gmail.com
Abstract
Ca0.5Sr0.5Nb2O6 (CSN), a novel compound with crystal structure similar to the well-
known orthorhombic columbite compound CaNb2O6 (CN) of alkaline earth niobates family was
synthesized by a simple co-precipitation technique. The synthesized material was characterized
by using X-ray diffraction (XRD) technique, transmission electron microscopy (TEM), UV-Vis
diffuse reflectance spectroscopy, FTIR spectroscopy, and photoluminescence emission
spectroscopy. It was found that the CSN compound stabilizes in P b c n space group with the
improved symmetry as compared to the CN. The lattice parameters of CSN were found to be
comparable with those of CN, and they are: a = 15.013Å, b = 5.757Å and c = 5.198Å. The
morphology of CSN and CN particles synthesized by a similar method was found to be identical.
However, in CSN stronger self activated luminescence than that of CN was observed. The self-
activated luminescence property of CSN was assigned to the niobate octahedral group [NbO6]7-
similar to the CN.
Keywords:
B. Chemical synthesis; C. Crystal structure; D. Optical spectroscopy; D. Luminescence
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1. Introduction:
The space group P b c n (space group no. 60) belongs to a large family of orthorhombic
centrosymmetric point group mmm. Calcium niobate (CaNb2O6, CN), a well known compound of
the alkaline earth niobates belongs to this space group. It is well known that the magnitude of
any property of the material is related to the atom types that make the solid, however, the
presence or absence of a property is often controlled by the symmetry of the solid [1]. Hence,
compounds like CaNb2O6, BaNb2O6 and SrNb2O6 although have similar atomic compositions,
their crystal structures and physical properties are different [2,3]. On the other hand, if
compositions and symmetries of materials are similar, the probability of getting similarities in
their properties at the same or different extent is expected to be more [4,5]. The novel material
undertaken in the present study i.e. Ca0.5Sr0.5Nb2O6 (CSN) shows structural and compositional
similarities with the CN. Therefore, the investigation of CSN under the light of known properties
and applications of CN will be very exciting.
The initial investigations on CN have been done for its very important applications in
holography and laser, as it possesses a strong source of coherent light and very high refractive
index ( 2.07 – 2.20) [6-8]. According to the literature, the CN possesses a strong, self-activated,
blue luminescence with the excitation by UV radiations of 253cm-1, even at the room
temperature due to the niobate octahedral group [NbO6]7- [9]. Later on, many investigations on
CN regarding the symmetry, size, shape, and from applications point of view have been reported.
These studies show that the CN compound is a low symmetry material. Nanoparticles of the CN
have irregular sphere like, rod like and fiber like morphologies and useful for the photo-catalytic
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H2 evolution from pure water under UV irradiation, stimulated Raman scattering laser and low
cost lamp phosphor [3,10-17].
As far as CSN compound is concerned, to the best of our knowledge the material is not
reported yet and therefore the investigation of properties and applications analogous to CN is a
prolong work. In the present study, however, some important properties such as structural,
morphological, optical characterizations and self activated luminescence of CSN are undertaken.
Now-a-days the synthesis of compounds by chemical route becomes highly popular
because, by this route the reduction in the size of particles can be easily achieved; and also the
route shows better composition control, better particle morphology control, and good powder
reactivity [18]. Thus in the present study CSN and CN compounds (for the comparison) were
synthesized by a chemical co-precipitation synthesis method.
2. Material and methods
The synthesis of Ca0.5Sr0.5Nb2O6 (CSN) compound, as mentioned, was carried out by a
co-precipitation method. The starting materials were Nb2O5, SrCl2.6H2O and CaCl2.2H2O. All
they were of AR grade. The method is as described below.
A stoichiometric amount of Nb2O5 was converted to NbF5 by reacting with minimum
amount of HF. The reaction was carried out in a Teflon beaker which was maintained at 70oC in
a water bath for 15h. During the reaction, Nb2O5 was completely dissolved in HF and forms a
transparent solution of NbF5. This solution was added to another solution which was prepared by
dissolving stoichiometric amount of CaCl2.2H2O and SrCl2.6H2O in distilled water. The resultant
solution was vigorously stirred with a magnetic stirrer for 3-4 h. While stirring, an excess
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quantity of concentrated HCl was added to the solution to dissolve calcium fluoride and
strontium fluoride formed during the reaction of NbF5 with CaCl2.2H2O and SrCl2.6H2O. A
mixture solution of ammonium oxalate and ammonium hydroxide was then added drop wise
until the pH of the solution reached to 9.5 to precipitate niobium as hydroxide and calcium
and/or strontium as oxalates. pH of the solution around 9.5 ensures completion of the reaction.
The precipitate was filtered out and washed several times with distilled water. The washed
powder was then dried at 70oC in an oven, for 24h, and finally calcined at 650oC and 800oC for 8
h.
The synthesis of CN compound was also carried out by the similar method as reported
elsewhere [19,20].
X-ray diffractions (XRD) data of the synthesized compounds of CSN and CN were
collected by using X-ray diffractometer (D8 ADVANCE, BRUKER, GERMANY) employing
Cu-Kα radiation (1.54Å) with step size of 0.02o and step time of 46.5s. The morphology of the
particles was observed by using transmission electron microscopy (TEM) (TECNAI G2 20
ULTRA-TWIN, FEL, NETHERLANDS). FT-IR, PL emission and UV-Vis spectra of CSN and
CN were obtained by using FT-IR spectrometer ( THERMO NICOLET, AVATAR 370, USA),
spectroflurometer (F-7000, HITACHI, JAPAN) and UV-VIS spectrometer (UV-VIS 1700,
SHIMATZU, JAPAN), respectively.
For the measurements of UV-Vis diffuse reflectance spectra, pellets of the compounds
were prepared by mixing the powder with a few drops of 1 wt% solution of polyvinyl alcohol
and then pressing isostatically under the pressure of 5-6 t for 5 min. The pellets were heat-treated
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at 800oC for 5 h to remove the binder. The pellets of CSN were also used for the measurement
of bulk density.
3. Results and Discussion
3.1. Structural and morphological
Fig. 1 shows the XRD patterns of synthesized precursors of CSN and CN calcined at
650oC for 8h. The high intensity XRD peak at 2θ ~ 29.3o is a characteristic feature of CN
compound which is clearly seen in Fig.1 [6,21]. The XRD pattern of CSN follows the same
pattern, confirming at the first sight that the structure of CSN is similar to the CN. Similarities
are also observed from Fig.2, which represents the TEM and SAED images of CSN and CN
nanoparticles. The morphology of nanoparticles of the both is likely to be an irregular sphere,
and they are agglomerated. The size of nanoparticles of the both CSN and CN compound as
visualized by the TEM images in Fig.2 is almost same and is ranging from 60-80nm. Beside the
XRD pattern, the poly-crystalline nature of these compounds is clearly confirmed from their
respective selected area electron diffraction (SAED) patterns (Fig. 2) as concentric rings along
with bright spots are observed. The spotty rings patterns are due to the reciprocal patterns of
reflections from a large numbers of crystallites under the view of SAED signal [1]. However,
individual identities of these compounds can be observed from the energy dispersive X-ray
(EDX) patterns shown in Fig. 3, as EDX patterns show different constituents of CSN and CN
compounds. Note that the signals of C and Cu were generated from the carbon coated copper
grid.
For the better clarity of peaks and for the structural analysis, the precursors of CSN and
CN were again heat- treated at 800oC for 8h; the XRD patterns of which are shown in Fig.4. The
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peaks pattern of CN as seen in Fig. 4 clearly matched with the JCPDS data file no. 31-289. CSN
being a novel compound its structural analysis was done by using Rietveld refinement software
FULLPROF. The lattice parameters of CN calculated by least square fit method and that of CSN
calculated by the refined method are given in Table 1. Although, Fig. 4 shows the identical XRD
peak patterns of CSN and CN, the careful observation differentiates these peak patterns. This can
also be realized from Table 1, in which the slight difference in lattice parameters ‘c’ of CSN and
CN is observed. The origin of this change may be associated to the slight difference in the sizes
of Sr and Ca ions.
Table 2 shows the coordinates and Wyckoff sites of atoms in CSN. From this table it is
observed that the Sr2+ ions occupy the same site as that occupied by the Ca2+ ions. This may be
due to comparable ionic radii of Ca+ 2(1.00Ǻ) and Sr+ 2(1.18Ǻ). The presence of Sr+ 2 ions
however can affect the Nb sites, which can be realized by comparing the cell structure of CSN
(Fig. 5) with the cell structure of CN [3]. Nb atoms in CSN are arranged more mannerly than
they are in CN, result in the increase in symmetry of the crystal structure of CSN as compared to
the CN.
Solid solution reaction CaNb2O6- SrNb2O6 is not reported in the literature unlike to
SrNb2O6 -BaNb2O6 and CaNb2O6- BaNb2O6 reactions. However, we assume that such a reaction
is there and CSN is the end product of such reaction. It is to be noted that the general phase of
CN is orthorhombic, and that of SN is monoclinic [3]. Therefore, in CN- SN reaction the CN
may be a dominant reactant over the SN as the product CSN is orthorhombic. Our prediction is
well supported by the finding on non-sensitivity of nanocrystalline SrNb2O6 powders to the
applied electric field intensity [3]. Thus, in CN-SN solid solution reaction, the orthorhombic
columbite structure is formed initially and Sr2+ ions get completely diffused into the lattice by
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sharing the sites with Ca2+ ions. Zhou and Liu [22] reported a similar observation on diffusion of
BaNb2O6 (BN) into the lattice of Bi0.5Na0.5TiO3 compound without any significant change in the
Bi0.5Na0.5TiO3 cell structure.
3.2. Establishing the formula Ca0.5Sr0.5Nb2O6
The chemical formula Ca0.5Sr0.5Nb2O6 was established getting corroboration from the
following ways:
For the synthesis of the compound the stoichiometric ratio of Nb2O5, SrCl2.6H2O and
CaCl2.2H2O was taken as 2:1:1. The synthesis was not carried out at high temperature therefore
there is no ablation of Ca2+ or Sr2+ ions during the calcinations. Since the synthesis method is
well- established [19], the final product after removal of bi-products such as water, oxygen, HCl
etc by the filtering process and the calcinations is Ca0.5Sr0.5Nb2O6.
Secondly, using the lattice parameters determined by Rietveld refinement and taking 4
molecules of Ca0.5Sr0.5Nb2O6 per unit cell (Fig. 5) the density was calculated. The value was
found to be 3.028 g cm-3. Experimentally, the bulk density measurement of Ca0.5Sr0.5Nb2O6 was
also carried out and it was found to be 2.82 g cm-3. Within experimental error the assessment of
theoretical and experimental density values is acceptable.
Finally, the quantification results obtained by using EDX shows that the values of atomic
% of Ca(K) and Sr(K) were 2.77 ± 0.7 and 3.01 ± 0.19 respectively. Considering the reliability
and uncertainty of EDX quantification analysis, the compositions of Ca2+ and Sr2+ ions in
Ca0.5Sr0.5Nb2O6 compound may be treated as equal. (Note that the elements from the carbon
coated copper grid C(K) and Cu(K) were also counted in the quantification result and together
with they carry atomic% ~ 44%).
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3.3. Optical characterization
Fig. 6 shows the FTIR spectra of the synthesized CN and CSN compounds. The intense
absorption is observed in the fingerprint range 1000-400 cm-1. FTIR spectrum of CN match with
the spectrum reported earlier by Mathai et al [23]. Accordingly, the absorption band 861.07 cm-1
is assigned to the ν1 (B2u) mode of the vibration, the band 743.84 cm-1 is assigned to the ν2 (B2u)
mode of vibration, the strong broad band 595.94 cm-1 is due to the broadening caused by
overlapping of the ν3 mode of vibrations i.e. ν3 (B1u), ν3 (B2u) and ν3 (B3u) symmetries, and
finally the band 484.36 cm-1 is due to the overlapping vibrations of ν4 (B1u) and ν4 (B3u) modes.
The corresponding absorption bands for CSN were found at 869.62, 747.33, 584.61, and 478.53
cm-1, respectively. These modes of vibrations are due to Sr-O and/or Ca-O stretching, Nb-O
stretching and O-Nb-O bending [23]. Some changes in band 5 (Fig. 6) are observed, which can
be assigned to the higher symmetry of CSN structure as compared to the CN.
Fig. 7 shows the UV-VIS diffuse reflectance spectra of CN and CSN nanoparticles. UV-
VIS spectra show that the % reflectance of CSN is smaller than that of CN. Corresponding to the
spectra shown in Fig. 7, the Kubelka-Munk (K-M) transformed spectra are shown in Fig. 8. The
K-M transformed spectra is a plot of [F(R∞)hν]1/n against hν. According to the K-M theory the
relation between these two is given by a straight line equation [24]:
ꢅ
ꢆ
ꢁ
)
[ꢀ ꢂꢃ ℎꢄ] = ꢇꢁℎꢄ − ꢈꢉ)
…1
where C is a constant, and n= ½ for the estimation of direct band gap energy.
ꢁ
)
Function ꢀ ꢂꢃ is called Kubelka-Munk function or remission and given by the equation
ꢐ
ꢊ
ꢋ
ꢁꢌꢍꢎ
)
ꢏ
ꢁ
)
=
ꢀ ꢂꢃ
=
…2
ꢑꢎ
ꢏ
where S and K are the Kubelka-Munk scattering and absorption coefficients respectively.
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The term R∞ is called limiting reflectance [25], and it is given by
ꢎ
ꢒꢓꢔꢕꢖꢗ
ꢂꢃ
=
…3
ꢎ
ꢘꢗꢙꢗꢘꢗꢆꢚꢗ
From extrapolations of the linear region, the values of optical band-gap energy were
determined. The direct (n=1/2) band-gap energies of CN and CSN are found to be 3.56 and 3.76
eV respectively. The corresponding absorption edges are observed at wavelength 348nm and 330
nm respectively for CN and CSN compounds. The obtained values of bandgap energies and their
absorption edges show good agreement with the reported values of alkaline earth niobates [2].
3. 4. Luminescence study
It is well known that the alkaline earth niobate CaNb2O6 exhibits luminescence property
when doped with activators like Eu3+, Yb3+ and Tm3+ [14-16,26]. Beside this, it possesses the
self-activated luminescence due to edge-shared [NbO6]7- octahedra group [13,27]. This
luminescent effects depend on the Nb-O-Nb bonding as the conduction band is composed of
Nb5+ 4d orbitals, and the valence band of O2- 2p orbitals between the corner sharing octahedra
[13,28]. The fact is also observed in our synthesized CN and CSN nanoparticles. The room
temperature emission (λex=254nm) spectra of CN and CSN are shown in Fig. 9. These spectra
show broad blue emission peaks at about 457 nm for the both compounds. The blue emission is
found stronger for CSN (more than twice the value of CN) than the CN.
In CSN, though the structural symmetry of the cell is improved by sharing the 4c sites
between Ca2+ and Sr2+ ions, the charge distribution between the Ca/Sr ions and the [NbO6]7-
octhedra may be anisotropic if Ca2+ ions occupy inner 4c sites and Sr2+ ions occupy edge shared
4c sites (Fig. 5) or vice-versa, because Ca atom lose its electron from 4s2 whereas Sr atom from
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5s2. Due to this charge anisotropy the probability of radiative transitions increases which leads to
the increase in emission intensity.
4. Conclusion
Ca0.5Sr0.5Nb2O6 (CSN) nanoparticles were successfully synthesized by the co-
precipitation method and they stabilize in orthorhombic columbite phase with the space group
P b c n. In the cell structure, Sr+2 ions get completely distributed and share the Wyckoff position
4c with the Ca+2 ions because of their comparative ionic radii. The morphologies of
nanoparticles of CN and CSN are found to be similar. The structure of the CSN was found more
symmetric than the CN. The luminescence of CSN was found to be intense than the CN. The
photoluminescence properties of CSN is assigned to the [NbO6] 7- octahedral similar to CN.
Because of similarities of CSN and CN, profound result in symmetry and self activated
luminescence, we believe that the CSN is a good alternative for CN in many applications like
laser host material, low cost lamp phosphor, photocatalytic activity etc, however further
investigation is required for the confirmation.
Acknowledgment
The authors thank to SAIF Kochi and Department of Physics, Savitribai Phule Pune
University, Pune, India for XRD and TEM facilities respectively. The authors also thank to Dr.
R. R. Patil, Institute of Science Nagpur, for fruitful discussion on luminescence.
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Captions of Figures
Fig. 1 X-ray diffraction patterns of precursors of CN and CSN calcined at 650oC, for 8h.
Fig. 2 TEM images of precursors of (a) CN and (b) CSN calcined at 650oC. Inset shows SAED
patterns, confirming polycrystalline nature of CN and CSN.
Fig. 3 Energy dispersive X-ray spectrum of (a) CN and (b) CSN showing their compositions
(signals of C and Cu were generated from the carbon coated copper grid).
Fig. 4 X-ray diffraction patterns of precursors of CN and CSN further heat treated at 800oC, for
8h ( insignificant unknown phase).
Fig. 5 Cell structure of CSN orthorhombic nanoparticles as visualized by Powder cell (PCW)
program.
Fig. 6 FT-IR spectra of 800oC heat treated compounds of CN and CSN.
Fig. 7 UV-VIS diffuse reflectance spectra of 800oC heat treated compounds of CN and CSN
nanoparticles.
Fig. 8 K-M transformed absorption spectra of CN and CSN nanoparticles for (n=1/2).
Fig. 9 PL emission (λex=254nm) spectra of 800oC heat treated compounds of CN and CSN at
room temperature.
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Table 1. Lattice parameters of CN and CSN
Compounds
a(Å)
b(Å)
c(Å)
CN
CSN
15.014
15.013
5.741
5.757
5.302
5.198
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Table 2. Refined atomic positions of Ca0.5Sr0.5Nb2O6 compound
Atom
Ca
Sr
Nb
O1
O2
O3
Wyck.
4c
4c
8d
8d
x
0
0
y
z
Occ
0.5
0.5
1
1
1
0.823(1)
0.823(1)
0.287(1)
0.048(10) 0.471(10)
0.391(6)
0.157(6)
0.25
0.25
0.305(1)
0.169(1)
0.063(3)
0.064(4)
0.248(3)
8d
8d
-0.343(6)
0.094(7)
1
Goodness-of-fit (square root of χ2) = 2.22
2

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Figure 1

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Figure 2

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Figure 3

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Figure 4

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Figure 5

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Figure 6

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Figure 7

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Figure 8

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Figure 9

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Study on a novel compound Ca0.5Sr0.5Nb2O6 is reported.
It possesses properties similar to the CaNb2O6.
Cell structure is more symmetric than the CaNb2O6.
The self luminescent intensity was found more than twice the intensity of CaNb2O6.