5
0 Rao et al.
Asian J. Chem.
nanocrystals have been studied by Dai et al. [6]. They observed
that lower pH and lower concentration of Mn resulted in uniform
nanospheres with narrow size and higher luminescence thermal
stability. The band gap value increased with increase in Mn
content from 4.47 to 4.78 eV [6].
observed such a increase intensity or shift towards blue light
region in Mn doped luminescent materials. In order to under-
stand these in this work, manganese has been selected as dopant
in place of calcium in CaWO
4
and initially the effect of Mn on
has been presented.
structural properties of CaWO
4
Pawlikowska et al. [7] synthesized pure and manganese
EXPERIMENTAL
doped calcium molybdates (Ca1-xMn
tungstates (Ca1-xMn MoO
solid state reaction and citrate-nitrate combustion methods. They
identified that with increase in Mn content in Ca1-xMn MoO
x 4
MoO ) as well as molybdato-
x
4
)
0.50(WO )
4
0.50) for 0 < x ≤ 0.15 by
To synthesize nano crystalline Ca1-xMn
x
WO
4
, each of
Na
2
WO
4
·2H
2
O, Ca(NO
3
)
2
·4H
2
O and MnCl
2
·4H
2
O were separ-
x
4
ately dissolved in deionised water and stirred for 0.5 h to obtain
clear homogeneous 1 M solution. Calcium nitrate and manganese
chloride are mixed and stirred for 1 h. This solution is added
dropwise to sodium tungstate at room temperature resulting
in the formation white precipitate under continuous stirring
for 2 h. The products were washed with water several times
and after final washing with ethanol were dried at 95 ºC for
band gap value decreased upto Mn concentration of 0.10 and
with further increase of Mn band gap increased. Whereas in
x 4 4
case of Ca1-xMn MoO )0.50(WO )0.50 band gap value decreased
with increasing Mn concentration. Li et al. [8] observed intense
2+
2+
green emission when Zn ions are replaced with Mn ions in
Zn GeO synthesized by facile solvothermal process with H O/
2
4
2
ethylene glycol as solvent. This kind of intense green emission
8
h in hot air oven. The powder thus obtained was subjected
to heat treatment at 500 ºC to get single phase.
4
6
2+
has been attributed to the d-d transition ( T
1
→ A
1
) of Mn
ions. They also elaborately studied the luminescence mechanisms
XRD patterns are recorded at room temperature in the 2θ
range of 10º to 80º using PanAlytical, X-Pert pro. XRD patterns
of all the samples have been refined according to Rietveld profile
refinement procedure using MAUD program 2.8 [12]. Fourier
transform infrared (FTIR) spectra measurements are made by
Shimadzu IR-Prestige 21 instrument in transmittance method
2+
of Zn
2
GeO
4
:Mn by using density functional theory (DFT).
Similarly, White et al. [9] observed a bright green light
2+
emission in Mn doped zinc silicate and phosphate where Mn
ions act as activators. Chan et al. [10] showed an increase in
photoluminescence intensities in LiZn1-xPO :Mn phosphors
with various concentrations of Mn (x = 0.02, 0.04,0.06, 0.08,
.1, 0.12, 0.14, 0.18, 0.22) upto x = 0.12 and then it decreased
as a result of concentration quenching. The emission lines are
4
x
with KBr pellet as IR window in the wave number region of
0
-1
4
000 to 400 cm .
2
+
6
6
attributed to transitions of Mn ion from ground level A
1
( S)
RESULTS AND DISCUSSION
X-ray diffraction studies of Ca1-xMn WO were illustrated
4
4
4
4
4
4
4
4
4
4
4
4
to E ( D), T
levels.
Zhang et al. [11] tried to explore the optical properties of
Mn-dopedAWO (A = Ca, Sr, Ba) nanorods synthesized using
2
( D), [ A
1
( G), E( G)], T
2
( G) and T
1
( G) excited
x
4
in Fig. 1. All diffraction peaks were found perfectly indexed
JCPDS standards, 41-1431) in conformity with scheelite type
tetragonal structure with space group I41/a [13]. The XRD
spectra confirm that the products are pure in phase with no
other characteristic peaks.
(
4
a modified template-directed methodology under ambient,
room-temperature conditions. They identified that presence
2+
of Mn not only substantially increases the photoluminescent
potential of a pristine tungstate material but also reinforces its
versatility by adding a desirable magnetic component to its
repertoire of properties. Broadband absorption in the range
from 350 to 450 nm has been observed by them in UV-visible
absorption spectra of as-prepared Mn-doped tungstate nano-
JR1
JR2
JR3
JR4
JR5
JR6
6
4
rods at room temperature, corresponding to A
1
→ T
1
transition
2+
associated with the 3d states of Mn ions. They also observed
that the emission peak positions were not affected by the incor-
poration of Mn ions, implying that the energy gap related to
2−
the blue emission withinWO
4
complexes is similarly unaltered
by the presence of Mn dopant. However, Mn-doped alkaline-
earth metal tungstate nanorod samples clearly showed far
greater and enhanced photoluminescent emission intensity as
compared with their undoped counterparts, all normalized in
terms of concentration. That is, as compared with pure CaWO
nanorods, Mn-doped CaWO nanorods yielded 1.8 times
4
0
10
20
30
40
50
(°)
Fig. 1. XRD patterns of Ca1-xMn
60
70
80
90
2θ
4
x
WO
4
greater luminescence intensity at the major peak position
located at 502 nm. The observed increase in luminescence
has been partially attributed to the intra-3d-shell transitions of
The structure of the unit cell manifests that the tungsten
atoms are coordinated with four oxygen atoms in order to form
tetrahedral [WO ] clusters, where the tetrahedral angles are mini-
mally distorted. While the calcium atoms are in coordination
with eight oxygen atoms, resulting in the formation of [CaO
clusters with a deltahedral configuration, a symmetry group
2+
Mn ions.
Based on the literature, it has been identified that clear
4
2+
understanding of the effect of Mn ions on the optical properties
of CaWO is necessary. This is in fact due to the changes
8
]
4
Rao, M. Jaganadha
