CHEM01200604004 Shri Sanyasinaidu Boddu - Homi Bhabha ...

lattice. Asymmetric ratio is ~ 12 for all the three tungstates. High value of asymmetric ratio
indicates that Eu 3+ ion occupies a highly asymmetric environment in MWO 4 (M = Ca, Sr, Ba)
crystal lattice. The intensity of Eu 3+ emission decreases with increase in the ionic size of the
alkaline earth metal cations. Similar effect was observed by Blasse, et al. for UO 6 emission in
the tungstate host [69]. Presence of ions with large radius around the luminescent center is
equivalent to soft surroundings. With increase in the softness of surroundings, stokes shift
increases and as a result the excited tungstate group decays by a non-radiative route to ground
state instead of transferring energy to lanthanide ions. This explains the decrease in
luminescence intensity when going from CWO 4 to SrWO 4 and then to BaWO 4 . It is also
known that with increase in the stiffness of the lattice, non-radiative process decreases.
Intensity (arb.units)
20000
16000
12000
8000
4000
5 D 0
5 D 0
7
F 1
7 F 2
5 D 0
λ exc
= 270 nm
7
F 3
CaWO 4
:Eu 3+
SrWO 4
:Eu 3+
BaWO 4
:Eu 3+
5
D 0
7 F 4
0
550 600 650 700 750
Wavelength (nm)
(a)
Intensity (arb.units)
(b)
80000
60000
40000
20000
0
λ em
: 615 nm
CaWO 4
:Eu 3+
SrWO 4
:Eu 3+
BaWO 4
:Eu 3+
200 250 300 350 400
Wavelength (nm)
Fig.108. (a) Emission spectra and (b) excitation spectra from CaWO 4 : Eu 3+ , SrWO 4 : Eu 3+ ,
BaWO 4 : Eu 3+ nanoparticles. (λ exc = 270 nm and λ em = 615 nm)
In the present case the ionic radii of metal ion increases while going from CaWO 4 to SrWO 4
and then to BaWO 4 . Hence, the stiffness of the lattice decreases in the same order and this
explains the observed maximum in the luminescence intensity from CaWO 4 as compared to
SrWO 4 and BaWO 4 . Values of lattice parameters further confirm this. The excitation spectra
corresponding to Eu 3+ emission is shown in Fig.108 (b). The patterns consist of broad peak
centered at 270 nm and sharp peaks above 310 nm. The broad band is assigned to the Eu-O
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