340
S.K. Gupta et al. / Journal of Alloys and Compounds 695 (2017) 337e343
excitation range from 230 to 300 nm. The spectral feature remains
same as a function of excitation wavelength but they do differ in
terms of intensity; 250 nm excited sample gives the maximum
emission output. The fluorescence spectrum has six characteristic
bands at 460, 480, 500, 520, 546 and 572 nm. The four main peaks
2 4
doping in Al site of normal spinel MgAl O .
Fig. 5(b) shows the total and angular momentum decomposed
DOS in the presence of neutral O vacancy and U atom doped in Mg
site. The spin-up and spin-down components are shown separately
in upper and lower panels, respectively. Overall nature of the VB
remains unchanged and an impurity band that appears 2.3 eV
ahead of VB maximum in the band-gap is mainly due to neutral
oxygen defect comprises of Mg-s and O-p states [32]. Moreover, an
additional impurity band appears just below the CB, mainly
contributed by U-f states in the spin-up component. Fermi level is
moved towards the CB compared to normal spinel structure.
(
(
460, 480, 500, and 520 nm) have a full width at half maxima
FWMH) of around 20 nm (Fig. 4(a).). Complete disappearance of
defect induced host emission of MAS in emission spectrum of
uranium doped MAS is an indicative of the fact that complete en-
ergy transfer takes place at 1.0 mol % of uranium ion concentration.
This we have tried to explain using DFT calculation in section 3.3.
Such vibronic progression with constant spacing is specific
Fig. 5(c) shows the total and angular momentum decomposed
DOS in the presence of O vacancy with charge þ1 (V
2
þ
þ1
signature of uranium in þ6 oxidation state in the form of UO
2
[13].
O
). Overall
Such uniformly distributed vibrational progression arises from
nature of the VB remains unchanged but two impurity bands
appear in the band-gap below the Fermi level. The impurity states
are present 2.5 eV and 5.1 eV above the VB maximum. Impurity
levels that are present near the VB maximum arises due to þ1
charged oxygen vacancy which is composed of s-states of Mg and
p-states of O [32]. U f-states has a significant contribution in the
spin-up component in that impurity band. Impurity band present
just at the Fermi level is solely contributed by the U f-states.
strong interaction of the ground state Raman active O¼U¼O sym-
3
metric stretching mode with the П
(
u
electronic triplet excited state
ꢁ1
generally observed between 780 and 900 cm ). These charac-
teristic emission peaks of MgAl :U (Fig. 4(a)) is due to LMCT
involving electronic transition from bonding oxygen orbital (
and ) to a non-bonding uranium 5f and 5f orbital [12]. UeO
in uranyl ion (UO ) has partial triple bond character and relatively
shorter bond length compared to single UeO bond in uranate ion
UO ). The position of first vibrational band (ʋ0-0) is most confir-
2 4
O
u g
s , s ,
p
u
p
g
d
ɸ
2
Fig. 5(d) shows the total and angular momentum decomposed
DOS in the presence of O vacancy with charge þ2 (V
þ2
(
6
O
). Overall
matory signature in deciding the number of oxygen around ura-
nature of the VB remains unchanged but three impurity bands
appear above VB maximum and below the Fermi energy. Impurity
bands near VB are composed of f-states of U and impurity states
just below the Fermi-level are composed of s-states of Mg and p-
states of oxygen which arises due to þ2 charged oxygen defect.
These impurity states are filled with electrons as it is situated just
below the Fermi energy. Impurities bands appear just below the CB
minimum are solely composed of f-states of U.
nium and bond order of UeO. And it is termed as zero phonon band
2
þ
(
ZPB). ZPB for UO can vary from 440 to 520 nm and the fact in our
2
spectrum it is observed at 460 and the subsequent vibrational
progression can be seen at room temperature is an indication of the
2
þ
.
fact U (þ6) stabilizes as UO
Fig. 4(b) shows the emission spectra for the uranium incorpo-
rated MgAl samples as a function of excitation wavelength from
00 to 400 nm. It is seen from the figure that as the excitation
2
2 4
O
3
U d and f-states contribute strongly in the lower part of CB as
well as in the defect states generated due to oxygen vacancies. In
our previous study [32] we have shown that photo-luminescence
wavelength increases and reached the limit of 330 nm the fine
uranyl structures vanishes and a broad band was observed centered
at around 525 nm which is the signature of uranate ion in octa-
properties of the MgAl
2 4
O are dominantly governed by the defect
6
ꢁ
hedral coordination (UO
6
).
states coming from the presence of oxygen vacancies (neutral and
charged). As a result photon energy transfer from host MgAl
2 4
O to
3.3. Efficient energy transfer from defect related emission of host to
dopant U is easy and preferable.
uranyl ion- A DFT study
2 4
In order to study the change in electronic structure of MgAl O
normal spinel with the oxygen defects and U atom doped in Mg
site, the total and angular momentum decomposed density of
states (DOS) is calculated and plotted in Fig. 5. Fig. 5(a) shows DFT
2 4
3.4. Local site occupancy of uranium in MgAl O spinel
2 4
calculated DOS of pure MgAl O (normal spinel) which shows
lower part of the valence band (VB) is mainly composed of s-states
of Al, Mg and upper part of VB comprises of p-states of Al, Mg and O.
On the other hand, lower part of conduction band (CB) is contrib-
uted by s and p states of Mg majorly as well as s and p states of Al.
The GGA-PBE calculated electronic band-gap is 6.0 eV, which is
lower compared to experimentally reported value of 7.8 eV (direct
In order to compare the emission profile of uranyl ion in a
magnesium aluminate spinel and that of pure uranyl ion compound
we have synthesized uranyl fluoride and recorded its fluorescence
spectrum. Fig. 6 shows the comparative emission spectra of uranyl
fluoride crystal systems and that of MgAl
2 4
O : U. The emission
spectrum of MgAl : U exhibit broad spectral features compared
2 4
O
band-gap at
G
point) measured from optical reflectivity experiment
to uranyl fluoride crystal; which is typical of uranyl ion in disor-
[35]. Underestimation of band-gap is a well known limitation of the
dered chemical surrounding. Secondly, the emission bands in
GGA [36e39]. In this study we focus on the change of the band-gap
due to presence of O vacancy (neutral and charged), so it is ex-
pected to cancel the GGA-PBE calculated band-gap error during
comparison.
2 4
MgAl O : U are blue shifted w.r.t to peak position of uranyl fluoride
which also indicates a distorted chemical environment for the
uranyl ion in magnesium aluminate compared to pure uranyl
2
þ
compound. This indicates that majority of UO
2
occupies relatively
2þ
In order to understand the change in electronic structure of
asymmetric Mg site (tetrahedral) in magnesium aluminate.
To get better insight into local structure and site of uranyl ion in
magnesium aluminate, we have conducted luminescence life time
measurements. The decay curves corresponding to the uranyl ions
in magnesium aluminate spinel are depicted in Fig. 7 under exci-
tation wavelengths of 250 nm monitoring emission at 501 nm.
The PL decay curve was fitted using bi-exponential model using
equation:
normal spinel MgAl
2
O
4
due to the oxygen vacancy, we calculated
0
þ1
DOS of V
O
(neutral O vacancy), V
O
(O vacancy of charge þ1) and
þ1
V
O
(O vacancy of charge þ2) and those results has already been
elaborated in our previous study [32]. In the defective unit-cell, U
atom is doped in Mg site and the change in the electronic DOS is
shown in Fig. 5(b), (c) and (d). Uranium doping was done prefer-
entially on Mg site as it is energetically favorable compared to