Effect of crystalline ordering on the luminescent properties of Eu3+-doped aluminum oxide nanophosphors

Article abstract of DOI:10.1016/j.jssc.2020.121427

Al₂O₃:Eu³⁺ nanophosphors were synthesized by the microwave assisted solvothermal technique. The structural and photoluminescence characteristics of alumina powders doped with 6.4 ?at.percent, 7.5 ?at.percent, 9.5 ?at.percent and 13 ?at.percent Eu, calcined at 900 ?°C, 1000 ?°C, 1100 ?°C and 1200 ?°C for 3 ?h, were investigated by X-ray diffraction, and reflectance, photoluminescent and X-ray fluorescence spectroscopies. The effects of the calcination temperature on the structure and, in turn of it on the photoluminescence properties are investigated. Luminescent decays curves were measured for the emission ascribed to the ⁵D₀-⁷F₂ transition in all the samples and a non-exponential behavior is observed. An increase in the average lifetime correlates well with the formation of the EuAlO₃ perovskite phase as the calcination temperature increases. Judd-Ofelt analyses were performed and some additional quantities were derived from them such as the radiative and nonradiative transition rates and the emission quantum ef?ciency. The decrease in the Ω₂ parameter value as the calcination temperature increases is compatible with a higher symmetry around the Eu³⁺ site as well as with a reduction in the Eu³⁺ covalence bonding within this ion into EuAlO₃ perovskite.

Full text of DOI:10.1016/j.jssc.2020.121427

Journal of Solid State Chemistry 288 (2020) 121427
Contents lists available at ScienceDirect
Journal of Solid State Chemistry
journal homepage: www.elsevier.com/locate/jssc
3þ
Effect of crystalline ordering on the luminescent properties of Eu -doped
aluminum oxide nanophosphors
I. Padilla-Rosales a,b, C. Falcony , R. Sosa , M. Aguilar-Frutis , G. Alarc oꢀ n-Flores , F. Gonz ꢀa lez
c
d
e
e
a,*
a
Departamento de Ingeniería de Procesos e Hidr aꢀ ulica, Universidad Aut ꢀo noma Metropolitana-Iztapalapa, C.P. 55-534, Ciudad de M ꢀe xico, M ꢀe xico
Programa de Nanociencias y Nanotecnología, Centro de Investigaci ꢀo n y de Estudios Avanzados del Instituto Polit ꢀe cnico Nacional, Av. IPN 2508, Col. San Pedro
b
Zacatenco, C.P. 07360, Ciudad de M ꢀe xico, M ꢀe xico
c
Departamento de Física, Centro de Investigaci ꢀo n y de Estudios Avanzados del Instituto Polit ꢀe cnico Nacional, Av. IPN 2508, Col. San Pedro Zacatenco, C.P. 07360, Ciudad
de M ꢀe xico, M ꢀe xico
Departamento de Física, Universidad Aut ꢀo noma Metropolitana-Iztapalapa, C.P. 55-534, Ciudad de M ꢀe xico, M ꢀe xico
d
e
Instituto Polit ꢀe cnico Nacional, Centro de Investigaci ꢀo n en Ciencia Aplicada y Tecnología Avanzada, Calzada Legaría 694, Col. Irrigaci ꢀo n, Miguel Hidalgo, C.P. 11500,
Ciudad de M ꢀe xico, M ꢀe xico
A R T I C L E I N F O
A B S T R A C T
3
þ
Keywords:
Al O :Eu nanophosphors were synthesized by the microwave assisted solvothermal technique. The structural
2 3
3þ
Al O :Eu nanophosphors
2 3
and photoluminescence characteristics of alumina powders doped with 6.4 at.%, 7.5 at.%, 9.5 at.% and 13 at.%
Photoluminescence
ꢀ
ꢀ
ꢀ
ꢀ
Eu, calcined at 900 C, 1000 C, 1100 C and 1200 C for 3 h, were investigated by X-ray diffraction, and
reflectance, photoluminescent and X-ray fluorescence spectroscopies. The effects of the calcination temperature
on the structure and, in turn of it on the photoluminescence properties are investigated. Luminescent decays
Luminescence decay
Crystalline ordering
Whole X-ray pattern fitting
5
7
curves were measured for the emission ascribed to the
0 2
D - F transition in all the samples and a non-exponential
behavior is observed. An increase in the average lifetime correlates well with the formation of the EuAlO
perovskite phase as the calcination temperature increases. Judd-Ofelt analyses were performed and
3
some additional quantities were derived from them such as the radiative and nonradiative transition rates and
the emission quantum efficiency. The decrease in the Ω parameter value as the calcination temperature increases
2
3þ
3þ
is compatible with a higher symmetry around the Eu site as well as with a reduction in the Eu covalence
bonding within this ion into EuAlO perovskite.
3
1. Introduction
spectral region suitable for an optical spectroscopic characterization.
5
7
3þ
0 1
Thus, if the D - F transition is dominant it is implied that Eu ion is in
Lanthanide ions are commonly used as activators in several host
a site with inversion symmetry; this transition shows three emission
peaks if Eu3 occupies a site with C
þ
, C2h, and D2h, point group sym-
lattices for their excellent luminescent properties. There is a huge num-
ber of applications in which they are used; for example, phosphors for
lighting, displays, lasers, scintillators, biosensors, etc. [1–4]. Among
lanthanides, the europium has been one of the most extensively used
1
metries. When the Eu3 ion occupies sites with C
þ
, D4h, D3d, C6h and D6h
4
point group symmetries two emission peaks are observed and only one
peak for higher point group symmetries [9–11].
3þ
because as trivalent ion (Eu ) exhibits a strong red emission ascribed to
On the other hand, alumina is an important material in many tech-
nological developments due to its excellent chemical and physical
properties. It is well known for its high hardness, high melting point and
good chemical stability [12,13]. Furthermore, alumina has been used as a
host matrix for several trivalent lanthanide ions because it has a wide
band gap and consequently an optical transparency ranging from the
ultraviolet to near infrared region [14].
5
7
3þ
its
D - F transition. Derived from this feature Eu -doped materials
0 2
have been recognized as efficient luminophores possessing a high po-
tential in some optical fields such as electroluminescence devices, optical
amplifiers, polychromatic displays and mercury-free lamps [5–8].
3
þ
The emission of Eu ion is routinely used as a probe of the local
3þ
structure in host lattices for the following reasons: (1) The Eu -ground
7
state ( F
0
) is non-degenerate, and (2) the electronic transitions among
Regarding the synthesis methods, the microwave assisted sol-
vothermal technique, unlike other techniques as sol gel, combustion, and
5
7
D
J
and FJ’ -whose energy values are well separated are found in the
*
Corresponding author.
E-mail address: fgg@xanum.uam.mx (F. Gonz ꢀa lez).
https://doi.org/10.1016/j.jssc.2020.121427
Received 2 February 2020; Received in revised form 27 March 2020; Accepted 3 May 2020
Available online 19 May 2020
0022-4596/© 2020 Elsevier Inc. All rights reserved.

I. Padilla-Rosales et al.
Journal of Solid State Chemistry 288 (2020) 121427
the conventional solvothermal, has the advantage of being a technique
with a rapid synthesis rate due to the energy transfer is over localized.
This characteristic results in a synthesis time reduction, and also an ho-
mogeneous heating is achieved over the entire solution. This technique
has other advantages such as its easy scalability, high reproducibility,
and good control of the particle size [15].
detector (Bruker, Lynxeye) [18]. The diffraction patterns were measured
ꢀ
ꢀ
ꢀ
between 15 and 108 , with a 2θ step of 0.020 and a counting time of
115.20 s per point, and subsequently refined by the Rietveld method,
implemented in the TOPAS software, version 4.2 [19] using the funda-
mental parameter approach [20]. In order to parametrize the whole
pattern fittings, a Lorentzian profile for modeling the average crystallite
size, lattice parameters, crystal symmetry, and ion positions into the
crystal cell, among others, were used.
The emission and excitation spectra, and the luminescent decay
curves were recorded with an Edinburgh Instruments FSP920 spectro-
fluorometer based on the method of single photon counting which
combines steady state and phosphorescence lifetime measurements. In
the latter, the electronics operate in multichannel scaling mode. The
spectrofluorometer is equipped with a 450 W xenon lamp as a CW light
source and an R928P PMT as a detector. In order to avoid the light
dispersion, a double monochromator (two coupled 0.3 m) was employed
to excite the samples. The emission was detected using a 0.3 m mono-
chromator. All the excitation and emission spectra were corrected for the
wavelength dependent responses of the Xe lamp and the detector,
respectively. For the measurements of the luminescence decay curves,
This work is focused in the investigation of the photoluminescent
properties dependence on the crystalline structure of nanosized
3
þ
Al
2 3
O
:Eu powders obtained by the microwave assisted solvothermal
3þ
technique and their further annealing. In these nanophosphors, Eu
shows important changes in its photoluminescence features when it is
incorporated into amorphous and different alumina crystal structures or
in EuAlO
content is investigated. High Eu content and high temperatures in the
thermal treatments give rise to the EuAlO crystallization, which pos-
sesses a perovskite-like structure. In the ABO general chemical formula
for like-perovskite compound, the A-larger cation, i.e. the Eu , possesses
a C point symmetry [16].
The dependence of the photoluminescence properties was studied as
a function of the annealing temperature and the Eu content, too.
. Also, the luminescence behavior as a function of the Eu3
þ
3
3
3
3
þ
s
ꢀ
ꢀ
Annealing temperatures range from 900 C to 1200 C, following in-
the excitation photons were provided by a 60 W
μF920H pulsed Xe flash
ꢀ
cremental steps of 100 C. Since, so far there is no detailed information in
lamp with a pulse width of ~1.5 s and a repetition rate of 100 Hz. The
μ
the literature about the luminescent dynamical measurements and their
dependence with the crystalline structure of the alumina, especially for
the chemical form EuAlO . For that reason, in this paper the structural
3
absorption spectra were obtained by using the diffuse reflectance tech-
nique. The measurements were performed in a Cary 5 spectrophotometer
equipped with a Praying Mantis accessory (Harrick Scientific Products,
Inc.) for diffuse reflection spectroscopy. The spectra were recorded in the
wavelength range from 200 nm to 800 nm. The morphology of the
samples was characterized by spherical aberration-corrected scanning
transmission electron microscopy (ARM200F, JEOL). For these studies,
the powder was ultrasonically dispersed in isopropanol and a drop of the
suspension was deposited on a carbon-coated copper grid.
and photoluminescence properties that includes the decay times are also
reported. As additional information the radiative properties also were
studied by the Judd-Ofelt theory (JO) which was used to predict different
parameters such as oscillator strengths, luminescence branching ratios,
excited state radiative lifetimes and quantum efficiencies. JOES, an
3þ
application software was used to analyses the Al
2 3
O Eu
emission
spectra in order to obtain the JO intensity parameters and derived
quantities [17].
3. Experimental results and discussion
In addition to photoluminescence spectroscopy, and photo-
3
þ
luminescence decay measurements the Al
2
O
3
:Eu powders were char-
3.1. Crystalline structure
acterized by X-ray diffraction, including the quantification of the phases
by the rietveld method and reflectance diffuse spectroscopy.
3
þ
The X-ray diffraction patterns of Al O :Eu powders doped at 6.4
2
3
ꢀ
ꢀ
at.%, 7.5 at.%, 9.5 at.% and 13 at.% Eu and calcined at 900 C, 1000 C,
1100 C and 1200 C are shown in Fig. 1. Figs. 1a and b depict the XRD
patterns of alumina powders doped with the lower contents of europium
ꢀ
ꢀ
2. Experimental details
2
.1. Synthesis
(6.4 at.% and 7.5 at.% Eu) which have similar characteristics when
ꢀ
calcined at 900 C. In both samples a mix of amorphous and γ-Al
O
2 3
:Eu3 powders were synthetized by the microwave assisted
þ
The Al
solvothermal technique. The solution was prepared with Al(NO
98%, Sigma-Aldrich, USA) and EuCl .6H O (99.9%, Sigma-Aldrich,
O
2 3
2 3
phases is found. The first crystalline phase to appear is γ-Al O (JCPDS
3
)
3
.9H
2
O
file No. 01-074-4629). By increasing the temperature of the thermal
ꢀ
(
3
2
treatment at 1000 C, a second crystalline phase (EuAlO ) begins to be
3
USA). Both precursors were dissolved in ethanol (ACS reagent, J.T.
Baker, USA) for 10 min until a homogeneous solution was reached. Four
different concentrations of europium, namely 0.012 mol/L, 0.015 mol/L,
formed, but a poor signal associated with this phase is observed. How-
ꢀ
ever, at 1100 C it is evident the formation of EuAlO , the narrower peaks
3
assigned to this phase become more pronounced and defined at this
0
.018 mol/L and 0.03 mol/L but maintaining a total (Eu þ Al) concen-
tration of 0.150 mol/L were used. Accordingly, the samples were pre-
pared using a total volume of 30 mL and the following Al(NO .9H O/
EuCl .6H O mass relations: 1.5532 g/0.1319 g, 1.5193g/0.1649 g,
.4855 g/0.1978 g and 1.3505 g/0.3298 g, respectively. Each one of the
temperature, being a clear evidence that the quantity of EuAlO increases
with the temperature. When the powders are calcined at 1200 C the wt%
3
ꢀ
3
)
3
2
fraction of amorphous component decreases substantially, conversely to
what happens to the EuAlO3 phase. For these samples a mix of amor-
phous, γ-Al O and EuAlO phases is observed.
3
2
1
2
3
3
mixed solutions were placed into a borosilicate vial, which was tightly
closed and placed into a microwave reactor. For the synthesis, the solu-
tion was kept at 160 C for 20 min. The as-synthesized product obtained
Fig. 1c and d shows the XRD patterns obtained from powders with 9.5
at% and 13 at.% Eu after being subjected to thermal treatments at 900 C,
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
1000 C, 1100 C, and 1200 C. In these figures, it may be observed the
changes in the crystalline structure produced when the temperature rises.
For these samples, with a higher Eu content and annealed at the lowest
after the solvothermal-microwave assisted procedure took place was a
ꢀ
gel, subsequently separated by centrifugation and dried at 200 C for 2 h.
ꢀ
ꢀ
Finally, the solid material was grinded and calcined for 3 h at 900 C,
temperature (900 C), a γ-Al O3 –delayed or absent formation is
2
ꢀ
ꢀ
ꢀ
1
000 C, 1100 C and 1200 C.
observed, which is more evident for the sample with 13 at.% Eu. In this
case, only an amorphous structure is observed. The same powders
ꢀ
ꢀ
ꢀ
2
.2. Characterization
calcined at 1000 C have peaks centered at about 45.5 and 66.6 , in 2θ,
corresponding to the tetragonal γ-Al phase, also at this temperature
peaks assigned to the EuAlO perovskite phase are present. The EuAlO
2 3
O
:Eu3 powders was established by X-ray
þ
The crystal structure of Al
diffraction performed in a Bruker D-8 Advance diffractometer fitted with
a CuK tube and a one-dimensional position-sensitive silicon strip
O
2 3
3
3
peaks become more clearly defined when both, the annealing tempera-
ture and the europium content increase. Powders with 9.5 at.% Eu
α
2

I. Padilla-Rosales et al.
Journal of Solid State Chemistry 288 (2020) 121427
ꢀ
ꢀ
ꢀ
Fig. 1. X-ray diffraction patterns from a) 6.4 at.%, b) 7.5 at.%, c) 9.5 at.% and d) 13 at.% Eu-doped alumina nanophosphors annealed at 900 C, 1000 C, 1100 C and
ꢀ
1200 C.
ꢀ
calcined at 1100 C have a very small signal associated with the amor-
phous phase. The amorphous component decreases as the annealing
EuAlO
3
unit cell was modeled with an orthorhombic symmetry described
3
þ
by the Pbnm space group (# 62), and a basis containing one Eu at (x, y,
¼), one Al at (½, 0, 0) and two O at (xO1, yO1, ¼) and at (xO2, yO2
ꢀ
3þ
2ꢁ
temperature increases until at 1200 C it finally seems to disappear.
,
Due to the presence of more than one amorphous or crystalline phase
in the powders, Rietveld analyses were performed in order to determine
the concentration of the phases, but also these analyses allow for quan-
tifying the average crystallite size and the cell parameters of all the
samples. Once each phase, crystalline or amorphous was identified, they
zO2). Initial values for the cell parameters and variable coordinates were
those ones reported in Ref. [22]. Quantifying amorphous components by
whole XRD pattern fitting procedures represents a challenge because the
factor structure which is necessary to weight the concentration of a
phase, and straightforwardly obtained when a crystalline unit cell is
known, is, for all practical purposes unavailable for amorphous phases.
Different approaches to overcome such difficulty have been proposed,
were described as follows. The γ-Al
2 3
O unit cell was modeled assuming a
tetragonal symmetry according to the space group I41/amd [21]; the
3

I. Padilla-Rosales et al.
Journal of Solid State Chemistry 288 (2020) 121427
but most of them require the use of internal or external standards [23]. In
order to avoid the use of those standards, we opted to model the
Table 2
Unit cell parameters and average crystallite size (nm) calculated for the crys-
talline phases in alumina powders with 6.4 at.%, 7.5 at.%, 9.5 at.% and 13 at.% of
contribution to the XRD pattern of the amorphous phase from an Al
2 3
O
ꢀ
ꢀ
ꢀ
ꢀ
Eu annealed at 900 C, 1000 C, 1100 C and 1200 C. The different space groups
s.p.) are indicated.
crystalline phase. The β-Al one was the best to describe that contri-
2 3
O
(
bution, and “amorphization” was given by reducing the crystallite size
through the refinement procedure until a reasonable agreement between
experimental and calculated X-ray diffraction intensities was reached. So
at.%
Eu
Annealing
Temperature
Lattice Parameters (Å)
a b c
Crystallite
Size (nm)
ꢀ
(
C)
2 3
that, the β-Al O unit cell was modeled with a monoclinic structure
2 3
γ-Al O (s.p. I41/amd)
symmetry described by the C2/m space group (# 12) and a basis con-
3
þ
2ꢁ
6.4
900
000
100
5.503(42)
5.568(11)
5.568(19)
5.539(4)
5.50(42)
5.568(11)
5.568(19)
5.539(4)
7.95(12)
7.977(31)
7.928(30)
7.936(5)
23
44
55
38
taining two Al at (xAl1, 0, zAl1) and at (xAl2, 0, zAl2) and tree O at (xO1
, zO1), (xO2, 0, zO2) and at (xO3, 0, zO3), (JCPDS file No. 04-002-2602)
24]. Parameters such as the unit cell dimensions, atomic positions and
,
1
1
0
[
1200
the size-crystallite-related were refined in order to achieve the best fit to
the observed pattern associated with the amorphous component. How-
ever, only the wt% fraction obtained from the Rietveld refinement for
this component was taken into account. The results about the phases
quantification and unit cell parameters are reported in Tables 1 and 2,
respectively.
EuAlO
3
(s.p. Pbnm)
–
4.939(68)
9
00
–
–
5
1
000
4.939
68)
7.43(11)
(
1
100
5.274(4)
5.290(2)
2 3
γ-Al O (s.p. I41/amd)
5.573(17)
5.556(7)
5.558(8)
5.581(48)
5.274(4)
5.290(2)
7.464(5)
7.462(3)
5
5
1200
00
7
.5
9
5.573(17)
5.556(7)
5.558(8)
5.581(48)
(s.p. Pbnm)
-
7.517(17)
7.442(7)
7.468(8)
8.150(44)
33.0
34.6
42.1
70.8
The results obtained from the Reitveld analyses on the weight per-
centages of the different phases, given in Table 2, allows a direct com-
1
1
000
100
8.056(16)
7.98(15)
7.90(14)
parison with the qualitative observations. On one hand, EuAlO
markedly increases as the annealing temperature and europium content
increase, on the other hand, the γ-Al concentration phase has a
3
phase
1200
EuAlO
3
9
00
-
-
-
2 3
O
1
1
1
000
100
200
5.250(9)
5.293(3)
5.281(2)
γ-Al
5.298(15)
5.263(3)
5.281(1)
10.5
42.4
66.6
ꢀ
reduction for powders calcined at 1100 C. When the annealing tem-
ꢀ
perature rises at 1200 C the amorphous and γ-Al
O
2 3
phases are reduced
for powders with Eu contents of 6.4 at.% and 7.5 at.%, and particularly
the amorphous phase suffers a drastic reduction in their weight per-
centages finally disappearing in the cases of samples with 9.5 at.% and
9.5
O
2 3
(s.p. I41/amd)
9
00
–
–
–
–
1
1
1
000
100
200
5.570(10)
5.569(9)
5.60(23)
5.570(10)
5.569(9)
5.60(23)
7.976(21)
7.934(23)
7.93(50)
79.1
92.2
24.0
13 at. % Eu. Fig. 2 shows representative plots from among all resulting
refinement analyses corresponding to Al powders doped with 9.5
2
O
3
EuAlO3 (s.p. Pbnm)
ꢀ
ꢀ
ꢀ
ꢀ
at.% Eu, calcined at 900 C, 1000 C, 1100 C and 1200 C, a good
agreement between experimental and calculated data is observed.
900
–
–
–
–
1
1
1
000
100
200
5.286(2)
5.570(10)
5.2920(4)
7.468(2)
7.465(8)
7.4589(5)
5.268(2)
5.273(1)
5.2762(3)
38.8
41.8
53.7
1
3
γ-Al
–
O
2 3
(s.p. I41/amd)
–
3
.2. STEM
9
00
–
–
1
000
5.66(35)
5.549(2)
5.549(2)
5.66(35)
5.549(2)
5.549(2)
7.97(96)
8.024(6)
8.024(6)
28.0
47.8
30.7
STEM observations of the annealed samples revealed that the pow-
1100
1
200
ders consist of agglomerates of nanoparticles, with individual sizes less
than 50 nm, several micrographs were obtained for a small set of samples
and their detailed characteristics were reported in a previous work [25].
However, annular dark-field imaging also was obtained by scanning
transmission electron microscopy (STEM) with high spatial resolution.
For illustrative purposes only two micrographs corresponding to
EuAlO
3
(s.p. Pbnm)
–
7.468(4)
7.461(7)
9
00
–
–
–
1
000
5.282(3)
5.293(4)
5.2942(4)
5.261(5)
5.272(4)
5.2769(3)
33.1
67.3
46.7
1100
200
1
7.4561(5)
3þ
ꢀ
Al
2 3
O :Eu powders doped with 7.5 at.% Eu calcined at 900 C and 1200
and c) but by Z-contrast images (Fig. 3b and d), the distribution of the
europium ion can be distinguished in both images. In powders calcined at
00 C, the Eu element is localized in an homogeneous way although in
an amorphous structure, unlike the sample calcined at 1200 C where
saturated areas of europium are observed in a crystalline structure, this
characteristic is responsible of the concentration quenching in the
emission intensity and probably also corresponds to areas where Eu is
in the EuAlO phase.
ꢀ
C are shown (Fig. 3). The morphology is similar between them (Fig. 3a
ꢀ
9
Table 1
ꢀ
Quantification of crystalline phases in powders with 6.4 at.%, 7.5 at.%, 9.5 at.%
ꢀ
ꢀ
ꢀ
ꢀ
and 13 at.% of Eu annealed at 900 C, 1000 C, 1100 C and 1200 C.
3þ
at.%
Eu
Phase quantification (wt. %)
Annealing
Temperature ( C)
Phases
found
Amorphous
Al
γ-Al
2
O
3
●EuAlO
3
3
ꢀ
2 3
O
6
7
9
.4
.5
.5
900
A þ γ
Aþ γ þ ●
Aþ γ þ ●
Aþ γ þ ●
A þ γ
90.9
80.4
76.9
64.9
88.6
71.6
56.4
56.1
100
86.7
80.9
0
9.1
18.9
21.7
33.6
11.4
26.8
41.8
35.3
0
11.0
14.8
88.5
0
0
0.7
1.4
1.5
0
1.5
1.8
8.6
0
1.3
3.7
11.5
0
1.1
9.6
27.0
1
1
1
000
100
200
3.3. UV–vis DR spectra
3þ
ꢀ
The UV–vis absorbance spectra of the Eu doped Al
2
O
3
powders
calcined at 900 C, 1000 C, 1100 C and 1200 C were obtained by the
diffuse reflectance technique and they are shown in Fig. 4. BaSO was
900
ꢀ
ꢀ
ꢀ
1
1
1
000
100
200
Aþ γ þ ●
Aþ γ þ ●
Aþ γ þ ●
A
4
used as a reference standard material for measuring the absorbance
spectra. All the samples exhibit a broadband centered at 260 nm corre-
900
1
1
1
000
100
200
Aþ γ þ ●
Aþ γ þ ●
γ þ ●
2-
sponding to the charge transfer from the, either the ligand-metal (O -
3
þ
6
Eu ) of the oxygen 2p orbital to the Europium 4f orbital or due to an 4f
7
1ꢁ
9
00
A
100
88.9
50.15
0
to 4f O transition. From these spectra it is also possible to observe a set
3
þ
1
3
1000
Aþ γ þ ●
Aþ γ þ ●
γ þ ●
10.0
40.3
73.0
of sharp peaks in the 350 nm–600 nm range ascribed to the Eu 4f-4f
1
1
100
200
transitions. The absorption peaks may be assigned to the electric dipolar
7
5
7
5
7
5
0 4 0 2 1 6
transitions: F → D at 362 nm, F → G at 377 nm, F → L at 394
4

I. Padilla-Rosales et al.
Journal of Solid State Chemistry 288 (2020) 121427
ꢀ
ꢀ
ꢀ
ꢀ
Fig. 2. Rietveld refinement plots of Al
2
O
3
powders doped with 9.5 at.% Eu, calcined at a) 900 C, b) 1000 C, c) 1100 C and d) 1200 C.
nm, 7F
→ D
5
at 415 nm, 7F
→ D
5
at 465 nm, F
7
→ D
5
at 526 nm this is
phenomenon has already been reported in a previous work but in pow-
ders calcined at 1000 C [25].
1
3
0
2
0
1
7
5
ꢀ
a magnetic dipolar transitions and F
1
→ D
1
at 535 nm.
Unlike Fig. 5(a), in Fig. 5(c) the emission spectra of alumina nano-
ꢀ
phosphors doped with 6.4 at. %, 7.5 at. % calcined at 1200 C and 9.5
ꢀ
3
.4. Photoluminescence properties
at.% Eu and 13 at.% Eu calcined at 1100 C are included, from this figure
5
7
0 2
it is possible to recognize a splitting into four peaks of the D - F tran-
3þ
The photoluminescence response of the alumina nanophosphors
doped with 6.4 at. %, 7.5 at. %, 9.5 at. % and 13 at. % of Eu calcined at
9
emission and excitation spectra and luminescence decay curves. Fig. 5
shows the emission spectra for all concentrations and annealing tem-
peratures upon excitation wavelength at 393 nm. Spectra are grouped
depending on their features; hence, those ones having similarities are
plotted together. The first group (Fig. 5(a)) includes emission spectra of
sition, this behavior is the result of the change in the Eu ion local
environment. In Fig. 5(d) it is possible to observe the emission spectra of
two samples that have the maximum Eu content (9.5 at.% and 13 at.%)
ꢀ
ꢀ
ꢀ
ꢀ
00 C, 1000 C, 1100 C and 1200 C were investigated through
ꢀ
calcined at 1200 C. In these samples, EuAlO is the dominant phase and
3
this fact is reflected in the emission spectra shape through the well-
defined peaks at 613 nm and 617 nm. Since the strongest emission of
the powders is mainly due to the 5D0
occupying non-centrosymmetric sites.
→
7
F2 transition, Eu should be
3þ
ꢀ
ꢀ
samples doped at all at.% Eu and calcined at 900 C and 1000 C, i.e., at
Fig. 6 shows the excitation spectra for alumina powders doped at 6.4
ꢀ
ꢀ
the lower annealing temperatures. All the emission spectra of this group
at.%, 7.5 at.%, 9.5 at.% and 13 at.% Eu calcined at 900 C, 1000 C, 1100
exhibit the splitting of the maximum band, ascribed to the 5
D
0
- F
7
ꢀ
ꢀ
2
C and 1200 C. All the excitation spectra were measured monitoring at
616 nm. They exhibit several sharp peaks associated with the following
transition, in two peaks at 613 nm and 616 nm, and in most cases the 613
nm emission peak is slightly higher. When these powders are calcined at
9
3
þ
transitions between the different electronic energy levels of the Eu
:
ꢀ
7
F0→ 5H6 (319 nm), F0 → D (362 nm), F → G (381 nm), F1 → L
7
5
7
5
7
5
00 C, the highest emission intensity is observed for the sample with the
4 0 3
6
7
5
7
5
7
5
larger amount of europium (13 at.% Eu). However, the emission intensity
in this sample is reduced when microstructural changes in the material
are induced by increasing the annealing temperature as we have
explained previously.
The reduction in the emission intensity may be associated with two
factors; the first of them is related to the well-known concentration
quenching effect, given by the actual Eu concentration (mass Eu/total
mass compound) increase into the samples, while the second one is
related to the changes in the crystalline structure induced by the tem-
perature increasing of the heat treatments. Actually, the latter
1 3 0 0 0
(395 nm), F → D (413 nm), F → D1 (464 nm) and F → D (526
nm) dipolar magnetic and F1 → D at 535 nm. The excitation spectra
7
5
1
were also grouped in four different sets. In Fig. 6(a) are depicted the
spectra that have a broad peak centered at around 270 nm, which is
ascribed to an Eu3 –O - charge transfer (CT) state. This excitation band
þ
2
3
þ
diminishes for higher temperatures and Eu concentration. In Fig. 6(b)
it is observed a shoulder in the charge transfer band around 300 nm,
which is more pronounced for the excitation spectra in Fig. 6(c). Since
surprisingly there is no report in which unambiguously the luminescent
5

I. Padilla-Rosales et al.
Journal of Solid State Chemistry 288 (2020) 121427
:Eu3 powders doped with 7.5 at.% Eu calcined at a) 900 C and b)
þ
ꢀ
Fig. 3. Annular dark-field imaging by scanning transmission electron microscopy (STEM) for Al
2
O
3
ꢀ
1200 C.
properties of Eu3 are presented in a single phase EuAlO
Fig. 6 d), in our opinion this band should be associated with an O
CT state in EuAlO
where the EuAlO
þ
compound (see
luminescence dynamics through
a bi-exponential decay model is
3
ꢁ
3þ
2
-Eu -
compatible with the presence of at least two different chemical envi-
. Despite this band has been observed in other reports
ronments around the Eu3 . Intriguingly the XRD patterns of these sam-
þ
3
2-
3
phase is formed, it was not associated with an O -
Eu charge transfer process. Yuya et al. have showed excitation spectra
for EuAlO phosphor where a broad band centered at around 295 nm is
present [26]. Analogous results have been reported by Yue Hu et al. in
ples exhibit a mix of amorphous, gamma and EuAlO phases. This group
3
3þ
have similar characteristics to those depicted in Fig. 7(c), however the
non-exponential behavior is more pronounced for these samples.
The experimental decay data are analyzed by fitting the experimental
data using the expression [28]:
3
3þ
2
Al O3:Eu system where a strong broadband in the wavelength range
from 255 nm to 320 nm is also present; however, they assume that the
change in this band from 255 nm to lower energies is related to the
ꢀ
ꢁ
X
t
IðtÞ ¼
a
i
exp ꢁ
(1)
3þ
τi
2
Eu -O - bond length reduction [27].
i
with
3
.4.1. Phtotoluminescence decay
The luminescence decay curves were obtained for alumina powders
X
a
i
¼ 1
(2)
doped at all at.% Eu concentrations, and annealing temperatures. Fig. 7
illustrates the behavior of the normalized luminescence decay curves for
all samples. The experimental decay curves show a strong dependence on
the Eu doping content and calcination temperature, but all of them can
be fitted by two single exponential decays. In this figure are shown the
photoluminescence decays plots ln I vs t. It is possible to observe essen-
tially four groups of decay times. Fig. 7(a) includes the powders with 6.4
at.%, 7.5 at.% and 13 at.% Eu calcined at 900 C and 1000 C, as well as
powders with 9.5 at.% Eu calcined at 900 C. In this first group, it is seen
the quenching concentration effect. This behavior has been associated
with an energy migration increase among Eu ions, as the europium
i
When the decay curve is not a single exponential, an average lifetime
can be calculated from the following equation:
3þ
P
2
i
ia
i
τ
τ
av ¼ P
(3)
ia
i i
τ
ꢀ
ꢀ
The average lifetime (
τ
avÞ values are included in Table 3. From these
ꢀ
results, it can be observed a reduction in the average decay times for
ꢀ
ꢀ
3þ
powders calcined at 900 C and 1000 C (Fig. 8(a)) as the Eu con-
centration increases. This behavior, attributed to an increase in the non-
radiative decay rate, and further supported by the decrease in the lumi-
nescence emission intensity, also when Eu concentration increases, is
well-correlated with a concentration quenching effect (Fig. 9). On the
3þ
3þ
3þ
content increases since the Eu -Eu inter-ionic distance is reduced.
This phenomenon has been reported in a previous work [25]. In Fig. 7(b)
the decay times have almost the same behavior, the description of the
3þ
6

I. Padilla-Rosales et al.
Journal of Solid State Chemistry 288 (2020) 121427
:Eu3 powders doped with a) 6.4 at.%, b) 7.5 at.%, c) 9.5 at.% and d) 13 at.% of Eu calcined at 900 C, 1000 C, 1100 C and
þ
ꢀ
ꢀ
ꢀ
Fig. 4. Absorption spectra of Al
2
O
3
ꢀ
1200 C.
Fig. 5. Emission spectra for Eu3þ-doped alumina nanophosphors with 6.4 at.%, 7.5 at.%, 9.5 at.% and 13 at.% of Eu calcined at 900 C, 1000 C, 1100 C and 1200 C.
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
other hand, powders calcined at 1100 C and 1200 C show a different
behavior, in these cases the decay time become slower (Fig. 8(b)) and
there is a tendency to a linear behavior decay related to the predomi-
3.4.2. Judd-Ofelt parameters
The Judd–Ofelt (JO) analysis is a useful tool for assessing the lumi-
nescence performance in lanthanides -doped materials [29,30]. The JO
intensity parameters are the conspicuous information derived from the
3
nance of the EuAlO .
7

I. Padilla-Rosales et al.
Journal of Solid State Chemistry 288 (2020) 121427
Fig. 6. Excitation spectra for Eu3þ-doped alumina nanophosphors with 6.4 at.%, 7.5 at.%, 9.5 at.% and 13 at.% of Eu calcined at 900 C, 1000 C, 1100 C and
ꢀ
ꢀ
ꢀ
ꢀ
1200 C.
X
2
Ω jΨJjjUðλÞjjΨ’J’j2
analysis that give relevant information about the local structure and
bonding around the lanthanide ions [31].
DED ¼ e
(5)
λ
λ¼2; 4;6
The emission spectra of a lanthanide ion produced by the radiative
relaxation from an excited state ΨJ to several lower lying states Ψ’J’ give
place to the different bands in such spectra. From the JO theory the
spontaneous emission probability A of the transition ΨJ→Ψ’J’ can be
expressed by the equation:
where Ω
λ
are the JO parameters, e is the elementary charge and
ðλÞ
2
jΨJjjU jjΨ’J’j are the squared reduced matrix elements. In the case of
3
þ
5
Eu most of the matrix elements are zero, except for the transitions D
0
7
5
7
5
7
(2)
→ F
U
2
, D
0
→ F
4
and D
0
→ F
6
for which those element are U ¼ 0.0032,
(4)
(6)
¼ 0.023 and U ¼ 0.0002, respectively.
ꢂ
ꢃ
3
2
6
4
π4
hð2J þ 1Þ
~
υ
nðn þ 2Þ
JOES software allows the implementation of JO analysis in a very
3
AðΨJ; Ψ’J’Þ ¼
D
ED þ n DMD
(4)
simple way from emission spectra of Eu3 -doped compounds, thus it is
possible to obtain important data related to the luminescent properties of
our compounds, such as the radiative lifetime, branching ratios, stimu-
lated emission cross section, luminescence quantum efficiency and op-
tical gain [17], some of these parameters obtained from the Judd-Ofelt
analyses are listed in Table 3. Since, the 5D - F transition was not
þ
3
9
Where h is the Planck’s constant, ~
υ is the average transition energy in
ꢁ1
cm , 2J þ 1 is the degeneracy of the initial state, n is the medium’s
refractive index, and DED and DMD are the electric and magnetic dipole
3
þ
7
strengths, respectively. As pointed above, the luminescence of Eu is
0
6
5
mainly the result of transitions from the D
0
excited electronic state to the
observed in the present case, its J–O associated parameters could not be
evaluated.
7
5
7
lower F
j
levels. The transitions from D
0
to F0,3,5 are strictly forbidden
by the Laporte’s rule, which implies that DED and DMD should be zero.
However, this selection rule is relaxed for lanthanides embedded in a
host, due to the transitions can be partly allowed by some phenomena
such as vibronic coupling, the non-centrosimetric distortion of the
lanthanide site ocupation, of via mixing of higher configurations into the
The observed lifetime obtained experimentally, takes into account
both radiative (A ) and non-radiative (A ) rates and it is mathematically
expressed as:
r
nr
1
τ
¼ A
(6)
nr þ A
r
4
f wavefunctions by the crystal-field effect [32].
5
7
The transition D
0 1
→ F is the only magnetic dipole transition that has
Theoretical radiative lifetime is obtained directly from the emission
spectrum and it is given by the following equation:
no electric dipole contribution, this transition is independent of the local
ꢁ
42
2
2
5
ion environment and its value is DMD ¼ 9:6 ꢂ 10 esu cm [33]. The D
0
7
ꢁ3
→
F
2,4,6 transitions are forced electric dipole allowed and the strength of
n
J
1
th
rad
1
τ
¼
(7)
all induced dipole transitions is determined by the following equation:
14:65 Jtot
8

I. Padilla-Rosales et al.
Journal of Solid State Chemistry 288 (2020) 121427
þ
Fig. 7. Photoluminescence decay curves for Al
2
O
3
:Eu3 powders calcined at different temperatures divided in four groups according to their behavior. (a) 6.4 at.%,
ꢀ
ꢀ
ꢀ
7
.5 at.%, 9.5 at.%, 13 at.% of Eu calcined at 900 C (b) 6.4 at.%, 7.5 at.% of Eu calcined at 1100 C and 9.5 at.% Eu calcined at 1000 C, (c) 6.4 at.% and 7.5 at.% of Eu
calcined at 1200 C, 9.5 at.% and 13 at.% of Eu calcined at 1100 C. (d) 9.5 at.% and 13 at.% of Eu calcined at 1200 C.
ꢀ
ꢀ
ꢀ
Table 3
3þ
ꢀ
ꢀ
ꢀ
ꢀ
JOES output data for alumina nanophosphors doped with 6.4 at.%, 7.5 at.%, 9.5 at.% and 13 at.% Eu calcined at 900 C, 1000 C, 1100 C and 1200 C.
6
.4 at.% Eu3þ 7.5 at.% Eu3þ
ꢀ
Temperature ( C)
900
7.208
4.773
0.132
0.592
1.912
2.111
473.783
284.464
69
1000
7.200
4.654
0.133
0.592
1.916
2.123
470.960
485.465
54
1100
6.984
5.053
0.134
0.578
1.930
2.128
469.865
350.627
63
1200
6.598
4.840
0.137
0.138
1.978
2.226
449.204
679.668
45
900
7.364
4.885
0.130
0.594
1.878
2.072
482.659
268.966
71
1000
7.050
4.815
0.134
0.584
1.930
2.137
468.004
783.090
41
1100
7.0129
4.960
0.134
0.580
1.929
2.133
468.840
1040.549
34
1200
6.483
5.026
0.137
0.549
1.980
2.231
448.147
562.515
50
ꢁ
ꢁ
20
20
2
Ω
Ω
2
4
[10
cm ]
2
[10
cm ]
5
7
β
exp( D
0
→ F
1
)
)
5
7
β
exp ( D
0
→ F
2
τ
τ
th .[ms]
av [ms]
A
R
A
NR
η
x 100 (%)
Asymmetric ratio
4.985
4.985
4.933
4.869
4.968
4.944
4.931
4.899
9.5 at.% Eu3þ
13 at.% Eu3þ
ꢀ
Temperature [ C]
900
1000
7.097
4.952
0.132
0.582
1.910
2.116
472.463
978.179
36
1100
6.751
4.760
0.137
0.572
1.980
2.204
453.574
815.541
40
1200
6.764
3.824
0.139
0.582
2.006
2.247
445.016
381.997
60
900
7.148
4.640
0.134
0.596
1.941
2.136
468.145
411.920
59
1000
7.137
5.013
0.131
0.581
1.898
2.105
475.107
1204.380
31
1100
6.936
5.215
0.132
0.567
1.909
2.123
470.944
280.664
70
1200
6.670
4.394
0.140
0.577
2.016
2.258
442.915
411.465
58
ꢁ
ꢁ
20
20
2
Ω
Ω
2
[10
cm ]
7.277
4.863
0.131
0.592
1.896
2.090
478.449
764.497
42
2
4
[10
cm ]
5
7
β
exp( D
0
→ F
1
)
5
7
β
0 2
exp ( D → F )
τ
τ
th . [ms]
av [ms]
A
R
A
NR
η
x 100 (%)
Asymmetric ratio
4.917
4.917
4.9
4.885
4.897
4.902
4.899
4.886
R
bonding and the chemical environment of the ion. The Ω
2
, for example, is
Where n is the refractive index, and Jk ¼ IK ð~
ν
Þd~
ν
is the integrated in-
3þ
5
7
a structure-sensitive parameter and depends on the covalence of the Eu
site [33]; an increase in the value of Ω indicates a high covalence
tensity of the D
0
→ F
κ
, transition, where κ ¼ 1, 2, 4, 6.
2
The Judd-Ofelt parameters provide information on the chemical
9

I. Padilla-Rosales et al.
Journal of Solid State Chemistry 288 (2020) 121427
ꢀ
ꢀ
Fig. 8. Behavior of the average time decay constants (listed in Table 3) as a function of the europium content in the nanophosphors calcined at (a) 900 C and 1000 C
ꢀ
(b) 1100 and 1200 C.
increases with the calcination temperature, nevertheless the branching
values are lower than 0.5, indicating a slow emission radiation. The
5
7
0 2
highest branching ratios are reported for the D → F transition, all of
them are higher than 0.5, meaning a strong emission radiation. In gen-
eral, high values for branching ratios implies the potential application in
laser creation [36]. β 5
7
F
2
values are reduced when the annealing
0
D →
temperatures increase. We speculate this behavior is related with the
EuAlO
3
3
perovskite phase formation and its fast emission radiation due to
þ
the Eu concentration quenching.
Luminescence quantum efficiency (or intrinsic quantum yield), η, is,
by definition, the ratio of the number of photons emitted to the number of
photons absorbed. For lanthanide ions it is also equal to the ratio of the
observed lifetime to the radiative lifetime [37].
τ
obs
rad
η
¼ τ
(10)
The
η
quantum yield (QY) values calculated using eq. (10) from the
τ
τ
rad obtained through the JOES software and the lifetimes measured
obs) are presented in Table 3. These values are an approximation that
(
Fig. 9. Integrated emission intensity from all the nanophosphors calcined at
allows to assess which one among the nanophosphor synthesized is the
ꢀ ꢀ ꢀ ꢀ
00 C, 1000 C, 1100 C and 1200 C.
9
most efficient. It corresponds to the sample doped at 7.5 at. % Eu calcined
ꢀ
at 900 C (QY ¼ 71%). At this temperature, the quantum yield for
3þ
2ꢁ
between the Eu ion and the surrounding O as well as low symmetry
powders doped at 6.4 at. % Eu (QY ¼ 69%) is closer to 71%. This result
3þ
at the Eu site [34]. On one hand, a variation in Ω
change in the local environment; on the other hand, Ω
to the viscosity and rigidity of the host matrix in which the ions are
2
is an indicator of the
3þ
confirms that the maximum radiative efficiency of the Eu ion corre-
4
and Ω are related
6
3þ
sponds to the sample possessing the lower symmetry around the Eu
ꢀ
ion, i.e., the samples calcined at 900 C. On the other hand, the QY for
powders calcined at higher temperatures (1100 and 1200 C) undergoes
3þ
located. The structural changes in the vicinity of the Eu ion in a short
and long range are related to Ω and Ω , respectively. In Fig. 10 ((a) and
b)) is illustrated the variation of the Ω2,4 parameters as a function of the
annealing temperature. The changes on Ω (Fig. 10 (a)) from high to low
ꢀ
2
4
a reduction, which is probably related with the formation of the crys-
talline phases.
(
2
Equations for radiative transition probability in the simplest form are:
values as calcination temperature increases show that the system shifts
from low to high symmetry, and that the chemical covalence is reduced.
Branching ratios, also shown in Fig. 10 ((c) and (d)), may be used to
predict the relative intensities of all emission lines originating from a
given excited state [15]. The theoretical branching ratio is obtained from
the JO theory according to the equation:
ꢄ
ꢅ
9
3
λ
2
2
λ
A
λ
¼ 8:034X10 ~
υ
n
λ
n þ 2
Ω
λ
U
(11)
λ
The asymmetric ratios also were obtained through the JOES software
and they are shown in Fig. 11. The trend is like that observed in Fig. 10
(a), that is because Ω and the hypersensitive ratio have similar physical
2
significance regarding the symmetry and the covalent or ionic nature of
th
k
the bonding between Eu3 and the surrounding ligands [34].
þ
A
k
β ¼ P
(8)
A
k
4
. Conclusions
These parameters also can be calculated from the emission spectra
because they are ratios of the integrated intensities,
Eu3 -doped aluminum oxide nanophosphors were synthetized by the
þ
J
k
microwave assisted solvothermal technique. The effect of the calcination
temperature and the concentration of Eu on the photoluminescent
properties were analyzed. Both the increment in the temperature and the
exp
βk ¼ J
(9)
tot
Experimental branching ratios were obtained directly from the
emission spectra since these parameters are the fraction of the total
photon flux from an upper to a lower level [35] In this case, theoretical
and experimental β for
Fig. 10 (c) and (d)). For the
concentration of Eu give place to the formation of the crystalline γ-Al
2 3
O
and EuAlO phases. The effect of the crystalline ordering on the lumi-
3
nescent properties were investigated by means of XRD diffraction and
Rietveld refinement analyses. They show that at high temperatures
5
7
5
7
0
D →
F
D
1
and
→
D
0
→
F
2
transitions are plotted
transition the branching ratio
5
7F
(
0
1
3
EuAlO becomes the dominant phase. However, photoluminescent
1
0

I. Padilla-Rosales et al.
Journal of Solid State Chemistry 288 (2020) 121427
3
þ
ꢀ
ꢀ
ꢀ
Fig. 10. Judd-Ofelt parameters for alumina nanophosphors doped with 6.4 at.%, 7.5 at.%, 9.5 at.% and 13 at.% Eu calcined at 900 C, 1000 C, 1100 C and 1200
ꢀ
2 4 0 1 0 2
C, a) Ω and b) Ω D → F D → F transitions.
, c) β for 5 and d) 5
7
7
associated plausibly with the charge transfer band of EuAlO
3
contrary to
the band at 250 nm, which is associated with the O -Eu charge transfer
in Al diminishes considerably as the annealing temperature increases.
2-
3þ
2 3
O
Interestingly, the nanophosphor exhibiting the highest emission intensity
corresponds to the sample with 13 at. % Eu3 and calcined at 900 C,
þ
ꢀ
which is completely amorphous. The decay time shows a strong depen-
dence on the Eu3 doping content and calcination temperature and it was
fitted by two single exponential decays in all cases, which is associated
with the presence of at least two different chemical environments around
þ
the Eu3 ion, a result that is supported by the XRD analysis. The pa-
þ
2
rameters obtained by the J-O theory, such as Ω and the asymmetric ratio
show that the system shifts from low to high symmetry, and that the
chemical covalence is reduced with the increase in calcination temper-
ature, a result that is compatible with the increase in symmetry around
the site of the Eu3 with the formation of the EuAlO
þ
phase. However, the
question about what are the Eu3 luminescent properties only in the
EuAlO phase remains open.
3
þ
3
Declaration of competing interest
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.
Fig. 11. Asymmetric ratio for alumina nanophosphors doped with 6.4 at.%, 7.5
3þ
ꢀ
ꢀ
ꢀ
at.%, 9.5 at.% and 13 at.% Eu calcined at 900 C, 1000 C, 1100 C and
ꢀ
1
200 C.
CRediT authorship contribution statement
results show that the emission intensity of the Eu3þ in EuAlO
phase is
3
reduced even when the order has been increased in the structure.
Nevertheless, a new excitation band appears at around 303 nm,
I. Padilla-Rosales: Formal analysis, Investigation, Writing - original
draft, Visualization. C. Falcony: Methodology, Resources, Writing -
1
1

I. Padilla-Rosales et al.
Journal of Solid State Chemistry 288 (2020) 121427
review & editing. R. Sosa: Validation. M. Aguilar-Frutis: Resources. G.
Alarc oꢀ n-Flores: Resources, Writing - review & editing. F. Gonz aꢀ lez:
Conceptualization, Methodology, Resources, Writing - original draft,
Visualization.
[16] C.E. Rodríguez-García, N. Perea-L oꢀ pez, G.A. Hirata, Near UV-Blue Excitable Green-
Emitting Nanocrystalline Oxide, Advances in Materials Science and Engineering
2
011 (2011), 790517.
[
[
[
17] A. Ciri ꢀc , S. Stojadinovi ꢀc , M. Sekuli ꢀc , M.D. Drami ꢀc anin, JOES: An application
ꢀ
3þ
software for Judd-Ofelt analysis from Eu emission spectra, J. Lumin. 205 (2019)
51–356.
3
18] W. Dabrowski, P. Grybos, P. Hottowy, K. Swientek, P. Wiacek, Integrated readout of
silicon strip detectors for position sensitive measurement of X-rays, Nucl. Instrum.
Methods Phys. Res., Sect. A 512 (2003) 213–219.
Acknowledgements
19] A. Coelho, TOPAS and TOPAS-Academic: an optimization program integrating
computer algebra and crystallographic objects written in Cþþ, J. Appl. Crystallogr.
Isela Padilla-Rosales is grateful to PRODEP-SEP for the granted
postdoctoral scholarships. The authors thank CONACyT-M ꢀe xico for
providing financial support under CB-154962 2010–01 and INFR-2011-
5
1 (2018) 210–218.
[20] R. Cheary, A. Coelho, A fundamental parameters approach to X-ray line profile
fitting, J. Appl. Crystallogr. 25 (1992) 109–121.
1-163250 projects. Also, special thanks to Daniel Bahena (Cinvestav-IPN)
[
21] C.M. G oꢀ mez, G. Del Angel, E. Ramos-Ramírez, I. Rangel-V ꢀa zquez, F. Gonz ꢀa lez,
A. Arrieta, A. V ꢀa zquez-Zavala, A. Bonilla-S ꢀa nchez, M. S ꢀa nchez-Cantú, Alumina
coating with TiO2 and its effect on catalytic photodegradation of phenol and p-
cresol, J. Chem. Technol. Biotechnol. 91 (2016) 2211–2220.
for STEM image acquisition, CICATA-LEGARIA for the technical assis-
tance and the XRD Laboratory (T-128) UAM-I for the XRD measurements,
respectively.
[22] P.J. Saines, M.M. Elcombe, B.J. Kennedy, Lanthanide distribution in some doped
alkaline earth aluminates and gallates, J. Solid State Chem. 179 (2006) 613–622.
References
[23] N.V.Y. Scarlett, I.C. Madsen, Quantification of phases with partial or no Known
crystal structures, Powder Diffr. 21 (4) (2006), 278–274.
[
[
1] G. Blasse, B.C. Gramaier, Luminescent Materials, Springer, Berlin, 1994.
2] Cees Ronda, Luminescence, from Theory to Applications, Wiley-VCH Verlag GmbH
[24] L.M. Foster, G.V. Chandrashekhar, J.E. Scardefield, R.B. Bradford, Phase diagram of
the system Na
J. Am. Ceram. Soc. 63 (1980) 509–512.
[25] I. Padilla-Rosales, J.G. Caba n~ as-Moreno, G.L. Jimenez, M.A. Frutis, G. Alarcon-
2 2 3 2 3 2 2 3 2 3
O⋅Ga O -Ga O and its relation to the system Na O⋅Al O -Al O ,
&
Co, KGaA, Weinheim, 2008.
3] G. Blasse, Luinescent materials: is there still news? J. Alloys Compd. 225 (1995)
29–533.
4] S. Basavarajappa Patil, et al., Enhanced photocatalytic activity and biosensing of
gadolinium substituted BiFeO nanoparticles, Chemistry 3 (2018) 9025–9033.
5] G. Hirata, N. Perea, M. Tejeda, J.A. Gonzalez-Ortega, J. McKittrick, Luminescence
study in Eu-doped aluminum oxide phosphors, Opt. Mater. 27 (2005) 1311–1315.
6] S. Kumar, R. Prakash, V. Kumar, G.M. Bhalerao, R.J. Choudhary, D.M. Phase,
[
[
[
[
5
Flores, C. Falcony, Near UV excitable Eu-doped alumina nanophosphors synthesized
by the microwave assisted solvothermal technique, Mater. Res. Express 4 (2017)
125007.
3
[26] Y. Onishi, T. Nakamura, S. Adachi, Solubility limit and luminescence properties of
3
þ
Eu ions in Al
2 3
O powder, J. Lumin. 176 (2016) 266–271.
3
þ
[27] Y. Hu, Q. Liu, G. Liu, Z. Zhou, Al
2 3
O :Eu spherical red-emitting phosphor:
3
þ
Surface and spectral studies of Eu doped
α
-Al
2
O
3
synthesized via solution
Synthesis by hydrothermal technique and luminescence characteristics, J. Alloys
Compd. 724 (2017) 969–974.
combustion synthesis, Adv. Powder Technol. 26 (2015) 1263–1268.
[
[
[
7] M. Nogami, T. Hayakawa, T. Ishikawa, Room- temperature photochemical hole
[28] M.H.V. Werts, R.T.F. Jukes, J.W. Verhoeven, The emission spectrum and the
burning in Eu3 - doped Al
þ
3þ
2 3 2
O -SiO glass, Appl. Phys. Lett. 75 (1999) 3072–3074.
radiative lifetime of Eu in luminescent lanthanide complexes, Phys. Chem. Chem.
8] H. Mack, R. Reisfeld, D. Avnir, Fluorescence of rare earth ions adsorbed on porous
vycor glass, Chem. Phys. Lett. 99 (1983) 238–239.
9] J. Zhou, L. Xie, J. Zhong, H. Liang, J. Zhang, M. Wu, Site occupancy and
luminescence properties of Eu3þ in double salt silicate Na3LuSi3O9, Opt. Mater.
Express 8 (2018) 736–743.
Phys. 4 (2002) 1542–1548.
[29] B.R. Judd, Optical Absorption Intensities of Rare-Earth Ions, Phys. Rev. 127 (1962)
750–761.
[30] G.S. Ofelt, Intensities of Crystal Spectra of Rare-Earth Ions, J. Chem. Phys. 37
(1962) 511–520.
[
10] J.C.G. Bünzli, G.R. Choppin, in: Lanthanide Probes in Life, Chemical and Earth
Sciences, Theory and Practice, Elsevier, Amsterdam, 1989, p. 219.
[31] Mithlesh Kumar, T.K. Seshagiri, S.V. Godbole, Fluorescence lifetime and Judd–Ofelt
5
parameters of Eu doped SrBPO , Physica B 410 (2013) 141–146.
[32] K. Binnemans, Interpretation of europium (III) spectra, Coord. Chem. Rev. 295
(2015) 1–45.
[
11] C. G o€ rller-Walrand and K. Binnemans, in: Handbook on the Physics and Chemistry
of Rare Earths, Rationalization of Crystal Field Parametrization, eds. K.A.
Gschneidner Jr and L. Eyring (North-Holland, Amsterdam), to be published.
12] E. D o€ rre, H. Hübner, B. Ilschner, N.J. Grand, Alumina: Processing, Properties and
Applications, Springer, Berlin, 1984.
[33] Lj Dacanin, S.R. Luki ꢀc , D.M. Petrovi ꢀc , M. Nikoli ꢀc , M.D. Drami ꢀc anin, Judd–Ofelt
3
þ
[
[
analysis of luminescence emission from Zn SiO :Eu nanoparticles obtained by a
2 4
polymer-assisted sol-gel method, Physica B 406 (2011) 2319–2322.
13] Y. Kim, S.M. Lee, C.S. Park, S.L. Lee, M.Y. Lee, Substrate dependence on the optical
[34] M. Ferhi, C. Bouzidi, K. Horchani-Naifer, H. Elhouichet, M. Ferid, Judd-Ofelt
3þ
properties of Al
1997) 3604–3606.
14] E.P. Gusev, M. Copel, E. Cartier, I.J.R. Baumvol, C. Krug, M.A. Gribelyuk, High
reolution depth profiling in ultrathin Al films on Si, Appl. Phys. Lett. 76 (2000)
76–178.
2
O
3
films grown by atomic layer deposition, Appl. Phys. Lett. 71
3 4
analysis of spectroscopic properties of Eu doped KLa(PO ) , J. Lumin. 157 (2015)
(
21–27.
[
[
[35] B. Di Bartolo and O. Forte, Advances in Spectroscopy for Lasers and Sensing
Chapter: 21 Publisher: Springer Netherlands.
[36] M.J.F. Digonnet, in: Marcel Dekker (Ed.), Rare-Earth-Doped Fiber Lasers and
Amplifiers, 2 edn, 2001. New York.
[37] C.K. Jørgensen, R. Reisfeld, Judd-Ofelt parameters and chemical bonding, J. Less
Common Met. 93 (1983) 107–112.
2 3
O
1
15] I. Bilecka, M. Niederberger, Microwave chemistry for inorganic nanomaterials
synthesis,, Nanoscale 2 (2010) 1358–1374.
1
2