Luminescence and absorbance of highly crystalline CaMoO4, SrMoO4, CaWO4 and SrWO4 nanoparticles synthesized by co-precipitation method at room temperature

Article abstract of DOI:10.1016/j.jallcom.2010.07.033

Highly crystalline CaMoO₄, SrMoO₄, CaWO₄ and SrWO₄ nanoparticles were successfully synthesized by the co-precipitation of mixtures of Ca(NO₃)₂4H₂O or Sr(NO₃)₂

Full text of DOI:10.1016/j.jallcom.2010.07.033

Journal of Alloys and Compounds 506 (2010) 475–481
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jallcom
Luminescence and absorbance of highly crystalline CaMoO , SrMoO , CaWO and
4
4
4
SrWO nanoparticles synthesized by co-precipitation method at room
4
temperature
a,∗
a
b
c,∗
Titipun Thongtem , Sukjit Kungwankunakorn , Budsabong Kuntalue , Anukorn Phuruangrat
,
c
Somchai Thongtem
a
Department of Chemistry, Faculty of Science, Chiang Mai University, Chiang Mai 50200, Thailand
Electron Microscopy Research and Service Center, Faculty of Science, Chiang Mai University, Chiang Mai 50200, Thailand
Department of Physics and Materials Science, Faculty of Science, Chiang Mai University, Chiang Mai 50200, Thailand
b
c
a r t i c l e i n f o
a b s t r a c t
Article history:
4 4 4 4
Highly crystalline CaMoO , SrMoO , CaWO and SrWO nanoparticles were successfully synthesized by
Received 12 December 2009
Received in revised form 7 July 2010
Accepted 7 July 2010
•
•
•
the co-precipitation of mixtures of Ca(NO3)2 4H2O or Sr(NO3)2, and Na2MoO4 2H2O or Na2WO4 2H2O
◦
dissolved in ethylene glycol at room temperature (30 C). Phases, morphologies, atomic vibrations
and optical properties were analyzed by X-ray diffraction, transmission electron microscopy, Fourier
transform infrared and Raman spectrophotometry, and ultraviolet–visible and photoluminescent spec-
troscopy. All products were proved to be MXO4 (M = Ca and Sr, and X = Mo and W) with body-centered
tetragonal scheelite structures, having round nanoparticles with the average sizes of 12.06 ± 1.65,
Available online 15 July 2010
Keywords:
Nanostructured materials
Co-precipitation
Luminescence
1
6.40 ± 2.44, 15.49 ± 2.19, and 15.40 ± 2.30 nm for CaMoO4, SrMoO4, CaWO4 and SrWO4, respectively.
Their ␯1(Ag), ␯3(Bg), ␯3(Eg), ␯4(Bg), ␯2(Ag) and ␯f.r.(Ag) vibration modes were also detected – being shifted
Optical spectroscopy
to lower wavenumbers from MMoO4 to MWO4, due to the change of efficient atomic mass of the oscil-
6
+
2−
2−
lating ions between X and O in the [XO4] complexes. Band gaps of CaMoO4, SrMoO4, CaWO4 and
SrWO4 were determined to be 5.07, 3.72, 5.40, and 4.47 eV, respectively. Photoluminescent (PL) emissions
were at 414, 413, 418, and 414 nm for CaMoO4, SrMoO4, CaWO4 and SrWO4, respectively.
©
2010 Elsevier B.V. All rights reserved.
1
. Introduction
Nanomaterials have interesting opto-electrical properties
which are different from their bulks. These properties are con-
trolled by uniform shape and narrow size distribution [10]. There
are a variety of methods used to synthesize metal molybdates
and tungstates such as hydrothermal process [11,12], microwave-
assisted synthesis [13] and cyclic microwave irradiation [14],
microwave-hydrothermal synthesis [2–4,15], and sonochemical
method [16]. In a solution system, solvent is important to control
dispersion of particles. Specifically, polyol as ethylene glycol (EG)
has been widely used for synthesizing of dispersion nanomaterials
because the reaction is able to efficiently proceed [16] – EG is one
of the efficient absorbers and stabilizers. In this report, we present
the synthesis of nanocrystalline alkaline earth metal molybdates
and tungstates (M = Ca and Sr) by co-precipitation method at room
temperature without the subsequent calcination at high tempera-
ture. This method is simple, low energy consumption and friendly
to the environment.
Molybdates and tungstates are important luminescent mate-
rials with scheelite-type tetragonal structure, belonging to I4 /a
space group with two formula units per primitive cell. Each of X
atoms (X = Mo and W) is surrounded by four equivalent O atoms
1
2−
composing the [XO4] tetrahedral configuration and each divalent
metal shares corners with eight adjacent O atoms of [XO4]2 tetra-
hedrons [1–5]. Alkaline earth metal molybdates and tungstates
are very interesting materials due to their structural properties,
−
great potential and promising applications. CaMoO4 and SrMoO4
have attracted particular interest in a variety of applications such
as hosts for lanthanide activated lasers, luminescence materials,
microwave applications and catalysts [5–7]. CaWO can be used for
luminescence, thermoluminescence, stimulated Raman scattering
behavior, and superior luminescence as blue phosphor (433 nm)
4
[
8]. CaWO and SrWO have attracted attention for using in oscil-
4 4
loscopes, and as scintillating material for detecting X- and ␥-rays
in medical applications [8,9].
2
. Experiment
•
Calcium nitrate tetrahydrate (99% Ca(NO3)2 4H2O), strontium nitrate (≥99%
∗ Corresponding authors. Tel.: +66 (0)53 943344; fax: +66 (0)53 892277.
E-mail addresses: ttpthongtem@yahoo.com (T. Thongtem),
phuruangrat@hotmail.com (A. Phuruangrat).
Sr(NO3)2), sodium molybdate dihydrate (≥99% Na2MoO4 2H2O) and sodium
•
•
tungstate dihydrate (99% Na2WO4 2H2O) were purchased from Sigma–Aldrich
Reagents Co., and used without further purification. Each 0.005 mole of these start-
0
925-8388/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2010.07.033

4
76
T. Thongtem et al. / Journal of Alloys and Compounds 506 (2010) 475–481
Fig. 1. XRD patterns of CaMoO4, SrMoO4, CaWO4 and SrWO4.
ing materials was separately dissolved in 15.00 mL ethylene glycol, used as a solvent.
After the starting materials were completely dissolved, white solutions formed.
Then the corresponding two solutions were mixed together with continuous stirring
eight O ions [1,5,19]. Their approximate crystallite sizes were cal-
culated from the (1 1 2) peaks of their corresponding XRD patterns
and Scherrer’s equation [18,20], as shown below.
◦
for 24 h at room temperature (30 C). Finally, white precipitates were synthesized,
◦
washed with distilled water and absolute ethanol, and dried in air at 80 C for 24 h.
ꢁk
L =
(2)
Their phases, morphologies, and atomic vibration were characterized by D-500
B cos ꢀ
Siemens X-ray diffractometer (XRD), using Cu K␣ radiation, the scanning 2ꢀ angle
◦
of 15–60 range, with a graphitic monochromator and a Ni filter, Bruker Tensor
The ꢁ, ꢀ, k, and B parameters are the wavelength of Cu K␣ radiation
(1.54056 Å) [21], Bragg angle of the (1 1 2) planes, a constant (0.89),
and full width at half maximum of the (1 1 2) peaks in radian. Their
approximate crystallite sizes are 14, 17, 18 and 20 nm for CaMoO4,
2
7 Fourier transform infrared (FTIR) spectrometer with diluted KBr, HORIBA JOBIN
YVON T64000 Raman spectrometer using 50 mW Ar laser with ꢁ = 514.5 nm, and
JEOL, JEM-2010 transmission electron microscope (TEM) carried out at 200 kV. The
samples for TEM analysis were dispersed in ethanol under ultrasonic radiation and
the dispersed samples were dropped on the supported carbon film coated on 3-
mm diameter fine-mesh copper grids. Their optical properties were recorded by
Lambda-25 Perkin-Elmer UV–visible, and LS50B Perkin-Elmer luminescence spec-
trophotometers.
SrMoO , CaWO and SrWO , respectively.
4
4
4
In the solution system, the MXO (M = Ca and Sr, X = Mo and W)
4
2+
nanoparticles precipitated – M cations as electron pair accep-
2−
tors (Lewis acid) reacted with XO4
anions as electron pair
donors (Lewis base). The reaction between these two species
3
. Results and discussion
2+
2−
(
M
←:XO4 ) proceeded to produce bonding. The lowest molec-
ular orbital energy of Lewis acid interacted with the highest
molecular orbital energy of Lewis base, and MXO4 nanoparticles
Fig. 1 shows XRD patterns of alkaline earth metal molybdate
and tungstate (M = Ca and Sr) nanocrystals. They show pure phases
were finally synthesized [4,15].
◦
of body-centered tetragonal system (a = b =/ c and ˛ = ˇ = ꢂ = 90 )
2−
Scheelite structure has [XO4]
molecular ionic units with
for CaMoO , SrMoO CaWO and SrWO structures with no detec-
2+
4
4,
4
4
strong X–O covalent bonds, which have weak coupling with M
tion of any impurities, comparing to the JCPDS database nos.
cations. Group theory calculation presents 26 different vibrations
8
0
5-0585 (CaMoO ), 85-0586 (SrMoO ), 41-1431 (CaWO ) and 85-
587 (SrWO ) [1,17]. M(NO ) xH O (M = Ca for x = 4, and M = Sr
ꢀ
ꢀ
4
4
4
for MXO crystal with zero wavevector (k = 0), represented in Eq.
4
•
4
3 2
2
(3) [2–5,12,19,20,22–24]:
•
for x = 0) and Na XO4 2H O (X = Mo, W), separately dissolved in
2
2
ꢃ = 3Ag + 5Au + 5Bg + 3Bu + 5Eg + 5Eu
For only Raman and IR-active modes, the vibration are reduced to
ꢃ = 3Ag + 4Au + 5Bg + 5Eg + 4Eu (4)
(3)
ethylene glycol, were mixed and stirred for 24 h at room tem-
perature, to form MXO4 (white precipitates) by the following
reaction.
in EG,24 h stirring
M(NO ) · xH O + Na XO · 2H O
−→
MXO + 2NaNO3
3
2
2
2
4
2
4
The 3Ag, 5Bg and 5Eg modes are Raman-active, and the 4Au
and 4Eu modes are IR-active. The 1Au and 1Eu correspond to
the zero frequency of acoustic modes. The 3Bu modes are silent.
The A and B modes are nondegenerate, and the E mode is dou-
bly degenerate. The “g” and “u” subscripts indicate the parity
+
(x + 2)H O
(1)
2
Their lattice parameters were calculated from the plane-spacing
equation for tetragonal structure and Bragg’s law for diffraction [18]
–
summarized in Table 1 and very close to their corresponding stan-
dard values [17]. The increase of lattice parameters from CaMoO4
to SrMoO , and CaWO to SrWO4 is related to ionic radii of alka-
Table 1
Lattice parameters of the products.
4
4
+
Lattice parameter
CaMoO4
SrMoO4
CaWO4
SrWO4
2
2+
line earth metals (Ca = 1.12 Å and Sr = 1.25 Å) [1]. For scheelite
structure, each of X (X = Mo and W) ions is in the tetrahedrons of
O ions, and the divalent M (M = Ca and Sr) ions are surrounded by
a (Å)
c (Å)
5.2172
11.4201
5.4103
5.2560
5.4202
12.0673
11.4007
12.0031

T. Thongtem et al. / Journal of Alloys and Compounds 506 (2010) 475–481
477
Fig. 2. Raman spectra of CaMoO4, SrMoO4, CaWO4 and SrWO4.
Table 2
under inversion in centrosymmetric crystals. The Ag, Bg and Eg
Raman wavenumbers of the products.
modes arise from the same motion of MXO . Thus, the 13 zone-
4
Lattice vibration
Wavenumber (cm−1)
center Raman-active modes are expected, presented by Eq. (5)
[
3–6,19,20,22–24]:
CaMoO4
SrMoO4
CaWO4
SrWO4
␯
␯
␯
␯
1(Ag)
3(Bg)
3(Eg)
4(Bg)
907
845
795
400
333
208
915
842
795
371
335
189
873
832
792
392
322
203
883
831
794
368
327
181
ꢃ = 3Ag + 5Bg + 5Eg
(5)
The Raman vibrational modes can be classified into two groups
the external and internal modes. The first is known as lattice
–
␯2(Ag)
␯f.r.(Ag)
2
+
phonon, corresponding to the motion of M cations relative to
2
−
the rigid [XO4]
tetrahedron units. The second belongs to the
vibration inside [XO4]2− tetrahedron units, and is considered as
2−
the stationary state of mass center. In free space, [XO4] tetra-
hedrons have Td symmetry [6,23,24], and are composed of four
internal modes (␯ (A ), ␯ (E ), ␯ (F ) and ␯ (F )), one free rotation
symmetry is reduced to S [5,23,24]. Raman spectra, recorded on
4
−
1
150–1000 cm , are shown in Fig. 2, and their six vibrations are
summarized in Table 2. Raman wavenumbers shifted from CaMoO4
to CaWO and SrMoO to SrWO , caused by the covalent bond for-
1
1
2
1
3
2
4
2
mode (␯f.r.(F )) and one translation mode (F ) [1,3–6,19,20,22–24].
1
2
4
4
4
2
−
6+
6+
2−
2−
When [XO4] tetrahedrons reside in scheelite structure, the point
mation between the Mo (or W ) and O ions in the [MoO4]
Fig. 3. FTIR spectra of CaMoO4, SrMoO4, CaWO4 and SrWO4.

4
78
T. Thongtem et al. / Journal of Alloys and Compounds 506 (2010) 475–481
Fig. 4. TEM images and SAED patterns of (a–d) CaMoO4, SrMoO4, CaWO4 and SrWO4, respectively.
and [WO4]2 complexes, which change the efficient mass of the
−
stretching at 657–966 and 648–950 cm and weak W–O bending
−1
−
1
oscillating atoms [25].
at 461 and 434 cm for CaWO and SrWO [20], respectively.
4
4
The internal ␯ (F ) stretching and ␯ (F ) bending modes are
TEM images (Fig. 4) present all product morphologies, which
have good narrow particle-sized distributions containing a num-
ber of round nanoparticles with uniform sizes, which improve
their luminescent properties. The nanoparticle sizes were mea-
sured from the TEM images, of which the size-distributions are
narrow and very close to normal curves, as shown in Fig. 5. The
average particle size and standard deviation of these products
are 12.06 ± 1.65, 16.40 ± 2.44, 15.49 ± 2.19, and 15.40 ± 2.30 nm for
3
2
4
2
actives in only IR frequencies. In fact, the stretching modes arise
from X–O anti-symmetric stretching vibrations in the [XO4]2
−
tetrahedron groups [1,3,4]. The FITR spectra of nanoparticles at
−1
the wavenumber range of 400–4000 cm
are shown in Fig. 3.
The strong X–O stretching vibration was detected as strong Mo–O
−1
stretching at 737–974 and 720–970 cm and weak Mo–O bend-
−1
ing at 423 and 411 cm for CaMoO and SrMoO , and strong W–O
4
4

T. Thongtem et al. / Journal of Alloys and Compounds 506 (2010) 475–481
479
Fig. 5. Particle-sized distribution curves of CaMoO4, SrMoO4, CaWO4 and SrWO4.
Fig. 6. The (˛hꢄ)2 vs hꢄ plots of CaMoO4, SrMoO4, CaWO4 and SrWO4.

4
80
T. Thongtem et al. / Journal of Alloys and Compounds 506 (2010) 475–481
Fig. 7. PL spectra of (a–d) CaMoO4, SrMoO4, CaWO4, and SrWO4, respectively.
CaMoO , SrMoO , CaWO and SrWO , respectively. SAED patterns
The four ligand p(␴) orbitals are compatible with the tetrahedral
4
4
4
4
(
inserted in TEM images) shape like fully concentric rings, imply-
representation for the a1 and t2 symmetries, and the eight ligand
ing that the products were polycrystalline. The interplanar spaces
were calculated from the diameters of the rings, and compared with
those of the JCPDS standard [17]. All SAED patterns show the same
p(␲) orbitals are for the t , t2 and e symmetries. The top occu-
1
pied state has t symmetry formed from O2p(␲) states. The lowest
1
unoccupied state has e symmetry formed from a combination of the
Mo4d(e) and O2p(␲) orbitals for metal molybdates, and W5d(e) and
O2p(␲) orbitals for metal tungstates, to give anti-bonding (*). The
hybridizations between the Mo4d for metal molybdates or W5d for
metal tungstates and O2p orbitals are specified as covalent bonding
between the ions. For ground state system, all one-electron states
below band gap (Eg) are filled to give a many-electron 1A1 state.
At the lowest excited state, there are one hole in the t1 (primarily
O2p(␲)) state and one-electron in the e (primarily Mo4d for metal
molybdates, and W5d for metal tungstates) state which give rise to
(
1 0 1), (1 1 2), (0 0 4), (2 0 0), (2 1 1), (2 0 4), (2 2 0), (1 1 6) and (3 1 2)
planes – in good accordance with the XRD results.
Fig. 6 shows the (˛hꢄ)2 vs hꢄ curves of CaMoO , SrMoO ,
4
4
CaWO4 and SrWO , which were calculated from their UV–visible
4
absorbance using the equation proposed by Wood and Tauc –
shown in Eq. (6) [2,12,19] below.
n
˛
hꢄ = (hꢄ − Eg)
(6)
where ˛ is the absorbance, h the Planck constant, ꢄ the photon
frequency, Eg the energy gap, and n the pure numbers associated
with the different types of electronic transitions. For n = 1/2, 2,
many-electron 1T1, T1, T2 and 3T2 states. Only 1T2 → A1 is the
3
1
1
probable or allowed transition [20,22–24].
3
/2 and 3, the transitions are the direct allowed, indirect allowed,
Fig. 7 shows photoluminescence (PL) spectra at room tempera-
ture of CaMoO4, SrMoO4, CaWO4 and SrWO4 using the excitation
wavelengths of 212, 290, 214, and 276 nm, respectively. These
spectra are rather broad covering the 350–600 nm range. Multi-
peaks of emission spectra were specified using Gaussian analysis
in combination with the origin program. They exhibited the emis-
sion peaks at 414, 413, 418, and 414 nm for CaMoO4, SrMoO4,
CaWO4 and SrWO4, respectively – in accordance with the emissions
of previous reports [7,8,10,11,27,29]. It is generally known that
the emission spectra of the metal molybdates and tungstates are
direct forbidden, and indirect forbidden, respectively. Each energy
gap was determined by extrapolation of each linear portion of
the curves to ˛ = 0. In the present research, the molybdates and
tungstates with scheelite-type tetragonal structure present the
direct allowed electronic transition (n = 1/2) [2,12,26], and the
energy gaps of CaMoO , SrMoO , CaWO and SrWO are 5.07, 3.72,
4
4
4
4
5
.40 and 4.47 eV, respectively. They are in good accordance with the
previous reports [5,19,26–28]. In fact, energy band gap depends on
several factors such as the electronegativity of transition metal ions,
connectivity of the polyhedrons, deviation in the O–X–O bonds,
2−
mainly influenced by charged transitions within the [XO4] com-
2−
plexes, including several factors such as distortions of the [XO ]2−
distortion of the [XO4]
tetrahedrons, growth mechanism, and
4
degree of structural order–disorder in the lattice [2,19].
tetrahedron groups caused by the different angles of O–X–O, par-
ticle sizes, crystalline degree, morphology and surface defects.
Generally, the emission phenomena of metal molybdates and
tungstates occur by the electronic charge transfer within the
XO4]2 units. The hybridization of the molecular orbital of [XO4]
−
2−
4. Conclusions
[
complexes was caused by the coupling between the O2p(␴) and
O2p(␲) ligand orbitals and Mo4d(t ) and Mo4d(e) orbitals for metal
Alkaline earth metal molydate and tungstate (M = Ca and
Sr) nanoparticles were successfully synthesized by the co-
2
molybdates, or W5d(t ) and W5d(e) orbitals for metal tungstates.
2

T. Thongtem et al. / Journal of Alloys and Compounds 506 (2010) 475–481
481
precipitation process. It is
a
simple method, low energy
[8] L. Sun, M. Cao, Y. Wang, G. Sun, C. Hu, J. Cryst. Growth 289 (2006) 231–235.
9] Z. Lou, M. Cocivera, Mater. Res. Bull. 37 (2002) 1573–1582.
10] D. Chen, G. Shen, K. Tang, H. Zheng, Y. Qian, Mater. Res. Bull. 38 (2003)
783–1789.
[11] J. Liao, B. Qiu, H. Wen, J. Chen, W. You, L. Liu, J. Alloy Compd. 487 (2009) 758–762.
[
consumption and friendly to the environment. XRD and SAED pat-
terns show that all products are body-centered tetragonal scheelite
structure. Their vibrations were studied by Raman and FTIR spec-
troscopy, of which the results are in accordance with the XRD and
SAED analyses. TEM revealed the morphologies of the products
which show the nanosized particles with narrow normal distribu-
tions. The direct allowed energy gaps of CaMoO , SrMoO , CaWO
[
1
[
12] J.C. Sczancoski, M.D.R. Bomio, L.S. Cavalcante, M.R. Joya, P.S. Pizani, J.A. Varela,
E. Longo, M.S. Li, J.A. Andrés, J. Phys. Chem. C 113 (2009) 5812–5822.
13] J.H. Ryu, J.W. Yoon, C.S. Lim, W.C. Oh, K.B. Shim, J. Alloy Compd. 390 (2005)
245–249.
[
[14] T. Thongtem, A. Phuruangrat, S. Thongtem, J. Ceram. Proc. Res. 9 (2008)
58–261.
4
4
4
2
and SrWO , determined using Wood and Tauc method, are 5.07,
4
[
[
[
15] L.S. Cavalcante, J.C. Sczancoski, R.L. Tranquilin, J.A. Varela, E. Longo, M.O.
Orlandi, Particuology 7 (2009) 353–362.
16] T. Thongtem, A. Phuruangrat, S. Thongtem, J. Ceram. Proc. Res. 9 (2008)
3
.72, 5.40, and 4.47 eV, respectively.
1
89–191.
17] Powder Diffract. File, JCPDS-ICDD, 12 Campus Boulevard, Newtown Square, PA
9073-3273, U.S.A., 2001.
Acknowledgement
1
This research was supported by the National Nanotechnology
Center (NANOTEC), a member of National Science and Technology
Development Agency (NSTDA), Ministry of Science and Technology,
Thailand.
[18] C. Suryanarayana, M.G. Norton, X-ray Diffraction. A Practical Approach, Plenum
Press, NY, 1998.
[
19] J.C. Sczancoski, L.S. Cavalcante, M.R. Joya, J.W.M. Espinosa, P.S. Pizani, J.A. Varela,
E. Longo, J. Colloid Interface Sci. 330 (2009) 227–236.
[20] A. Phuruangrat, T. Thongtem, S. Thongtem, J. Alloy Compd. 481 (2009) 568–572.
[
[
21] F.Q. Dong, Q.S. Wu, Y.P. Ding, J. Alloy Compd. 476 (2009) 571–574.
22] A. Phuruangrat, T. Thongtem, S. Thongtem, J. Cryst. Growth 311 (2009)
References
4076–4081.
[
23] T. Thongtem, A. Phuruangrat, S. Thongtem, Appl. Surf. Sci. 254 (2008)
7581–7585.
[
[
[
[
[
1] A. Phuruangrat, T. Thongtem, S. Thongtem, J. Ceram. Soc. Jpn. 116 (2008)
05–609.
2] L.S. Cavalcante, J.C. Sczancoski, R.L. Tranquilin, M.R. Joya, P.S. Pizani, J.A. Varela,
E. Longo, J. Phys. Chem. Solid 69 (2008) 2674–2680.
3] L.S. Cavalcante, J.C. Sczancoski, J.W.M. Espinosa, J.A. Varela, P.S. Pizani, E. Longo,
J. Alloy Compd. 474 (2009) 195–200.
4] L.S. Cavalcante, J.C. Sczancoski, L.F. Lima Jr., J.W.M. Espinosa, P.S. Pizani, J.A.
Varela, E. Longo, Cryst. Growth Des. 9 (2009) 1002–1012.
6
[24] T. Thongtem, S. Kaowphong, S. Thongtem, Appl. Surf. Sci. 254 (2008)
7765–7769.
[25] T.T. Basiev, A.A. Sobol, Y.K. Voronko, P.G. Zverev, Opt. Mater. 15 (2000) 205–216.
[26] M.A.M.A. Maurera, A.G. Souza, L.E.B. Soledade, F.M. Pontes, E. Longo, E.R. Leite,
J.A. Varela, Mater. Lett. 58 (2004) 727–732.
[27] J.H. Ryu, B.G. Choi, J.W. Yoon, K.B. Shim, K. Machi, K. Hamada, J. Lumin. 124
(2007) 67–70.
[28] R. Lacomba-Perales, J. Ruiz-Fuertes, D. Errandonea, D. Martínez-García, A.
Segura, Europhys. Lett. 83 (2008) (arti. no. 37002).
5] J.C. Sczancoski, L.S. Cavalcante, M.R. Joya, J.A. Varela, P.S. Pizani, E. Longo, Chem.
Eng. J. 140 (2008) 632–637.
[
[
6] T. Thongtem, A. Phuruangrat, S. Thongtem, Mater. Lett. 62 (2008) 454–457.
7] J. Liu, J. Ma, B. Lin, Y. Ren, X. Jiang, J. Tao, X. Zhu, Ceram. Int. 34 (2008) 1557–1560.
[29] Z. Chen, Q. Gong, J. Zhu, Y.P. Yuan, L.W. Qian, X.F. Qian, Mater. Res. Bull. 44
(2009) 45–50.