Luminescence of Eu-doped langanite nanopowders synthesized by a citrate sol-gel method

Article abstract of DOI:10.1016/j.jallcom.2010.07.206

Eu-doped langanite nanopowders were prepared by a citrate sol-gel method and annealed in air at various temperatures between 700 °C and 1000 °C. For annealing temperatures up to 800 °C, only the langanite phase is observed. For higher annealing temperatur

Full text of DOI:10.1016/j.jallcom.2010.07.206

Journal of Alloys and Compounds 507 (2010) 470–474
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jallcom
Luminescence of Eu-doped langanite nanopowders synthesized by
a citrate sol–gel method
S. Georgescua,∗, A.M. Voiculescua, O. Tomaa, S. Nastasea, C. Mateia, M. Osiacb
a National Institute for Laser, Plasma and Radiation Physics, 409 Atomistilor Street, Magurele, Jud. Ilfov, 077125, Romania
b Faculty of Physics, University of Craiova, Romania
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 7 June 2010
Received in revised form 23 July 2010
Accepted 27 July 2010
Available online 8 August 2010
Eu-doped langanite nanopowders were prepared by a citrate sol–gel method and annealed in air at
various temperatures between 700 ◦C and 1000 ◦C. For annealing temperatures up to 800 ◦C, only the
langanite phase is observed. For higher annealing temperatures, part of the langanite powder transforms
in perovskite (LaGaO3), as evidenced in both XRD and luminescence spectra. Regardless the annealing
temperature, the lifetime of 5D0 remains close to 1 ms. The reddish color of the powders due to color cen-
ters associated to oxygen defects intensifies with increasing annealing temperature. The best efficiency
of the Eu-langanite nanopowders was obtained for the sample annealed at 700 ◦C (∼60% of the efficiency
of the bulk material).
Keywords:
Langanite
Nanopowder
Sol–gel
© 2010 Elsevier B.V. All rights reserved.
Eu3+
Luminescence
ent ions, Ga3+ and Nb5+ (Ta5+) (also, with equal probability). Ga3+
1. Introduction
occupies the remaining positions (C and D). The structural differ-
The search of new and efficient phosphors for solid-state light-
ing is a problem of great interest in present. One of the problems
to be solved is the improvement of the spectral matching between
the emission of the LED and the absorption of the phosphor. Though
efficient, the rare-earth doped phosphors present narrow absorp-
tion lines in near UV or blue where efficient LEDs are available. A
possible solution is the use of partially disordered crystals as hosts
for rare-earth ions. As a result of the disorder, the absorption lines
are wider. Such hosts could be the crystals from the langasite fam-
ily (generic—LGX), namely langasite (LGS—La3Ga5SiO14), langanite
(LGN—La3Ga5.5Nb0.5O14) and langatate (LGT—La3Ga5.5Ta0.5O14).
LGS, LGN and LGT crystallize in the P321 space group, symme-
try class 32 and are isostructural with the calciumgallogermanate
(Ca3Ga2Ge4O14) [1]. The general formula is A3BC3D2O14 where A
represents the dodecahedral positions (distorted Thompson cubes),
B represents the octahedral positions and C and D represent the
tetrahedral positions. La3+ occupies the position A. The local sym-
metry at this site is C2 [2]. In contrast with LGS where Ga3+ and
Si4+ share with equal probability the tetrahedral positions D, in
LGN (LGT) the octahedral positions B are occupied by two differ-
ence between LGS and LGN (or LGT) consists in the placement of the
shortest distance positions randomly occupied around the A site:
four D positions in the plane perpendicular on the C2 axis for LGS
and two B positions along the C2 axis in LGN and LGT. Besides, there
is a larger charge difference between Nb5+ (or Ta5+) and Ga3+ than
between Si4+ and Ga3+ [3,4].
5
When excited in the near UV (395 nm, transition 7F0 → L6),
Eu3+-doped LGX crystals show bright red luminescence which sug-
gests the possibility to use these materials as red phosphors. The
luminescence properties of LGS, LGT, and LGN Eu-doped crystals
were discussed in Refs. [3–7].
Obtaining the powder of LGX starting with single crystal growth
and subsequent milling is a rather expensive approach. Other
methods resulting directly in powders would be of interest. Such
a method, involving lower temperature synthesis, is the sol–gel
approach. There are only few papers regarding the sol–gel synthe-
sis of LGX compounds [8–12] and only one (in our best knowledge)
concerning Eu-doped nanopowder (Eu-doped LGN) [9]. In Ref. [9]
the as-obtained Eu-doped LGN powder is annealed at temperatures
between 800 ◦C and 1100 ◦C for 5 h and no phase transition was
observed.
In this paper, encouraged by the previous results (ours and from
the literature), we synthesized, using the citrate sol–gel method
described in Refs. [9–11], Eu-doped LGN nanopowders. The effect
of thermal treatments is examined using XRD and optical spec-
troscopy (diffuse reflectance and luminescence spectra, kinetics of
∗
Corresponding author at: National Institute for Laser, Plasma and Radiation
Physics, Solid-State Quantum Electronics Laboratory, 409 Atomistilor Street, 077125
Magurele, Ilfov, Romania.
E-mail address: serban.georgescu@inflpr.ro (S. Georgescu).
0925-8388/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2010.07.206

S. Georgescu et al. / Journal of Alloys and Compounds 507 (2010) 470–474
471
the luminescent levels 5D0 and 5D1). A comparison is made with
the results obtained on crystalline Eu:LGN [7].
2. Experimental
The Eu (3 at.%)-doped LGN powder was obtained using the citrate sol–gel
method [9–11]. For preparing the nanoparticles, La(NO3)3·6H2O, Ga(NO3)3·5H2O,
Eu(NO3)3·6H2O, Nb(OH)5 and citric acid were used as precursors, in the correspond-
ing ratios. First of all, Nb(OH)5 was synthesized by fully dissolving Nb2O5 in HF,
followed by the addition of ammonia. The white powder of Nb(OH)5 was then dis-
solved in an aqueous solution of citric acid. The resulting sol was stirred until a clear
solution has been achieved. Then, an aqueous mixture containing La3+, Ga3+ and
Eu3+ nitrates has been added to the Nb-citrate system, with constant stirring. The
final solution was stirred for another 1 h, heated in an oven at 80 ◦C, until a very vis-
cous gel has been formed. The brown-yellow colloid was dried at 110 ◦C, and then
calcined in air at temperatures between 700 ◦C and 1000 ◦C.
XRD measurements were performed on a Shimadzu 6000 diffractometer (Cu
K␣). The fluorescence of the Eu:LGN was excited using a ScienceTech Xe-Hg 350-W
lamp with suitable filters. The fluorescence spectra were measured at room temper-
ature using a Horiba Jobin-Yvon 1000M monochromator, an S-20 photomultiplier
and an SR830 lockin amplifier from Stanford Research Systems. For decay mea-
surements, the luminescence was excited with the second harmonics of a Nd-YAG
laser and analyzed with an Ortec MCS-PCI multichannel scaler card. The experi-
mental setup for diffuse reflectance measurements was described elsewhere [5]. All
measurements were performed at room temperature.
Fig. 2. Luminescence spectra of Eu-doped LGN powders annealed at various tem-
peratures. The spectra are not corrected for spectral sensitivity of the experimental
apparatus. The main transitions are indicated.
spectra in Fig. 2 were not corrected for the spectral sensitivity.
7
As one can see, the main luminescent transition is 5D0 → F2 (at
∼16,250 cm−1). No significant modification in the shape of the
luminescence spectrum is observed when the annealing temper-
ature increases from 700 ◦C to 800 ◦C. For higher temperatures, the
luminescence spectra reflect the transition from pure LGN phase to
the mixture between LGN and LaGaO3 phases. The modifications
3. Results and discussion
In Fig. 1 are shown the XRD patterns for Eu-doped LGN pow-
ders annealed at 800 ◦C and 1000 ◦C. For comparison, the XRD
pattern of the powder obtained by milling a Eu-doped LGN sin-
gle crystal is also shown. For the sample annealed at 800 ◦C, all
the observed diffraction lines belong to the LGN phase. For higher
annealing temperatures (1000 ◦C), besides the diffraction lines cor-
responding to the LGN phase, other diffraction lines, identified as
belonging to LaGaO3 phase are observed (denoted with stars in
Fig. 1). It results that for the Eu-LGN powders obtained using this
sol–gel method, the LGN phase is not stable at higher annealing
temperatures and transforms in LaGaO3, possibly due to the evap-
oration of gallium oxide. No such transformation was noted in Refs.
[8–12].
are evident in the luminescence bands corresponding to 5D0 → F1
7
7
and 5D0 → F2 transitions. For example, the luminescence peak at
16,925 cm−1 (∼590 nm), which dominates the 5D0 → F1 transition
7
in the luminescence spectrum of the sample annealed at 1000 ◦C,
is characteristic for Eu3+ in LaGaO3 [13].
The diffuse reflectance spectra (in rapport with the ‘white’
BaSO4) of the Eu-LGN samples are given in Fig. 3. The absorption
lines corresponding to the transitions from 7F0 and 7F1 (popu-
lated at room temperature) are visible. The baselines of the diffuse
reflectance spectra are tilted towards shorter wavelengths, more
tilted for higher annealing temperatures (i.e. the reddish coloration
of the samples is intensified with the annealing temperature). The
red coloration of the crystals from the langasite family is related to
point defects involving oxygen [14–16].
The Scherrer formula enables the calculation of the average size
of the coherence domain of the LGN grains. For the sample treated
at 800 ◦C, the size of the coherence domain is about 90 nm, while
for the sample annealed at 1000 ◦C it increases to 160 nm. The cal-
culations were carried off using the two most intense diffraction
lines around 2ꢀ = 30◦, both characteristic to the LGN phase.
The luminescence spectrum (Fig. 2) shows the lines originat-
ing in 5DJ levels (J = 0, 1, and 2). The fluorescence was excited at
The thermal treatments could modify the neighborhood of the
Eu3+ ion. In order to estimate the asymmetry ratio R2, defined
as the ratio between the area of the electric-dipole transition
7
7
(5D0 → F2) and the area of the magnetic-dipole one (5D0 → F1),
the fluorescence spectra should be corrected for the spectral sen-
sitivity of the experimental apparatus and the superposition with
the luminescence lines from 5D1 and 5D2 should be eliminated.
This elimination, performed by adjusting the phase of the lockin
amplifier, is possible due to the large difference between the life-
time of 5D0 (close to 1 ms) and lifetimes of the higher levels (tens
of microseconds), as obtained from the decay measurements. The
5
395 nm (transition 7F0 → L6). Since the sensitivity of our exper-
imental apparatus is higher for higher wavenumbers, in order to
see better the luminescence lines from 5D1 and 5D2 levels, the
results are shown in Fig. 4. Since the areas of the 5D0 → F5,6
7
transitions are very small [7], only 5D0 → FJ (J = 0, 1, 2, 3, and
7
4) lines are shown. We obtained R2 = 5.13 0.02 for the sample
annealed at 700 ◦C and 5.06 0.02 for the sample annealed at
800 ◦C. For the Eu-LGN powder obtained by milling the single
crystal, R2 = 4.68 0.02 [7]. The reduction of the asymmetry ratio
with the annealing temperature is related to the improving of the
local symmetry produced by the increase of the particle size. For
higher annealing temperature part of the LGN particles transforms
in LaGaO3 which has a different local symmetry (Cs) at the Eu3+
position [17].
As a measure of the luminescence efficiency, we represented
the total areas of the luminescence spectra of the powders func-
tion of the annealing temperature; the area of the luminescence
Fig. 1. XRD patters for Eu-doped powders annealed for five hours in air at 800 ◦
C
and 1000 ◦C as well as of the Eu:LGN crystal powder. The diffraction lines denoted
by * belong to the LaGaO3 phase.

472
S. Georgescu et al. / Journal of Alloys and Compounds 507 (2010) 470–474
Fig. 3. Reflectance spectra of Eu-doped LGN powders annealed at various temperatures.
spectrum of the bulk material (obtained by milling the single crys-
tal) is also shown. The represented areas were normalized to the
area of the bulk material luminescence spectrum. We note that the
Eu-concentration is the same in the nanopowders and in the bulk:
3 at.% in rapport with La. In order to obtain reproducible results
from the comparison of the luminescence spectra areas, a mechan-
ically stable experimental setup was designed; the powder samples
were placed in the same position at every measurement, and their
density was maintained the same. The results are given in Fig. 5.
Every experimental point in this figure represents an average of
several measurements.
The efficiency of the nanopowders is inferior to the bulk mate-
rial. The best result was obtained for the sample annealed at
700 ◦C. Usually, the efficiency increases with the temperature
of the thermal treatment due to the improvement of the crys-
tallinity and the removal of the adsorbed impurities. The decrease
of efficiency with annealing temperature observed in this case
could be related to the reddish color of the powders. This color
which intensifies with the temperature of the thermal treatment
screens the pump radiation, reducing the absorption of the UV
light, decreasing the emission intensity. The increase in inten-
sity of the red coloration is stronger in the temperature range
700–900 ◦C and weaker between 900 ◦C and 1000 ◦C (see Fig. 3).
Thus, in the range 900–1000 ◦C the decrease of the efficiency
due to the red coloration can, in principle, be compensated by
the favorable effect of the elimination of adsorbed impurities
Fig. 5. Luminescence intensity (area of the luminescence spectra) of the Eu:LGN
powders function of the annealing temperature; for comparison, the area of the
luminescence spectrum of the bulk material (normalized to unity) is also given.
and improvement of crystallinity. The competition of these fac-
tors is complicated in this case by the transformation of LGN in
perovskite.
The decay curves of the 5D0 level for pumping in 5D1 (pump
transition 7F1 → D1) for the samples annealed at 700 ◦C and 800 ◦C
5
Fig. 4. Luminescence spectra of the samples annealed at 700 ◦C (black line) and 800 ◦C (gray line). The spectra were corrected for the spectral sensitivity of the experimental
apparatus and the superposition with the lines from higher 5DJ levels was eliminated by adjusting the phase of the lockin amplifier.

S. Georgescu et al. / Journal of Alloys and Compounds 507 (2010) 470–474
473
of its particles to agglomerate; it is therefore difficult to find its
value.
From the risetime of the 5D0 decays, the lifetime of the 5D1
level can be estimated. For both samples its value is approximately
50 ␮s. For the Eu-LGN samples annealed at higher temperatures,
though part of the material transforms in LaGaO3, the lifetimes
do not change significantly: 930 ␮s and 67 ␮s for the sample
annealed at 900 ◦C and 845 ␮s and 64 ␮s, for the 1000 ◦C sam-
ple. The non-exponential character of the 5D0 decay could be due
to the nonuniformity of the sites occupied by Eu3+ ions in nano-
crystallites and to the influence of the impurities still present on
the grain surfaces. For the bulk Eu:LGN [7], the decay of 5D0 is expo-
nential with ꢁ = 1050 ␮s; the efficiency lifetime for 5D1 is 63 ␮s, not
far from the values obtained for sol–gel samples.
4. Conclusions
Fig. 6. Decay curve for the luminescence of 5D0 level in Eu-doped LGN powders
annealed at 700 ◦C. The risetime is due to the lifetime of the 5D1 level. Black circles:
experiment; continuous gray line: three-exponential fit.
Eu-doped LGN powders were obtained from a citrate sol–gel
synthesis and thermally treated at temperatures between 700 ◦C
and 1000 ◦C. For the annealing temperatures up to 800 ◦C, only the
langanite phase is observed. For higher annealing temperatures,
part of the langanite transforms in LaGaO3. The modifications are
visible in both XRD and luminescence spectra. Besides the lumi-
nescence of 5D0, luminescence from higher 5DJ levels is observed
in the luminescence spectra. The asymmetry ratio decreases from
5.13 0.02 for the sample annealed at 700 ◦C to 5.06 0.02 (800 ◦C)
denoting the improvement of the local symmetry with the increase
of the particles’ size.
are given in Figs. 6 and 7. The risetime observed in Figs. 6 and 7 is
due to the lifetime of 5D1.
The decay of 5D0 can be very well fitted with a three-exponential
expression
ꢀ
ꢁ
ꢀ
ꢁ
ꢀ
ꢁ
−t
−t
−t
I(t) = I1 exp
+ I2 exp
+ I3 exp
(1)
ꢁ1
ꢁ2
ꢁ3
with the first exponential corresponding to the rising part of the
curve (I1 < 0) and the last two mimicking its falling part. The val-
ues of I2, ꢁ2, I3, ꢁ3 can be used to calculate the ‘efficiency’ lifetime
according to ꢁeff = (I2ꢁ2 + I3ꢁ3)/(I2 + I3) which results directly from
the definition of the ‘efficiency’ lifetime.
For pumping in the 5L6 level (395 nm), the greatest efficiency
of the emission of Eu:LGN powders was obtained for the sample
treated at 700 ◦C. The efficiency obtained represented about 60% of
the bulk material efficiency.
We obtained the following results: ␶eff(5D0) = 760 ␮s for the
sample annealed at 700 ◦C and ␶eff(5D0) = 930 ␮s for the sample
annealed at 800 ◦C. Our results are in the millisecond domain and
very different from the lifetime obtained in Ref. [9] (9.2 ns). For the
present time, we are not able to explain this difference.
The radiative lifetime can be obtained from the calibrated lumi-
nescence spectra given the probability of the magnetic-dipole
transition [18]. However, the calculation of the magnetic-dipole
transition probability would require the knowledge of the refrac-
tive index of the sample. For the case when the crystallites size
is smaller than the radiation wavelength, an average refractive
index (between the refractive indexes of the crystal and the
medium) was introduced [19]. This average index of refraction
depends on the compactness of the powder and on the tendency
As a result of thermal treatment in air, the Eu-doped LGN pow-
ders become reddish, the coloration being intensified for higher
temperatures.
For annealing temperatures between 700 ◦C and 1000 ◦C the
lifetime of 5D0 level remains in the millisecond domain while the
lifetime of 5D1 varies from 50 ␮s to 67 ␮s.
Acknowledgement
This workwas supportedbythe NationalCouncilof theScientific
Research in Universities (CNCSIS) in the frame of the Project (IDEI)
ID 812.
References
[1] B.V. Mill, A.V. Butashin, G.G. Kodzhabagian, E.L. Belokoneva, N.V. Belov, Doklady
Akademii Nauk SSSR 264 (1982) 1385.
[2] V.N. Molchanov, B.A. Maksimov, A.F. Kondakov, T.S. Chernaya, Yu.V. Pisarevsky,
V.I. Simonov, JETP Lett. 74 (2001) 222.
[3] S. Georgescu, O. Toma, A.M. Chinie, L. Gheorghe, A. Achim, A.S. Stefan, Opt.
Mater. 30 (2008) 1007.
[4] S. Georgescu, O. Toma, A.M. Chinie, L. Gheorghe, A. Achim, A.S. Stefan, J. Lumin.
128 (2008) 741.
[5] S. Georgescu, A.M. Voiculescu, O. Toma, C. Tiseanu, L. Gheorghe, A. Achim, C.
Matei, Optoelectron. Adv. Mater.–Rapid Commun. 3 (2009) 1379.
[6] S. Georgescu, A.M. Voiculescu, C. Matei, O. Toma, L. Gheorghe, A. Achim, Roma-
nian Rep. Phys. 62 (2010) 128.
[7] S. Georgescu, O. Toma, A.M. Voiculescu, L.Gheorghe, A. Achim, C. Matei, S. Hau,
to be published.
[8] X.Q. Zhang, D.R. Yuan, S. Guo, J. Crystal Growth 308 (2007) 80.
[9] F.P. Yu, D.R. Yuan, X. Zhao, S.Y. Guo, X.L. Duan, L.M. Kong, X.Q. Wang, J. Crystal
Growth 310 (2008) 3256.
[10] F.P. Yu, D.R. Yuan, X.L. Duan, L.M. Kong, X.Z. Shi, S.Y. Guo, L.H. Wang, X.F. Cheng,
X.Q. Wang, J. Alloys Compd. 459 (2008) L1.
[11] F.P. Yu, D.R. Yuan, L.M. Kong, X.L. Duan, S.Y. Guo, X.Q. Wang, X. Zhao, Nanotech-
nology 19 (2008) 045705.
[12] F.P. Yu, D.R. Yuan, S.Y. Guo, X.L. Duan, X.Q. Wang, L.M. Kong, X.Q. Zhang, X. Zhao,
Cryst. Res. Technol. 43 (2008) 522.
Fig. 7. Decay curve for the luminescence of 5D0 level in Eu-doped LGN powders
annealed at 800 ◦C. The risetime is due to the lifetime of the 5D1 level. Black circles:
experiment; continuous gray line: three-exponential fit.
[13] X. Liu, J. Lin, J. Appl. Phys. 100 (2006) 134306.

474
S. Georgescu et al. / Journal of Alloys and Compounds 507 (2010) 470–474
[14] T. Taishi, T. Hayashi, N. Bamba, Y. Ohno, I. Yonenaga, K. Hoshikawa, Physica B
401–402 (2007) 437.
[17] I. Garcia-Rubio, J.A. Pardo, R.I. Merino, R. Cases, V.M. Orera, J. Lumin. 86 (2000)
147.
[15] T. Taishi, T. Hayashi, T. Fukami, K. Hoshikawa, I. Yonenaga, J. Crystal Growth
304 (2007) 4.
[18] M.H.V. Werts, R.T.F. Jukes, J.W. Verhoeven, Phys. Chem. Chem. Phys. 4 (2002)
1542.
[16] G.M. Kuzmicheva, E.N. Domoroschina, V.B. Rybakov, A.B. Dubovsky, E.A. Tyun-
ina, J. Crystal Growth 275 (2005) e715.
[19] R.S. Meltzer, S.P. Feofilov, B. Tissue, H.B. Yuan, Phys. Rev.
R14012.
B 60 (1999)