Characterization of dense lead lanthanum titanate ceramics

Materials Chemistry and Physics 124 (2010) 1051–1056
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Materials Chemistry and Physics
journal homepage: www.elsevier.com/locate/matchemphys
Characterization of dense lead lanthanum titanate ceramics prepared from
powders synthesized by the oxidant peroxo method
Alexandre H. Pinto a , Flavio L. Souza b , Adenilson J. Chiquito c , Elson Longo d ,
Edson R. Leite a , Emerson R. Camargo a,∗
a
LIEC-Laboratório Interdisciplinar de Eletroquímica e Cerâmica, Departamento de Química, UFSCar-Universidade Federal de São Carlos,
Rod.Washington Luis km 235, CP 676 São Carlos, SP 13565-905, Brazil
b
Centro de Ciências Naturais e Humanas, Universidade Federal do ABC, Rua Santa Adélia 166, Bangu, Santo André, SP 09210-170, Brazil
c
Departamento de Física, UFSCar-Federal University of São Carlos, Rod.Washington Luis km 235, CP 676 São Carlos, SP 13565-905, Brazil
d
Instituto de Química de Araraquara, UNESP-Universidade Estadual Paulista, Rua Francisco Degni, CP 355 Araraquara, SP 14801-907, Brazil
a r t i c l e
i n f o
Article history:
Received 10 December 2009
Received in revised form 14 July 2010
Accepted 8 August 2010
Keywords:
Oxides
Chemical synthesis
Crystal structures
Dielectric properties
a b s t r a c t
Nanosized powders of lead lanthanum titanate (Pb1−x Lax TiO3 ) were synthesized by means of the oxidantperoxo method (OPM). Lanthanum was added from 5 to 30% in mol through the dissolution of lanthanum
oxide in nitric acid, followed by the addition of lead nitrate to prepare a solution of lead and lanthanum
nitrates, which was dripped into an aqueous solution of titanium peroxo complexes, forming a reactive
amorphous precipitate that could be crystallized by heat treatment. Crystallized powders were characterized by FT-Raman spectroscopy and X-ray powder diffraction, showing that tetragonal perovskite
structure is obtained for samples up to 25% of lanthanum and cubic perovskite for samples with 30%
of lanthanum. Powders containing 25 and 30% in mol of lanthanum were calcined at 700 ◦ C for 2 h,
and in order to determine the relative dielectric permittivity and the phase transition behaviour from
ferroelectric-to-paraelectric, ceramic pellets were prepared and sintered at 1100 or 1150 ◦ C for 2 h and
subjected to electrical characterization. It was possible to observe that sample containing 25% in mol of
La presented a normal behaviour for the phase transition, whereas the sample containing 30% in mol of
La presented a diffuse phase transition and relaxor behaviour.
© 2010 Elsevier B.V. All rights reserved.
1. Introduction
Isomorphic substitution of lead by lanthanum induces some
interesting changes in the physical properties of lead titanate.
One of the most studied effects is the dependence between structural characteristics of the lanthanum-modified lead titanate and
the lanthanum concentration [1]. In fact, Pb1−x Lax TiO3 solid solutions (thereafter referred to as PLT) have been used as model to
understand various phenomena related to the cubic-to-tetragonal
transition [2]. However, it is well known that nanosized particles
generally display properties that differ from those observed in bulk
material, and for this reason alternative synthetic routes to the traditional solid-state reaction have been developed to obtain a wider
number of compounds at nanometric scale [3–7]. Particularly, PLT
and PbTiO3 have been synthesized by several wet-chemical routes
[8–10], and recently one of us developed a new synthetic route
∗ Corresponding author. Tel.: +55 16 3351 8090; fax: +55 16 3351 8350.
E-mail addresses: fl[email protected] (F.L. Souza), [email protected]
(A.J. Chiquito), [email protected] (E. Longo), [email protected] (E.R. Leite),
[email protected] (E.R. Camargo).
0254-0584/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.matchemphys.2010.08.030
called the “oxidant peroxo method”, sometimes referred by the
abbreviation OPM [11–17]. This wet-chemical technique of synthesis is characterized by the fundamental oxy-reduction reaction
between lead (II) ions and some water soluble peroxo complexes
that results in the formation of an amorphous and highly reactive precipitate. This precipitate is free of common contaminants,
such as halides or graphitic carbon, usually found in materials
synthesized by others chemical routes. Since the OPM precipitate
is formed at molecular-level, its composition can be efficiently
controlled, and because of its reactivity, it was observed that the
crystallization occurs at temperatures below than those reported
for PbTiO3 and PZT systems synthesized by solid-state reaction.
Moreover, the OPM technique uses water as solvent and a relatively simple experimental apparatus, without the necessity of dry
atmosphere or toxic compounds.
Previously, OPM route was used with success to prepare several
lead-based compositions, for instance lead titanate, lead hafnate,
lead zirconate and specially several compositions of lead zirconate
titanate [11–16]. In a recent paper, we prepared and characterized
dense ceramics bodies of PZT, with a Zr:Ti molar ratio of 50 to 50,
from powders prepared by the OPM route, demonstrating that OPM
powders can be efficiently used to prepare high quality ferroelectric
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A.H. Pinto et al. / Materials Chemistry and Physics 124 (2010) 1051–1056
Fig. 2. Raman spectra of PLT powders calcined at 900 ◦ C for 1 h with different compositions collected at room temperature.
Fig. 1. XRD patterns collected at room temperature of PLT powders calcined at
900 ◦ C for 1 h with different compositions. The L0 pattern refers to pure lead titanate
tetragonal phase and the L30 pattern to cubic phase.
ceramic samples [17]. Although lead lanthanum titanate materials
have been extensively studied because of its interesting structural
and electrical properties, only recently we developed the OPM technique to be used with rare earth elements. In this paper, we are
reporting the synthesis of lead lanthanum titanate (PLT) powders
by the oxidant-peroxo method (OPM) with up to 30% in mol of
lanthanum, the sinterization of dense ceramics bodies at different
temperatures and their electrical characterization.
2. Experimental
2.1. Synthesis
All of the chemical reagents were used as received without any purification. Lead
lanthanum titanate with nominal composition of Pb1−x Lax TiO3 , with x up to 0.30,
was prepared by the OPM route [11–17] through the addition of 1 g (0.02 mol) of titanium metal powder (99.7%, Aldrich, Germany) into a mixture of 90 mL of hydrogen
peroxide (analytical grade, Synth, Brazil) and 60 mL of ammonia aqueous solution
(analytical grade, Synth-Brazil). This solution was kept at rest in an ice-water bath
for approximately 5 h, resulting in a yellow transparent aqueous solution of soluble peroxytitanate [Ti(OH)3 O2 ]− ion with concentration of 0.14 mol L−1 . Lanthanum
oxide (99.5%, Merck, Germany) previously heat treated at 400 ◦ C for 1 h to eliminate
carbonate was weighted and added into 30 mL of diluted nitric acid solution at pH = 2
(analytical grade, Synth, Brazil). The volume was corrected to 50 mL with distilled
water, and lead nitrate (99.5%, Riedel-de Haën, Germany) was added to complete
the stoichiometric ratio of each sample composition. Finally, the lanthanum and
lead nitrates solution was slowly dripped into the peroxytitanate solution under
stirring and cooled with ice-water bath, resulting in a vigorous evolution of gas. An
orange precipitate was immediately formed and the solution lost its yellow colour.
This precipitate was filtered and washed with acetone to eliminate the adsorbed
water and nitrate ions. This washed amorphous precipitate was dried at 70 ◦ C for
5 h and ground using a mortar. Amounts of 0.30 g of this powder, thereafter referred
Fig. 3. Transmission electron microscopy image of the PLT powder with 25% of
lanthanum (L25) calcined at 900 ◦ C for 1 h.
to as “precursor”, were calcined at 700 ◦ C for 2 h under a heating rate of 10 ◦ C min−1
using closed alumina boats.
2.2. Sinterization
Pellets in form of disks with 6.8 mm of diameter and 1.9 mm of thickness were
prepared with the calcined powders and pressed using an isostatic pressing with
load of 32 tons for 1 min. After pressing, the pallets were sintered at two different temperatures (1100 and 1150 ◦ C) in a tubular oven for 2 h with heating rate of
10 ◦ C min−1 . In order to avoid lead volatilisation, samples were sintered using closed
Table 1
Lattice parameters of PLT samples calcined at 900 ◦ C for 1 h with different compositions.
Sample
L5 900
L10 900
L20 900
L25 900
L30 900
Lattice parameters (Å)
Angles
a
b
c
c/a
˛=ˇ=
3.9018
3.9099
3.9367
3.9261
3.9256
3.9018
3.9099
3.9367
3.9261
3.9256
4.0676
4.0395
3.9599
3.9284
3.9256
1.0425
1.0331
1.0059
1.0006
1.0000
90.00◦
90.00◦
90.00◦
90.00◦
90.00◦
Cell volume (Å3 )
Symmetry
61.924
61.752
61.369
60.554
60.496
Tetragonal
Tetragonal
Tetragonal
Tetragonal
Cubic
A.H. Pinto et al. / Materials Chemistry and Physics 124 (2010) 1051–1056
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Fig. 4. FESEM images (A) of the fractured surface of the SL30B ceramic sample sintered at 1150 ◦ C for 2 h, and (B) of the L30 powder calcined at 700 ◦ C for 2 h. (C) FESEM
image of the fractures surface of the SL30B sample showing a larger area.
zirconia boats and immersed in PLT powders previously prepared by solid-state
reaction.
2.3. Characterization
The precursor and all of the calcined powders were characterized by Raman
spectroscopy with Fourier transform (FT-Raman) and powder X-ray diffraction
(XRD). Raman spectra were collected at room temperature between 100 and
950 cm−1 using a FT-Raman Bruker RFS 100/S spectrometer, using the 1064 nm line
of an yttrium aluminium garnet (YAG) laser. The X-ray powder patterns were also
collected at room temperature in the 2 range from 10◦ to 80◦ with step scan of
0.02◦ using a Rigaku D/MAX 2500 diffractometer with a rotary anode (Cu K␣ radiation) operating at 150 kV and 40 mA. Crystallographic coherence lengths (crystallite
size) were calculated according to Scherer’s equation [18], adjusting each peak with
a Lorentzian function to determine the full width at half maximum (FWHM). The
(1 0 1) reflexion of monocrystalline silicon was used as FWHM reference, and the
crystallite size was estimate from the average of three peaks. The morphology of
powder and the surface of fractured sintered PLT ceramic samples were characterized by high-resolution field emission scanning electron microscopy (FESEM,
Zeiss SUPRA 35). For electrical measurements, gold contacts were deposited on
the samples surfaces and the dielectric constant (ε ) measured at different frequencies (10 and 100 kHz) and different temperature ranges during the heating
and cooling modes (Pb0.75 La0.25 TiO3 samples were measured from 300 to 700 K and
Pb0.70 La0.30 TiO3 from 50 to 300 K) using a Keithley 3330 (LCZ) meter and a Agilent
4263B (LCR) Meter coupled with Helio Cryo CCS model.
3. Results and discussion
Usually, samples of PLT are prepared by the conventional solidstate reaction. In this traditional method, PbO, La2 O3 and TiO2 are
weighed according to the planned stoichiometry and mixed by ball
milling. On the other hand, the OPM approach to the synthesis of
the PLT is based on a molecular level reaction between soluble
compounds, following a “bottom-up” strategy of synthesis. First,
it means that less energy is necessary to assembly nanosized crystalline particles and second, final materials are more homogeneous
and uniform than those obtained by traditional milling and firing
process. In the OPM route, the oxy-reduction reaction between lead
ions and the peroxo titanium complexes results in a precise and
stoichiometric precipitate with the desired Pb:La:Ti mole ratio. This
precipitate can be described, at least, as a complex mixture of amorphous and hydrated oxides PbO2 , TiO2 and La2 O3 that will result in
crystallized PLT after the firing step, as described by Eq. (1).
Pb(OH)4 2− + La(H2 O)6 3+ + [Ti(OH)3 O2 ]−
→ {PbO2 ·TiO2 ·La2 O3 }nH2 O → Pb1−x Lax TiO3
(1)
3.1. Formation of the PLT
In La modified PbTiO3 , La3+ replaces Pb2+ rather than Ti4+ and
to keep the electrical neutrality, site vacancies should be created in
the crystalline structure [2]. Another possibility is to reduce some
Ti4+ to Ti3+ to compensate the excess of positive charge introduced
by the lanthanum ions. Therefore, the properties of PLT can be
influenced by the presence of vacancies and, in some extension,
also by the Ti3+ concentration. However, isomorphic substitution
of lead by lanthanum atoms in the A site of the perovskite induces
some changes in the structural properties of PbTiO3 host material. For instance, when the lanthanum content is higher than
25% in mol, a diffuse character of the ferroelectric-to-paraelectric
phase transition (DPT) is observed, which can be described by
the tetragonal-to-cubic transition. From this point of view, some
researchers reported that the modification in PbTiO3 induced by
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Fig. 5. (A) Relative dielectric constant in function of the temperature for SL25B and SL30B samples. (B) Temperature dependence of the relative dielectric constant of the SL25B
sample collected at different frequencies. (C) Temperature behaviour of the inverse of the dielectric constant at 100 kHz of the SL25B sample. (D) Fitting of the (1/ε − 1/εm )
vs (T − Tm ) at 100 kHz of the SL25 sample.
the introduction of lanthanum ions results in structural changes
directly related to the nature of the phase transition. In the OPM
procedure to synthesize PLT, La and Pb ions are added together at
molecular level, which should drive to a homogeneous distribution of La in substitution of Pb in the A-site. However, considering
the oxidizing environment that results by the presence of peroxo
group, the electrical neutrality should be obtained mainly because
of vacancies in the cation site than because of the presence of Ti3+
or oxygen vacancies.
Fig. 1 shows the XRD patterns of PLT samples with different
amounts of lanthanum, ranging from 5% of lanthanum up to 30%
of lead substituted, all of them calcined at 900 ◦ C for 1 h. It is
possible to observe the structural modification from tetragonal to
cubic phase as the amount of lanthanum is increased, with the
sample containing 5% of lanthanum (L5) showing tetragonal and
the sample containing 30% of lanthanum (L30) the cubic structure
[19–23]. Table 1 shows the lattice parameters and cell volumes
calculated using the Unit Cell software [24], where is possible to
observe that the tetragonality parameter (c/a ratio) decreases lin-
Table 2
Experimental parameters extracted by Fig. 5, (Tc ) transition temperature, CurieWeiss temperature (Tcw ) inverse of dielectric constant (1/εm ) and Curie-Weiss
constant (C).
Frequency (Hz)
Tc (K)
Tcw
1/εm
C
120
1K
10 K
100 K
378
378
378
378
442
442
442
442
1.12E−4
1.13E−4
1.13E−4
1.15E−4
3.2E−6
3.4E−6
3.5E−6
3.5E−6
early with the increases of lanthanum amount until the unit for
L30 sample. Neves et al. [1] found similar results for PLT samples prepared by traditional solid state reaction and after Rietveld
refinements, they pointed out that the defects occur mainly in A
site shared by lanthanum and lead ions, discarding defects in B
site and oxygen vacancies, which is in agreement with our discussion based in chemical considerations regarding the OPM route.
The XRD patterns of Fig. 1 show that samples L5 and L10 are indubitably tetragonal and sample L30 is cubic. Pattern L20 can also be
assigned as tetragonal when (0 0 1) and (2 0 0) reflection peaks are
observed in detail. It is important to note that the diffraction peaks
(0 0 2) and (2 0 0) are clearly separated in the L20 pattern, indicating the tetragonal structure of the sample with 20% of lanthanum.
However, if XRD is the most popular and widely accepted technique to characterize crystalline structures, sometimes its relative
low sensibility makes impossible to assert the correct structure. In
this way, Raman scattering spectroscopy has been employed with
success to check the presence of secondary phases at levels of 1%
Table 3
Experimental parameters extracted by Fig. 6, (Tc ) transition temperature, inverse of
dielectric constant (1/εm ), diffuseness parameter () and Curie-Weiss constant (C*).
Frequency (Hz)
Tc (K)
PLT 30 sintered at 1100 ◦ C for 2 h
10 K
256
100 K
263
◦
PLT 30 sintered at 1150 C for 2 h
10 K
258
100 K
265
1/εm
C*
1.4E−4
1.3E−4
1.23
1.26
1.54E−6
1.48E−6
1.16E−4
1.22E−4
1.51
1.55
1.41E−6
1.39E−6
A.H. Pinto et al. / Materials Chemistry and Physics 124 (2010) 1051–1056
1055
Fig. 6. (A) Temperature dependence of the relative dielectric constant of SL30A (1100 ◦ C) and SL30B (1150 ◦ C) samples measured at different frequencies. (B) Critical parameter
() fitted by ln(1/ε − 1/εm ) as function of ln(T − Tm ) at 100 kHz for the SL30A, and (C) the critical parameter () for SL30B sample.
in mass or even less, or to assign correctly some crystalline phases
that are not easily distinguishable by XRD [25].
To check the correct structural characteristic of the L25 sample,
all of powders were subjected to FT-Raman analysis and their spectra in the range from 100 to 800 cm−1 are shown in Fig. 2, using the
spectrum of pure lead titanate (L0) as reference for tetragonal and
the L30 for cubic structure. The first impression of Fig. 2 is the gradual changing of the phonon modes between these two extremes.
The general shape of L0 spectrum is easily identified in the L5 and
L10 spectra, confirming the tetragonal symmetry of these two samples. The broadening of the Raman peaks observed in these spectra
indicates that the incorporation of La in the lattice results in structural disorder together to the change in the crystalline structure
[26]. The tetragonal-to-cubic transition could be evaluated through
the merging of the E(3TO) and A1(3TO) modes and the vanishing
of the A1(2TO) mode.
Fig. 3 shows a representative transmission electronic (TEM)
image of the L25 sample calcined at 900 ◦ C for 1 h. One of the most
outstanding features of the OPM powders is its reactivity and it
is interesting to note the presence of necks between particles of
approximately 40 nm with rounded sides, confirming the partial
sintering of these nanometric powders.
phase, pressed and sintered at different temperatures for 2 h
(1100 ◦ C for L30, thereafter referred to as SL30A, and 1150 ◦ C for
L30 and L25, referred to as SL30B and SL25B, respectively). It is
well known that pure lead titanate ceramics spontaneously crack
on cooling after sintering, which is attributed to the large lattice
anisotropy of PbTiO3 . On the other hand, the substitution of Pb by
La in PLT makes possible to obtain dense ceramics without cracking
with the sinterability enhanced due to the formation of vacancies
in the crystalline lattice, which facilitates the interatomic diffusion
during the sintering process [27–29]. The apparent densities of the
sintered ceramics were measured as 94–99% of theoretical density,
calculated by ratio between the density of the single crystal and
the geometrical density of the sintered bodies. These values are
relatively high when compared to those reported previously for
similar compositions. Fig. 4 shows the FESEM images of the fractured surface of SL30A sintered sample (A) and (C), and the image
of the precursor powder (B). A high degree of densification with
fairly uniform grains sizes could be observed, despite the fact that
the precursor powder used to prepare the pressed pellets seems
agglomerated.
3.2. Sintered bodies
Fig. 5A shows the dependence of the relative dielectric permittivity with temperature of the SL25B and of the SL30A samples
measured at 10 kHz, where is possible to observe that the Curie
point (Tc ) is shifted to lower temperatures when the amount of
lanthanum is increased from 25% (SL25B) to 30% (SL30B), which
Considering that L25 and L30 samples are in the limits of the
tetragonal-to-cubic transition, powders with these two compositions were calcined at 700 ◦ C for 2 h to crystallize the perovskite
3.3. Electric and dielectric properties
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A.H. Pinto et al. / Materials Chemistry and Physics 124 (2010) 1051–1056
is related to the decrease in tetragonality with the amount of
lanthanum [27–31]. The dielectric constant of the SL25B sample
measured in function of the temperature at four different frequencies is shown in Fig. 5B, and as expected for a ferroelectric material
that follows the Curie-Weiss law in the vicinity of the transition
temperature, a sharp transition between tetragonal (ferroelectric)
and cubic (paraelectric) phases was observed [17–19,32]. When the
Currie-Weiss temperature (To ) is smaller than the phase transition temperature (Tc ), a first-order transition is observed, whilst
a second-order transition is observed when To = Tc [33]. In order
to determine the ferroelectric-to-paraelectric phase transition, the
dependence of the temperature with the inverse of the dielectric constant (1/ε − 1/εm vs T − Tm ) was presented in Fig. 5C. The
dielectric stiffness was fitted at a narrow temperature range near
to the ferroelectric-to-paraelectric phase transition temperature
(Tc ) and the Currie-Weiss temperature (To = 368 K) calculated was
lower than the transition temperature (Tc = 378 K), indicating a
first-order phase transition. The Curie-Weiss constant (C) value
was also estimated fitting the experimental data (Fig. 5D), and the
values obtained for SL25B at different frequencies presented similar values around 3.5 × 10−6 . There is an empirical relationship for
the Curie-Weiss law (ε = C*/(T/To ) ) that describes the diffuseness
parameter () of the phase transition. The value calculated for the
SL25A sample measured at different frequencies and temperatures
was of 0.9987, with close to the unit, indicating a sharp transition [34]. It is possible to note a non-diffuse character and the
measurements at different frequencies showed that the Curie temperature Tc is independent of the frequency. These results lead to
conclude that the use of the OPM method to prepare PLT powders
with La up to 25% results in ceramic samples with sharp transition between ferroelectric and paraelectric phases. Previous studies
[29,30,35] reported that PLT with more than 20% of La prepared by
different synthetic routes show either a diffusive phase transition
or a beginning of a relaxor behaviour. However, the SL25A sample presented a discernible deviation from the Curie-Weiss law at
Tcw = 408 K, which is remarkably higher than the transition temperature (Tc = 378 K) measured at 100 kHz during heating (Fig. 5C).
This deviation in the Currie-Weiss law was also observed in different frequencies, as summarized in Table 2. Several authors also
observed this deviation from the Curie-Weiss law at Tcw = Tb (Tb is
known as Burns temperature) and is attributed to the presence of
locally polar region [36–38].
Fig. 6A shows the temperature dependence of relative dielectric
constant of the SL30A and SL30B samples measured at different frequencies during the heating and cooling modes. The data revealed
that the maximum of dielectric constant shifts toward higher temperature and its magnitude decrease as the frequency is increased.
Strong frequency dispersion at temperatures around the transition temperature (Tm ) was also observed, showing clearly that the
modification induced on the PbTiO3 crystal lattice by the addition
of lanthanum leads the material to a relaxor state [30,35,38]. It is
known that the introduction of vacancies in the cation sites affects
the long-range order of PLT ceramics, particularly at higher La compositions. Recently, Lee et al. [39] identified that vacancies in the
B-sites are primarily responsible to induce relaxor behaviour in
Pb1−x Lax TiO3 perovskites. One of the main features of the phase
transition in relaxor ferroelectrics is the broadening of the transition, termed as diffuse phase transition (DPT) that was observed
only for SL30 samples. Based on the empirical relationship proposed by Curie-Weiss to describe the diffuseness parameter () of
a phase transition, it was possible to obtain a more definite conclusion about the relaxor behaviour of the PLT samples prepared
from OPM powders fitting the experimental data for PLT samples
sintered at different temperature and frequency (Fig. 6B and C),
indicating a diffuse transition (Table 3), with diffuseness parameter
1 < ≤ 2, in good agreement with the literature [34].
4. Conclusions
It was demonstrated that it is possible to prepare nanosized
powders of lead lanthanum titanate (Pb1−x Lax TiO3 ) by means of
the oxidant peroxo method (OPM). Tetragonal phase was observed
by X-ray diffraction in the samples with lanthanum up to 25% in mol
and cubic for samples with 30% of lanthanum, which was confirmed
by Raman spectroscopy. The dielectric measurements indicated
a normal behaviour for the phase transition from ferroelectric to
paraelectric for the sample with 25% in mol of lanthanum, whereas,
a diffuse phase transition and relaxor behaviour were observed to
the sample containing 30% in mol of lanthanum.
Acknowledgements
The financial support from FAPESP (Projects 07/50904-2 and
07/58891-7), FINEP, CNPq and CAPES is gratefully acknowledged.
The INCTMN/CMDMC also provided support for this work.
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