application of nano structured ca doped ceo2 for ultraviolet filtration
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Application of nanostructured Ca doped CeO2 for ultraviolet filtration
Laurianne Truf fault a,*, Minh-Tri Ta a, Thierry Devers a, Konstantin Konstantinov b,Valerie Harel a, Cyriaque Simmonard a, Caroline Andreazza c, Ivan P. Nevirkovets b,Alain Pineau c, Olivier Veron a, Jean-Philippe Blondeau a
a Institute PRISME, site de Chartres, EA 4229 Universite d’Orle ans, 21 rue de Loigny la Bataille, 28000 Chartres, Franceb Institute for Superconducting & Electronic Materials, University of Wollongong, NSW 2522, AustraliacCentre de Recherche sur la Matie re Divise e, UMR 6619-CNRS, 1b rue de la Fe rollerie, 45071 Orle ans Cedex 2, France
1. Introduction
Recently, CeO2 has been the subject of many studies regarding
its use as a catalyst [1], polishingagent [2], or potential material for
ultraviolet (UV) filtration [3,4]. In the UV radiation range reaching
the Earth’s atmosphere, the ultraviolet type B sub-range (UVB,
290–320 nm) is already well filtered by nanostructured TiO2 in
sunscreen cosmetic products. The ultraviolet type A (UVA)
radiation is divided into two domains. The first one, called ‘‘short
UVA’’, comprises the most energetic and thus the most harmful
type of UVA radiation, whose wavelengths are between 320 and
340 nm. These wavelengths are implicated in skin cancers [5]. The
second domain, called ‘‘long UVA’’, comprises the less energetic
radiation, whose wavelengths are between 340 and 400 nm. This
domain of UVA radiation is responsible for early skin aging. The
need for new materials able to filter the ‘‘short UVA’’ radiation hasincreased in the field of cosmetic products. With a band-gap of
3.2 eV, good transparency in the visible range, and no known
toxicity, nanostructured CeO2 appears to be a promising inorganic
material for use as a UV filter in sunscreen cosmetic products. In
several previous studies [6,7], the doping of CeO2 with different
elements such as Zn and Mg has been successfully used to shift the
material’s band-gap value because of their effects on electronic
transitions.
Another significant problem for the pure CeO2 is its photo-
catalytic activity. As a result, it could oxidise under light and
degrade the other compounds present in the cream. This
characteristic makes the pure material incompatible with use in
cosmetic products. In fact, the CeO2 fluorite type structure is not
stable, because the Ce4+ ionic radius is not large enough to reach
the ideal value of 0.732 for the ionic radius ratio, r (Mn+)/r (O2À) , ofa
metallic element (M) in an MO8 coordination oxide. Thus, Ce4+ has
the tendency to be easily transformed into Ce3+, which has a larger
ionic radius. This reaction is accompanied by release of oxygen to
equilibrate the charges, which leads to the above-mentioned
negative effect.
A number of papers [8–10] have reported that doping with
divalent elements can reduce the photocatalytic activity of CeO2,and that the most efficient of these is Ca. The replacement of Ce4+
by a cation with a lower valence and a larger ionic radius, such as
Ca2+, stabilises the fluorite structure [10]. Although several results
have been already published regarding the effects of Ca doping,
there are few studies that are devoted to the effects of doping over
a large concentration range.
Different chemical methods can be used for the synthesis of
pure or doped CeO2. Among them, the electrochemical deposition
method [11], hydrothermal synthesis [12–14], the pyrrolidone
solution route [15,16], thesol–gel method[17,18], the soft solution
method [8–10], and the co-precipitation technique [7,19] can all be
Materials Research Bulletin 45 (2010) 527–535
A R T I C L E I N F O
Article history:
Received 11 September 2009
Received in revised form 20 January 2010
Accepted 4 February 2010
Available online 12 February 2010
Keywords:
A. Nanostructures
B. Chemical synthesis
C. X-ray diffraction
C. Electron microscopy
D. Optical properties
A B S T R A C T
Calcium doped CeO2 nanoparticles with doping concentrations between 0 and 50 mol% were
synthesized by a co-precipitation method for ultraviolet filtration application. Below 20 mol% doping
concentration, the samples were single-phase. From 30 mol%, CaCO3 appears as a secondary phase. The
calculated CeO2 mean crystallite size was 9.3 nm for the pure and 5.7 nm for the 50 mol% Ca-doped
sample. Between 250 and 330 nm, the absorbance increased for the 10, 30, and 40 mol% Ca-doped
samples compared to the pure one. The band-gap wasfound to be 3.20 eV for the undoped, and between
3.36 and3.51 eV forthe doped samples.The blue shiftsare attributedto thequantumconfinement effect.
X-ray photoelectron spectroscopy showed that the Ce3+ atomic concentration in the pure sample was
higher than that of the 20 mol% Ca-doped sample.
ß 2010 Elsevier Ltd. All rights reserved.
* Corresponding author.
E-mail address: [email protected] (L. Truffault).
Contents lists available at ScienceDirect
Materials Research Bulletin
j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / m a t r e s b u
0025-5408/$ – see front matterß 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.materresbull.2010.02.008
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listed. The co-precipitation method has several advantages: it is
simple, cost-efficient, and gives reproducible results.
In this study, we have used the co-precipitation method to
synthesise calcium-doped CeO2 powders with doping concentra-
tions in the range of 0–50 mol%. We have studied systematically
the effects of doping on the structural and optical properties of
CeO2.
2. Experimental procedures
2.1. Synthesis of pure and Ca-doped CeO 2
Pure and calcium-doped CeO2 powders were synthesized by the
co-precipitation method. For the synthesis of the pure material, a
1.15 mol L À1 cerium nitrate solution (Ce(NO3)3Á6H2O, Alfa Aesar,
99.5%) was mixed with 5 mol L À1 sodium hydroxide (NaOH, Alfa
Aesar, 98%) at ambient temperature. The resulting precipitate was
recovered by centrifugation and washed three times with
deionised water. A 27% (w/w) hydrogen peroxide solution was
then added at a temperature of 50 8C. The oxidised precipitate was
centrifuged and washed with deionised water before filtration
with a folded filter and calcination at 500 8C for 6 h in a porcelain
crucible (VWR) under air. The calcium doped CeO2 powders weresynthesized by adding a calcium chloride solution (CaCl2, Alfa
Aesar, 97%) to the initial solution with a varying concentration,
depending on the expected calcium doping molar concentration.
Beige powders were obtained at the end of the experimental
procedure.
2.2. Analyses used
2.2.1. TGA–DTA
Before calcination, the pure sample was characterized by
thermogravimetric analysis (TGA) and differential thermal analy-
sis (DTA) with a TG–DTA 92-18 Setaram instrument. The sample
was heated from 20 to 1000 8C at a rate of 10 8C/min under argon.
2.2.2. FTIR
Fourier transform infrared (FTIR) spectra of the pure sample
before and after calcination (mid-infrared source) were collected
using a Vertex 70 Fourier transform infrared spectrometer from
Bruker in attenuated total reflection (ATR) mode in the range of
400–4000 cmÀ1 with a resolution of 4 cmÀ1.
2.2.3. XRD
The crystalline structure of the pure and doped samples was
identified by X-ray diffraction (XRD) using the Cu Ka wavelength
(l = 1.5418740 A) of an X’Pert Pro X-ray diffractometer from
PANalitycal in the Bragg-Brentano configuration. The samples
were analysed in the range of 20–1008 with a step of 0.0048 and a
time per step of 90 s. X’Pert HighScore + software was used toanalyse the data. The Scherrer formula presented below was used
for the most intense peak, which was fitted by a pseudo-Voigt
function, to determine the mean crystallite size:
Tc ¼kl
B cosu ; with B ¼ Bobs À Bstd; (1)
where Tc is the mean crystallite size, k is a constant shape factor
(set at 0.9 in our experiments;a value suitable fora cubiccrystal),l
is the wavelength of the incident X-rays, Bobs is the observed full-
widthat half-maximum (FWHM) of the considered peak, Bstd is the
instrumental contribution to the FWHM, and u is the value of the
diffracted angle. Rietveld type refinement was used to determine
the lattice constants.
2.2.4. TEM
The morphology and the particle sizes were characterizedusing
a CM 20 transmission electron microscope (TEM) from Philips. The
samples were dispersed in methanol by ultrasonication. A drop of
the suspension was then laid on a carbon-coated grid and dried
under a lamp to let the methanol evaporate. The accelerating
voltage used in TEM was 200 kV. A statistical grain size analysis
was realised from the TEM images by measuring the diameter, or
the biggest dimension for non-spherical particles, of at least 200
particles per sample. Selected area electron diffraction (SAED) was
performed to determine the crystallinity of the structure. The
interplanar spacings were evaluated from the SAED patterns using
the following formula:
lL ¼ Rd; (2)
where lL is a constant of the microscope, R is the ringradius, and d
is the interplanar spacing. The constant of the microscope was
calculated by measuring the radius of a gold standard pattern
whose interplanar spacings are well known.
2.2.5. UV–vis absorption spectroscopy
The absorption spectra of the samples were recorded with a V
530 ultraviolet–visible spectrophotometer from Jasco in the rangeof 200–1000 nm using quartz cells 1 cm in length. The samples
were dispersed in ethanol at a concentration of 7 Â 10À4 mol L À1
(3 mg in 25 mL) by ultrasonication for 30 min. Some pure ethanol
was taken as a reference. The absorption coefficient, a, was
calculated from the absorption spectra using the following
equation:
a ¼2303Â 103 Á A Á r
l Á c ; (3)
where A is the absorbance, r is the real density of CeO2 (set at
7.28 g cmÀ3 for our calculations), l is the length of the curve, and c
is the concentration of the CeO2 suspension. The band-gap values
were calculated by plotting (ahn)2 as a function of hn, where hn is
the photon energy. The intersection of the extrapolated linearportions with the abscissa axis gives the band-gap value.
2.2.6. XPS
X-ray photoelectron spectra (XPS) of the pure and 20 mol%
calcium-doped samples were collected using a SPECS system
installed in a high-vacuum chamber with the base pressure below
10À8 mbar; the X-ray excitation was provided by Al Ka radiation
with the photon energy hn = 1486.6 eV at a high voltage of 12 kV
and a power of 120 W. The spectra were collected at the pass
energy of 20 eV in the fixed analyser transmission mode.
Thepowder underanalysis was dusted onto an adhesive carbon
tape. An identical carbontape with a reference Cu sample on it was
used to determine the charge shift. The peak positions for Ce3+ and
Ce4+
obtained in this way are in good agreement with thosereported in the literature [20].
It is known that the XPS spectrumof pure CeO2 has six peaks for
the 3d line due to strong hybridization of the oxygen 2p valence
band with the Ce 4f orbital, which makes quantitative analysis of
the reduction of Ce atoms from the 4+ to the 3+ state extremely
complicated [21,22]. We have chosen the following method to
determine relative concentrations of the Ce3+ and Ce4+ cations
from the Ce 3d5/2 line. First, the background was subtracted using
the Shirley approximation, and then the 3d5/2 peak structure was
fitted by five components (i.e., three peaks originating from the 4+
state, and two peaks originating from the 3+ state) using the
commercial CasaXPS2.3.15 software package. The relative atomic
concentrations of the cations under question were determined as
the ratio of the respective peak areas (i.e., the total area of thethree
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calcium-doped sample. The dependence of the CeO2 mean
crystallite size as measured by XRD on the calcium doping
concentration is presented in Fig. 5. The graph shows that the CeO2
mean crystallite size decreases with increasing doping concentra-
tion. Nevertheless, the decrease seems to reach a threshold from a
doping concentration of 30 mol%. Basically, the addition of a
dopant into a crystalline structure affects the crystallite growth
kinetics. Before inserting itself into the CeO2 structure, the calcium
is first located between the CeO2 grain boundaries and thus
disturbs the normal growth of the CeO2 crystallites. We can
distinguish two domains in Fig. 5, corresponding to two modes of
the CeO2 crystal size change. The first domain corresponds to a
calcium doping concentration between 0 and 30 mol%. In this
domain, the CeO2 mean crystallite size decreases sharply with the
calcium doping concentration. Above 30 mol%,the beginning of the
second domain, the decreasing of the CeO2 crystallite size becomes
less pronounced because of the secondary phase formation.
In Fig. 6, we present TEM images of the pure CeO2 and the
50 mol% calcium-doped CeO2 nanoparticles. Despite the ultra-
sonication, both images show that the crystallites tend toagglomerate and form aggregates. This tendency has already been
reported by Phoka et al. [16]. Basically, nanoparticles have a
natural tendency to agglomerate for two main reasons. First, the
agglomeration is a more stable configuration from an energetic
point of view. Then, nanoparticles tend to agglomerate to allow for
crystallite growth. The results presented in Table 1 indicate that
the mean crystallite sizes measured from the TEM images differ at
most by 1 nm from those obtained by XRD. This means that the
TEM results are consistent with those obtained by XRD. The
crystallite size histograms of pure CeO2, 20 mol% calcium-doped,
and 50 mol% calcium-doped nanoparticles are shown in Fig. 7. For
the three samples, the crystallite size is between 2 and 20 nm. The
calcium doping causes a reduction in the number of crystallites
belonging to the size range from 10 to 20 nm. It is noteworthy thatthe mean crystallite size for the 50 mol% Ca-doped sample
obtained from the TEM images is bigger than that for the
20 mol% Ca-doped sample. This result seems at first to be
inconsistent with the XRD results. However, the 50 mol% Ca-
doped sample contains the CaCO3 phase, whose crystallite size is
on average bigger than that of CeO2. Since it is hardly possible to
distinguish the CeO2 crystallites from the CaCO3 crystallites on a
TEM image, we suggest that our measurements involve CaCO3
crystallites, which thus explains the bigger mean crystallite size
Fig. 4. Dependence of the CeO2 lattice parameter on the calcium doping
concentration.
Fig. 5. Dependence of the CeO2 mean crystallite size as measured by XRD on the
calcium doping concentration.
Fig. 6. TEM images of pure CeO2 (a) and 50 mol% Ca-doped CeO2 (b) nanoparticles.
Table 1
Comparison of mean crystallite size measured by XRD to mean particle size
measured by TEM for pure CeO2, 20mol% Ca-doped CeO2, and 50mol% Ca-doped
CeO2.
Sample Mean crystallite
size measured
by XRD (nm)
Mean particle
size measured
by TEM (nm)
Standard deviation
for TEM results
CeO2–0 mol% Ca 9.3 8.3 2.3
CeO2–20 mol% Ca 6.8 5.9 1.6
CeO2–50 mol% Ca 5.7 6.3 1.7
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obtained for the 50 mol% Ca-doped sample. The SAED pattern of
pure CeO2 nanoparticles is presented in Fig. 8. The interplanarspacings measured from this pattern correspond to the CeO2
structure, and confirm the purity of the CeO2 pure sample.
Next, we consider the absorbance curves of the pure and
calcium-doped samples (see Fig. 9). The absorbance curve of theCeO2 pure sample is composed of one large band, whose maximum
is located at around 315 nm. For CeO2, the fundamental absorption
is due to a charge transfer between the full 2p (O) orbital and the
empty 4f (Ce) orbital [3,6,15], which corresponds to an experi-
mental band-gap value of 3.19 eV for the bulk [6]. For nanomater-
ials with particle sizes down to a few nanometers, the band-gap
value is modified because of the quantum confinement effect. For
spherical nanoparticles with an infinitely high potential energy
Fig. 7. Crystallite size histograms of (a) pure CeO2, (b) 20 mol% Ca-doped, and (c) 50 mol% Ca-doped nanoparticles.
Fig. 8. SAED pattern of pure CeO2
nanoparticles.
Fig. 9. Absorbance curves of 20 mol% (a), 50 mol% (b), 0 mol% (c), 30 mol% (d),
40 mol% (e) and 10 mol% (f) calcium doped CeO2
nanoparticles.
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outside the sphere, the band-gap value is dependent on theparticle
radius R, and can be determined from the following formula [27]:
E ¼ E g þh2
8R2
1
meþ
1
mh
À
1:8e2
4pee0R(4)
where E g is thebulk band-gap, R is theradiusof thenanoparticles, me
andmharethe effective massesof theelectronand hole,respectively,
ande
is the relative dielectric constant of CeO2. The above equationindicates that the band-gap value increases with decreasingparticle
radius R. This phenomenon can be observed on the absorbance
curves of the calcium-dopedsamples. These curves are composed of
one large band, but the maximum absorption is located at a lower
wavelength (around 300 nm) than for the pure sample. This means
that the doping causes a blue shift of the maximum absorption. This
blue shift canbe quantifiedby calculatingboththe theoretical band-
gap values fromthe above equation and the experimental band-gap
values from the absorbance curves. Fig. 10(a)–(e) shows the band-
gap value extraction for the 0, 10, 20, 30, 40, and 50 mol% calcium-
doped samples, respectively. We have calculated the theoretical
band-gap values for each sample by taking E g = 3.15 eV,
me = mh = 0.4 m, where m is the mass of a free electron, and
e = 24.5 [28], andby replacingR by themean crystallitesize obtained
from the XRD results. Fig. 11 presents the calculated band-gap as a
function of the calcium doping concentration, and Fig. 12 presents
the experimental band-gap as a function of the calcium doping
concentration.
The calculated band-gap value of pure CeO2 with a mean
crystallite size of 9.3 nm is 3.160 eV. From Fig. 13 one can see that
this value increases with decreasing mean crystallite size, as
expected according to Eq. (4). The experimental band-gap values
are always higher than the theoretical ones, indicating that the
mean crystallite size may have been over-valued for all the
samples. The observed difference between the theoretical and the
Fig. 10. Plot of (ahn)2 as a function of energy for the 0 mol% (a), 10 mol% (b), 20 mol% (c), 30 mol% (d), 40 mol% (e), and 50 mol% (f) Ca-doped CeO2
samples.
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experimental values can thus be explained by the fact that the
crystallite size chosen for the calculations is an average. As shown
by the crystallite size histograms obtained from the TEM images,
the crystallite size is in reality between 2 and 20 nm.
The experimental band-gap value of the pure CeO2 is 3.20 eV(369 nm), andis higher than the bulk experimentalvalue. This blue
shift of the band-gap value for CeO2 nanoparticles (which has
already been reported [3,15,16]) results in a change in the
electronic band structure due to the quantum confinement effect
[16]. For the calcium-doped CeO2 nanoparticles, all the values are
higher than 3.20 eV. This means that the calcium doping has
increased the blue shift that already exists for pure CeO2
nanoparticles compared to CeO2 bulk.
As one can infer from Fig. 9, there is no indication of any
dependence of the absorbance intensity on the calcium doping
concentration. Indeed, between 250 and 330 nm, the 10, 30, and
40 mol% Ca-doped samples absorb more UV radiation than the
pure sample. The most harmful UVA radiation, i.e., the short-
wavelength UVA radiation, is thus better filtered below 330 nmwith these doping concentrations. Among the three samples cited
above, the 10 mol% one is the most efficient between 265 and
325 nm. In fact, several factors affect the absorption properties of
the doped sample in opposite directions. First, the doping with
calcium should make the ceria unit cell more stable by tending to
the value of 0.732 for ideal ionic radius ratio, r (Mn+)/r (O2À), of a
MO8 coordination oxide. We could thus expect that the absorption
properties of the calcium-doped samples are decreased. As a result,
the calcium-doped samples should be less sensitive to the UV
radiation. Thus, the more calcium the sample contains, the more
the absorbance should decrease. The crystallite size should affect
the absorption capacities of the samples as well.The XPS spectra of the pure and the 20 mol% calcium-doped
samples were measured before ion bombardment [cf. Fig. 14(a)
and (c)], and after ion bombardment [cf. Fig. 14(b) and (d)]. The
peaks in the energy interval between approximately 877 and
903 eV belong to the Ce 3d5/2 level. There are three peaks (situated
at 882–883, 889–890, and 898–899 eV) that may be attributed to
the cerium (IV) oxidation state, whereas the other two peaks
(situated at 881–882 and 885–886 eV) may be attributed to the
cerium (III) state [20]. The peaks from the different oxidationstates
overlap, making analysis of the structure extremely complicated.
As is mentioned above, we performed deconvolution of the peak
structure using the CasaXPS2.3.15 software package. In Fig. 14, the
experimental spectra are shown as ‘‘noisy’’ curves, whereas the
‘‘smooth’’ dashed and solid peaks, obtained by fitting theexperimental peak structure, characterize the Ce3+ and Ce4+ ions,
respectively. The white line that fits the experimental curve
corresponds to the sum of all the components.
Using the components that belong to a definite oxidation state,
one can quantify the relative concentrations of the Ce3+ and Ce4+
ions according to the relations:
%Ce3þ ¼ACe3þ
ACe3þ þ ACe4þ
 100; %Ce4þ ¼ACe4þ
ACe3þ þ ACe4þ
 100; (5)
where ACe3þ and ACe4þ denotethe total areaof the Ce3d5/2 peaks for
the (III) and (IV) oxidation states, respectively.
The calculation shows that the concentration of Ce3+ ions in the
pure sample (37%) is higher than that in the doped sample (21%)before ion bombardment. This means that the Ce4+ relative
concentration in the calcium-doped sample (79%) is higher than
that in the pure one (63%). The oxygen concentration is higher in
the calcium-doped sample than in the pure one, too. We can
conclude that the doped sample better approaches the CeO2 ideal
stoichiometrybecause it contains moreCe4+ ions and moreoxygen,
and that the calcium doping has successfully made the CeO2
structure more stable. Also, since thepuresample is not as stableas
the doped sample due to its higher Ce3+ relative atomic
concentration, we can suppose that this material will be more
easily excited by the UV radiation and react more strongly to this
excitation.
After the ion bombardment, the relative concentration of Ce3+
has increased in both the pure (up to 51%) and in the doped (up to
Fig. 11. Plot of the calculated band-gap as a function of the calcium doping
concentration.
Fig. 12. Plot of the experimental band-gap as a function of the calcium doping
concentration.
Fig. 13. Plot of the calculated band-gap as a function of mean crystallite size.
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39%) samples. Nevertheless, the Ce3+ relative ionic concentration
remains higher after the bombardment in the pure sample than in
the doped one. Possibly, the ion bombardment caused the
observed reduction by changing Ce4+ ions into Ce3+ ions. In this
case, the energy provided by the Ar beam could have broken the
Ce-O bonds, leading to the formation of Ce3+ ions.
4. Conclusions
Pure and calcium doped CeO2 nanoparticles with a calcium
doping concentration between 0 and 50 mol% have been success-
fully synthesized by the co-precipitation method. The calcium
doping modifies the structural and optical properties of pure CeO2.Above a 30 mol% calcium doping concentration, the samples
contain a CaCO3 secondary phase and are not suitable for a use as a
cosmetic product. The calcium doping causes a decrease in the
mean crystallite size and increases the absorbance for the 10, 30,
and 40 mol% Ca-doped samples between 250 and 335 nm. The
10 mol% Ca-doped sample is the most efficient between 265 and
325 nm. A blue shift of the absorption is observed first for the pure
CeO2 nanoparticle sample compared to the bulk CeO2, and thenfor
the doped samples compared to the pure sample. This blue shift
allows for better screening of short UVA, the most harmful UVA
wavelengths which are involved in skin cancers. Since the Ce3+
relative atomic concentration has been found to be higher in the
pure sample than in the doped samples, we can also conclude that
the calcium doping successfully made the structure more stable.
Another advantage of the calcium doping is the cost. Indeed,
since cerium (III) nitrate hexahydrate is around five times more
expensive than calcium chloride, doping CeO2 with calcium allows
one to decrease the final cost of the nanoparticle product.
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Fig. 14. XPS spectra of pure powder [panels (a), (b)], and 20 mol% calcium-doped powder [panels (c), (d)]. Spectra (a) and (c) are taken for the samples before ion
bombardment;spectra (b)and (d)are takenafterthe bombardment.The black dashedand solid linesrepresent fitted peaksfor theoxidation states(III) and(IV), respectively;
the white line represents the sum of all the components. Dotted lines are for the Ce 3d 3/2 components (not considered here).
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