Mixed valence cerium substitution in Gd2-xCexTi2O7+δ pyrochlores

Article abstract of DOI:10.1016/j.elspec.2019.05.004

Pyrochlore-type oxides having the formula Gd_1.6Ce_0.4Ti₂O_7+δδ have been synthesized by ceramic and co-precipitation methods and annealed at 1300 °C under different environments. Rietveld refinement of powder X-ray diffraction (XRD)patterns indicated that the products adopt the pyrochlore-type oxide structure with Ce ions substituting for Gd ions. Analysis of Ce 3d X-ray photoelectron spectroscopy (XPS)spectra confirmed the presence of both Ce³⁺ and Ce⁴⁺, which was corroborated by analysis of Ce L₃-edge X-ray absorption near edge spectroscopy (XANES)spectra. Incorporation of Ce⁴⁺ imparts disorder on the Ti octahedra as observed by analysis of Ti K-edge XANES spectra. Due to the presence of Ce⁴⁺ in the A-site of the pyrochlore-type structure, oxygen is likely intercalated into the vacant anion 8b site to charge balance the system. This intercalation results in an increase of the average Ti coordination number from 6 to between 6 and 7, which leads to the formula unit being better represented as Gd_1.6Ce_0.4Ti₂O_7+δδ.

Full text of DOI:10.1016/j.elspec.2019.05.004

Accepted Manuscript
Title: Mixed Valence Cerium Substitution in
Gd2 Ce Ti O
Pyrochlores
−x
x
2
7+␦
Authors: Jeremiah C. Beam, Atikin G. Hehn, Andrew P.
Grosvenor
PII:
S0368-2048(19)30013-1
DOI:
Reference:
https://doi.org/10.1016/j.elspec.2019.05.004
ELSPEC 46855
To appear in:
Journal of Electron Spectroscopy and Related Phenomena
Received date:
Revised date:
Accepted date:
31 January 2019
11 May 2019
12 May 2019
Please cite this article as: Beam JC, Hehn AG, Grosvenor AP, Mixed Valence Cerium
Substitution in Gd Ce Ti O Pyrochlores, Journal of Electron Spectroscopy and
2
-x
x
2
7+delta
Related Phenomena (2019), https://doi.org/10.1016/j.elspec.2019.05.004
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1
Mixed Valence Cerium Substitution in Gd2-xCe
x
Ti O7+δ Pyrochlores
2
Jeremiah C. Beam, Atikin G. Hehn, Andrew P. Grosvenor*
Department of Chemistry, University of Saskatchewan, Saskatoon, Saskatchewan, Canada, S7N 5C9
*
Author to whom correspondence should be addressed
E-mail: andrew.grosvenor@usask.ca
Highlights:



Variation of the Ce oxidation state in Gd2-xCe
XPS and XANES confirmed the presence of Ce and Ce
Increase in Ce oxidation state is charge balanced by O intercalation
x
Ti
2
+
O
7+δ was investigated
3
4+
ABSTRACT: Pyrochlore-type oxides having the formula Gd1.6Ce0.4Ti
2
O7+δ have been synthesized by
ceramic and co-precipitation methods and annealed at 1300 °C under different environments. Rietveld
refinement of powder X-ray diffraction (XRD) patterns indicated that the products adopt the pyrochlore-
type oxide structure with Ce ions substituting for Gd ions. Analysis of Ce 3d X-ray photoelectron
3
+
4+
spectroscopy (XPS) spectra confirmed the presence of both Ce and Ce , which was corroborated by
-edge X-ray absorption near edge spectroscopy (XANES) spectra. Incorporation of Ce4+
analysis of Ce L
3
imparts disorder on the Ti octahedra as observed by analysis of Ti K-edge XANES spectra. Due to the
4
+
presence of Ce in theA-site of the pyrochlore-type structure, oxygen is likely intercalated into the vacant
anion 8b site to charge balance the system. This intercalation results in an increase of the average Ti
coordination number from 6 to between 6 and 7, which leads the formula unit being better represented as
Gd1.6Ce0.4Ti
2
O
7+δ
.
Keywords: solid-state chemistry, nuclear waste sequestration, x-ray absorption spectroscopy

2
1
. Introduction
The proliferation of high-level nuclear waste represents a challenging environmental concern[1].
Ceramic waste forms have been proposed as a solution to this problem with the radioactive waste
products being incorporated into a crystalline structure[1]. A variety of crystal structures and
compositions have been studied because of the ability of these structures to incorporate different waste
products [2–5]. Pyrochlore-type oxides, with the formula A
2
B
2
7
O , have been proposed as candidates to
incorporate actinide waste products [1,6,7]. Pyrochlore-type oxides are attractive materials due to the
resistance of these materials to leaching and radiation damage [8].
Pyrochlore-type oxides (space group F d -3 m) have a very broad range of chemical compositions,
with over 500 synthetic compositions being known [9–11]. The 8-coordinate A-site is normally
occupied by an ion in the 3+ oxidation state (Figure 1). However, 4+ ions have been shown to be
stabilized in the A site by the addition of a 2+ ion to maintain charge balance [12,13]. Similarly, the 6-
coordinate B-site is normally occupied by a 4+ ion, but both 3+ and 5+ ions have been shown to be
stabilized in the B site by the incorporation of either oxygen vacancies or A-site cation vacancies
[
10,14]. The pyrochlore-type structure can form when the radius ratio of A- to B-site cations is between
.78 and 1.46 while the defect fluorite structure can form when the radius ratio is ≤ 1.46 [7,15]. The
1
pyrochlore-type structure is a derivative of the fluorite crystal system with ordered oxygen vacancies
while the defect fluorite structure has randomly distributed oxygen vacancies. The 48f anion position,
which is shared between the A- and B- site vertices, is the only atomic position in the structure that is
not located at a special position. Thus, changes in the 48f x-coordinate are able to describe distortions
of the A- and B-site polyhedra from the ideal pyrochlore structure[7].

3
Figure 1. Pyrochlore crystal structure of Gd1.6Ce0.4Ti
magenta. TiO
2
O
7+δ. GdO
8
and CeO
8
polyhedra are depicted in
6
octahedra are depicted in cyan. The structure is projected along the (2 2 2) plane and
contains eight formula units.
Cerium is often used as a surrogate for plutonium due to the similarities in the radii and redox properties
of these elements [16,17]. Previous work has suggested that when cerium is substituted into Gd
2
Ti
2
7
O ,
the solubility limit is reached at Gd1.6Ce0.4Ti , it only substitutes for gadolinium, and is only present in
2
O
7
the 3+ oxidation state[18]. Additionally, it has been shown that post-synthetic treatment of Ce
2
Zr
2
7
O by
sodium hypobromite results in partial oxidation of Ce3+ to Ce , which results in an excess of oxygen
4+
being present in the structure with the final stoichiometry being Ce
2
Zr
2
O7+δ[19]. This excess oxygen has
been suggested to occupy the normally vacant anion 8b site, which is coordinated to the Zr ions. In this

4
work, it is shown that when Ce is incorporated into Gd
2
Ti
2
O
7
, it only substitutes for Gd in the A-site and
is present in both 3+ and 4+ oxidations states. The increase in average A-site oxidation state likely results
in an oxygen-intercalated pyrochlore (i.e. Gd1.6Ce0.4Ti 7+δ). The average Ce oxidation state has also been
shown in this study to vary depending on the synthesis conditions and/or annealing atmosphere.
. Experimental Section
2
O
2
2
.1 Synthesis and Powder XRD
.1.1. Synthesis of Gd1.6Ce0.4Ti
7+δ pyrochlore-type materials were synthesized using Gd
O as precursors. Stoichiometric amounts of the precursors were thoroughly mixed using
a mortar and pestle and pressed into pellets at ~6 MPa. The pellets were loaded into an alumina boat,
placed into a tube furnace, and heated to 1300 °C under an atmosphere of N (g) (PraxAir; 99.99%) or
air at a rate of 100 °C/hr and held at this temperature for 72 hours. Samples that were heated under air
were quench cooled after the 72-hour heating cycle. Samples heated under N (g) were allowed to cool
2
2
O7+δ Materials by the Ceramic Method
Gd1.6Ce0.4Ti
2
O
2
O
3
, TiO
2
, and CeO or
2
Ce(NO
3
)
3
·6H
2
2
2
to room temperature over 12 hours. The samples were then reground, pelleted, and heated under the
same conditions for an additional 72 hours. All samples produced by this method were brown in color.
2
.1.2. Synthesis of Gd1.6Ce0.4Ti
Samples synthesized by the co-precipitation method (designated as “p” below) were synthesized by
dissolving Gd(NO ·6H O (Alfa Aesar, 99.9%), Ce(NO ·6H O, and titanium (IV) tertbutoxide
Sigma-Aldrich, deposition grade) in 1 M nitric acid, followed by the addition of NH OH (concentrated;
8 wt. %) dropwise until the pH reached 10 and a white precipitate formed. The precipitate was filtered
2
O7+δ Materials by a Co-precipitation Method
3
)
3
2
3
)
3
2
(
4
2
and washed with deionized water and allowed to dry in air overnight before being pressed into a pellet
and heated under the same conditions as the ceramic samples described above. All samples produced
by this method were brown in color. Table 1 contains a list of the sample names and starting materials,

5
synthesis method, and annealing atmospheres used to synthesize all pyrochlore-type materials discussed
herein.
2
.1.3. Synthesis of Standard Materials
Gd Ce , a standard sample containing Ce , was synthesized using Gd
and CeO (Sigma-Aldrich; 99.9%). Stoichiometric amounts of the precursors were thoroughly mixed
4
+
2
2
O
7
2
3
O (Alfa Aesar; 99.99%)
2
using a mortar and pestle and pressed into pellets at ~6 MPa. The pellets were loaded into an alumina
boat, placed into a tube furnace, and heated to 1300 °C under air at a rate of 100 °C/hr and held at this
temperature for 72 hours. The pellets were removed from the furnace and quench cooled in air.
3
+
CePO
4
, a standard sample containing only Ce , was synthesized by dissolving 1.197 g of
Ce(NO ·6H
3
)
3
2
O (Alfa Aesar; 99.5%) in 50 mL of deionized water and adding 0.20 mL of H PO (Fisher
3
4
Scientific, 85%). This solution was allowed to reflux overnight and the solution was filtered to collect
a white precipitate, which was annealed under air at 1100 °C for 3 days and quench cooled to room
temperature.
Table 1. List of pyrochlore-type samples showing differences in synthesis methods, starting materials,
and annealing atmospheres
Sample Name
Synthesis Method Starting Materials
Annealing
Atmosphere
Gd1.6Ce0.4Ti
2
2
2
O
7+δ
O
7+δ
O
7+δ
Ceramic
Gd
TiO
2
O
2
3
3
3
, Ce(NO
3
3
)
3
3
,
,
N
2
2
3
+
(
Ce /N
2
)
Gd1.6Ce0.4Ti
Ceramic
Gd
TiO
2
O
2
, Ce(NO
)
N
4
+
(
Ce /N
2
)
Gd1.6Ce0.4Ti
Ceramic
Gd
2
O
, CeO
2
, TiO
2
Air
4
+
(
Ce /air)
p-Gd1.6Ce0.4Ti
2
O
7+δ
Co-precipitation
Gd(NO
Ti(OC
3
)
H
3
, Ce(NO
3
)
3
, N
2
3
+
(
Ce /N
2
)
4
9
)
4

6
2
.1.4. Powder X-ray Diffraction
Powder XRD patterns were collected using a Panalytical Empyrean powder XRD equipped with a
Co Kα1,2 (1.79 Å) source. High quality power X-ray diffraction patterns were collected from Gd Ti
7+δ (Ce /air), and p-
) samples. The patterns were collected using a 2θ range of 5-120° and a step
2
2
7
O ,
3
+
4+
4+
Gd1.6Ce0.4Ti
Gd1.6Ce0.4Ti
2
2
O
7+δ (Ce /N
2
), Gd1.6Ce0.4Ti
2
O
7+δ (Ce /N
2
), Gd1.6Ce0.4Ti O
2
3
+
O
7+δ (Ce /N
2
size of 0.017 °. The patterns were analyzed by performing a Rietveld refinement. The parameters
included in the refinement were scale factor, zero shift, lattice parameter, Caglioti scattering coefficients,
O 48f x-coordinate, and overall isotropic thermal factor (BOVL). The patterns were refined using the
X’Pert HighScore Plus software program [20].
2
.2 XPS
4
+
4+
XPS spectra from Gd1.6Ce0.4Ti
2
O
7+δ (Ce /N
2
), Gd1.6Ce0.4Ti
2
O
7+δ (Ce /air), and Gd1.6Ce0.4Ti
2
O
7+δ
3
+
(
Ce /N ) were collected using a Kratos AXIS Supra X-ray photoelectron spectrometer equipped with
2
an Al Kα (1487 eV) X-ray source located in the Saskatchewan Structural Sciences Center, University
of Saskatchewan. High resolution Ce 3d and C 1s XPS spectra were collected using a pass energy of 40
eV, a step size of 0.05 eV, and an appropriately sized energy window. All spectra were collected with
the charge neutralizer turned on and were calibrated by setting the binding energy of the C 1s peak from
adventitious carbon to 284.8 eV. The XPS spectra were analyzed using the CasaXPS software program
[
21].
2
.3 XANES
The Ti K-edge and Ce L -edge XANES spectra were collected using beamlines located at the
3
Advanced Photon Source synchrotron radiation facility, Argonne National Laboratory. Spectra were
collected using the 20-BM and 9-BM beamlines. Both beamlines were equipped with a Si (1 1 1)
monochromator. Samples were prepared as powders sealed between layers of Kapton tape and collected
in partial fluorescence and transmission modes using a 13-element Ge detector and N -filled ionization
2

7
chambers, respectively. The Ti K-edge XANES spectra were calibrated using Ti metal (4966 eV) and
the Ce L
edge XANES spectra was 0.7 eV or 0.5 eV when collected using the 20-BM beamline or the 9-BM
beamline, respectively. The resolution of the Ce L -edge XANES spectra was 0.8 eV or 0.6 eV when
3
-edge XANES spectra were calibrated using Cr metal (5989 eV). The resolution of the Ti K-
3
collected using the 20-BM beamline or the 9-BM beamline, respectively. All XANES spectra were
calibrated, normalized, and analyzed using the Athena software program [22]. Linear Combination
Fitting of the Ce L -edge XANES spectra was also performed using the Athena software program. Error
3
in the LCF data was determined by manually fitting each component to a composition where the
intensities matched that of the experimental spectra.
3
. Results and Discussion
3
.1. Structure
X-ray diffraction patterns from the Ce-substituted Gd1.6Ce0.4Ti O7+δ pyrochlore-type materials
2
3
+
show that the products are majority single-phase (Figure 2); however, the Gd1.6Ce0.4Ti
2
O
7+δ (Ce /N )
2
4
+
and Gd1.6Ce0.4Ti
), while the Gd1.6Ce0.4Ti
Rietveld refinement of the XRD patterns from the Gd1.6Ce0.4Ti
2
O
7+δ (Ce /N
2
) samples contained a small amount of TiO
2
(2 wt. %) and CeO
7+δ (Ce /air) sample contained a moderate amount of CeO (7 wt. %).
7+δ series was performed to determine
2
(2 wt.
4+
%
2
O
2
2
O
if there were any changes in the pyrochlore-type crystal structure based on changes in starting materials,
synthesis methods, or annealing atmosphere. The model system was based on Ce substituting for Gd in
the A-site. The Rietveld refinements performed using this structural model are shown in Figure 2 and
Figure S1 in the Supporting Information section. The refinement results (Table 2) indicate that the
experimental patterns agreed well with the model system. Upon closer inspection of the XRD patterns
of the pyrochlore-type materials (Figure 3), a shift in peak position to lower 2 θ values (larger unit cell)
was observed for samples that were annealed in N
2
(g) when compared to the unsubstituted pyrochlore
3
+
3+
(Gd
2
Ti
2
O
7
). Substitution of Ce for Gd should result in an expansion of the pyrochlore lattice as the

8
3
+
3+
ionic radius of Ce (1.14 Å) in an 8-coordinate environment is larger than Gd (1.05 Å) [23]. However,
the XRD patterns of samples that were annealed in air were shifted to higher 2θ values (smaller unit
cell) when compared to the XRD pattern from the unsubstituted pyrochlore material (Table 2). This
3+
4+
could indicate that there is partial oxidation of Ce to Ce , which would result in a contraction of the
4+
3+
lattice due to the smaller size of Ce (0.97 Å) compared to Gd in an 8-coordinate environment [23].
4
+
4+
Another possibility is that Ce substitutes for Ti in the B-site. While there was a small amount of TiO
2
(2%) present in these materials, Ce substituting for Ti in the B-site would result in a large amount of
TiO (12%) present in the final product. Also, attempts to refine the diffraction patterns to include the
2
possibility of Ce substituting for Ti in the B-site were unsuccessful (Figure S2), indicating that Ce only
substitutes for Gd in the A-site.

9
8000
7000
6000
5000
4000
3000
2000
1000
0
Gd1.6Ce0.4Ti O (Ce3+, N2)
2 7+d
(
222)
Calculated
(111)
(044)
(
004)
(
113)
(
113)
(226)
(444)
(115)
(135)
1000
500
0
-500
10
20
30
40
50
60
70
80
2q (degrees)
Gd1.6Ce0.4Ti O (Ce4+/ air)
2
7+d
*
*
Gd1.6Ce0.4Ti O (Ce4+/ N2)
2
7+d
Gd1.6Ce0.4Ti O (Ce3+/ N2)
2
7+d
p-Gd1.6Ce0.4Ti O (Ce3+/ N2)
2
7+d
Gd Ti O
2
2
7
10
20
30
40
50
60
70
80
2
q (degrees)
3
+
Figure 2. (Top) XRD pattern and Rietveld refinement of Gd1.6Ce0.4Ti
O
2 7
(Ce /N
2
).
(
Bottom) XRD
Patterns of cerium-substituted pyrochlores synthesized using different annealing conditions and starting
materials. CeO and TiO impurities are indicated by (*).
2
2

1
0
Table 2. Rietveld refinement parameters for Gd2-xCe
x
Ti O7+δ[24].
2
Parameters
Gd
2
Ti
2
O
7
Gd1.6Ce0.4Ti
2
O
7+δ
Gd1.6Ce0.4Ti
2
O
7+δ
Gd1.6Ce0.4Ti
(Ce /air)
2
O
7+δ
p-
3+
4+
4+
(
Ce /N
2
)
(Ce /N
2
)
Gd1.6Ce0.4Ti
O
2 7+δ
3
+
(
Ce /N )
2
a (Å)
10.1843(5) 10.197(1)
10.1956(9)
0.420(5)
1.0(4)
10.175(1)
0.42(1)
10.1911(8)
0.426(7)
0.7(4)
48f O (x)
0.420(5)
0.(2)
0.421(1)
1.7(4)
B
R
R
OVL
1.(1)
wp
7.17403
5.23556
1.87759
8.08379
6.05142
1.7845
8.3056
14.71565
11.75847
1.56624
10.1271
6.8719
exp
5.4437
χ2
2.32785
2.17178
(222)
4+
Gd1.6Ce0.4Ti O (Ce / air)
2
7+d
4+
Gd1.6Ce0.4Ti O (Ce / N )
2
7+d
2
3+
Gd1.6Ce0.4Ti O (Ce / N )
2
7+d
2
3+
p-Gd1.6Ce0.4Ti O (Ce / N )
2
7+d
2
Gd Ti O
2
2
7
35.0
35.5
36.0
2
q (degrees)
Figure 3. Zoomed in view of the (222) powder XRD peak from the pyrochlore-type materials showing
the change in peak position that occurred when the annealing atmosphere was changed. The red line
indicates the kα
1
peak position for Gd
2
Ti
2
7
O .

1
1
3
.2. X-ray Photoelectron Spectroscopy
Analysis of the power XRD patterns from these materials indicated that the lattice constants
varied depending on the synthetic conditions used, which could result from a variation in the Ce
oxidation state. The samples annealed in air were exposed to an oxidizing environment that could
4
+
result in the formation of Ce . While the Ce-containing precursor used to form the
4
+
4+
Gd1.6Ce0.4Ti
2
O
7+δ (Ce /N
2
) sample did not contain any Ce to begin with, and was annealed in
O, contained both NO -
which are oxidizing agents, which could lead to the partial oxidation of Ce3+ to Ce4+.
an inert environment, the starting material, Ce(NO
3
)
3
·6H
2
3
and H O,
2
Additionally, it is likely that the N (g) used contained a small amount of water. The effect of
2
3
+
4+
these different oxidizing conditions on the ratio of Ce /Ce in substituted pyrochlores is not
known. Ce 3d XPS spectra have been shown to be sensitive to changes in Ce oxidation state.[25]
Ce 3d XPS spectra were collected from the pyrochlore-type materials and appropriate standards
to study how the Ce oxidation state changes depending on synthesis conditions and are shown
in Figure 4. The spectrum from CePO
4
is presented to show the spectral features that would be
expected from a Ce -bearing material. The spectrum consists of two pairs of spin-orbit split
peaks and associated shake-up satellites. The Ce 3d5/2 component is labeled u and the associated
shake-up satellite peak is labeled u , while the Ce 3d3/2 component is labeled u ’ and the shake-
up satellite is labeled u ’ in Figure 4. Ligand to metal charge transfer shake-up peak satellites
result from the donation of a valence electron from neighboring O atoms into the conduction
band of the Ce atoms [26]. The Ce 3d XPS spectrum from CeO is also presented in Figure 4
3
+
0
1
0
1
2
4
+
and is consistent with what would be expected for spectra from Ce -bearing materials, with
three pairs of spin-orbit split peaks and associated shake-up satellites that result from the
4
+
0
1
2
different final states of Ce (i.e., 4f , 4f , and 4f ) [27]. The Ce 3d5/2 components are labeled as

1
2
v
o
, v
1
, and v
2
, and the Ce 3d3/2 components labeled as v
0
’, v
1
’, and v
2
’ in Figure 4 [23, 24]. The
4
+
peak labeled as v
2
’ is diagnostic of the presence of Ce [25].
v0
v2
CeO2
v '
2
v '
0
v3
v1
v '
v '
3
1
u1
u0
CePO4
u '
0
u '
1
4
+
Ce /N
2
4
+
Ce /air
3
+
Ce /N
2
920
910
900
890
880
Binding Energy (eV)
4
+
4+
Figure 4. Ce 3d XPS spectra from of Gd1.6Ce0.4Ti
2
O
7+δ (Ce /N
2
), Gd1.6Ce0.4Ti O7+δ (Ce /air), and
2
3
+
Gd1.6Ce0.4Ti
spectra from Ce (CeO
2
O
7+δ (Ce /N ) materials synthesized using different conditions. The Ce 3d XPS
2
4
+
3+
2
) and Ce (CePO ) standards are also shown.
4
The Ce 3d XPS spectra from the Ce-substituted pyrochlore-type materials can be compared
to the spectra from the two standard materials to determine the relative amount of each oxidation
3
+
4+
state present based on the difference in peak intensities and binding energies of the Ce and Ce
3
+
spectral features. In the Gd1.6Ce0.4Ti
2
O
7+δ (Ce /N
2
) sample, the u
o
/u1 and
u
o
’/u
1
’ intensity ratios

1
3
are similar to that of CePO . This indicates that the primary oxidation state of Ce is 3+ in this
4
4
+
sample; however, some Ce is also present, which can be observed by the presence of peak v
2
’.
is higher than the intensity of
, indicating that there is more Ce in this sample when compared to Gd1.6Ce0.4Ti
Ce /N ). This is corroborated by an increase in the peak height of v ’. This indicates that
materials synthesized using a Ce containing precursor contain some Ce after annealing in air.
4
+
In the Gd1.6Ce0.4Ti
2
O
7+δ (Ce /N
2
) sample, the intensity of peak v
0
4
+
peak u
0
2
O
7+δ
3
+
(
2
2
4
+
3+
4
+
When comparing the spectra from the Gd1.6Ce0.4Ti
2
O
7+δ (Ce /N
2
) and Gd1.6Ce0.4Ti
2
O
7+δ
4
+
4+
(
Ce /air) samples, the peaks associated with Ce are higher in intensity in the sample annealed
in air. This indicates that the materials annealed under air have more Ce4+ than the materials
annealed under N
2
(g).
3
.3. XANES
3
.3.1. Ce L
Ce L -edge XANES spectra have been shown to be sensitive to changes in oxidation state
and these spectra were collected to confirm the Ce 3d XPS results presented above. The Ce L
3
-edge XANES
3
3
-
3
+
4+
edge XANES spectra from Ce (CePO
Figure 5. The Ce L -edge spectrum from Gd
spectrum from CePO consists of one feature. The feature in the Gd
4
) and Ce (Gd
Ce consists of four main features while the
Ce spectrum labeled B
2
Ce
2
O
7
) standard materials are shown in
3
2
2
O
7
4
2
2
O
7
1
is a pre-edge feature that results from the 2p → 4p quadrupolar transition. Features B2,
3
B , and
B
4
result from dipolar 2p → 5d transitions in which the final state 4f occupancies change to 4f2,
1
0
3+
4
f , and 4f , respectively [28]. The Ce component is similar in energy to feature B and results
2
from the excitation of a 2p electron to a 5d state [28]. The Ce L
3
-edge XANES spectra from the
Gd1.6Ce0.4Ti O
2
7+δ materials are shown in Figure 6. The materials synthesized using a Ce3+
3
+
precursor have a higher intensity of the Ce feature compared to the materials synthesized using

1
4
4
+
(g) have a higher intensity Ce3+
a Ce precursor. Additionally, the materials annealed under N
2
feature compared to the materials annealed under air.
B2
Ce3+
Ce4+
B3
B4
B1
5
700 5710 5720 5730 5740 5750 5760
Energy (eV)
3
+
4+
Figure 5. Ce L
3
-edge XANES spectra of Ce (CePO
4
) and Ce (Gd
2
Ce
2
O ) standards.
7

1
5
Gd1.6Ce0.4Ti O (Ce4+/ N2)
2
7+d
Ce3+
Gd1.6Ce0.4Ti O (Ce3+/ N2)
2 7+d
Gd1.6Ce0.4Ti O (Ce4+/ air)
2
7+d
p-Gd1.6Ce0.4Ti O (Ce3+/ N2)
2
7+d
Ce4+
5700
5710
5720
5730
5740
5750
5760
Energy (eV)
Figure 6. Comparison of Ce L
3
-edge XANES spectra from the Gd1.6Ce0.4Ti O7+δ materials.
2
The Ce L
linear combination fitting (LCF) method involving two standards, CePO
results are shown in Figure 7 and in Table 3. The LCF results indicate that the p-
3
-edge XANES spectra from the Gd1.6Ce0.4Ti
2
O7+δ materials were fitted using a
4
and Gd Ce , and the
2
2
O
7
3
+
3+
Gd1.6Ce0.4Ti
2
O
7+δ (Ce /N ) sample contained the most Ce (~70 %) but that there was still a
2
4
+
moderate amount of Ce (~30 %) present (Figure 7a). The ceramic sample, Gd1.6Ce0.4Ti
2
O
7+δ
3
+
3+
4+
(
Ce /N
2
) (Figure 7b), contained similar amounts of Ce (~67%) and Ce (~33%) as the sample
synthesized by the co-precipitation method. Additionally, when the starting material was
3
+
changed to CeO
2
(Figure 7d) and the sample was annealed under N
2
(g), the Ce (~48%) and
4
+
Ce (~52%) content were roughly equal. Changing the annealing atmosphere to air (Figure 7c),
4
+
resulted in a large increase in the Ce (~75%) content of the material. While this sample had 7%
present, as discerned from the XRD pattern, it does not account for all of the Ce4+
CeO
2

1
6
determined by analysis of the Ce L XANES spectrum, which indicates that a substantial amount
3
4
+
of Ce is present in this sample. The results of the LCF fits were consistent with the XPS analysis
of these materials presented above.
a)
7+d (Ce3+/ N
Gd1.6Ce0.4Ti O (Ce4+/air)
2 7+d
p-Gd1.6Ce0.4Ti
2
O
2
)
c)
Fit
Fit
Residual
Ce4+
Ce3+
Residual
4+
Ce
3+
Ce
5
700 5710 5720 5730 5740 5750 5760
Energy (eV)
5700 5710 5720 5730 5740 5750 5760
Energy (eV)
b)
3+
d)
4+
Gd1.6Ce0.4Ti O (Ce /N2)
Gd1.6Ce0.4Ti O (Ce /N2)
2
7+d
2
7+d
Fit
Residual
Fit
Residual
4
+
Ce
Ce
4
+
Ce
Ce
3
+
3
+
5
700 5710 5720 5730 5740 5750 5760
Energy (eV)
5700 5710 5720 5730 5740 5750 5760
Energy (eV)
Figure 7. Fitted Ce L
3
-edge XANES spectra from Gd1.6Ce0.4Ti O7+δ materials synthesized using
2
different conditions.

1
7
Table 3. LCF results from the Ce L -edge XANES spectra. All values have an error of ±5%.
3
3
+
4+
Sample description
Ce (%)
Ce (%)
3
+
Gd1.6Ce0.4Ti
2
2
O
O
7+δ (Ce /N
2
)
)
67
52
33
48
4+
Gd1.6Ce0.4Ti
7+δ (Ce /N
2
4
+
Gd1.6Ce0.4Ti
2
O
7+δ (Ce /air)
25
70
75
30
3+
p-Gd1.6Ce0.4Ti
2
O
7+δ (Ce /N )
2
3
.2 Ti K-edge XANES
4
+
3+
Avariation in the Ce /Ce ratio in these materials would suggest that a change in the local
coordination environment would occur as a result in a variation in the O stoichiometry to charge
balance the system. Ti K-edge XANES spectra were collected to study how the changes in the
4
+
3+
Ce /Ce ratio affected the Ti coordination environment. The Ti K-edge XANES spectra from
7+δ materials are presented in Figure 8. Ti K-edge XANES spectra have been
the Gd1.6Ce0.4Ti O
2
shown to be sensitive to both changes in oxidation state and coordination number [29,30]. There
are two distinct regions present in Ti K-edge XANES spectra, which are referred to as the pre-
edge (< 4977 eV) and the main-edge (> 4977 eV) regions. The pre-edge region results from 1s
→
3d quadrupolar transitions and consists of three distinct features, which are labeled as A
1
, A
2
,
and A
3
in Figure 8. Feature A
1
and A
2
are primarily due to the excitation of 1s electrons to crystal
*
4+
field split 3d t2g and e
g
states when studying Ti in an octahedral environment [29–31]. Feature
A
3
results from the excitation of 1s electrons to 3d orbitals of next-nearest-neighbor Ti atoms
that are hybridized with Ti 4p states on the absorbing atom through bridging O 2p orbitals [29–
1]. The intensities of the pre-edge features are very sensitive to changes in coordination number
3

1
8
due to changes in the amount of d- and p-orbital overlap that occurs in different coordination
environments [29].
The energies of the pre-edge features of the pyrochlore materials are consistent with Ti4+
in a 6-coordinate environment [29]. However, all pre-edge features are more intense in the
spectra from the Ce-substituted pyrochlore materials in comparison to the unsubstituted
pyrochlore, Gd
2
Ti
2
7
O , as shown in Figure 8. While it has been reported that an increase in the
pre-edge intensity is indicative of a decrease in the coordination number from 6, a 6-coordinate
centrosymmetric environment with distorted octahedral geometry represents a minimum
intensity of this pre-edge feature [29,32]. The loss of inversion symmetry as the distorted
octahedral geometry shifts to higher or lower coordination results in an increase of the amount
of 3d and 4p overlap. Thus, any change in coordination that breaks the inversion symmetry of
the distorted octahedral geometry will result in an increase in the intensity of the pre-edge peak
due to the increase in p-orbital character, which results in an increase in the dipolar character of
the transition [32].

1
9
Gd Ti O
7
2
2
3
+
Gd1.6Ce0.4Ti O (Ce / N2)
2
7+d
4
+
Gd1.6Ce0.4Ti O (Ce / N2)
2
7+d
4
+
Gd1.6Ce0.4Ti O (Ce / air)
2
7+d
3
+
p-Gd1.6Ce0.4Ti O (Ce / N2)
2
7+d
A3
A2
A1
4965
4970
4975
4980
Energy (eV)
4960
4970
4980
4990
5000
5010
5020
Energy (eV)
Figure 8. Ti K-edge XANES spectra of the Gd2-xCe
x
Ti O7+δ materials synthesized under different
2
synthetic conditions. The inset shows an expanded view of the pre-edge region.
Incorporation of Ce4+ into the pyrochlore structure must be charged balanced either by
cation vacancies or by the intercalation of oxygen into vacant anion sites. There are two vacant
anion sites in the pyrochlore structure, the 8b and 32e sites.[33] The 8b site is tetrahedrally
coordinated to four B-site ions in the pyrochlore structure, while the 32e site is octahedrally
coordinated to 3 A-site ions and 3 B-site ions [19]. Previous studies of the Ce
2
Zr O7+δ system
2
showed that when δ ≤ 0.2, the intercalated oxygen occupies the 8b site, but also displaces oxygen
from the 8a site.[19] When δ > 0.2, the site occupancy of 8a increases as the 8b site is saturated[19].

2
0
This led to an increase in the average coordination number of the Zr ions to somewhere between
and 7[19].
6
4
+
Ce incorporation into the pyrochlore structure could result in the intercalation of oxygen
into the vacant 8b site surrounding the Ti atoms, resulting in 7-coordinate Ti, which charge
balances the system. The intercalation of these O atoms around the Ti coordination environment
increase the amount of overlap between the Ti 3d and 4p orbitals, resulting in an increase in the
intensity of the pre-edge features. The lattice parameters, as determined by Rietveld Refinement,
do not follow a linear trend as expected. If Vegard’s Law was followed, the materials with the
highest concentration of Ce4+ would have the smallest lattice constant, and the materials with
4
+
lowest concentration of Ce would have the largest lattice constant. This deviation could be due
to the competing effect of oxygen intercalation which should result in expansion of the lattice and
4
+
lattice contraction due to Ce incorporation. These competing effects are the likely reason as to
why the lattice constants reported in Table 2 do not follow Vegard’s Law.
4
. Conclusions
Cerium substitution in Gd
single-phase products having the formula, Gd1.6Ce0.4Ti
2
Ti
2
O
7
pyrochlore-type materials results in the formation of
7+δ, with Ce substituting for Gd in the A-
2
O
site. The Ce exists in the A-site with both 3+ and 4+ oxidation states present, which could be
charge balanced by the incorporation of excess oxygen into the vacant anion 8b site. The relative
3
+
4+
amounts of Ce and Ce can be varied based on the oxidation state of the Ce-containing starting
material, the synthetic method used, and the annealing environment. The versatility of the
pyrochlore structure to incorporate multiple oxidation states of the same element could be
important for the sequestration of plutonium, which primarily occurs in +3 and +4 oxidation states.

2
1
5
. Acknowledgements
This project has been funded through a Discovery Grant awarded to APG from the Natural
Sciences and Engineering Research Council (NSERC) of Canada. The Canadian Foundation for
Innovation is thanked for funding the purchase of the PANalytical Empyrean X-ray diffractometer
Kratos AXIS Supra XPS used in this work. JB acknowledged support from the CLS Graduate and
Post-Doctoral Student Travel Support Program. Dr. Xiaoxuan Guo, Mr. Derek Holzscherer, Ms.
Sarah McCaugherty, and Mr. Arthur Situm from the Department of Chemistry, University of
Sakatchewan are thanked for help collecting the XANES spectra presented in this study. Dr.
George Sterbinsky, Dr. Tianpin Wu, Dr. Zou Finfrock, and Dr. Matthew Ward are thanked for
their assistance in carrying out XANES experiments at the APS. The CLS is supported by NSERC,
the National Research Council of Canada, the Canadian Institutes of Health Research, the Province
of Saskatchewan, Western Economic Diversification Canada, and the University of Saskatchewan.
Sector 20 (CLS@APS) facilities at the Advanced Photon Source are supported by the US
Department of Energy - Basic Energy Sciences, the Canadian Light Source and its funding
partners, and the Advanced Photon Source. Use of the Advanced Photon Source, an Office of
Science User Facility operated for the U.S. Department of Energy (DOE) Office of Science by
Argonne National Laboratory, was supported by the U.S. DOE under Contract No. DE-AC02-
0
6
6CH11357.
. Declaration of Interest
The authors declare no competing financial interest.
Supporting information available: Rietveld refinement patterns, example of Rietveld refinement
with Ce substituting in the B-site of the pyrochlore structure.

2
2

2
3
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