Phase Evolution in the CaZrTi2O7–Dy2Ti2O7 System: A Potential
Host Phase for Minor Actinide Immobilization
Zirconolite is considered to be a suitable wasteform material for the immobilization of Pu and other minor actinide species produced through advanced nuclear separations. Here, we present a comprehensive investigation of Dy3+ incorporation within the self-charge balancing zirconolite Ca1–xZr1–xDy2xTi2O7 solid solution, with the view to simulate trivalent minor actinide immobilization. Compositions in the substitution range 0.10 ≤ x ≤ 1.00 (Δx = 0.10) were fabricated by a conventional mixed oxide synthesis, with a two-step sintering regime at 1400 °C in air for 48 h. Three distinct coexisting phase fields were identified, with single-phase zirconolite-2M identified only for x = 0.10. A structural transformation from zirconolite-2M to zirconolite-4M occurred in the range 0.20 ≤ x ≤ 0.30, while a mixed-phase assemblage of zirconolite-4M and cubic pyrochlore was evident at Dy concentrations 0.40 ≤ x ≤ 0.50. Compositions for which x ≥ 0.60 were consistent with single-phase pyrochlore. The formation of zirconolite-4M and pyrochlore polytype phases, with increasing Dy content, was confirmed by high-resolution transmission electron microscopy, coupled with selected area electron diffraction. Analysis of the Dy L3-edge XANES region confirmed that Dy was present uniformly as Dy3+, remaining analogous to Am3+. Fitting of the EXAFS region was consistent with Dy3+ cations distributed across both Ca2+ and Zr4+ sites in both zirconolite-2M and 4M, in agreement with the targeted self-compensating substitution scheme, whereas Dy3+ was 8-fold coordinated in the pyrochlore structure. The observed phase fields were contextualized within the existing literature, demonstrating that phase transitions in CaZrTi2O7–REE3+Ti2O7 binary solid solutions are fundamentally controlled by the ratio of ionic radius of REE3+ cations.
Introduction<!>Materials
Synthesis<!>Materials
Characterization<!>Systematic
Examination of Phase Evolution
in the Ca1–xZr1–xDy2xTi2O7 (0.10 ≤ x ≤ 1.00) System<!><!>Systematic
Examination of Phase Evolution
in the Ca1–xZr1–xDy2xTi2O7 (0.10 ≤ x ≤ 1.00) System<!><!>Phase
Field Confirmation by High-Resolution
Transmission Electron Microscopy<!><!>XAS Investigations of Dy Oxidation State and
Coordination<!><!>XAS Investigations of Dy Oxidation State and
Coordination<!><!>XAS Investigations of Dy Oxidation State and
Coordination<!>Discussion
within Context of Existing Literature<!><!>Discussion
within Context of Existing Literature<!>Conclusions<!>
<p>Zirconolite
structured materials have been widely studied for the
immobilization of actinide-rich radioactive waste streams, due to
excellent chemical alteration resistance and radiation tolerance.1−5 This includes use as a major constituent of the various SYNROC assemblages
for the disposition of high-level actinide-rich wastes derived from
nuclear fuel reprocessing.6,7 The CaZrTi2O7 parent structure has been shown to accommodate U, Pu,
Np, and Cm and is therefore a suitable host matrix for minor actinide
(MA) species such as Am.8−11 Natural analogue specimens have also been shown to
retain ∼20 wt % U/Th over geological timescales.12 The parent structure ABC2O7 is derivative of the anion deficient fluorite structure type and
is closely related to the pyrochlore A2B2O7 family of minerals. The ideal zirconolite unit cell is composed
of planes of corner-sharing CaO8 and ZrO7 polyhedra,
interleaved by hexagonal tungsten bronze (HTB)-style motifs along
the [001] direction.13 Ti4+ cations
are distributed across three distinct sites in the HTB plane: two
Ti4+ sites are arranged in TiO6 octahedra and
one Ti4+ is 5-fold coordinated in a 50% statistically coordinated
trigonal bipyramidal site. In this idealized structure description,
cation and HTB layers are integrated 1:1 along [001], related by a
180° rotation along the c* axis. Due to the
two-layered structure, stoichiometric CaZrTi2O7 is referred to as the zirconolite-2M polytype, with reference to
the monoclinic symmetry of the unit cell; this polytype has previously
been demonstrated to crystallize over the compositional range CaZrxTi3–xO7 for 0.8 < x < 1.3.14</p><p>As the zirconolite structure has three distinct cation
acceptor
sites, the solubility of REE3+/Ac4+ species
is extensive. The incorporation of Ce4+/U4+/Pu4+ species within the Zr4+ site in zirconolite is
accommodated by several structural transitions, first from zirconolite-2M
to the zirconolite-4M polytype.8,9,15 The zirconolite-4M polytype is described by Coelho et al. as an
admixture of zirconolite-2M and pyrochlore; four HTB-type layers interlaced
with Ca/Zr polyhedra (zirconolite), and Ca/Ti polyhedra (pyrochlore),
resulting in unit cell doubling along the c-axis
from ∼11 to 23 Å.16 Further
isovalent substitution of cations within the Zr4+ site
promotes a structural transformation from zirconolite-4M to pyrochlore,
although it should be noted that this does not occur for the corresponding
CaZr1–xThxTi2O7 solid solution, for which the intermediate
4M phase does not form.17 Cubic pyrochlore-structured
materials (parent structure A2B2O7—space group Fd3̅m; Z = 8) have attracted significant interest in
many areas of solid-state chemistry, with titanate and zirconate pyrochlores
(A2Ti2O7 and A2Zr2O7, respectively) developed as potential wasteforms
for actinides, due to high radiation stability.18−24 The rare earth pyrochlore structure is derived from the fluorite
(AO2) superstructure, with one-eighth of the oxygen atoms
replaced by vacancies, and the A3+ and B4+ cations
in 8- and 6-fold coordination with oxygen, respectively. These cations
are ordered along the [110] direction, resulting in the unit cell
adopting cubic symmetry. The phase stability of A2B2O7-type structures is dependent on the ionic radius
ratio of the A and B cations; the ordered cubic pyrochlore structure
is stable in the range 1.46 < rA/rB < 1.78. Compounds with rA/rB below this range adopt
a disordered defect-fluorite structure, while compounds with rA/rB > 1.78 crystallize
with monoclinic layered perovskite-related structure.19</p><p>The present study aims to systematically evaluate
the phase transitions
in the CaZrTi2O7–Dy2Ti2O7 system with the progressive accommodation of
Dy3+, acting as a surrogate for actinide species such as
Pu3+, Am3+, Cm3+, and Np3+. These data are expected to complement existing data for closely
related zirconolite solid solutions Ca1–xZr1–xGd2xTi2O7, Ca1–xZr1–xY2xTi2O7, Ca1–xZr1–xNd2xTi2O7, Ca1–xZr1–xSm2xTi2O7, and Ca1–xZr1–xCe2xTi2O7+δ.4,25−28 While the incorporation of Dy3+ within the zirconolite
structure has not been previously reported, the Dy2Ti2O7 pyrochlore end member has attracted notable
interest given its applications as a spin-ice compound, due to prominent
geometric frustration.29,30 Furthermore, Dy has been previously
used as a surrogate for Am for the fabrication of AmN and (Am-Pu)N
compounds, on the basis of ionic radii constraints and expediency
(Am3+ = 1.09 Å; Dy3+ = 1.03 Å in 8-fold
coordination).31,32</p><!><p>All materials
used were fabricated by a conventional solid-state synthesis route
from component oxides, targeting the solid solution Ca1–xZr1–xDy2xTi2O7 (0.10 ≤ x ≤ 1.00, Δx = 0.10). Precursors
CaTiO3 (Sigma-Aldrich, 99.9%), ZrO2 (Sigma-Aldrich,
99.9%), TiO2 (anatase—Sigma-Aldrich, 99.9%) dried
at 180 °C, and Dy2O3 (Alfa Aesar, 99.9%)
dried at 800 °C were weighed according to the targeted composition,
to yield 3 g batches. The oxide reagents were added to a ZrO2-lined milling jar and homogenized with Y-stabilized ZrO2 milling media and isopropanol for 20 min, using a Fritsch Pulverisette-23,
operating at 25 Hz for 20 min. For each composition, the powder slurries
were discharged, sieved to separate milling media, and dried at 80
°C overnight to evaporate excess solvent. Approximately 0.5 g
of each composition was prepared for sintering by first compacting
into the walls of a hardened steel die, under 3 tonnes of uniaxial
pressure, forming powder compacts 13 mm in diameter. The pellets were
then placed onto a zirconia crucible and sintered in air at 1400 °C
(Δ5 °C min–1) for 24 h. Once cooled,
the pellets were reground using a pestle and mortar, repressed into
pellets, and subjected to a second sintering regime for a further
24 h at 1400 °C (Δ5 °C min–1) to
promote phase purity. After the second sintering step, the pellets
were recovered from the furnace for analysis.</p><!><p>Powder
X-ray diffraction (XRD) was conducted using a Bruker D2 Phaser fitted
with a Lynxeye position-sensitive detector. Data were acquired in
the range 5° ≤ 2θ ≤ 80° (Δ0.02°)
using Cu Kα radiation (λ = 1.5418 Å, Ni Filter),
operating at 30 kV and 10 mA. Phase identification was achieved using
the PDF4+ database. Quantitative phase analysis and unit cell dimensions
were calculated by Rietveld analysis of powder XRD data, using the
Bruker TOPAS software package. Prior to analysis by scanning electron
microscopy (SEM), sintered pellets were mounted in cold setting resin
and polished to a 1 Î¼m optical finish. SEM data were collected
using a Hitachi TM3030 operating with a 15 kV accelerating voltage
at a working distance of 8 mm. Energy-dispersive X-ray spectrometry
(EDS) for semiquantitative compositional analysis was conducted using
a Bruker Quantax 70 spectrometer. EDS mapping was performed over an
area of 140 × 105 μm2 for approximately 10 min.</p><p>X-ray absorption spectroscopy (XAS) at the Dy L3-edge
was conducted at the Photon Factory Synchrotron Facility (Tsukuba,
Japan) using beamline BL-27B, in a conventional transmission configuration.
Spectra were collected at the Dy L3-edge (7790 eV) for
specimens corresponding to nominal composition x =
0.10, 0.30, 0.60, and 1.00. Spectra were collected between 7590 and
8540 eV at the following steps, with a count time of 1s/step: 5 eV
(7590–7760 eV), 0.5 eV (7760–7860 eV), 1 eV (7860–7940
eV), and 2 eV (7940–8540 eV). Samples were finely ground and
dispersed in poly(ethylene glycol), to a concentration corresponding
to one absorption length, and pressed into 13 mm pellets. Spectra
were collected alongside Dy2O3, Dy2TiO5, and Dy2Ti2O7 reference
compounds, representing Dy3+ in 6-fold, 7-fold, and 8-fold
coordination with oxygen, respectively. Calibration of all XAS data
was performed by aligning the Dy2O3 reference
compound from this dataset to a previously collected Dy2O3 standard that was calibrated using a Co foil (edge
position 7709 eV). All edge positions were chosen as the peak of the
first derivative. Data reduction and fitting of the Dy L3 edge EXAFS were performed using Artemis and Athena with FEFF6.33 Best-fit models were informed by pyrochlore
and zirconolite-2M structures from Farmer et al.34 and Whittle et al.,35 respectively.
The F-test methodology for EXAFS was employed to test whether the
addition of various shells was valid and statistically improved the
fit, the results of which are denoted by α.36 For α, >67% is equal to 1σ and >95% is
equal
to 2σ in terms of standard deviation.</p><p>Raman spectra were
collected for each sample using a Horiba-XploRA
Plus system, operating with a 532 nm air-cooled Ar+ laser
line, with a laser power of 5 mW. Powdered specimens were flattened
onto a glass slide and placed incident to the laser line, with spectra
collected in the range 100–1100 cm–1. Transmission
electron microscopy (TEM) was undertaken using either a JEOL F200
or an FEI Tecnai T20, both operating at 200 keV, with both micrographs
and electron diffraction (ED) patterns taken using a CCD camera. Specimens
were prepared for TEM via the crushed grain powdered
route, whereby a small amount of powder was rapidly ground in isopropanol
and the resultant solution pipetted onto a holey lined Cu mesh grid.</p><!><p>The phase evolution of Ca1–xZr1–xDy2xTi2O7 ceramics was analyzed by powder
X-ray diffraction (Figure 1). Three distinct phase fields were identified, corresponding
to mixtures of zirconolite-2M (C2/c), zirconolite-4M (C2/c), and cubic
pyrochlore (Fd3̅m). The x = 0.10 composition was found to form single-phase zirconolite-2M,
characterized by the doublet at 2θ = 30.5° corresponding
to the (221) and (40-2) reflections, the (004) reflection at 2θ
= 31.9°. Unit cell dimensions (Table 1) as determined by Rietveld analysis were
in agreement with those reported for closely related Ca1–xZr1–xGd2xTi2O7 solid solutions.25 A structural transformation from zirconolite-2M
to the zirconolite-4M polytype was observed in the compositional range
0.20 ≤ x ≤ 0.30, as characterized by
the appearance of intense supercell reflections at 2θ = 7.8
and 31.1°, attributed to the (002) and (008) reflections in the
zirconolite-4M structure, respectively. A representative section of
the microstructure for the sample corresponding to x = 0.20 is given in Figure 2. Two distinct phases were distinguished by variation in backscattered
electron contrast, identified to be zirconolite-2M and zirconolite-4M,
in agreement with powder XRD data. The phase labeled A was determined by EDS analysis to be zirconolite-2M. The phase labeled B was consistent with zirconolite-4M, appearing brighter than
the bulk matrix, given the expected higher solubility of Dy3+ in the 4M polytype. The average composition of both zirconolite
phases was derived from semiquantitative EDS analysis and is in general
agreement with targeted nominal stoichiometry (Table 2). The zirconolite-2M phase accounted for
just 19.0 ± 0.8 wt % of the overall phase assemblage when targeting x = 0.30, yet, a two-phase mixture of zirconolite-4M and
cubic pyrochlore was observed in the range 0.40 ≤ x ≤ 0.50; hence, zirconolite-4M was not isolated as a single
phase in the solid solution. This was unsurprising, as the 4M phase
has only previously been reported to crystallize in single phase over
a narrow compositional range, sensitive to preparation route, ionic
radii of dopant/host site, and targeted solid solution regime of REE3+/Ac4+.8,9,16 The cubic pyrochlore phase was evidenced by the appearance of reflections
at 2θ = 15.1 and 29.2°, corresponding to reflections in
the (111) and (113) plane, appearing at x = 0.40.
The microstructure of the x = 0.50 sample is shown
in Figure 3 and clearly
displays a microstructure dominated by two phases. The phases were
confirmed by EDS analysis to be zirconolite-4M and cubic pyrochlore
(labeled as A and B in Figure 3, respectively) Additional
reflections were not observed for 0.60 ≤ x ≤ 1.00, indicating complete Dy2O3 substitution
within the zirconolite-pyrochlore mixture, and confirmed by SEM analyses
(Figure 4).</p><!><p>Powder X-ray
diffraction data for Ca1–xZr1–xDy2xTi2O7 specimens in the compositional
range (0.0 ≤ x ≤ 1.0). The (002) and
(008) zirconolite-4M reflections are highlighted by closed circles
(●). Pyrochlore reflections are indexed by relevant (hkl) indices.</p><p>Backscattered electron
micrograph of the x = 0.20
composition, with EDS spectra of zirconolite-2M and zirconolite-4M
polytype phases.</p><p>Backscattered electron
micrograph of the x = 0.50
composition, with EDS spectra of zirconolite-4M and cubic pyrochlore
phases.</p><p>Backscattered electron micrographs of the x =
0.60, 0.70, 0.80, and 0.90 compositions, demonstrating phase-pure
pyrochlore.</p><p>Indicates phase purity.</p><!><p>Further evidence
of the structure transformation from zirconolite-2M
to cubic pyrochlore was inferred by Raman data, collected in the range
100–1100 cm–1 (Figure 5). The position and intensity of vibrational
modes for zirconolite-2M (x = 0.10) are in excellent
agreement with our previous observations for nominal CaZrTi2O7 synthesized under identical conditions.37 The low symmetry of the monoclinic 2 M unit
cell resulted in many active Raman vibrational modes. The dominant
symmetric stretching vibration at 780 cm–1 was attributed
to TiO6 octahedra.25 Raman active
modes in the range 100–700 cm–1 have not
been previously deconvoluted and assigned to individual vibrational
modes; however, it is accepted that the spectrum consists of internal
vibrations of the TiO6, CaO8, and ZrO7 polyhedral groups.26 As the nominal concentration
of Dy3+ was increased for 0.10 ≤ x ≤ 0.40, a significant degree of broadening occurred in the
spectral range 200–600 cm–1, attributed to
the disorder induced by the accommodation of Dy3+ in the
Ca2+ and Zr4+ sites, and associated polytype
transformation to zirconolite-4M. A notable abatement of the dominant
780 cm–1 mode was also noted, an artifact we have
recently observed in the Raman spectra of the CaZr1–xCexTi2O7 zirconolite solid solution, for which a similar polytype
transformation from zirconolite-2M to zirconolite-4M also occurred.38 Factor group analysis has previously determined
that the general cubic pyrochlore structure A2B2O(1)6O(2) gives rise to six Raman active vibrational modes:
Ag, Eg, and 4F2g.39 A reasonable fit for the end member Dy2Ti2O7 was achieved using a combination of pseudo-Voigt
profile functions (Figure S1). Six modes
were deconvoluted, corresponding to approximate wavenumbers: 206,
306, 331, 517, 547, and 697 cm–1. Bands for the
Dy2Ti2O7 spectrum have also been
assigned according to calculated wavenumbers for A2Ti2O7 pyrochlores (A = Y, Sm, Gd, Yb) by Gupta et
al., accounting for slight variations in position, determined by rA/rTi.40 The acquired Raman spectra are in excellent
agreement with Y2Ti2O7, Gd2Ti2O7, Sm2Ti2O7, and Dy2Ti2O7 specimens formed
by similar processing routes.28,41,42</p><!><p>Raman
spectra for Ca1–xZr1–xDy2xTi2O7 in compositional range 0 ≤ x ≤
1.00.</p><!><p>The phase fields identified
by powder XRD and SEM analyses were further evidenced by high-resolution
transmission electron (HR-TEM) microscopy, coupled with selected area
electron diffraction (SAED). As electron diffraction analysis allows
variations in stacking sequence to be distinguished, polymorphic transitions
in REE3+/Ac4+-doped zirconolites can be reconciled.
As presented in Figure 6a, high-resolution TEM analysis of the x = 0.3 composition
shows the layered structure of zirconolite-4M, as has been previously
reported.16 The bright-field HRTEM image
presented in Figure 6a was captured with the beam orientated down the [110] zone axis,
as is shown by the indexed electron diffraction pattern in Figure 6b. "Streaked"
reflections can be observed in Figure 6b, as identified by the left-pointing green arrows,
and clearly discernible ordered reflections, identified with right-pointing
blue arrows. Such reflections are the direct consequence of the stacking
sequence observed in Figure 6a, induced through the varied layer spacings indicated in
this micrograph (i.e., ∼3 and ∼7 Å).
This area shows a highly ordered stacking sequence in what is a grain
of the zirconolite-4M structure, leading to the strong reflections
observed, while the varied layer spacings produced the "streaked"
reflections. The reduced contrast is likely induced by a layer rich
in Dy, similar to Nd-doped zirconolite studies that have been reported,
although high-angle annular dark-field imaging would be required to
confirm this hypothesis. The dark-field micrograph presented in Figure 7a shows a two-layered
band structure down the [110] zone axis, imaged with the objective
aperture positioned over the diffuse reflection indicated by the right-pointing
red arrow. The two-layered bands have spacings of ∼23 and ∼11
Å, representing a doubling of the unit cell along the c-axis. As described by Coelho et al., these imperfections
are commonplace throughout an indexed zirconolite-4M structure and
indicate the presence of both 4M and 2M spacings within a single-crystal
grain. In contrast to Figure 6, this area contained nonuniform domains of varied spacing,
suggesting variations in the level of 4M ordering within each crystal
for the x = 0.30 sample. Analysis of the x = 0.60 composition through electron diffraction confirmed
the formation of the pyrochlore structure, as presented in Figure 8a,b for the [111]
and [211] zone axis patterns, respectively. No evidence of a zirconolite-2M
or 4M phase was detected throughout the grains observed, confirming
the phase transition to a pure pyrochlore phase at x = 0.60, in agreement with powder XRD and SEM observations.</p><!><p>(a) Bright-field
TEM micrograph of the x = 0.3
sample, showing the zirconolite-4M structure, with the electron beam
positioned down the [100] zone axis and (b) a [100] zone-axis electron
diffraction pattern indexed to the zirconolite-4M structure. Streaked
reflections are identified by left-pointing green arrows.</p><p>(a) Dark-field TEM micrograph of the x = 0.3 sample,
showing the zirconolite-4M structure, with the electron beam positioned
down the [110] zone axis and the objective aperture over the diffuse
reflection indicated by the arrow in (b) a [110] zone-axis electron
diffraction pattern indexed to the zirconolite-4M structure.</p><p>Zone-axis electron diffraction patterns of the x = 0.6 sample with the electron beam positioned down the
(a) [111]
and (b) [211] zone axes. Both patterns are indexed to the pyrochlore Fd3̅m structure.</p><!><p>Dy L3-edge X-ray absorption near
edge structure (XANES) spectra were collected for zirconolite-2M,
zirconolite-4M, and cubic pyrochlore (corresponding to x = 0.10, 0.30, and 0.60, herein referred to as Dy-titanates for clarity)
alongside Dy2O3, Dy2TiO5, and Dy2Ti2O7 reference compounds,
containing Dy3+ in 6-, 7-, and 8-fold coordination, respectively
(Figure 9). Experimental
XANES spectra at the Dy L3-edge of all reference compounds
and Dy-titanates were characterized by three distinct features (labeled
as A, B, and C in Figure 9 and Table S2). Primarily, the white line crest (feature A) was composed of a single intense feature for all compounds
at the overlapping edge position of 7792.5 ± 0.3 eV. The major
contribution to this feature arises from dipole allowed 2p3/2 → 5d3/2 electronic transitions.43 Theoretically, absorption spectra at the Dy L3-edge also comprise a weak pre-edge feature; however, this cannot
be resolved by conventional XAS due to 2p core-hole lifetime broadening.
Nevertheless, this feature can be observed with complementary techniques
such as resonant inelastic scattering spectroscopy (RIXS).44 Second, a weak yet discernible feature (labeled
as B in Figure 9 and Table S2) was also clearly
distinguished (we note that this was most prominent in Dy2Ti2O7, Dy2O3, and the
sample corresponding to x = 0.60). Finally, a post-edge
resonance peak (feature C in Figure 9 and Table S2)
was observed for all compounds, with maxima between 7830.8 and 7831.8
eV.</p><!><p>XANES data collected for x = 0.10, x = 0.30, and x = 0.60, alongside Dy2O3, Dy2TiO5, and Dy2Ti2O7 reference compounds, with features A, B, and C labeled. Insets on the right
show enlargements of features B and C.</p><!><p>There were several qualitative trends noted in
the XANES spectra
of the reference compounds and Dy-titanates. Primarily, it was clear
that the edge position (7792.5 ± 0.3 eV) and energy position
of feature A were very similar for all reference compounds
and Dy-titanates, indicating that all samples contained Dy in the
same oxidation state. As the reference compounds all contained Dy
uniformly as Dy3+, it was therefore considered that Dy
entered the zirconolite-pyrochlore solid solution entirely as Dy3+. Further confidence in Dy oxidation state assignment was
informed from bond valence sum analyses, the results of which are
summarized in Table 3. The speciation of Dy3+ is encouraging, as this remains
comparable to Am3+; previously, the synthesis of Am2Ti2O7 by calcination in air between
1200 and 1300 °C has been shown to result in the complete reduction
of Am4+ to Am3+.45 Feature B presented a variation that may be dependent
on the coordination of Dy3+ cations. This is evidenced
by the reference compounds (Dy2TiO5 and Dy2O3) having different energy maxima (∼7812.4
eV) compared to the Dy-titanates x = 0.10, 0.30,
0.60, and 1.00 (Dy2Ti2O7) (∼7815.9
eV) (Figure 9 and Table S2). This shift in maxima position could
be attributed to an increase in O coordination of the Dy3+ atoms, as the feature maximum for the 6-fold Dy2O3 is lower than that of the 8-fold Dy2Ti2O7; however, a more comprehensive systematic analysis
of L1- and L2-edges would be needed to confirm
this trend. Moreover, the intensity of feature B present
in Dy-titanates was also observed to vary as a function of Dy concentration
and thus changing structure type. This can clearly be seen when comparing
feature B intensity between zirconolite-2M (i.e., x = 0.10) in which Dy3+ was targeted
equimolar across both the 8- and 7-fold sites, and pyrochlore (i.e., x = 0.60) in which Dy3+ cations occupy only one 8-fold coordinated site. This is qualitative
evidence that suggests that there is an agreement between the targeted
self-balancing charge substitution scheme for zirconolite and Dy3+ being split between two crystallographically distinct sites
at a lower concentration. Qualitative trends were also observed for
feature C, in which a decrease in maxima (7830.8 eV)
intensity was observed in correlation with increased Dy doping of
the Dy-titanates. Additionally, a shift in maxima position of ∼0.9
eV is seen between the Dy-titanate samples and the reference compounds
(Dy2O3 and Dy2TiO5) possibly
indicating a change in Dy coordination environment. A similar coordination-related
energy shift in this feature has also been noted in the Dy and Sm
L3-edges for other complex materials and has been proposed
as the result of increased average Ln–O bond distance.43,46 This qualitative trend is broadly consistent with EXAFS analyses
(discussed below) whereby a slight decrease in the average Dy–O
bonds was observed with increasing Dy concentration (i.e., when Dy was modeled as the absorbing atom in zirconolite-2M, 4M,
and pyrochlore).</p><p>Fitting
of the EXAFS region provided insight into the local structure
of Dy3+ including the immediate coordination environment
and structure over a range of up to ∼4.5 Å from the central
Dy atom (Figure 10; Table 4). Analysis
of the Dy2Ti2O7 reference compound
(i.e., x = 1.00) produced a good
fit (R-factor = 0.0155) that consisted of 2 O atoms
at 2.19(2) Å, 6 O atoms at 2.45(2) Å, 6 Ti atoms at 3.57(1)
Å, 6 Dy atoms at 3.58(1) Å, 12 O atoms at 3.92(5) Å,
6 O atoms at 4.51 Å, and an O–Ti multiple scattering pathway
at 4.79 Å with a degeneracy of 24. This model is in excellent
agreement with the expected Dy2Ti2O7 structure as previously determined by single-crystal X-ray diffraction.34 Fitting of the Dy2Ti2O7 reference compound informed the fitting parameters of the x = 0.60 compound, as from XRD and TEM analyses, this was
confirmed to also adopt the cubic pyrochlore structure, despite partial
occupancy of Ca and Zr on the A site. An excellent fit (R-factor = 0.0076) to the data was produced with a similar model to
the Dy2Ti2O7 standard, albeit with
a lower coordination of Dy3+ atoms and fewer long-range
order shells fitted. The best-fit model consisted of 2 O atoms at
2.21(2) Å, 6 O atoms at 2.43(2) Å, 6 Ti atoms at 3.53(1)
Å, 3.6 Dy atoms at 3.59(2) Å, and 12 O atoms at 3.93(5)
Å. The number of second shell Dy···Dy paths was
reduced from 6 to 3.6 (x = 1.00 and 0.60, respectively),
which is in line with the reduced concentration of Dy in the x = 0.60 sample relative to the x = 1.00
(Dy2Ti2O7) compound. As the coordination
shell only contains 3.6 Dy, this leaves a remaining degeneracy of
2.4 that is made up of Ca and Zr; however, attempts to fit these atoms
into the EXAFS fits proved unsuccessful, with the addition resulting
in significantly worse fits (see Supporting Information for further details). This may be due to the limited data obtained
of the samples (k-range could only be fit to 12),
and therefore, a much higher k-range may be needed to deconvolute
the Ti, Dy, Ca, and Zr that all manifest at a similar point in the
EXAFS spectrum. Indeed, such limitations have been observed previously
in different systems with different elements (U in iron (oxyhydr)oxides);
however, the need for a high k-range, and the addition
of molecular dynamics modeling, to deconvolute many overlapping shells
in the EXAFS has been clearly shown.47,48</p><!><p>Dy L3 edge XAS spectra for Dy2Ti2O7 (i.e., x = 1.0), x = 0.6, x = 0.3, and x = 0.1 samples
(where x refers to the structure Ca1–xZr1–xDy2xTi2O7). Left: k3-weighted EXAFS. Right: Fourier transform of k3-weighted EXAFS, using a Hanning window function.
Black lines are data, and red lines are the best modeled fits for
the data.</p><p>The amplitude
reduction factor (S02) for
all samples was 0.95; N is the degeneracy; σ2 is the Debye–Waller
factor; R is the interatomic distance; α is
the result of the F-test indicating the confidence that adding the
path improves the fit (>67% is equal to 1σ and >95 % is
equal
to 2σ in terms of standard deviation).</p><p>Indicates that the path was parameterized
using O1 and Ti1 parameters.</p><p>Indicates that the parameters were
linked.</p><p>Indicates that the
parameters were
linked. The general formula relating to "x" is Ca1–xZr1–x,Dy2xTi2O7.</p><!><p>Fitting of the
EXAFS for the x = 0.1 sample to
a zirconolite-2M model produced a good fit (R-factor
= 0.0040) with the model consisting of 4 O atoms at 2.28(1) Å,
4 O atoms at 2.43(1) Å, 2 Ti atoms at 3.33(3) Å, 4 Ti atoms
at 3.52(2) Å, 1 Zr at 3.58(10) Å, and 12 O at 4.13 Å.
This is in agreement with the expected zirconolite-2M structure. The x = 0.3 sample proved the most challenging to fit, due to
the composition adopting the complex zirconolite-4M structure. The
zirconolite-4M structure is composed of both pyrochlore and zirconolite-2M
structural units, resulting in a system of mixed coordination environments
for the Dy3+ ions. Consequently, a fit that represented
a mixture of the zirconolite-2M and pyrochlore structure was attempted,
using a pyrochlore CIF file as the basis. A good fit was obtained
(R-factor = 0.0028) which consisted of 5 O atoms
at 2.29(5) Å, 3 O atoms at 2.41(2) Å, 6 Ti atoms at 3.49(1)
Å, 0.6 Dy atoms at 3.59(4) Å, and 8 O atoms at 4.05(4) Å.
The reduced occupancy of the distal O shell (8 instead of 12) is likely
representative of the lower degree of long-range order in the structure.
The degeneracy of the Dy shell is concordant with expected occupancy
of the zirconolite-2M structure. As with the previous samples and
as discussed above, the Ca and Zr atoms could not be fit due to the
limited k-range obtained.</p><p>While EXAFS fitting
suggests that the Dy3+ cations are
8-fold coordinated by O atoms in all compositions, this coordination
environment is only expected to be the sole environment for the Dy2Ti2O7 pyrochlore reference compound.
Upon doping the zirconolite-2M structure with Dy3+, it
is expected that the Dy atoms will occupy the Ca (8-fold) and Zr (7-fold)
sites, with a possible preference for the Ca site on the basis of
compatible ionic radii (Ca2+ = 1.12 Å and Dy3+ = 1.027 Å in 8-fold coordination).49 In the x = 0.1 sample, an even split of 4 O atoms
at 2.28(1) Å (Zr site) and 4 O atoms 2.43(1) Å (Ca site)
may suggest that Dy is equally distributed across the Ca (8-fold)
and Zr (7-fold) sites. Indeed, altering the total occupancy of the
first O shell to 7.5 (expected in a system with a 50:50 split of 7-
and 8-fold sites) by setting the degeneracy of the aforementioned
O atoms to 3.75 each produces an equally valid fit, albeit with a
slightly worsened R-factor of 0.0073, with a similar effect being
observed in the x = 0.3 composition (increased R-factor of 0.0032). This suggests that Dy3+ may
be doped in equal amounts across the Ca and Zr sites, in good agreement
with the targeted self-charge balancing substitution regime, and moreover,
the qualitative observations of features B and C in the XANES region.</p><!><p>Several systematic
CaZrTi2O7–REE3+Ti2O7 solid solutions have been reported
in the wider literature, the published phase fields from which are
summarized in Table 5 and Figure 11, with
descriptions below. A comparison of the ionic radii of lanthanides
and minor actinides is provided in Table S1. The radius ratio necessary to stabilize the cubic pyrochlore structure
in A2B2O7 ceramics is clearly demonstrated
by varying the size of the A3+ site, in agreement with
the model proposed by McCauley.50 The bracketed
areas displayed along the x-axis in Figure 11 represent the dominant phase present at
a given substitution value.</p><!><p>Visual summary of phase fields reported in
CaZrTi2O7–REE3+Ti2O7 solid
solutions with nominal formulation Ca1–xZr1–xREE2xTi2O7 (Key: 2M = zirconolite-2M; 4M =
zirconolite-4M; Py = cubic pyrochlore; NT = Nd2Ti2O7 (monoclinic double-layered perovskite, space group P21)).</p><p>Key: 2M = zirconolite-2M;
4M = zirconolite-4M;
Py = cubic pyrochlore; Pe = perovskite; NT = Nd2Ti2O7 (monoclinic double-layered perovskite, space
group P21).</p><p>Indicates phase purity.</p><!><p>Gd3+: Zhang et
al. synthesized
the Ca1–xZr1–xGd2xTi2O7 solid solution (0.0 ≤ x ≤
1.0) by oxide synthesis, with bulk pellets sintered at 1400 °C
for 48 h.25 The zirconolite-2M phase was
present over the compositional range 0.0 ≤ x ≤ 0.4, while the 4M polytype was stabilized between 0.2 ≤ x ≤ 0.5. The cubic pyrochlore phase was formed in
compositions for which x ≥ 0.4, and present
as a single phase above x = 0.6. These phase fields
are near-identical to those reported in the present work, which is
unsurprising given the relative ionic radii of Gd3+ and
Dy3+ cations (1.053 and 1.027 Å, respectively).</p><p>Sm3+: Similar phase fields
were reported by Jafar et al. in the Ca1–xZr1–xSm2xTi2O7 (0.0 ≤ x ≤ 1.0) system, fabricated by solid-state reaction at 1300
°C (24 h).28 Zirconolite-2M was found
to coexist alongside a minor perovskite phase when targeting x = 0.10, with mixtures of zirconolite-2M, 4M, and/or perovskite
formed in the compositional range 0.20 ≤ x ≤ 0.35. Cubic pyrochlore and zirconolite-4M were reported
for x = 0.40 and 0.50, with single-phase pyrochlore
observed beyond x ≥ 0.60.</p><p>Y3+: Jafar et al. also synthesized
corresponding Ca1–xZr1–xY2xTi2O7 compositions with a sintering temperature of 1300 °C.26 No perovskite phases were identified at any
interval, with single phase 2M and 4M formed at x = 0.10 and 0.40, respectively, with the intermediate compositions
composed of a mixture of polytypes. At x = 0.60,
zirconolite-4M was found to coexist with pyrochlore; however, cubic
pyrochlore was stabilized as a single phase beyond x ≥ 0.70.</p><p>Nd3+:
The phase fields observed
when doping Nd3+ into the structure were observed to deviate
from the conventional 2M → 4M → Py phase evolution,
as zirconolite-2M, 4M, or cubic pyrochlore was not formed as a single
phase at any compositional interval.51 Rather,
co-mixtures of 2M, 4M, and/or perovskite were present between 0.10
≤ x ≤ 0.40, progressing to mixtures
of cubic pyrochlore and perovskite between 0.50 ≤ x ≤ 0.70. As the end-member Nd2Ti2O7 structure is not capable of adopting the cubic pyrochlore
structure on the basis of ionic radii (Nd3+ = 1.109 Å),
a monoclinic double-layered perovskite structure was preferentially
formed beyond x = 0.90.</p><p>Ce3+: Meng et al. synthesized
the corresponding Ce solid solution; however, as Ce is known to adopt
both Ce3+ and Ce4+ oxidation states in zirconolite,
the targeted composition allowed for nonideal oxygen stoichiometry,
and thus denoted as Ca1–xZr1–xCe2xTi2O7+δ.4 Moreover,
Ce 3d XPS data confirmed that Ce was consistently distributed as ∼50%
Ce3+, inferring that the excess positive charge could be
compensated by cation vacancies. Unfortunately, the solid solution
was not progressed beyond x = 0.40, but, given the
relatively large radius of Ce3+ in comparison to other
REE3+ cations, the formation of single-phase zirconolite-4M
was detected at a lower concentration, corresponding to both x = 0.20 and 0.30. When targeting x = 0.40,
a co-mixture of pyrochlore and perovskite was formed.</p><!><p>Zirconolite-structured materials are a
candidate wasteform for
the immobilization of Pu and other highly radioactive minor actinide
species that may be derived from future advanced reprocessing cycles
for spent nuclear fuel. To this end, the novel Ca1–xZr1–xDy2xTi2O7 solid solution was fabricated
by a conventional solid-state route, with Dy3+ deployed
as an inactive surrogate cation to replicate the partitioning behavior
of minor actinides such as Am3+. XRD, SEM, TEM-ED, and
XAS techniques were used to characterize a series of distinct phase
transformations, with Dy3+ cations fully immobilized in
the zirconolite-2M phase at a concentration corresponding to x = 0.10, followed by progressive mixtures of zirconolite-2M,
4M, and/or pyrochlore in the compositional interval 0.20 ≤ x ≤ 0.50. Increasing the nominal Dy3+ concentration
beyond x = 0.60 resulted in the formation of single-phase
pyrochlore, successfully forming the end-member Dy2Ti2O7. Analyses of the Dy L3 XANES and
EXAFS regions confirm uniform Dy3+ speciation, consistent
with previously observed Am3+ under similar processing
conditions, and determine that the coordination environment of Dy
cations was consistent with occupation in zirconolite-2M, zirconolite-4M,
and pyrochlore-structures when targeting x = 0.10,
0.30, and 0.60, respectively. Given the exceptional radiation stability
and chemical durability of zirconolite and pyrochlore solid solutions,
it is expected that trivalent minor actinide species could be successfully
accommodated in solid solution at any compositional interval in the
CaZrTi2O7–Dy2Ti2O7 system.</p><!><p>Deconvolution
of Raman spectrum obtained for Dy2Ti2O7 (Figure S1); deconvoluted EXAFS
spectra for all samples (Figure S2); ionic radii for relevant lanthanides
and minor actinides (Table S1); XANES features from Figure 9 and corresponding energies
(Table S2); and EXAFS fitting parameters for a range of fitting models
(Table S3) (PDF)</p><p>ic1c03816_si_001.pdf</p><p>The authors
declare no
competing financial interest.</p>