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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

Tailored Perovskite Waste Forms for Plutonium Trapping Vanessa Proust, Renaud Jeannin, Frankie D. White, and Thomas E. Albrecht-Schmitt* Department of Chemistry and Biochemistry, Florida State University, Tallahassee, Florida 32306, United States

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ABSTRACT: Perovskite ceramics have been extensively studied as host matrixes for radionuclide entrapment for nuclear waste disposal. As an expansion of these investigations, cerium, neodymium, and plutonium were incorporated into a perovskite phase, ACu3FeTi3O12 (A = Nd, Ce, Pu), using sol−gel methods under oxidizing and reducing atmospheres. The targeted materials contained varying levels of Ce3+ and Nd3+ on the A site, yielding potential compositions of Nd1−xCexCu3FeTi3O12 (x = 0, 0.1, 0.2, 0.3, 0.4, 0.8). However, interrogation of these materials shows that the maximum Ce3+ loading is achieved near x ≈ 0.2. A single composition with plutonium was targeted, Nd0.9Pu0.1Cu3FeTi3O12, in order to properly model more realistic loading levels for a repositorydestined material. These compounds were characterized using powder X-ray diffraction with Rietveld refinements of the structures and by a variety of spectroscopic techniques. The data suggest that, in order to achieve Pu3+ substitution onto the A sites in the Nd0.9Pu0.1Cu3FeTi3O12, a reducing atmosphere must be employed. Otherwise, the redox activity of plutonium results in substitution onto multiple sites in the material as well as the formation of secondary phases such as TiO2.

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and the first two sites are available for Ln3+ and An3+/4+ substitution.21−24 Hollandite, with the general formula AxByC8−yO16, is an especially good candidate for incorporating 90 2+ Sr and 135,137Cs+ on the A site.14,25 Monazites can accommodate almost all actinide cations, typically in the 3+ state, and are among the most resilient materials known, especially with respect to high radiation tolerance.20,26,27 Finally, pyrochlores appear to be the least sensitive ceramic to processing conditions in terms of deleterious redox reactions during radionuclide incorporation.28−36 As a part of demonstrating the utility of plutonium immobilization in these ceramics, Yamazaki et al. showed that the substitution of Pu3+ on the A sites of the zirconatetype pyrochlore (La1−xPux)2Zr2O7+y (x = 0−1) was possible, but the measured solubility limit was 10 mol %, which is well within the incorporation range needed for a viable waste form because loads above ∼5% lead to the potential risk of criticality in a repository.33 However, recent studies have shown contrasting results that yielded the possibility that plutonium might be incorporating onto both A and B sites because of partial oxidation of Pu3+ to Pu4+,37−41 depending on whether oxidizing or reducing conditions are employed during materials preparation.42−44 Typically the goal is to avoid multiple site substitutions, because in many cases it is the resilience of the transition-metal oxide network that is providing stability with respect to radiolytic and environmental degradation. Finally, perovskites are among the most widely investigated phases for immobilizing actinides.13,45−63 Owing to the high coordination number of the A site, it can be occupied by large monovalent, divalent, trivalent, and tetravalent cations.

INTRODUCTION Nuclear energy is the only currently viable method of producing large amounts of electricity with a small carbon footprint irrespective of geographic location. However, this energy production has led to the generation of >90000 tons of used nuclear fuel in the U.S. alone. While some of this fuel can be recycled two or three times, resilient materials are ultimately needed that capture radionuclides for disposition in repositories, especially for plutonium.1−6 A large variety of materials have been proposed for this purpose, chiefly borosilicate and borophosphate glasses.7−12 However, an issue with utilizing vitrified waste forms for plutonium is that it has poor solubility in these glasses and tends to precipitate as PuO2 inclusions, thus weakening the waste form itself and, more importantly, not becoming an integral component of the glass.7−12 One solution to plutonium storage is its incorporation into ceramic materials that are known on the basis of mineral analogues to be capable of retaining radionuclides in the long term.5,13−16 Ceramic materials have the additional advantage of tenaciously retaining an array of cations with widely varying ionic radii and valences within their lattices.17,18 For immobilization of radiolanthanides and actinides, singlephase and stable polyphase ceramics have been considered as tailor-made hosts and are important alternatives to the use of glass matrices for containment. These ceramic materials include zirconolite (CaZrTi2O7), hollandite (BaAl2Ti6O16), monazite (LnPO4; Ln = La to Gd), pyrochlore (A2B2O7), and members of the perovskite family (CaTiO3), as well as many others.18−20 Each of these material types have different advantages. In the case of zirconolite (CaZrTi2O7), Ca2+, Zr4+, and Ti4+ cations are present on 8-, 7-, and 6-fold coordination sites, respectively, © XXXX American Chemical Society

Received: October 12, 2018

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DOI: 10.1021/acs.inorgchem.8b02832 Inorg. Chem. XXXX, XXX, XXX−XXX

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The morphological observations, as well as semiquantitative elemental analysis of the samples, were performed using a scanning electron microscope coupled with an energy-dispersive X-ray spectrometer (FEI Nova nanoSEM 400 microscope with an Oxford INCA x-sight EDS detector). The powder samples were placed on a double-sided adhesive carbon in aluminum holders.

However, the B site is preferentially occupied by smaller sixcoordinate cations as occurs with Mn3+/4+ and Ti4+ in CaCu3Mn4O12, ThCu3(Mn23+/Mn24+)O12, and Bi2/3Cu3Ti4O12.46,51,56 In these phases the Ca2+, Th4+, and Bi3+ cations are present on the A sites. Thus, these ceramics have been extensively probed for their ability to accommodate a variety of radionuclides on both the A and B sites.14,63,64 Herein, we expand on this line of inquiry by probing the substitution of Cen+ and Pun+ (n = 3, 4) into the perovskite family of materials with the composition ACu3FeTi3O12 (A = Nd, Ce, Pu) using sol−gel methods and both oxidizing and reducing processing conditions.

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RESULTS AND DISCUSSION Synthesis, Structural Analysis, and Thermal Stability of ACu3FeTi3O12 (A = Ce, Nd). Powder X-ray diffraction patterns from NdCu3FeTi3O12 samples were collected from 700 to 1200 °C in 100 °C intervals in air and compared with room-temperature diffraction data, as shown in Figure 1 and

EXPERIMENTAL SECTION

Caution! 242Pu (t1/2= 3.733 × 105 y) presents potential health risks owing to its α and γ radiation. All studies were conducted in a Category II Nuclear Hazard Facility. All experiments were carried out with approved safety operating procedures. Synthesis. A nitrate sol−gel process was used to synthesize NdCu3FeTi3O12, Ce2/3Cu3FeTi3O12, Ce/Pu-doped NdCu3FeTi3O12, and PuCu3FeTi3O12. Nd(NO3)3·6H2O (99.9%, Alfa Aesar), Ce(NO3)3·6H2O (99.9%, Matheson Coleman and Bell), Cu(NO3)2· 2.5H2O (98.2%, Fisher), Fe(NO3)3·9H2O (99.5%, Fisher), Ti(OCH(CH 3 ) 2 ) 4 (99.999%, Sigma-Aldrich), 2-methoxyethanol (CH3OCH2CH2OH, anhydrous, 99.8%, Sigma-Aldrich), and acetylacetone (CH3COCH2COCH3, >99%, Sigma-Aldrich), were used as starting materials as received from the manufacturer. The synthesis was carried out in two steps by the addition of a titanium isopropoxide in a mixed solution of 2-methoxyethanol and acetylacetone with a molar ratio (Ti:2-methoxyethanol:acetylacetone) of 1:6.5:1.5 into a second solution containing the nitrate of Nd, Ce, Cu, Fe, or Pu and 2-methoxyethanol according to the molar ratio (Ti:2-methoxyethanol) of 1:19 and adjusted to meet the final stoichiometry of the NdCu3FeTi3O12, Nd0.33Ce0.66Cu3FeTi3O12, Ce/ Pu-doped NdCu3FeTi3O12, and PuCu3FeTi3O12 samples. A brown gel with a 0.1 M lanthanide concentration was obtained after stirring for 7 h with gradual heating from 23 to 40 °C. The final powder samples were obtained by heat treatment of the gel from 700 to 1200 °C in air and/or argon−5% H2 reducing atmosphere in an alumina crucible. The plutonium(III) nitrate was prepared by dissolution of 242PuO2 (50 mg) with concentrated nitric acid and a few drops of hydrofluoric acid to aid in dissolution under fuming conditions under N2. The solution was evaporated to a residue and dissolved with 1 M nitric acid with the addition of 0.5 M hydroxylamine hydrochloride to ensure the presence of only PuIII. This procedure was repeated three times.65 The final samples were stored in a glovebox to avoid oxidation. The PuIII oxidation state was confirmed by solution UV− vis−NIR absorption spectroscopy. X-ray diffraction (XRD) patterns were obtained using a PANalytical X’Pert Pro diffractometer or a Siemens D500 θ−2θ goniometer using Cu Kα radiation. The samples were prepared with a back-loading technique to obtain a flat surface for phase analysis in the Bragg−Brentano geometry. On the basis of 2θ diffraction angles in the range from 10 to 100° with increments of 0.033 and 0.016°, the peak positions and the phase identification obtained were characterized and compared with the Joint Committee on Powder Diffraction Standards (JCPDS) database. The structures were also refined using Rietveld methods with the Bail fit66 using X’Pert highscore plus software. X-ray photoelectron spectroscopy (XPS) was carried out using a PHI 5100 series instrument from PHI Inc. The instrument is equipped with an unmonochromated Mg Kα source (1253.6 eV) and Al Kα source (1486.6 eV), and the pass energy of the analyzer was fixed at 89.45 eV. The photoelectron takeoff angle was 45° relative to the sample surface and 0°. The samples were deposited onto graphitecoated stubs. The spectra were collected by 15−150 sweeps without voltage. Shirley background was subtracted from all spectra before performing peak fitting to a Gaussian−Lorentzian function.

Figure 1. Powder X-ray diffraction patterns of NdCu3FeTi3O12 at room temperature after sintering at 1000 and 1200 °C in air.

Figure S1. The data obtained at 700 °C show a poorly crystallized material and the presence of several different phases (see Figure S1). However, further heating to 800 °C leads to a color change of the material from light gray to chocolate brown, and the diffraction pattern changes to reveal significantly improved crystallinity and data that are consistent with NdCu3FeTi3O12 present as a single phase (ICSD file No. 200954). Continued heating to 1200 °C leads to narrowing of the line widths and slight shifts of the diffraction lines to lower angles as the material continues to improve in crystallinity, mosaicity decreases, and defects are annealed. However, heating beyond 1200 °C leads to loss of crystallinity, and only amorphous material is present by 1300 °C when the sample is heated in air. The lattice parameters of NdCu3FeTi3O12 have been refined by least-squares methods using the pseudo-Voigt function. The results of these Rietveld refinements are provided in Table 1 and compared with literature values. Good agreement between known values and those obtained from these refinements are found.50,53,59 Further characterization of the elemental composition using semiquantitative EDS methods yields a composition of Nd1.0Cu3.2Fe1.1Ti3.1O12.3 from material heated to 900 °C with estimated esd’s of 5% on the metal content (see Table S1) and much lower precision on the oxygen content. NdCu3FeTi3O12 crystallizes in the body-centered, cubic space group Im3̅m (or Im3̅)50 and like all perovskite-like compounds can be described as a three-dimensional framework created by corner sharing of MO6 (M = Fe, Ti) B

DOI: 10.1021/acs.inorgchem.8b02832 Inorg. Chem. XXXX, XXX, XXX−XXX

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also see Table 1 and Table S1), the thermal stability window of this material is quite narrow, and at both higher and lower temperatures (800 and 1000 °C), diffraction peaks corresponding to CeO2, CuO, and Fe2TiO5 are all apparent, and by 1100 °C the measurements indicate that the material has devolved primarily into a mixture of CeO2 and poorly crystallized perovskite-like compounds. On the basis of these data, it is apparent that Ce2/3Cu3FeTi3O12 is less stable than NdCu3FeTi3O12.54,62,66 This raises the question of whether a more stable material can be obtained by doping Ce3+ into NdCu3FeTi3O12, and thus attempts were made to prepare Nd1−xCexCu3FeTi3O12 (x = 0.1, 0.2, 0.3, 0.4, 0.8). The PXRD patterns of these compounds are provided in Figure S3. These data indicate that the prepared samples are highly crystalline with the expected structure type, and all peaks were indexed and corresponded to the XRD pattern expected for the NdCu3FeTi3O12 structure type. Trace impurities from cubic CeO2 are observed at the highest doping levels of 0.4 and 0.8. Thus, it appears that incorporation of Ce3+ into NdCu3FeTi3O12 maximizes near x = 0.4 and that additional cerium leads to the formation of CeO2. No other oxides of neodymium, titanium, or iron were identified in these samples. The replacement of Nd3+ (1.27 Å) with the larger Ce3+ (1.34 Å) cation creates additional expected changes in the lattice.66 Figure 3 shows noticeable shifts in the diffraction lines to lower angles as the cerium content is increased, as would be expected for expanding unit cells. This is an important result, because under these conditions the cerium is capable of oxidizing to Ce 4+ and the possibility of incorporation of this cation onto the B sites might then be possible. In principle this is less likely because Ce4+ prefers coordination numbers higher than 6, but six-coordinate Ce4+ is certainly known. However, the ionic radius of Ce4+ is much smaller than that of the trivalent cations with an ionic radius of 1.14 Å,66 and the shift to a smaller volume cell would instead be observed. As a cautionary note, careful examination of the (220) line near 2θ ≈ 34° provides evidence that at x = 0.1, 0.2 a single phase is likely present, as shown in Figure S4. However, at x = 0.3 two peaks in this region are observed, where the higher angle peak is likely indicative of NdCu 3 FeTi 3 O 12 being present with little to no Ce 3+ incorporation. These data suggest that the Ce3+ incorporation

Table 1. Summary of Rietveld Refinements for NdCu3FeTi3O12 (1) and Ce2/3Cu3FeTi3O12 (2) a (Å) V (Å3) space group Rwp Rexp Rp GOF

2a

1b

1c

2c

7.401 405.33 Im3̅m

7.436 411.17 Im3̅m

7.4282(1) 409.87(2) Im3̅m 5.62 5.37 6.60 1.10

7.4096(2) 406.81(4) Im3̅m 4.47 4.44 4.21 1.13

a

References 53 and 59. bReference 50. cThis study.

octahedra. The formula can be rewritten as (Nd,Cu3)(Fe,Ti3)O12 to recapture a quadrupling of the original ABO3 formula. In this structure type the A sites are occupied by Nd3+ and the B sites by Fe3+ and Ti4+. The grouping of Cu2+ with Nd3+ suggests that the Cu2+ ions are in icosahedral sites. However, while there are 12 oxygen atoms around each Cu2+ cation, and thus it is on an A site, only 4 of these contacts would constitute Cu−O bonds, and the environment is a classical Jahn−Tellerdistorted, square-planar Cu2+ coordination environment with long axial contacts as shown in Figure 2.50 The use of these more compositionally and structurally complex perovskites is thought to increase radiation-damage resistance. When ionic radii mismatches are present in these materials or nonideal processing techniques are employed, one of the primary mechanisms of strain reduction is the formation of vacancies on the A sites, often yielding materials with composition A2/3Cu3M4O12.56,62 Attempts to intentionally prepare Nd2/3Cu3FeTi3O12 or Nd2/3Cu3Ti4O12 via sol−gel methods failed to yield a single-phase material, and substantial amounts of Fe2TiO5 and other oxides were identified from PXRD studies. However, replacement of Nd3+ with the larger Ce 3 + cations does allow for the preparation of Ce2/3Cu3FeTi3O12 as a single phase (within PXRD detection limits). However, attempts to prepare either the stoichiometric phase, CeCu3FeTi3O12, or a phase with lower levels of Ce3+, such as Ce1/2Cu3FeTi3O12, all yielded non-negligible amounts of impurities (see Figure S2). Although the PXRD pattern obtained from burnt amber Ce2/3Cu3FeTi3O12 obtained at 900 °C is indicative of a relatively pure material (referenced to ICSD file No. 238896;

Figure 2. Views of the crystal structure of ACu3FeTi3O12 (A = Ce, Nd, Pu) depicted in polyhedral (a) and ball-and-stick (b) representations. The icosahedral A sites shown in orange are occupied by Ln3+ or Pu3+. The octahedral B sites, shown in blue, contain statistically distributed Fe3+ and Ti4+ cations. Jahn−Teller-distorted Cu2+ cations located on A sites are best described as square planar but include longer Cu···O contacts in this figure. C

DOI: 10.1021/acs.inorgchem.8b02832 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 4. X-ray powder diffraction patterns of NdCu3FeTi3O12 (bottom), Nd0.9Pu0.1Cu3FeTi3O12 prepared in air (middle), and Nd0.9Pu0.1Cu3FeTi3O12 prepared under Ar and 5% H2 (top) at 900 °C.

Figure 3. SE M i ma ges o f (a) NdCu 3 FeTi 3 O 1 2 , (b ) Nd 0.9 Ce 0.1 Cu 3 FeTi 3 O 12 , (c) Nd 0.7 Ce 0.3 Cu 3 FeTi 3 O 12 , and (d) Ce2/3Cu3FeTi3O12.

more homogeneous material. As was observed with cerium, attempts to prepare Pu2/3Cu3FeTi3O12 result in mixtures of compounds. XPS Studies of NdCu3FeTi3O12 and Ce2/3Cu3FeTi3O12. XPS wide-scan spectra collected from samples of NdCu3FeTi3O12 and Ce2/3Cu3FeTi3O12 are provided in Figure S6. The individual peaks for each element were fit and confirm the presence of Nd3+, Cu2+, Fe3+, and Ti4+. However, the primary goal of these studies was to determine if cerium undergoes Ce3+ → Ce4+ oxidation during materials preparation as a way of modeling potential Pu3+ → Pu4+ oxidation that was noted to occur during the preparation of Nd0.9Pu0.1Cu3FeTi3O12 and Pu2/3Cu3FeTi3O12 even under a reducing atmosphere. The Ce(3d) XPS profile of Ce2/3Cu3FeTi3O12, as shown in Figure 5, consists of two 3d3/2−3d5/2 spin−orbit-split doublets. The highest binding energy peaks at 884.9 and 903 eV result from the Ce 3d94f1−O 2p6 final state, and the lowest binding energy peaks at 881.5 and 898.9 eV are consistent with Ce 3d94f2−O 2p5.68−70 The

level is overestimated in the previous data and that the maximum loading is near 20%. The SEM images of Nd 1 − x C e x C u 3 F e T i 3 O 1 2 ( x = 0 .1 , 0. 2, 0 .3 ) an d Ce2/3Cu3FeTi3O12 are provided in Figure 3 and show that particles synthesized by this sol−gel method occur as relatively homogeneous, but irregular, aggregates approximately 2 μm in size with a slight increase in grain size as the cerium content is increased. Incorporation of Plutonium into NdCu3FeTi3O12. The goal of the above studies was to incorporate two different lanthanides into a single phase to demonstrate viability prior to attempts at plutonium incorporation. The choice of both Ce3+ and Nd3+, as opposed to other lanthanides, was quite intentional because it has been shown that the ionic radius of Nd3+ most closely matches that of Pu3+.67 However, unlike Nd3+, plutonium is redox active, and thus the incorporation of a second lanthanide that is capable of undergoing oxidation is also useful. While plutonium starting materials with plutonium in several oxidation states were explored, only the reactions that started with Pu3+ yielded tractable data. A material with a composition of Nd0.9Pu0.1Cu3FeTi3O12 was targeted because the maximum Ce3+ loading was near 20% and even loadings of this amount are not realistic in repository scenarios as previously mentioned. The synthesis of this material was explored by heating in air, where plutonium oxidation is likely, and heating under a reducing atmosphere, as was previously performed in the cerium substitution studies. The PXRD patterns of Nd0.9Pu0.1Cu3FeTi3O12 prepared under these conditions are shown in Figure 4 and compared with the calculated pattern of NdCu3FeTi3O12. Regardless of the synthetic conditions employed, Nd0.9Pu0.1Cu3FeTi3O12 forms alone with minor CuO and PuO2 impurities. The difference between the two samples is that, when oxidizing conditions are employed, low levels of TiO2 are also observed and there is broadening of the (220) peak at 2θ ≈ 34° as shown in Figure S5. These latter data are suggestive of partial oxidation to PuIV, incorporation into the B site where Fe/Ti reside, and extrusion of TiO2. The material prepared under reducing conditions does not reveal the presence of TiO2 and possesses narrower diffraction lines that are all indicative of a

Figure 5. Ce(3d) XPS spectrum and fitting of Ce2/3Cu3FeTi3O12. D

DOI: 10.1021/acs.inorgchem.8b02832 Inorg. Chem. XXXX, XXX, XXX−XXX

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Doping of Ce3+ onto the Nd3+ sites can be achieved with a maximum loading near 20% cerium content. Additional Ce3+ is not incorporated into the materials and instead leads to the formation of CeO 2 even under reducing conditions. Nd0.9Pu0.1Cu3FeTi3O12 has been prepared by similar methods. While the formation of a PuO2 impurity is observed regardless of whether heating is performed in air or under a reducing atmosphere, reducing conditions improves the crystallinity of the final product. More importantly, under oxidizing conditions TiO2 forms during heating likely because of oxidation of Pu3+ to Pu4+ and doping onto the B sites into the material, which is undesirable.

presence of the low-energy peak at 916.2 eV and contribution in the shape of two 3d3/2−3d5/2 spin−orbit doublets at 888.1, 900.1, 903.1, 908.9, and 916.2 eV, are characteristic of the presence of low levels of Ce4+ with 3d94f0−O 2p6, Ce 3d94f2− O 2p4, and Ce 3d94f1−O 2p5 final states. However, these data do not suggest Ce4+ incorporation into Ce2/3Cu3FeTi3O12; rather, a nearly exact match exists between these Ce4+ features and known values for CeO2. This suggests the presence of a CeO2 impurity in the sample that is present below the PXRD detection limits. XPS spectra were also acquired from samples of Nd1−xCexCu3FeTi3O12 (x = 0.1, 0.2, 0.3, 0.4, 0.8), and the Ce(3d) profiles are provided in Figure 6. These spectra

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b02832. Additional powder X-ray diffraction data, XPS spectra, and EDS analyses (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail for T.E.A.-S.: [email protected]. ORCID

Vanessa Proust: 0000-0002-8558-7167 Frankie D. White: 0000-0002-8644-1231 Thomas E. Albrecht-Schmitt: 0000-0002-2989-3311 Notes

The authors declare no competing financial interest.

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Figure 6. Ce(3d) XPS spectra and fitting of (a) Nd 0 . 8 Ce 0 . 2 Cu 3 FeTi 3 O 1 2 , (b) Nd 0 . 7 Ce 0 . 3 Cu 3 FeTi 3 O 1 2 , (c) Nd0.6Ce0.4Cu3FeTi3O12, and (d) Nd0.2Ce0.8Cu3FeTi3O12.

ACKNOWLEDGMENTS This work was supported by the Center for Actinide Science and Technology (CAST), an Energy Frontier Research Center (EFRC) funded by the U.S. Department of Energy (DOE), Office of Science, Basic Energy Sciences (BES), under Award Number DE-SC0016568. We thank Drs. E. Lochner, T. Somasundaram, and L. J. van de Burgt for their help with the sample analysis.

composed of two multiplets corresponding to the spin−orbitsplit 3d5/2 and 3d3/2 states. In comparison to the spectra obtained from Ce2/3Cu3FeTi3O12, the Gaussian-fitted profiles of Nd0.8Ce0.2Cu3FeTi3O12 are primarily characteristic of the Ce3+. The four highest peaks are associated with the pairs of spin−orbit doublets from Ce3+ located at about 881.7, 885.6, 898.7, and 903.6 eV resulting from Ce 3d94f1−O 2p6 and Ce 3d94f2−O 2p5 states. The three low-intensity peaks at 887.7, 901.8, and 905.7 eV can be assigned to the spin−orbit-split 3d5/2 and 3d3/2 from Ce4+ contribution resulting from Ce 3d94f0−O 2p6, Ce 3d94f2−O 2p4, and Ce 3d94f1−O 2p5 final states. As expected from the PXRD data, the XPS spectra of samples with higher loadings of Ce3+ all show increased levels of Ce4+. Again, this consisted of a maximum doping level of ∼20% cerium and with excess cerium forming CeO2. Moreover, the satellite peak from Ce4+ appears at ∼916.2 eV, as in the same samples where diffraction lines from CeO2 were evident. Taken together, this leads to the conclusion that excess Ce4+ does not substitute onto the B sites in these materials and is simply excluded.

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REFERENCES

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CONCLUSIONS NdCu3FeTi3O12 and Ce2/3Cu3FeTi3O12 with perovskite-like structures were successfully synthesized as single-phase materials by sol−gel methods and heat treatment. The latter phase shows decreased thermal stability with respect to NdCu3FeTi3O12. These phases decompose above 1200 °C. E

DOI: 10.1021/acs.inorgchem.8b02832 Inorg. Chem. XXXX, XXX, XXX−XXX

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