The effect of Ce content on structure and stability of Gd1-xCexPO4: Theory and experiment

Accepted Manuscript Title: The effect of Ce content on structure and stability of Gd1−x Cex PO4 : theory and experiment Authors: Xiaofeng Zhao, Yuxiang Li, Yuancheng Teng, Lang Wu, Peng Bi, Xiaoyong Yang, Lili Wan PII: DOI: Reference:

S0955-2219(18)30674-5 https://doi.org/10.1016/j.jeurceramsoc.2018.11.009 JECS 12161

To appear in:

Journal of the European Ceramic Society

Received date: Revised date: Accepted date:

6 August 2018 30 October 2018 2 November 2018

Please cite this article as: Zhao X, Li Y, Teng Y, Wu L, Bi P, Yang X, Wan L, The effect of Ce content on structure and stability of Gd1−x Cex PO4 : theory and experiment, Journal of the European Ceramic Society (2018), https://doi.org/10.1016/j.jeurceramsoc.2018.11.009 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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The effect of Ce content on structure and stability of Gd1-xCexPO4: theory and experiment

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Xiaoyong Yang a, Lili Wang b

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Xiaofeng Zhao a, Yuxiang Li , a, Yuancheng Teng, a, Lang Wu a, Peng Bi a,

a

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State Key Laboratory For Environment-friendly Energy Materials, School of

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Materials Science and Engineering, Southwest University of Science and

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Technology, Mianyang 621010, PR China b

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Institute of Computer Application, China Academy of Engineering Physics,

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Mianyang 621900, PR China

*

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Corresponding author.



Yuancheng Teng: E-mail address, [email protected]

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

Yuxiang Li: E-mail address, [email protected]

Abstract: The structure distortion, bonding character, defect stability and chemical stability of Gd1-xCexPO4 (x = 0, 0.25, 0.5, 0.75 and 1) have been investigated by combined with experimental and first principles calculations to understand the

effect simulate actinides on the structural stability of Gd-monazite. The result shows that the Gd1-xCexPO4 (x = 0-1) ceramics could be prepared at 1450 oC, and Ce3+ is easily incorporated into Gd-monazite lattice. The structural analysis indicates that the symmetry of [GdO9] polyhedron decreases with increasing Ce content. The evolution

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of Gd defect stability and chemical stability in Gd1-xCexPO4 (x = 0-1) is tightly associated with the distortion of [GdO9] polyhedron, in which a greater distortion will

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induce more unstable. The best chemical stability in pH = 3 and 11 leachates is CePO4, followed by GdPO4, Gd0.75Ce0.25PO4, Gd0.5Ce0.5PO4, and finally

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Gd0.25Ce0.75PO4, revealing the GdPO4 monazite is perfectly suitable to immobilize

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actinides with relative low dose ratio.

1. Introduction

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Key words: Gd-monazite; Ce; structure; defect stability; chemical stability

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Since immense amounts of energy were found by neutron-bombarded Uranium

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(U), the nuclear energy has been attracted considerable attentions as it is well known for a huge potential to replace traditional fuels[1-3]. The fissile materials, like U235 and Plutonium (Pu239), are indispensable part in nuclear reactors and/or nuclear

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weapons because of they possess the ability to sustain a nuclear fission chain reaction[4, 5]. Generally, the U and Pu would be chemically separated and recovered from spent fuel to fabricate the MOX fuel further for use[6]. However, the residual actinides and some long-lived fission products in spent fuel should be isolated from

the biosphere due to their large radiation dose and long half-life. Incorporation is an effective method for long-term storage of radioactive waste [7-21], which is defined as fixing the nuclides into the waste form lattice to reducing the potential for migration.

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The unique immobilization challenge of waste streams is actinide-bearing. Actinides generally have very long half-lives, which could produce a large number of

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alpha ions (α) through decay. During geological storage, α ions carry the huge kinetic

energy (M eV) to cause the atomic disorder through cascade collisions. As a potential

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engineered waste forms to replace glass, the self-healing properties of ceramics allow

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the disordered atoms to return to the lattice position at a specific temperature, thereby

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avoiding amorphization [22-25]. Considering the design cost and conditions for

of

research

in

recent

30

years.

Phosphate

minerals

(britholites,

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focus

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geological disposal, high actinide tolerance for ceramic waste forms has always the

monazite/brabatite) are a promising candidate for the specific immobilization of

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long-life radionuclides since the natural phosphate-based minerals often present high

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weight loadings in actinides (up to 15 wt % in ThO2 or UO2) as well as strong resistances to aqueous corrosion and to radiation damages[18-21, 24, 26-33]. The synthesis processes of these phosphates have been extensively researched, including

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the preparation, the process, and the optimization of specific properties required[30, 31]. Moreover, the trivalent (Pu, Am, Cm) or tetravalent (Th, U, Np, Pu) actinides could be designed into these phosphate matrices based on their redox properties [20, 24, 26, 27, 29, 33].

GdPO4 monazite has attracted considerable attentions due to its high structural flexibility, chemical durability, and radio-resistance, as well as the attractive neutron absorbers of Gd (48800 barns) [27, 34-41]. Last three decades, the properties of GdPO4 monazite in terms of neutron absorber and the solubility of actinides have

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been extensively researched. For instance, Terra et.al [41] once prepared the hydrated GdPO4·H2O monazite by reacting high purity dissolved salts with phosphoric acid,

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and verified that GdPO4 is an excellent candidate for a chemically stable, water-insoluble neutron absorber for inclusion in spent nuclear fuel canisters.

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Recently, studies of solid solutions, such as GdPO4-YbPO4 and GdPO4-DyPO4, have

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revealed the high solubility of simulate actinides in GdPO4 structure [12, 42, 43]. It

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also revealed that the ionic radius of rare earths affect the final phase (monazite or

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xenotime) via the polyhedral deformation form GdO9 to ReO8 (Re = Yb and Dy),

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which proved that the ionic radius of actinides plays an important role on the final structural deformation. While, the ionic radius of minor actinides (like Pu3+: 1.01 Å)

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is relative large difference with the Gd3+ (0.938Å), which will cause a great structural

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distortion for GdPO4 with ultimate effects on phase stability. Numerous researches [44, 45] have indicated that the phase stability of solid solutions is largely dependent on their structural deviation from the lattice strain. Therefore, it is very necessary to

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study the effect of minor actinides on GdPO4 structural distortion. However, the discrepancy of structural characteristics for simulate actinides in monazite often encounters using X-ray diffraction (XRD), energy-dispersive spectrometry (EDS),

and Raman spectrum, especially the analysis results of bonding characters and atomic positions. First principles calculation is a practical method to study structural characteristic, chemical bonding and thermodynamic stability of materials [4, 39, 46-50]. Blanca

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Romero[51] predicted the structural and thermodynamic parameters of monazite-type ceramics using first principles calculation. In present work, Ce3+ (ionic radius: 1.02 Å)

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was employed as the surrogate of Pu3+ due to their similar electron structure and ionic

radius. The serial monazite ceramics Gd1-xCexPO4 (x = 0-1) were prepared by the solid

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phase sintering method. The evolution of Gd1-xCexPO4 (x = 0, 0.25, 0.5, 0.75 and 1)

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structures was analyzed by combined with experimental study and first principle

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calculations. In addition, the properties of Gd1-xCexPO4 (x = 0, 0.25, 0.5, 0.75 and 1)

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monazite in terms of the structural stability, chemical bonding, and defect stability

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and chemical stability were investigated. Several literatures have shown that the valence of Ce in monazite is trivalent prepared by using the cerium oxalate

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decahydrate (Ce2(C2O4)3·10H2O) as raw materials [52-55].

2. Experimental Section

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2.1 Synthesis

The Gd1-xCexPO4 (x = 0, 0.25, 0.5, 0.75 and 1) were designed and prepared using

the solid phase sintering method following their chemical stoichiometric ration. The reagents: gadolinium oxide (Gd2O3), ammonium dihydrogen phosphate (NH4H2PO4), and Ce2(C2O4)3·10H2O were used as raw materials. The raw materials were mixed

evenly by a planetary ball mill. Then, they were calcined at 600 oC for 2 h in air by a muffle furnace for CO2, CO, H2O and NH3[56]. The 8-10 wt.% of polyvinyl alcohol (PVA) was added in the calcined powder (600 oC) to forming the granules. Then, the cylindrical green bodies were prepared in a steel mold and then were further

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densification by cold isostatic pressing at 200 MPa. Later, the green bodies were heated at 520 oC for 2 h in air to excluding the PVA. Lately, The Gd1-xCexPO4 (x = 0,

4 h in a muffle furnace with heating rate of 5 oC/min.

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2.2 Characterization

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0.25, 0.5, 0.75 and 1) monazite were prepared by pressureless sintering at 1450 oC for

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The X-ray diffraction (XRD) patterns of the Gd1-xCexPO4 (x = 0, 0.25, 0.5, 0.75

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and 1) monazite powders were obtained with an X′Pert PRO Roentgen diffractometer

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system using Cu Kα rays (λ = 1.5418 Å) and a Bruker D8 Advance diffractometer (40

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kV and 40 mA) with a Bragg-Brentano θ/2θ configuration. The powder XRD patterns were detected using a step size of 0.0167 and an integration time of 2 s across the

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angular range 10o – 110o. The lattice parameter was refined by the Rietveld method

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using Fullprof program. Lastly, peak profile fitting was done using Pseudo-Voigt functions. Moreover, samples were observed with an Ultra55 field emission scanning electron microscope (FESEM). The chemical compositions of samples were analyzed

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by an Oxford IE450X-Max80 energy-dispersive spectrometry (EDS). 2.3 Leaching Tests Before leaching, the surfaces of samples were handled by polishing processes and an annealing treatment process to obtain a geometric surface area. Fig S1 shows the

SEM image of the polished surfaces of monazite ceramic. By the ASTM-C1220 methods [11, 19], the leaching tests of Gd1-xCexPO4 (x = 0, 0.25, 0.5, 0.75 and 1) monazite ceramics were performed in polytetrafluoroethylene vessel using at 90oC and pH = 3 and 11. The regular leaching intervals are as 0-1, 1-3, 3-7, 7-14, 14-21,

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21-28, 28-35 and 35-42 days. The hydrochloric acid and ammoniato were used to adjusting the pH of leachates.

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The ionic concentrations of Gd and Ce in all leachates were determined by an

Agilent 7700x inductively coupled plasma-mass spectroscopy (ICP-MS). To analyze

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the leaching activity of Gd and Ce, Their normalized elemental leach rate (LR,

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calculated from the following equations:

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g·m-2·d-1)[19] and the normalized elemental mass loss (NL, g·m-2) [26] were

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𝐿𝑅 =

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𝑁𝐿 =

𝐶1 × 𝑉

𝑓 × 𝑆 × ∆𝑡

𝐶1 × 𝑉 𝑓×𝑆

(1)

(2)

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where C1 is the concentration of the measured element (Gd or Ce) (g·m-3), V the

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volume of the leachate (m3), f the mass fraction of Gd or Ce in the sample, S the geometric surface area of sample (m2), t the duration of the leaching experiment in days (d).

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2.4 Computational Methods The current calculations within density functional theory （ DFT ） were performed by the CASTEP database. The electronic exchange-correlation interactions are described by the local-density approximation (LDA) of spin polarized scheme

(LSDA). Due to the strong correlation of the f shell of Ce/Gd, a 6 eV of Hubbard energy (U = 6 eV) was assigned for these elements as a more appropriate approximation to treat the on-site correlation of the 4f orbital, which has been analyzed and discussed in previous studies [51, 57]. The ultra-soft pseudo-potentials

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are used as describing the interactions between valence electrons and the ionic core, where 11 valence electrons for Ce (4f1 5s2 5p6 5d1 6s2), 18 for Gd (4f7 5s2 5p6 5d1 6s2),

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5 for P (3s2 3p3) and 6 for O (2s2 2p4) are performed. The plane wave energy cutoff of 650 eV was used for theses electronic wave functions. The Monkhorst–Pack scheme

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for 5×5×5 K-point sampling is used in the first irreducible Brillouin zone. The total

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energies are converged to 5×10-5 eV/atom. The atomic positions and lattices are

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optimized until that the forces are converged to within 10 m eV/Å.

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Through the direct substitution of Gd atom in GdPO4, the structure model of

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Gd1-xCexPO4 (x = 0, 0.25, 0.5, 0.75 and 1) monazite was designed to study the structural change of [LnO9] (Ln = Gd or Ce). The density of the total (DOS) and

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partial (PDOS) density of these structures were analyzed to detect the Ce-O and Gd-O

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chemical bonding. In order to compare the binding energy of Gd in Gd1-xCexPO4 (x = 0, 0.25, 0.5, 0.75 and 1) at ground state temperature, their formation energies have been calculated with a 2×1×1 supercell. The element formation energies can be

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defined in the following equation: 𝐸𝐹 = −(𝐸𝑉𝑁−1 − 𝐸 𝑁 + 𝐸𝑥 ) 𝑥

(3)

where 𝐸 𝑁 is the calculated energy of the defect free cell; 𝐸𝑉𝑁−1 is the calculated 𝑥 energy of the cell with the vacancy defect for cerium or gadolinium ions; N denotes

the number of atoms in the defect free cell. 𝐸𝑥 is the calculated energy of the cerium or gadolinium element in the chosen reference state from Ce/Gd molecule.

3. Results and discussion

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3.1 XRD and SEM analysis The Gd1-xCexPO4 (x = 0, 0.25, 0.5, 0.75 and 1) monazite ceramics were prepared

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by pressureless sintering at 1450 oC for 4h. The phase compositions of these

specimens characterized with the XRD patterns are represented Fig. 1. The patterns of these XRD peaks indicate that all specimens are presented as a single phase.

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Moreover, it can be noted that the main peaks are gradual shift from 31.31 to 31.15.

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Since Ce has relative larger ionic radius compared with Gd, the interplanar spacing of

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this series of solid solution will gradually increase with Ce content, leading that the θ corresponding to XRD peak position shifts to a lower angle (based on the Bragg's

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law). Hence, it can be concluded that the Ce is successfully incorporated in GdPO4

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lattice to form the Gd1-xCexPO4 (x = 0, 0.25, 0.5, 0.75 and 1) monazites. The surface SEM image of the Gd1-xCexPO4 (x = 0, 0.25, 0.5, 0.75 and 1)

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monazite ceramics prepared by pressureless sintering at 1450 oC for 4h are presented in Fig. 2. It can be seen in Fig 2 that the grains of all ceramic samples are closely

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packed. Only few pores exit at grain boundary. Moreover, the grain sizes increase slightly when the Ce incorporated in GdPO4 lattice, leading the grain size between 0.5-1 μm. The atomic% of A, B, C, D and E given in Table 1 are consistent with the expected stoichiometric ratio for Gd1-xCexPO4 (x = 0, 0.25, 0.5, 0.75 and 1) monazites.

3.2 The effect of Ce content on the structure of Gd1-xCexPO4 (x = 0, 0.25, 0.5, 0.75 and 1) monazites 3.2.1 Structural analysis Fig. 3 shows the Gd1-xCexPO4 (x = 0, 0.25, 0.5, 0.75 and 1) monazite lattice

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structures. For GdPO4 monazite, the Gd atoms are coordinated with nine oxygens for building a low symmetry [GdO9] polyhedron. The [GdO9] polyhedrons are usually

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described as an equatorial pentagon[38] and linked by irregular [PO4] tetrahedron to forming a chain along the [100] direction (c axis). This arrangement leads to the

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GdPO4 having a very low symmetry that induces perfectly structural flexibility. The

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presence of structural flexibility is generally correlated to the capability to

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accommodate radionuclide, including the incorporation of divalent, trivalent and

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tetravalent cations.

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Fig. S2–S6 presents the refined XRD pattern of Gd1-xCexPO4 (x = 0, 0.25, 0.5, 0.75 and 1) monazites including the values Rp, Rwp and Chi2. The refined lattice

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parameters of Gd1-xCexPO4 (x = 0, 0.25, 0.5, 0.75 and 1) monazites are presented in

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Table 2. The lattice parameters from refined XRD are agreement with the previously experimental reported values and current DFT calculations within 2.68 % deviation [10, 56]. This deviation between experiments and DFT calculations may be caused by

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the limitations of the LDA approximation, which underestimates the equilibrium bulk modulus [10]. The detailed structural information with atomic positions, isotropic displacement factors biso and occupancies of Ce and Gd of these monazite specimens are presented from Table 3. This refinement of the occupancy information reveals that

Ce easily occupies the expected Gd site, as would be expected from the first principle calculation. Moreover, there is no distinct displacement for the Gd atoms when Ce atoms were incorporated in GdPO4 lattice. The evolutions of Gd1-xCexPO4 (x = 0, 0.25, 0.5, 0.75 and 1) lattice parameters,

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lattice volume and  angle are presented in Fig. 4. Noted in Fig. 4 that the lattice

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parameters (a, b and c) gradually increase with the concentration of Ce. The line

evolution of these lattice parameters shows that the Gd1-xCexPO4 (x = 0, 0.25, 0.5, 0.75 and 1) has the property of the continuous substitutional solid solution. The

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increase in unit cell parameters can be attributed to the incorporated of Ce3+ ions at

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the Gd site (ion radii of Ce3+ and Gd3+ are 1.02Å and 0.938Å, respectively). This

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transition of lattice parameters also generates an expansion of the unit volume. The expansion ratio of the volume for composition with x = 0.25, 0.5, 0.75 and 1 are

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1.92%, 2.70%, 5.61% and 7.42%, respectively. However, the  angle gradually shifts

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to lower direction, revealing that the doped Ce3+ induces the structure to transform from GdPO4 to CePO4. To investigate the Ce-induced lattice damage of Gd-monazite,

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the expansion ratios of lattice parameters (a, b, c) were analyzed based on the equation: = (lattice constant x1 − lattice constant x2)/ [lattice constant x1 ×(x2 – x1)].

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The average expansion ratios of a, b, and c for Gd1-xCexPO4 are 2.08%, 2.33%, and 2.00%, respectively. This results show that the doped Ce preferentially causes lattice damage of Gd-monazite along the b axis, which is similar to the damage effects of Am-doped LaPO4 monazite [58]. Simultaneously, note that the c-axis has the smallest lattice damage.

The structural analysis shows that the c-axis is the link orientation of [LnO9] (Ln = Gd and/or Ce) polyhedrons for Gd1-xCexPO4 (x = 0, 0.25, 0.5, 0.75 and 1) monazites, see Fig 3. Hence, it is very necessary to investigate the effect of [LnO9] polyhedrons on this lattice damage. Due to XRD information only shows statistical mixed

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placeholder for Gd and Ce, the bond lengths of Gd-O and Ce-O for Gd1-xCexPO4 (x = 0, 0.25, 0.5, 0.75 and 1) monazite structures were analyzed by DFT calculation; and

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the results are given in Table 4. After Ce substituted for Gd, most Gd-O and Ce-O bond-lengths increase slightly. This trend is correlated with the increase of the lattice

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parameter. However, there is no definite evolution regular for Gd-O and Ce-O

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bond-lengths by a careful analysis of the bond-length distribution. Calculated by

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Table 4, the difference value of the bond lengths (B) for [LnO9] (the maximum

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length minus the minimum length) is obtained and presented in Fig. 5. Note in Fig 5

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that a distinct distortion was observed for [GdO9] polyhedron when the Ce atom is incorporated in Gd-monazite lattice. The symmetry of the [GdO9] polyhedron

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decreases as an increase of Ce content. While, the [CeO9] polyhedron has a relative

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smaller distortion compared with [GdO9]. Hence, it can be concluded that the distortion of [GdO9] polyhedron plays an important role to resist the expansion of c

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axis.

3.2.2 Chemical bonding In order to understand the role of Gd site substitution on the chemical bonding, the total (DOS) and partial (PDOS) density of states of Gd1-xCexPO4 (x = 0, 0.25, 0.5, 0.75 and 1) matrix were calculated and gathered in Fig 6. Compared with outer

electron of Ce (4f15s25p65d16s2) and Gd (4f75s25p65d16s2), the major difference is the f orbit. However, the f orbit plays as minor role in chemical bonding but has a considerable impact on band gap [59-62]. Note that the shape of DOS curves of Ce-4f1 is different from Gd-4f7 since the minor peaks of Ce-4f1 and Gd-4f7 locate at

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-1.0 to 0.5 eV and -6.0 to 8.3 eV, respectively. Comparing the DOS charts for Gd1-xCexPO4 (x = 0, 0.25, 0.5, 0.75 and 1), no significant difference are observed

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except the region of -16.2 eV to -13.5 eV. Thus, it can be inferred that the Ce3+ partially substituting for Gd3+ has little effect on the chemical bonding property in

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GdPO4 monazite. For the composition of Ln-O (Ln = Ce or Gd) chemical bonding,

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the major contribution comes from the O-2p with the Ln-5d, which produces the π

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and π* bonding states in -9.0 to 0.0 eV and 4.0 to 12.0 eV. The bond population of

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Gd1-xCexPO4 (x = 0, 0.25, 0.5, 0.75 and 1) are presented in Table 5, where a high value

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of the bond population indicates a covalent bond while a low value indicates an ionic interaction. With such a scheme, it can be noted in Table 5 that the [PO4] tetrahedron

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exhibits covalent bonding character and Ln-O bonds is the ionic nature.

3.2 The effect of Ce content on the defect stability of Gd1-xCexPO4 (x = 0, 0.25, 0.5,

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0.75 and 1)

To investigate the influence of doped Ce on the binding energy of Gd in

Gd1-xCexPO4 (x = 0, 0.25, 0.5, 0.75 and 1) monazites, the defect formation energies of Gd were calculated by the first principles calculation. Fortunately, since a doubly-negative background charge compensating for vacancy defect, the

Gd1-xCexPO4 (x = 0, 0.25, 0.5, 0.75 and 1) monazite produced an insulator compound so it could deal with any charged defect [10]. The Gd vacancy defect energies were calculated in Gd1-xCexPO4 (x = 0, 0.25, 0.5, 0.75 and 1), along with calculating the isolated Gd single atom energy. The calculated energies were then combined

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according to the defected Equation (3) and presented in Table 6, where more negative values indicate that the elements are more stable. The calculated defect formation

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energy of Gd for Gd1-xCexPO4 (x = 0, 0.25, 0.5, 0.75) are -15.6738 eV, 15.3624 eV, -14.8979 eV and -14.8169 eV, respectively. It can be seen that the defect formation

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energy of Gd in Gd1-xCexPO4 (x = 0, 0.25, 0.5, 0.75) monazite gradually go up with an

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increase of Ce content, showing the stability of Gd in these compositions gradually

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weakened. The evolution of the stability for Gd is strongly associated with the

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induces more unstable for Gd.

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distortion of [GdO9] polyhedron, in which the greater distortion of [GdO9] polyhedron

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3.3 The effect of Ce content on the chemical stability of Gd1-xCexPO4 (x = 0, 0.25,

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0.5, 0.75 and 1)

The influence of doped Ce on GdPO4 monazite is decided not only by the defect

formation energy but also by the case with which Gd3+ is detached from the lattice by

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leachates. Moreover, the chemical stability of waste form in leachates is a key factor when evaluate the safe disposal of waste forms. In this regard, the chemical stability of Gd1-xCexPO4 (x = 0, 0.25, 0.5, 0.75 and 1) monazite ceramics were detected in pH = 3 and 11 leachates. Fig. 7 shows the normalized elemental leach rates (LR) of Gd and

Ce for Gd1-xCexPO4 (x = 0, 0.25, 0.5, 0.75 and 1) monazites in pH = 3 leachates at 90 o

C with regular intervals. As clearly shown in this figure, a fast element leaching is

observed during the first 7 days. It could be due to the presence of the original surface defect area in the samples that could result from local decomposition at the surface

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during surface polishing and heat treatment. After 7 days of dissolution, the dissolution is congruent and a steady state is reached. Moreover, the similar leaching

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behaviors of Ce and Gd show that [GdO9] and [CeO9] polyhedrons would be destroyed in the same time, implying the similar bonding nature of Ce-O and Gd-O.

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Fig. 8 presents the LRCe and LRGd of Gd1-xCexPO4 (x = 0, 0.25, 0.5, 0.75 and 1)

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monazites in pH = 11 leachates at 90 oC with regular intervals. Similar to the leaching

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evolution of pH = 3 solution, the LRCe and LRGd for all samples quickly decreased in

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the first 7 days and achieve stability at 21 - 42 days. Note in Fig. 8 and Fig. 9 that the

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Gd1-xCexPO4 (x = 0, 0.25, 0.5, 0.75 and 1) monazites have a higher stability in pH = 11solution compared with that in pH = 3 solution. After 42 days of pH = 3 dissolution,

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the normalized elemental leach rates are found between 2.1  10-3·g·m-2·d-1 (for

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Ce0.75Gd0.25PO4) and 8.8  10-5·g·m-2·d-1 (for CePO4), which is agreement with the lower range of normalized leach rates reported for highly durable ceramic materials proposed for radionuclides [26, 34, 36, 63-65]. The high durability of Gd1-xCexPO4 (x

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= 0, 0.25, 0.5, 0.75 and 1) monazites are confirmed under pH = 3 and pH =11 solutions. Moreover, it is worth noting that the chemical stability of Gd-monazite ceramic decrease when the simulated nuclide (Ce) is incorporated in Gd-monazite lattice.

Fig. 9 presents the normalized elemental mass loss (NL) of Gd and Ce for Gd1-xCexPO4 (x = 0, 0.25, 0.5, 0.75 and 1) monazites in pH = 3 leachates at 90 oC with regular intervals. As clearly shown in this figure, the evolutions of NL(Gd) and NL(Ce) present a linear growth trend after initial 7 days, which reveals that Gd and Ce are

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released in pH = 3 leachates. It could be concluded that the decrease of the LR value of leached samples observed in pH = 3 solutions is due to the establishment of a

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steady state regime of dissolution through diffusion phenomena. The diffusion

phenomena are confirmed by analyzing Fig 11. As shown in Fig.11, the obvious

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corrosion morphology is observed on the surface of sample leached in pH = 3 solution.

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However, no gelatinous phases or newborn compounds are found on the

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solid-solution interface. Fig. 10 presents the NL(Gd) and NL(Ce) for Gd1-xCexPO4 (x =

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0, 0.25, 0.5, 0.75 and 1) monazites in pH = 11 leachates at 90 oC. The dissolution is

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clearly characterized by precipitation of Gd and Ce during 28 to 42 days. The striped precipitation is observed on the leached monazite surface (see Fig. 11). As it was

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reported for other monazites, the formation of LnPO4·nH2O crystals was evidenced in

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such experimental conditions onto the surface of the leached ceramics [11, 18, 66]. It can be concluded that the low-soluble precipitation prevents the release of elements in

A

pH = 11 solutions after 28 days consequently to passivation phenomena. As mentioned above, a steady state regime of dissolution process is established

at 7 - 42 days for samples leached in pH = 3 solutions and at 7-14 days for samples leached in pH = 11 solutions. Since only the dissolution reaction (𝐿𝑛𝑃𝑂4 = 𝐿𝑛3+ + 𝑃𝑂43− ) is considered to study the effect of Ce content on the stability of Gd1-xCexPO4

(x = 0, 0.25, 0.5, 0.75 and 1) monazites, the 42 and 7 days are selected as the reaction for acid and alkaline leaching, respectively. Directly comparing the LRCe and LRGd in acid (42 day) and alkaline (7 day) leachates, the stability of Ce is obviously better than Gd, especially for the Gd0.25Ce0.75PO4, Gd0.5Ce0.5PO4 and Gd0.75Ce0.25PO4

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monazite. This is mainly coming from the greater distortion of [GdO9] induces the lower structural stability. In addition, one can be observed that the LRGd increases with

SC R

Ce content, which agrees to the evolution of defect formation energy. This good agreement between leaching experiment and defect stability calculation confirms the

U

distortion of [LnO9] is the main responsibility for the effect on Ln leaching activity.

N

Comparing the chemical stability of samples under acid leachate when Ce is doped in

A

GdPO4, the Gd0.75Ce0.25PO4 has the lowest average normalized elemental leach rates

M

(LR = 1.2695×10-3 g·m-2·d-1) and the lowest mass loss factor (see Fig. 9), which

ED

shows that GdPO4 monazite is perfectly suitable for immobilization the minor actinides with a low dose ratio. Moreover, this result also shows that the chemical

PT

stability has the same influencing factor (intrinsic structure) with the defect stability,

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which offers a feasible solution to predict the chemical stability of monazite by calculating the defect formation energy.

A

4 Conclusions

GdPO4 monazite represents one candidate to immobilize the minor actinides since its high structural flexibility, high chemical durability, and attractive neutron adsorption properties of Gd. To understand the effect of minor actinides on structural

stability of GdPO4, the structure, chemical bonding, defect stability and chemical stability of Gd1-xCexPO4 (x = 0, 0.25, 0.5, 0.75 and 1) were investigated by combined with experimental study and first principles calculation, in which the Ce3+ was used as the surrogate for Pu3+. The result shows that the doped Ce preferentially causes lattice

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damage of Gd-monazite along the b axis, while the c-axis has the smallest lattice damage. The structural analysis shows that the c-axis is the link orientation of [LnO9]

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(Ln = Gd and Ce) polyhedrons, revealing the [LnO9] polyhedrons occur distortion to resist the expansion of c axis. The [GdO9] polyhedron has a relative higher distortion

U

compared with [CeO9], and the symmetry of the [GdO9] polyhedron gradually

N

decreases as an increase of Ce content. The evolution of the defect stability and

A

chemical stability for Gd is strongly associated with the distortion of [GdO9]

M

polyhedron, in which the greater distortion could induce more unstable. The best

ED

chemical durability in all leachates (pH = 3 and 11) is CePO4 ceramic, followed by GdPO4, Gd0.75Ce0.25PO4, Gd0.5Ce0.5PO4, and finally Gd0.25Ce0.75PO4, revealing GdPO4

PT

monazites are perfectly suitable for immobilization actinides with a low dose ratio.

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Moreover, both LRCe and LRGd of Gd1-xCexPO4 (x = 0, 0.25, 0.5, 0.75 and 1) ceramics are lower than 2×10-3 m·d-1 in pH = 3 and 11 leachates after 42 day, showing all the samples have the high stability. The present work also offers a feasible solution to

A

predict the evolution of chemical stability of monazite via calculating the defect formation energy.

Acknowledgments

We sincerely appreciate the financial support of the National Natural Science Foundation of China (No.11275158, No.11705152), and Longshan academic talent research supporting program of Southwest University of Science and Technology (No.17LZX407).

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A

CC E

PT

ED

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A

N

U

SC R

Nuclear Materials, 465 (2015) 550-555.

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A

CC E

PT

ED

M

N

A

processed by solid sintering at 1450 oC for 4 h

U

Fig. 1 The XRD pattern of Gd1-xCexPO4 (x = 0, 0.25, 0.5, 0.75 and 1) monazites

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Fig. 2 The surface SEM image of the Gd1-xCexPO4 (x = 0, 0.25, 0.5, 0.75 and 1)

monazite ceramics sintered at 1450 oC for 4h: (a) GdPO4, (b) Gd0.75Ce0.25PO4, (c)

A

CC E

PT

ED

M

A

N

U

Gd0.5Ce0.5PO4, (d) Gd0.75Ce0.25PO4, (e) CePO4

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Fig. 3 GdPO4 structure (a), Gd0.75Ce0.25PO4 structure (b), Gd0.5Ce0.5PO4 structure (c),

N

Gd0.25Ce0.75PO4 structure (d), CePO4 structure (e), [GdO9] and [CeO9] polyhedrons

A

CC E

PT

ED

M

A

connection (f).

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Fig. 4 the evolution of lattice parameters for Gd1-xCexPO4 (x = 0.25, 0.5, 0.75 and 1)

A

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and  angle

PT

monazites versus the substitution ratio x, along with the variation of lattice volume

IP T SC R U

N

Fig. 5 The difference value of the bond lengths (B) of [LnO9] (Ln = Ce or Gd) for

A

Gd1-xCexPO4 (x = 0, 0.25, 0.5, 0.75 and 1) monazites; B is maximum length minus

A

CC E

PT

ED

M

minimum length.

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A

CC E

monazites

PT

Fig. 6 Total and partial density of states of Gd1-xCexPO4 (x = 0, 0.25, 0.5, 0.75 and 1)

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Fig. 7 The normalized elemental leach rate of Gd (a) and Ce (b) for Gd1-xCexPO4 (x =

A

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0, 0.25, 0.5, 0.75 and 1) monazites in pH = 3 leachates at 90 oC with regular intervals.

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Fig. 8 The normalized elemental leach rate of Gd (a) and Ce (b) for Gd1-xCexPO4 (x =

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0, 0.25, 0.5, 0.75 and 1) monazites in pH = 11 leachates at 90 oC with regular

A

intervals.

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Fig. 9 The normalized elemental mass loss of Gd (a) and Ce (b) for Gd1-xCexPO4 (x =

A

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0, 0.25, 0.5, 0.75 and 1) monazites in pH = 3 leachates at 90 oC with regular intervals.

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Fig. 10 The normalized elemental mass loss of Gd (a) and Ce (b) for Gd1-xCexPO4 (x

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= 0, 0.25, 0.5, 0.75 and 1) monazites in pH = 11 leachates at 90 oC with regular

A

intervals.

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Fig. 11 The microstructures of monazite ceramic (Gd0.5Ce0.5PO4) surface after being

A

CC E

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ED

M

A

N

U

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leached in pH =3 (a) and pH = 11 (b).

Table 1 The EDS (atom%) of the Gd1-xCexPO4 (x = 0, 0.25, 0.5, 0.75 and 1) monazite ceramics sintered at 1450 oC for 4h Site

Gd

Ce

P

O

GdPO4

A

15.69

―

15.00

69.32

Gd0.75Ce0.25PO4

B

12.53

4.82

16.27

65.37

Gd0.75Ce0.25PO4

C

8.52

7.24

14.65

Gd0.75Ce0.25PO4

D

4.49

12.08

14.91

CePO4

E

―

18.70

16.72

69.59 68.52 64.58

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U N A M ED PT CC E A

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Atom%

Table 2 The lattice parameters and lattice volume of Gd1-xCexPO4 (x = 0, 0.25, 0.5, 0.75 and 1) monazites by experiments and DFT calculations

Gd0.25Ce0.75PO4

β (Å)

V(Å3)

XRD

6.65046

6.84304

6.33429

103.999

279.708

DFT

6.62552

6.68103

6.30725

104.183

275.919

XRD

6.68789

6.89088

6.37180

103.869

285.086

DFT

6.62933

6.82931

6.31413

103.697

277.429

XRD

6.70372

6.90827

6.38621

103.765

287.258

DFT

6.68284

6.88791

6.34896

103.682

283.920

XRD

6.76162

6.97921

6.44075

103.607

295.413

DFT

6.71229

6.94457

6.38787

103.432

289.610

XRD

6.79755

7.02242

6.47221

103.466

300.458

DFT

6.77507

6.44860

103.305

298.399

7.01836

ED PT CC E A

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c (Å)

M

CePO4

b (Å)

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Gd0.5Ce0.5PO4

a (Å)

U

Gd0.75Ce0.25PO4

Lattice parameters

N

GdPO4



A

Composition

Table 3 The structural information of Gd and Ce atoms of Gd1-xCexPO4 (x = 0, 0.25, 0.5, 0.75 and 1) monazites showing atomic positions, isotropic displacement factors biso and occupancies Composition

Atom

x

y

z

Biso

GdPO4

Gd

0.28148

0.15524

0.09597

1.731

Gd0.75Ce0.25PO4

Gd

0.28145

0.15528

0.09598

1.477

Ce

0.28145

0.15528

0.09598

1.477

Gd

0.27914

0.15754

0.09337

2.695

Ce

0.27914

0.15754

0.09337

2.695

0.500

Gd

0.28209

0.15797

0.09827

1.320

0.750

Gd

0.28209

0.15797

0.09827

1.320

0.250

Ce

0.280220

0.157841

0.09998

1.061

1.000

A

CC E

PT

ED

M

A

CePO4

IP T

1.000 0.750 0.250 0.500

SC R

U

Gd0.25Ce0.75PO4

N

Gd0.5Ce0.5PO4

Occupancy

Table 4 The bond lengths of Gd-O and Ce-O of one [LnO9] (Ln = Gd or Ce) for Gd1-xCexPO4 (x = 0, 0.25, 0.5, 0.75 and 1) monazites GdPO4

(Å)

[GdO9]

[GdO9]

[CeO9]

[GdO9]

[CeO9]

[GdO9]

[CeO9]

[CeO9]

Ln-O1

2.35147

2.33186

2.43263

2.29246

2.43094

2.34190

2.43094

2.43974

Ln-O2

2.35240

2.35068

2.43971

2.45352

2.44523

2.37141

2.44314

2.46225

Ln-O3

2.36467

2.37708

2.45460

2.4668

2.45523

2.39272

2.46158

2.47029

Ln-O4

2.39966

2.38515

2.48385

2.49282

2.46373

2.40920

2.46852

2.49508

Ln-O5

2.41475

2.43120

2.48903

2.50347

2.49493

2.44294

2.47956

2.50726

Ln-O6

2.43139

2.43824

2.51115

2.54259

2.49667

2.48235

2.56608

2.54864

Ln-O7

2.45249

2.53413

2.51986

2.56958

2.51401

2.48834

2.58294

2.57105

Ln-O8

2.54264

2.56195

2.58972

2.63247

2.59547

2.58170

2.64383

2.63901

Ln-O9

2.72857

2.73382

2.70549

2.76701

2.80345

2.74359

2.77086

M ED PT CC E A

Gd0.25Ce0.75PO4

N

U

SC R

Gd0.5Ce0.5PO4

A

Gd0.75Ce0.25PO4

2.71082

CePO4

IP T

Length

Table 5 The bond population of Gd1-xCexPO4 (x = 0, 0.25, 0.5, 0.75 and 1) monazites by DFT calculations Composition

Population (Ce-O)1-9

(P-O)1-4

GdPO4



0.08 - 0.24

0.62 -0.67

Gd0.75Ce0.25PO4

0.05 - 0.31

0.04 - 0.22

0.63 - 0.69

Ce0.5Gd0.5PO4

0.08 - 0.32

0.06 - 0.22

0.62 - 0.68

Gd0.25Ce0.75PO4

0.07 - 0.32

0.06 - 0.23

0.62 - 0.68

GdPO4

0.06 - 0.28



0.63 - 0.69

SC R

U N A M ED PT CC E A

IP T

(Gd-O)1-9

Table 6 The formation energies of Gd for Gd1-xCexPO4 (x = 0, 0.25, 0.5, 0.75 and 1) monazites by DFT calculations Composition

Defect deformation energy (eV)

Gd0.75Ce0.25PO4

-15.3624

Gd0.5Ce0.5PO4

-14.8979

Gd0.25Ce0.75PO4

-14.8169

CePO4

–

SC R

-15.6738

A

CC E

PT

ED

M

A

N

U

GdPO4

IP T

[GdO9]