Synthesis and structures of high-entropy pyrochlore oxides

Journal Pre-proof Synthesis and structures of high-entropy pyrochlore oxides Zhen Teng, Lini Zhu, Yongqiang Tan, Sifan Zeng, Yuanhua Xia, Yiguang Wang, Haibin Zhang

PII:

S0955-2219(19)30852-0

DOI:

https://doi.org/10.1016/j.jeurceramsoc.2019.12.008

Reference:

JECS 12921

To appear in:

Journal of the European Ceramic Society

Received Date:

28 October 2019

Revised Date:

3 December 2019

Accepted Date:

4 December 2019

Please cite this article as: Teng Z, Zhu L, Tan Y, Zeng S, Xia Y, Wang Y, Zhang H, Synthesis and structures of high-entropy pyrochlore oxides, Journal of the European Ceramic Society (2019), doi: https://doi.org/10.1016/j.jeurceramsoc.2019.12.008

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Synthesis and structures of high-entropy pyrochlore oxides

Zhen Tengab, Lini Zhua, Yongqiang Tana, Sifan Zenga, Yuanhua Xiaa, Yiguang Wangc* and Haibin Zhanga*

a

Innovation Research Team for Advanced Ceramics, Institute of Nuclear Physics and

b

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Chemistry, China Academy of Engineering Physics, Mianyang, 621900, China.

Department of Materials Science, Fudan University, 220 Handan Road, Shanghai

200433, China.

Institute of Advanced Structure Technology, Beijing Institute of Technology, Haidian

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c

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District Beijing, 100081, China.

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* Corresponding authors: Y. G. Wang, and H. B. Zhang,

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Abstract

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E-mail address: [email protected], [email protected] .

Multicomponent oxides with pyrochlore structure (A2Zr2O6O’) containing up to 7

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different cations with equiatomic amounts have been successfully synthesized, which broadens pyrochlore solid solutions to high entropy pyrochlore oxides. XRD and Raman results indicate that all compositions possessed single-phase pyrochlore structure and the HAADF-STEM images with corresponding EDS mapping demonstrate that all cations were randomly and homogeneously distributed. This new

class of high-entropy pyrochlore oxides may open a new research direction in pyrochlore-based materials.

Keywords: High-entropy oxides, Pyrochlore (A2B2O6O’), Zirconate, Multicomponent oxides.

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1. Introduction

Entropy manipulation has become a novel tool for properties modification of

traditional ceramics [1-5]. Since Rost et al successfully synthesized the

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entropy-stabilized single-phase (Mg0.2Zn0.2Cu0.2Co0.2Ni0.2)O in 2015 [6], a series of

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high-entropy ceramics with different crystal structures including fluorite oxides [7, 8], spinel oxides [9], perovskite oxides [10], monosilicates [11], carbides [12] and

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diborides [13] have been fabricated in the past few years. In addition, those

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high-entropy ceramics and their derivatives usually exhibit some unique properties such as much lower thermal conductivity [14, 15], higher mechanical strength [12,

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16], excellent electrochemical properties [17, 18], colossal dielectric constant [19]. Pyrochlore oxide is an important class of structural/functional ceramic and has a wide

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range of applications, such as solid oxides ion conductors [20-22], thermal barrier coatings [23-25] and a promising host matrix for the immobilization of minor actinides [26-28]. The ideal pyrochlore has the formula A2B2O6O’, where A sites can be rare earth elements and B sites can be transition metals or post-transition metals. The larger A3+ cation is coordinated to eight oxygen atoms (two 8b oxygen ions and

six 48ƒ oxygen ions). The smaller B4+ cation is coordinated to six oxygen ions located on the 48ƒ. There is an oxygen vacancy at 8a, which is surrounded by four B4+ cations. It is feasible to extend pyrochlore solid solution to high entropy pyrochlore oxides due to a large number of possible cations at both A and B sites employing the high-entropy design concept, the high-entropy pyrochlore oxides may be endowed with higher functional tunability or unexpected physical properties, compared with

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traditional pyrochlore oxides with one or two cations in the leading position. Nevertheless, the synthesis of high-entropy pyrochlore oxides containing up to 7 different cations has not been reported so far.

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Herein, we successfully synthesized a series of multicomponent, equiatomic

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single-phase, pyrochlore oxides via solid-state reaction. The phase composition, Raman spectroscopy, micro-scale EDS elemental mapping and atomic-resolution

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STEM with corresponding EDS mapping were investigated. The results clearly

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demonstrate that the seven oxides are single-phase pyrochlore oxides and all the rare earth elements are homogeneously distributed at the A sites.

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2. Experimental procedure

2.1. Synthesis of high entropy pyrochlore oxides

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In this study, seven compositions were synthesized. For the sake of facilitating presentation, we named them: #A1ZO, #A2ZO, #A3ZO, #A4ZO, #A5ZO, #A6ZO, #A7ZO, where “A and number” refers to corresponding elements types located at A sites; “ZO” refers to Zirconium and Oxygen element. The targeted stoichiometry for each composition is listed in Table 1.

Multicomponent pyrochlore oxides were synthesized using solid-state reaction method. Pyrochlore oxides were synthesized from the following oxides: ZrO2, La2O3, Nd2O3, Sm2O3, Eu2O3, Gd2O3, Dy2O3, Ho2O3 (Aladdin, 99.99% purity). All the powders were firstly preheated at 120 ℃ for 6 h to remove the moisture. Then the raw powders were weighted precisely with designed molar ratios and mixed by ball milling for 10 h with ethanol as milling medium. After drying, the milled powders

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were compacted into 3/4 inch-diameter pellets and cold-isostatic pressed at 200 MPa for 5 min. Finally, the pellets were sintered at 1500 ℃ for 40 h in air and cooled to room temperature naturally.

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2.2. Characterization

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X-ray diffraction (XRD) using Cu Kα radiation (λ = 1.54 Å) was used to identify the crystal structure and phase composition of oxides with a step scan of 0.02° and 10 s

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per step. XRD refinements were performed by Rietveld refinement software

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(Fullprof). Raman spectra were recorded on a confocal Raman spectroscopy system (Renishaw, RM-1000) using a He-Ne laser (λ = 532nm) in the range of 200-800 cm-1.

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The elemental distribution of samples was carried out on the field emission scanning electron microscope (SEM, Zeiss Supra 50VP, Germany) with energy dispersive

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spectroscopy (Oxford EDS, with INCA software) X-Ray spectrometer. The local homogeneity and elements distribution were characterized by aberration-corrected scanning transmission electron microscopy (AC STEM, Titan Cubed Themis G2300) with an accelerating voltage of 200 kV, which equipped an energy-dispersive X-ray spectroscopy detector.

3. Results and discussion Fig. 1(a) displays the XRD patterns of the seven compositions. As suggested by the presence of the typical super-lattice diffraction peaks (111), (311), (331), (511) and (531) [29], all compositions exhibit a single-phase, pyrochlore structure rather than a defect fluorite structure which generally shows a disordered arrangement of A and B cations. The structural stability of pyrochlore is mainly determined by the ratio

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between the average ionic radius of A and B cations (rA/rB) [30]. The defect fluorite structure is stable when the ratio (rA/rB) smaller than 1.46 and a pyrochlore structure is

favored as the ratio (rA/rB) range of 1.46 to 1.78. The ionic radius of Zr4+ in the

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six-fold coordination is 0.72 Å, while the ionic radius of Gd3+, Eu3+, Sm3+, Nd3+, La3+,

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Dy3+ and Ho3+ are 1.053 Å, 1.066 Å, 1.079 Å, 1.109 Å, 1.16 Å, 1.027 Å and 1.015 Å respectively in eight-fold coordination [31]. The calculation of the ratio (rA/rB) of

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seven compositions range from 1.46 to 1.52 and the as-synthesized oxides would

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present with pyrochlore structure theoretically. Fig. 1(a) clearly testify the authenticity of ratio (rA/rB) rule for high-entropy pyrochlore oxides.

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Furthermore, as shown partially enlarged in Fig. 1(a), the diffraction peaks shift to the lower angle (#A1ZO - #A5ZO) when the larger ionic radius atoms were doped into A

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sites and shifts to the higher angle (#A5ZO - #A7ZO) when doping smaller ionic radius elements. This is because the lattice parameter increase/decrease when the larger/smaller ionic radius of elements are successfully introduced into A sites according to the Vegard’s rule [32]. The structure analysis of seven pyrochlore oxides are refined by the Rietveld method

and the XRD patterns together with refinement results for a typical high-entropy pyrochlore oxides (Gd1/7Eu1/7Sm1/7Nd1/7La1/7Dy1/7Ho1/7)2Zr2O7 are shown in Fig. 1(b). The results of the refinement are listed in table 2. It can be found that the fit curve matches well with the XRD data. The Rp and Rwp factor of the seven pyrochlore oxides are around 1.5 and 2, which means the Rietveld results are reliable. Moreover, when larger ionic radius elements were doped into A sites, the lattice parameters

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gradually increased from 10.53931 Å to 10.66488 Å. The lattice parameters decreased

remarkably from 10.66488 Å to 10.58564 Å when smaller ionic radius elements were doped into A sites.

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In addition, the intensity of (111) is very sensitive to the position of oxygen 48ƒ(x, 1/8,

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1/8) [33] and the parameter of x ranges from 0.3125 (ideal pyrochlore) to 0.375 (defect fluorite) with the increase of disorder for A and B cations. As shown in Table 2,

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the x-parameter of seven oxides are around 0.34, which also demonstrates that the

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seven oxides are pyrochlore structure. It is also consistent with the previous reports which contain one rare earth element [28, 34].

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It is commonly known that Raman spectroscopy is sensitive to the vibrations of oxygen cation and can provide the disorder of local structure and pyrochlore exhibits

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six Raman active vibration modes and distributed as [35, 36]: Γ(Raman) = A1g + Eg + 4F2g

where the modes listed are all optical. For an ideal-pyrochlore, the A and B cations are at the center of reciprocal lattice and they do not contribute to the Raman active vibration modes. Thus the six Raman active vibration modes are associated with

oxygen cation, including one mode (F2g ) assigned to the oxygen cation at 8a and five modes (A1g + Eg + 3F2g ) for the oxygen cation at 48ƒ [37]. There is only one vibration mode (F2g ) that is assigned to the oxygen cation at 8c for the ideal fluorite. Fig.2 displays the Raman spectra of seven oxides and a representative pyrochlore oxide of #A7ZO with corresponding Gaussian peak fit. It is clear that all oxides show four typical Raman vibration modes of pyrochlore, including Eg , F2g (1), A1g ,

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F2g (2) in the range of 200-800 cm-1 without new Raman vibration modes[38]. When different elements were introduced into A sites, only the shift of four Raman vibration

earth elements are introduced into A sites.

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modes can be observed. It indicated that the order of local structure and all the rare

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Fig. 3 shows the detailed Raman shift with corresponding error bars of #A1-7ZO pyrochlore oxides after Gaussian peak, providing a visual comparison of four Raman

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modes. The four Raman modes shift to a lower frequency (red shift) from #A1ZO to

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#A5ZO and then shift to a higher frequency (blue shift) from #A5ZO to #A7ZO. As we know, the Raman modes shift to a lower/higher frequency, suggesting a

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weakening/strengthening of these chemical bonds [37]. However, we note that the shift of Raman modes presents a remarkably similar tendency with XRD patterns. The

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results convey that the red/blue shift of Raman models is mainly caused by the increase/decrease of lattice parameter and possesses long-range ordering of the local structure. Consequently, it can be concluded that the increase/decrease of lattice parameter caused by larger/smaller ionic radius of elements introduced into A sites leads to the elongation/shorten of related chemical bonds, and finally results in the

red/blue shift of Raman models. To better understand the elemental distribution in oxides, the cross-section SEM and corresponding EDS mapping were performed. As shown in Fig. 4, all elements were homogeneously dispersed in the matrix without any segregation or clustering, verifying compositional uniformity in the micro-scale. And the EDS mapping are in good agreement confirming the phase-purity of pyrochlore oxides.

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Atomic-resolution HAADF-STEM images and corresponding EDS analysis were performed to examine the local chemical homogeneity and elements distribution of A

site cations. As shown in Fig. 5 (a), the HAADF-STEM images exhibit sharp lattice

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fringes along the [110] zone axis, which means the excellent lattice integrality and the

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compositional homogeneity on an atomic-scale. The measured lattice parameter by STEM image is 10.63192 Å, which is close to the refinement result by XRD of

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10.58564 Å. It is interesting to note that the diffraction spots of STEM image exhibit

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changes in sizes and arranged on a certain distribution rule. Therefore, based on the crystal structure of #A1ZO, the schematic diagram of the pyrochlore structure of

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#A7ZO by VEATA is presented in Fig. 5 (b). As marked with yellow diamond frame, it is clear that four vertices of the diamond are filled with the smaller B sites atoms of

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Zr element, the center of diamond is composed of the bigger A sites atoms of rare earth elements and the center of four sides are formed by the alternately arranged of big A sites atoms and smaller B sites atoms along the [110] direction. It is the ordering arrangement of A and B atoms that caused the different sizes of diffraction spots. Therefore the HAADF-STEM images reveal an ordering of A and B atoms in

the space structure and all the rare earth elements were doped into A sites successfully on an atomic scale. The atomic structure models are marked with different colors in the insert magnified image in Fig. 5 (a). In order to examine the local distribution of A-site constituent, the energy-dispersive X-ray spectroscopy was performed. The eight dissimilar elements mapping of Gd, Eu, Sm, Nd, La, Dy, Ho, Zr are shown in Fig. 5 (c). As can be seen, all the elements were

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distributed homogeneously and randomly in the observed region on an atomic-scale

without any segregation or clustering. Furthermore, in Fig. 5 (d), the superposition of HAADF and Zr shows that the Zr elements overlap with diffraction spots partly and

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regularly, which is consistent with the previous discussion. The same phenomenon in

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the superposition of Zr with Gd and Dy can be observed.

Therefore, by comprehensive consideration of the analysis of XRD patterns, Raman

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spectroscopy, EDS mapping and STEM images with corresponding EDS mapping, it

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can be concluded that rare earth elements are equally and randomly doped into the A sites and led to the ordering of pyrochlores with high configurational entropies.

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Conclusions

In this paper, we successfully further extend traditional pyrochlore solid solutions to

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high entropy pyrochlore oxides with multicomponent (contain 2-7 component), equiatomic and high configurational entropies (on the A cation sublattices) by solid-state reaction. The XRD patterns and EDS mapping found that all the compositions possessed single-phase solid solutions of pyrochlore structure on the macro aspect. Meanwhile, the Raman and HAADF-STEM images with corresponding

EDS mapping confirm that all rear earth elements are randomly and homogeneously distributed in the A sites on the micro aspect. It is promising that this unique structure and tunability of compositions may lead to potential applications on thermal barrier coatings and nuclear materials.

Acknowledgments

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Haibin Zhang is grateful to the Foundation by the Recruitment Program of Global

Youth Experts and the Youth Hundred Talents Project of Sichuan Province. This work is supported by the Science Development Foundation of China Academy of

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Engineering Physics and National Natural Science Foundation of China (No.

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11905194).

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Figure captions

Fig. 1 (a) XRD patterns of #A1-7ZO pyrochlore oxides and (b) XRD patterns together with Rietveld fit of #A7ZO. Fig. 2 Raman spectra of #A1-7ZO pyrochlore oxides. The inset shows Gaussian peak fit of the #A7ZO.

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Fig. 3 Raman shift with corresponding error bars of four Raman modes of #A1-7ZO pyrochlore oxides.

Fig. 4 Cross-section SEM images and corresponding EDS sample mapping of

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#A1-7ZO pyrochlore oxides.

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Fig. 5 STEM images and EDS analysis of a representative pyrochlore oxide, #A7ZO. (a) Atomic-resolution HAADF-STEM images with different magnification along the

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[110] direction. The inset shows a higher magnification with overlay structural model

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and atomic structure. (b) The simulated model of ideal pyrochlore structure along the [110] zone axis using VESTA. (c) Elemental mapping of #A7ZO composes of eight

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dissimilar elements (Gd, Eu, Sm, Nd, La, Dy, Ho, Zr). (d) the superposition of

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HAADF and Zr, Zr and Gd, Zr and Dy.

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Table. 1 Composition and corresponding abbreviation of seven oxides. Composition

#A1ZO

Gd2Zr2O7

#A2ZO

(Gd1/2Eu1/2)2Zr2O7

#A3ZO

(Gd1/3Eu1/3Sm1/3)2Zr2O7

#A4ZO

(Gd1/4Eu1/4Sm1/4Nd1/4)2Zr2O7

#A5ZO

(Gd1/5Eu1/5Sm1/5Nd1/5La1/5)2Zr2O7

#A6ZO

(Gd1/6Eu1/6Sm1/6Nd1/6La1/6Dy1/6)2Zr2O7

#A7ZO

(Gd1/7Eu1/7Sm1/7Nd1/7La1/7Dy1/7Ho1/7)2Zr2O7

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Abbreviation

Table. 2 Refined parameters of seven oxides. Rp

Rwp

Lattice parameter (Å)

x-parameter

rA/rB

#A1ZO

1.43

1.93

10.53931

0.34093

1.4645

#A2ZO

1.30

1.89

10.55246

0.33365

1.4735

#A3ZO

1.39

1.95

10.56785

0.33438

1.4826

#A4ZO

1.88

2.43

10.59656

0.33682

1.4975

#A5ZO

1.69

2.34

10.66488

0.33931

1.5179

#A6ZO

1.66

2.20

10.61128

0.33311

1.5030

#A7ZO

1.34

1.87

10.58564

0.33133

1.4899

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Composition