Heavy ion irradiation-induced microstructural evolution in pyrochlore Lu2Ti2O7 at room temperature and 723 K

Journal of Solid State Chemistry 231 (2015) 159–162

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Heavy ion irradiation-induced microstructural evolution in pyrochlore Lu2Ti2O7 at room temperature and 723 K Q.R. Xie, J. Zhang n, X.N. Dong, Q.X. Guo, N. Li College of Energy, Xiamen University, Xiamen, Fujian 361005, China

art ic l e i nf o

a b s t r a c t

Article history: Received 29 April 2015 Received in revised form 7 July 2015 Accepted 4 August 2015 Available online 5 August 2015

Polycrystalline pyrochlore Lu2Ti2O7 pellets were irradiated with 600 keV Kr3 þ at room temperature and 723 K to a fluence of 4  1015 ions/cm2, corresponding to an average ballistic damage dose of 10 displacements per atom in the peak damage region. Irradiation-induced microstructural evolution was examined by grazing incidence X-ray diffraction, and cross-sectional transmission electron microscopy. Incomplete amorphization was observed in the sample irradiated at room temperature due to the formation of nano-crystal which has the identical structure of pyrochlore, and the formation of nano-crystal is attributed to the mechanism of epitaxial recrystallization. However, an ordered pyrochlore phase to a swelling disordered fluorite phase transformation is occurred for the Lu2Ti2O7 sample irradiated at 723 K, which is due to the disordering of metal cations and anion vacancies. & 2015 Elsevier Inc. All rights reserved.

Keywords: Phase transformation Nano-crystal Ion irradiation

1. Introduction Pyrochlore (belongs to a space group , no. 227, in International Tables for Crystallography) is a superstructure of the fluorite (MO2), except that there are two cation sites and one-eighth of missing anions [1]. The pyrochlore compounds with composition have attracted more attention in recent years [2]. The large A3 þ cation is at 16d site, eight-coordinated and located within a distorted cubic coordination polyhedron, while the smaller B4 þ cation is at 16 c site, six-coordinated and located within a distorted octahedron [3]. The (3 þ ,4þ) pyrochlores are of interest in nuclear waste management because of their ability to incorporate trivalent lanthanides and tri- and tetravalent actinides. Incorporation of actinides into ceramics is an important issue for the immobilization of actinide-bearing waste streams. Pyrochlore is a potential candidate for the immobilization of high level waste (HLW) and dismantled plutonium due to their wide incorporation ability for actinides and rare earth elements [4–6]. On the other hand, the naturally occurring pyrochlores are often found to be metamict (aperiodic) as a result of radiation damage from uranium and thorium transmutation [7]. An important aspect of assessing the physical and chemical durability of pyrochlore as a nuclear waste form is to understand atomic-scale changes caused by α-decay event damage [8,9]. Irradiation damage, especially amorphization, generally decreases the chemical durability of nuclear waste form in the repository environment because of the n

Corresponding author. E-mail addresses: [email protected], [email protected] (J. Zhang).

http://dx.doi.org/10.1016/j.jssc.2015.08.005 0022-4596/& 2015 Elsevier Inc. All rights reserved.

lower thermodynamic stability and higher impregnation rate of the irradiation damaged materials. During the past two decades, heavy ion irradiation experiments, substituting for short half-life actinide-doping (such as 244Cm), were used to simulate α-decay event in waste form, due to its ease of handling the irradiation samples over a much shorter period of time. The radiation susceptibility of nuclear form pyrochlore-Lu2Ti2O7 had been examinaed, and previous liquid nitrogen (LN2, 77 K) irradiation and in-situ TEM studies demonstrated that the pyrochlore ceramic can become fully amorphous under 1 dpa [3,10]. However, the pyrochlore-Lu2Ti2O7 can not become completed amorphous in our bulk sample irradiated at room temperature, even under a peak dpa of ∼10 because of the nano-crystals formation. Furthermore, previous studies pay their attention to the radiation effects below critical temperature, while few irradiation-induced evolutions of pyrochlore-A2Ti2O7 above critical temperature are reported. In this study, the pyrochlore Lu2Ti2O7 samples, which have the lowest critical temperature Tc ¼473 K in pyrochlore-A2Ti2O7 [3], were irradiated with 600 keV Kr3 þ ions to a fluence of 4  1015 ions/cm2 at RT and 723 K. The grazing incidence X-ray diffraction (GIXRD), and ex-situ TEM were implemented to determine the microstructural evolution induced by Krypton ion irradiation under (RT) and above critical temperature (723 K) in bulk samples.

2. Experimental Polycrystalline pyrochlore Lu2Ti2O7 pellets were prepared by traditional ceramic processing. The Lu2O3 (99.99% pure) and TiO2 (99.99% pure) powers were first heated at 1000 °C for 10 h to

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remove moisture and other volatile impurities. Stoichiometric amounts of the reactants were weighed to acquire the composition of Lu2Ti2O7. Firstly, the thoroughly ground mixtures were heated in the pellet form at 1300 °C for 24 h. In order to attain a better homogeneity, the products obtained after first heating were reground, pelletized, and sintered at 1450 °C for 48 h. Then, pellets were cut with a diamond saw, and the specimens were polished to a 0.5 μm diamond finish. These pellets were examined with normal X-ray Diffraction, and found possess the pyrochlore structures. The measured density of these samples was found to be 7.038 g/cm3, approximately 96% of their theoretical value (g/cm3). The well-polished samples were irradiated with 600 keV Kr3 þ ions at room temperature (RT) and 723 K in the Ion Beam Materials Laboratory at Los Alamos National Lab, using a 200 kV Danfysik High Current Research Ion Implanter. The irradiation fluence is 4.0  1015 Kr3 þ /cm2. The 600 keV Kr3 þ ions were implanted at normal incidence with an average flux of  1.0  1012 Kr/cm2/s. The displacement damage and projectile depth profile of 600 keV Kr3 þ ions in pyrochlore Lu2Ti2O7 were estimated using the Monte Carlo program SRIM [11], where the threshold displacement energies of 40 eV were arbitrarily assumed for all atoms in the oxide ceramic. The simulated displacements per atom (dpa) and Kr ion concentration for the fluence of 4.0  1015 Kr3 þ /cm2 are shown in Fig. 1. The irradiation peak damage range, Rp, was estimated to be approximately 120 nm with a longitudinal straggling, ΔRd, of 60 nm. Pristine and irradiated samples were characterized using grazing incidence X-ray diffraction (GIXRD). GIXRD measurements were performed using a Rigaku Ultima IV Advanced X-ray diffractometer, with Cu Kɑ radiation, α  2θ geometry, parallel beams. The α  2θ scans were performed using a step size of 0.02° and a dwell time of two seconds. X-ray patterns were recorded at a fixed glancing incidence angle of α ¼ 0.5° to investigate only the irradiation layer, and the implanted ion chemical effects can be ignored at the fluence of 4.0  1015 Kr3 þ /cm2 (Kr concentration is still less than 0.3 at% in the examined region). The penetration depth of X-ray is within 200 nm at incidence angle α ¼0.5° [12,13]. Integrated diffraction peak intensities were measured by fitting the diffraction patterns with pseudo-Voigt profiles to estimate the amorphization fraction. The cross-sectional TEM specimen was mechanically polished to a thickness below 10 μm using a diamond lapping film, followed by thinning to electron transparency using 4 keV Ar þ ion milling. TEM observations of the sample irradiated at room temperature were performed using a Tecnai F30 instrument operating with an accelerating voltage of 300 kV, and

the sample irradiated at 723 K were performed using a Tecnai G2 F20 S-Twin operating with an accelerating voltage of 200 kV. Selected area electron diffraction (SAED) patterns were used in this study to obtain electron diffraction patterns from small sample regions (diameter of ∼150 nm).

3. Results and discussion Fig. 2 shows the GIXRD patterns obtained from pristine Lu2Ti2O7 and Lu2Ti2O7 subjected to 600 keV Kr3 þ ions to a fluence of 4.0  1015 Kr3 þ /cm2 at room temperature and 723 K. The incidence angle of X-ray is 0.5°, and the penetration depth of X-ray is estimated less than irradiation layer. The diffraction patters were normalized and offset for clarity. The diffraction patterns from the pristine sample are composed of principal diffraction maxima (labeled as P(222), P(400), P(440)) and supperlattice reflections (labeled as P(111), P(311), P(331), P(511), P(531)). After irradiation to fluence of 4.0  1015 Kr3 þ /cm2 at RT, all the diffraction patterns remain, while the diffraction peaks shift to smaller 2 angle, which is indicating a slightly swelling of lattice parameters. Moreover, an obvious diffusion halo shown up with a center at the 2θ ¼32°, which indicates an amorphous phase formed in the examination area. Therefore, the GIXRD observations imply that the sample irradiated at RT was partially amorphous with ∼75% of amorphization in the basis of diffraction intensities calculation. As for irradiation at 723 K, the supper-lattice diffraction intensities diminish significantly. Besides, a set of diffraction patterns (marked with stars) are shown up on smaller 2θ side of pristine pyrochlore diffraction maxima, which can be indexed as (111), (200), and (220) plans of swelling defect fluorite phase. Therefore, a pyrochlore to a partial defect fluorite phase transformation was observed in the pyrochlore Lu2Ti2O7 irradiated by 600 keV Kr3 þ to a fluence of 4.0  1015 Kr3 þ /cm2 at 723 K. Fig. 3 shows the cross-section TEM micrograph and corresponding SAED diffraction patterns of Lu2Ti2O7 samples subjected to 600 keV Kr3 þ ion irradiation to a fluence of 4  1015 ions/cm2 at RT. The cross-section TEM image Fig. 3(a) demonstrates two distinctive diffraction contrasts due to the 600 keV Kr irradiation damage in pyrochlore Lu2Ti2O7, and the thickness of damage layer is ∼150 nm, in a reasonable agreement with the ion range of 600 keV Kr ions predicted by SRIM, Rp þ ΔRp (180 nm) shown in Fig.1. Furthermore, the SAED pattern Fig. 3(b) implies that nanosize crystals were formed in the damage layer, while Fig. 3

Normalized Intensity (a.u.)

P

0.35

8

0.25

0.20

6

0.15 4 0.10

Ion Concentration (at.%)

Displacement Damage (dpa)

0.30

2

Normalized Intensity (a.u.)

10

F P

F

2 Theta (°)

*

F(200)

F(220)

*

P

0.05

P

P

*

723 K

RT

P

P P

P

P

Pristine

0.00

0 0

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200

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400

Depth(nm) Fig. 1. SRIM simulation results of displacement damage and implanted Kr3 þ ion concentration as a function of depth for 600 keV Kr3 þ ion irradiation in Lu2Ti2O7 to a fluence of 4.0  1015 ions/cm2.

10

20

30

40

50

60

2 Theta (°) Fig. 2. GIXRD patterns of pristine Lu2Ti2O7 pellets and Lu2Ti2O7 irradiated by 600 keV Kr3 þ to a fluence of 4.0  1015 ions/cm2 at room temperature and 723 K (the incident angle of X-ray α¼ 0.5°).

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Fig. 3. Cross-section TEM micrograph (a), SAED diffraction patterns (b) and (c), HRTEM image (d) of Lu2Ti2O7 samples subjected to 600 keV Kr3 þ ion irradiation with a fluence of 4  1015 ions/cm2 at room temperature.

(c) indicates that the observation substrate layer was still single crystal. Moreover, Fig.3(d) shows the high magification TEM images of the surface damage layer, where the nano-crystals are observed with a diameter of∼10 nm, and the inset Fast Fourier Transform (FFT) image confirm nano-crystals are formed within the high magnification TEM observation area. Meanwhile, the Moire frings marked with arrow in Fig. 3(d), resulting from the interference between different crystals, were also observed. Fig. 4 shows the cross-section TEM micrograph and corresponding SAED patterns of Lu2Ti2O7 samples irradiated by 600 keV Kr3 þ at 723 K with a fluence of 4  1015 ions/cm2. Fig. 4(b,c) are corresponding SAED diffraction patterns of Lu2Ti2O7 samples from irradiated layer and substrate respectively. As a superstructure of fluorite, the diffraction pattern from pyrochlore contains two subsets of intensity maxima in Fig. 4(c). One is from the primary fluorite sub-cell; the other is from the supercell of ordered pyrochlore. After ion irradiation at 723 K, the diffraction spots from supercell disappeared, while the diffraction spots from primary fluorite sub-cell still existed in Fig.4(b), which means the ordered pyrochlore structure transfers to disordered fluorite structure. The formation of nano-crystals has been observed in many synthetic oxides ceramic irradiated by energetic ions and natural compounds suffered alpha-decay over geologic time [14–16]. According to a study on mechanism of nano-crystal formation, the

ion irradiation-induced nano-crystal formation occurs near the critical amorphization temperature Tc, which is a result of the competition between the formation of amorphous and crystal recovery [14,17,18]. Generally speaking, the simultaneous recovery processes are mainly associated with epitaxial recrystallization at the crystalline/amorphous interface or nucleation-growth recrytallization in bulk of amorphous volume [19]. At room temperature irradiation, the quenching rate after thermal spike induced by damage cascade is relatively faster, and the disordered zones are partially reserved to form amorphization. Meanwhile, point defects created by incident ions have a high mobility due to irradiation-enhanced diffusion, this leads to a rapid defect recovery of amorphous regions along the crystalline/ amorphous interface. Thus, nano-crystals can be formed around the undamaged crystalline domains with epitaxial recrystallization, and have an orientation related to the pristine crystalline matrix. In Fig. 4(d), within a small region of 40 nm, the nanocrystals have a similar orientation with a slightly deviation due to the local anisotropic strain field created by the formation of nanocrystals and amorphous volumes. Meanwhile, the existence of same orientation nano-crystals further testifies that epitaxial recrystallization dominates the formation of the nano-crystal, while nucleation-growth recrystallization mechanism can be ignored due to the low irradiation temperature.

Fig. 4. Cross-section TEM micrograph (a) and corresponding SAED diffraction patterns (b and c) of Lu2Ti2O7 samples subjected to 600 keV Kr3 þ ion irradiation to a fluence of 4  1015 ions/cm2 at 723 K.

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As for irradiation at 723 K (much higher than critical amorphous temperature), amorphous and nano-crystals were not discovered. Both GIXRD and TEM results above confirmed that the Kr ion irradiation induced the phase transformation of ordered Lu2Ti2O7 to a disordered fluorite structure. The pyrochlore Lu2Ti2O7 can be described as two interpenetrating face-centered cubic (fcc) cation sublattices and a single cubic (sc) oxygen sublattice with one-eighth of missing anions. The periodic configuration of cations and anions results in the ordered arragement of cations and oxygen vacancies. However, the ordered to disordered phase transformation of Lu2Ti2O7 induced by ion irradiation is apparently due to the rearrangements of atoms both on cation and anion sublattices. Upon the ion irradiation, the cation sublattices and oxygen vacancies become disordered. Once the cation antisites and anion vacancies fully disordered at certain irradiation dose, the pristine pyrochlore phase will transform into fluorite Lu2Ti2O7. The two interpenetrating fcc cation sublattices turned into a fcc M cation sublattices, where each M site is occupied by a probability of 50% Lu and 50% Ti, and each O site is occupied by O atom by a probability of 87.5%. Thus, the pyrochlore to fluorite phase transformation is a rearrangement process of cation and anion sublattices, and a similar experimental results were observed under 200 keV He ions irradiated at room temperature [20].

4. Conclusions Pyrochlore Lu2Ti2O7 were irradiated with 600 keV Kr3 þ ions to a fluence of 4  1015 ions/cm2 at RT and 723 K. At RT, ∼75% of amorphization was achieved according to the GIXRD results, and nano-crystals with diameters of 10 nm were formed. The formation of nano-crystals can be attributed to the mechanism of epitaxial recrystallization. However, amorphization was not observed by Kr3 þ irradiation at 723 K, and a pyrochlore underwent to a swelling fluorite structure. The phase transformation was attributed to the rearrangement of both cations ions and oxygen vacancies.

Acknowledgments This work was sponsored by the National Natural Science Foundation of China with Grant no. 11205128. The authors thank Y. Q Wang at Ion Beam Materials Lab in LANL for providing ion irradiation support.

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