Fabrication and phase transition of La2−xLuxZr2O7 transparent ceramics

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ScienceDirect Journal of the European Ceramic Society 34 (2014) 3951–3958

Fabrication and phase transition of La2−xLuxZr2O7 transparent ceramics Zhengjuan Wang a,b , Guohong Zhou a,∗ , Xianpeng Qin a , Fang Zhang a , Jianping Ai a,b , Peng Liu a,b , Shiwei Wang a,∗ a

State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, China b University of Chinese Academy of Sciences, Beijing 100049, China Received 24 January 2014; received in revised form 20 May 2014; accepted 25 May 2014 Available online 21 June 2014

Abstract A series of transparent ceramics with the composition of La2−x Lux Zr2 O7 (x = 0−2.0) were prepared by solid-state reactive sintering in vacuum. With the increase of Lu content (x), phase transition from pyrochlore to defective fluorite occurred and a two-phase region existed in the range of x = 0.6−1.2. Grain sizes of the pyrochlore phase dominated samples (x < 0.5) were 11−14 ␮m, and that of the defective fluorite phase dominated samples were larger than 60 ␮m. However, grain sizes of the samples in the two-phase region were smaller than 3 ␮m. The La0.8 Lu1.2 Zr2 O7 ceramic with the smallest grain size (∼2.5 ␮m) reached a highest in-line transmittance of 72.4% at 1100 nm among all the samples. © 2014 Elsevier Ltd. All rights reserved. Keywords: La2−x Lux Zr2 O7 ceramics; Vacuum sintering; Phase transition; Grain size

1. Introduction Lanthanide zirconates (Ln2 Zr2 O7 ) are incorporated into the A2 B2 O7 oxides, which are pyrochlore structure in most cases until the ordering of the two cations (A3+ , B4+ ) is disturbed and defective fluorite structure appears.1 The structure can be decided by the ionic radius ratios (rA /rB ). Generally, the pyrochlore structure is favored when the lanthanide ionic radius is larger than that of Gd3+ , and the smaller lanthanide ions tend to form defective fluorite structure.2,3 The order–disorder transition from pyrochlore structure to defective fluorite structure can also occur at a transition temperature.4 Besides, phase transition can happen from cubic pyrochlore to monoclinic under increasing pressure for Ln2 Zr2 O7 (Ln = Ce, Nd, Gd). The cubic and monoclinic phases coexist over a wide pressure range which increases with the ionic radius of Ln.5 Materials with Ln2 Zr2 O7 composition have been drawing the attention of researchers for a diversity of applications,

∗

Corresponding authors. Tel.: +86 021 52414320; fax: +86 021 52415263. E-mail addresses: sic [email protected] (G. Zhou), [email protected] (S. Wang). http://dx.doi.org/10.1016/j.jeurceramsoc.2014.05.046 0955-2219/© 2014 Elsevier Ltd. All rights reserved.

such as fast-ion conductors, photocatalysts,6 thermal barrier coatings (TBCs),5,7–10 matrices for immobilization of highly active radionuclides from nuclear wastes11 and host materials for scintillators.12 In previous researches, most of the Ln2 Zr2 O7 compounds with two lanthanide cations in Ln position were conventional ceramics fabricated at relatively lower temperatures compared to transparent ceramics. And what they focused on were crystal structure, changes on lattice parameters, oxygen position and so on, which would be applied in the field of fast-ion conductors, photocatalysts, TBCs and immobilization of actinide species in nuclear wastes. In other words, Ln2 Zr2 O7 compounds with two lanthanide cations have been rarely fabricated into transparent ceramics up to now and the corresponding characteristic and application on scintillators have received little attention. In our previous work, LaGdZr2 O7 transparent ceramic was fabricated and this transparent ceramic is a promising candidate for host materials of ceramic scintillators.13 Thus, further study on fabrication and properties of Ln2 Zr2 O7 transparent ceramics is of great value. Ln2 Zr2 O7 compounds with two different cations in Ln position include single and multi phase structure. For example, previous studies14–18 showed that (La1−x Ndx )2 Zr2 O7 (x = 0–1.0), (Ndx Gd1−x )2 Zr2 O7 (x = 0–1.0), (Sm1−x Lax )2 Zr2 O7

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(x = 0–1.0), and Gd2−y Cey Zr2 O7 (y = 0–2.0) were all pyrochlore structure, and Ybx Gd2−x Zr2 O7 (x = 0, 1.0 and 2.0) were all defective fluorite structure. Besides, in Sm2−x Dyx Zr2 O7 system, phase transition from pyrochlore to defective fluorite occurred when Dy3+ concentration increased to 40 mol% (x = 0.8).6 However, there are also two-phase regions in some compounds. In 2009, the crystal structures of La2−x Yx Zr2 O7 (0.0 ≤ x ≤ 2.0) and the stability regions of the pyrochlore and fluorite solid solutions were studied by Whittle et al. using neutron powder diffraction and electron microscopy.19 A two-phase region was found to exist and the limit composition of stable pyrochlore in the solid solution was close to La0.8 Y1.2 Zr2 O7 . Similar phenomenon was found for (La1−x Ybx )2 Zr2 O7 (x = 0–1) system, of which the phase structure transformed from pyrochlore to a two-phase region (pyrochlore and fluorite) and to fluorite.10 According to the rule of Subramanian et al.2 , pyrochlore phase is favored when the ionic radius ratio (rA /rB ) of the cations lies within the range of 1.46–1.78. When rA /rB is smaller than 1.46, anion-deficient fluorite is the stable structure. Besides, a monoclinic structure is formed when rA /rB is higher than 1.78. The above systems with Ln = La–Nd, Nd–Gd, Sm–La, Gd–Ce and Sm–Dy are all consistent with this rule except for La2−x Yx Zr2 O7 and (La1−x Ybx )2 Zr2 O7 with two-phase regions. This can be contributed to the large difference of the ionic ˚ radius of the two cations in Ln position (CN = 8: rLa 3+ = 1.160 A, 3+ 3+ 3+ ˚ ˚ rY = 1.019 A, rYb = 0.985 A). In the present work, La ˚ CN = 8) and Lu3+ (rLu 3+ = 0.977 A, ˚ CN = 8) (rLa 3+ = 1.160 A, with the largest difference in ionic radius were chosen as two lanthanide cations in Ln position. Here, A = La3+ and Lu3+ , B = Zr4+ ˚ CN = 6). Based on Subramanian’s theory,2 we (rZr 4+ = 0.720 A, could conjecture that La2−x Lux Zr2 O7 are pyrochlore structure when x < 1.189, and defective fluorite structure when x > 1.189 (when x = 1.189, rA /rB = 1.46). Whereas, a two-phase region would appear according to the analysis of La2−x Yx Zr2 O7 19 and (La1−x Ybx )2 Zr2 O7 10 . As both pyrochlore (space group: Fd-3m) and defective fluorite (space group: Fm-3m) structure belong to cubic system, ceramics with these structures could be made into transparent ceramics via solid-state reactive sintering in vacuum. On the other hand, except for transparency, higher density and effective atomic number are essential to obtain higher Xray or ␥-ray stopping power for scintillators.20 The doping of Lu with higher atomic number into La site can improve the density and effective atomic number of La2 Zr2 O7 , therefore making them more possible to be used as host materials for scintillators. The purpose of this study was to investigate the influence of ionic radius ratios on phase stability in La2−x Lux Zr2 O7 and verify the above conjecture, and the results will also help to find out a new type of ceramic scintillators. La2−x Lux Zr2 O7 (x = 0–2.0) ceramics were fabricated by solid-state synthesis and vacuum sintering technology. The crystal structure, phase composition, microstructure and in-line transmittance of the transparent ceramics were investigated. Furthermore, factors that give rise to light scattering and affect transparency in transparent ceramics, such as grain boundaries, residual pores, secondary phases, birefringence21 and surface finishment, were discussed.

2. Experimental procedure The La2−x Lux Zr2 O7 (x = 0, 0.2, 0.4, 0.6, 0.8, 1.0, 1.2, 1.4, 1.6, 1.8 and 2.0) samples were synthesized through a solid-state reaction using commercial powders of La2 O3 (99.99%), Lu2 O3 (99.99%) and ZrO2 (99.99%) as starting materials. Firstly, stoichiometric amounts of the powders with different Lu contents were weighed and mixed by ball milling using zirconia balls as grinding media in ethanol for 20 h. After that, the mixed powders were dried and sieved through 200-mesh screen. The powders were bidirectional pressed into pellets (Ø20 × 2.5 mm) at 20 MPa. Then the pellets were cold isostatically pressed at 200 MPa. The compact pellets were pre-sintered at 1400 ◦ C for 3 h in air to remove residual organics, and then sintered in vacuum at 1830 ◦ C for 6 h. Finally, the samples were annealed at 1500 ◦ C for 5 h in air. The final ceramics were polished on both sides to a thickness of 1 mm for test. The phase compositions of the resultant ceramics were analyzed by X-ray diffraction (XRD, Bruker D8 Focus diffractometer, Germany) with Cu K␣ radiation (λ = 0.15418 nm) in the range of 2θ = 10–80◦ . Morphologies of the thermally etched surfaces of the ceramics were observed with scanning electron microscopy (FE-SEM, JEOL, JSM-6700F, Tokyo, Japan). The phase composition was evaluated by energy disperse spectroscopy (EDS, Oxford X-MaxN 80, Cambridge, UK) on FEI Magellan 400. The in-line transmittance was measured using a spectrophotometer (Varian Inc., Cary 5000, USA) in the range of 200–1100 nm. The average grain sizes of the final ceramics were determined from the average linear intercept length multiplied by a statistical factor 1.56.22 The densities of La2−x Lux Zr2 O7 ceramics were measured by the Archimedes method in distilled water. 3. Results and discussion Fig. 1 shows a photograph of the polished La2−x Lux Zr2 O7 (x = 0–2.0) ceramics vacuum sintered at 1830 ◦ C for 6 h and annealed at 1500 ◦ C for 5 h. Each pellet is about 1 mm thick. With the increase of Lu content, the transparency first increases and then decreases, and finally increases again. XRD patterns of La2−x Lux Zr2 O7 (x = 0–2.0) transparent ceramics are shown in Fig. 2. Based on the XRD data, the corresponding lattice parameters were calculated and listed in Fig. 3. An obvious phase transition is found as Lu content increases (Fig. 2, left part). Compared with pyrochlore La2 Zr2 O7 (PDF#71-2363), as x increases from 0 to 0.4, single pyrochlore structure is observed with the superlattice peaks corresponding to (3 3 1) and (5 1 1) reflections. As x further increases, the diffraction peaks shift to higher angle, indicating that the lattice parameter decreases (Fig. 3, pyrochlore region) in agreement with the Vegard’s law. This is ascribed to the fact that the ionic radius of Lu3+ is smaller than that of La3+ . Two sets of diffraction peaks, corresponding to pyrochlore and defective fluorite phase respectively, were observed in the range of x = 0.6–1.2. The proportion of defective fluorite phase becomes higher when x increases. Accordingly, the proportion of pyrochlore phase becomes lower. From the partial enlarged XRD

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Fig. 1. Polished La2−x Lux Zr2 O7 (x = 0−2.0) ceramics sintered at 1830 ◦ C for 6 h in vacuum and annealed at 1500 ◦ C for 5 h (1 mm thick).

Fig. 2. XRD patterns of La2−x Lux Zr2 O7 (x = 0−2.0) ceramics.

patterns (Fig. 2, right part), the diffraction peaks corresponding to the two phases shift for x = 0.6–1.2 samples, indicating the compositions of the two phases varies with Lu content. The lattice parameters of the two phases also change, as shown in Fig. 3

Fig. 3. Plot of lattice parameter against Lu content in different phase regions.

(two-phase region). When x ≥ 1.4, there is hardly any pyrochlore phase and the defective fluorite phase (compared with defective fluorite Gd2 Zr2 O7 , PDF#80-0471) becomes the dominant phase. Similarly to x = 0–0.4, the diffraction peaks shift to higher angle and the lattice parameters linearly decrease as x increases (Fig. 3, defective flourite region). For x = 2.0 sample, the diffraction peaks are similar to Lu4 Zr3 O12 (PDF#77-0738, rhombohedral). It can be inferred that Lu2 Zr2 O7 did not form with a single defective flourite phase. Instead, a new phase (Lu4 Zr3 O12 ) appeared with a small amount of detectable secondary phase (the small diffraction peaks in Fig. 2 for x = 2.0 sample). The accurate composition remains to be further confirmed. The appearance of a two-phase region illustrated that Subramanian’s theory is not available when there is large difference in ionic radius in Ln position. The final La2−x Lux Zr2 O7 ceramics were polished on both sides and thermal etched in a muffle furnace at 1500 ◦ C for 5 h. The surface microstructures and grain sizes of the ceramics were investigated. Fig. 4 shows the backscattered electron images (BEI) of thermal-etched surfaces of x = 0–0.5 ceramics. For x = 0 and 0.2 samples, the growth of crystal grain was complete with almost straight grain boundaries. However, zigzag grain boundary is observed for x = 0.4 sample and many smaller grains (the

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Fig. 4. Backscattered electron images (BEI) of thermal-etched surfaces of La2−x Lux Zr2 O7 ceramics: (a) x = 0, (b) x = 0.2, (c) x = 0.4 and (d) x = 0.5.

lighter grains) distribute on the grain boundaries (Fig. 4c), which may be a secondary phase because there are small peaks in XRD patterns (Fig. 2, x = 0.4 sample). To confirm the second phase, x = 0.5 sample was fabricated and the thermal-etched surface is shown in Fig. 4d. Obviously, the size of the small grains increases compared to x = 0.4 sample. EDS analysis of the large grains (darker color) and small grains (lighter color) showed two La/Lu ratios, 3.85/1 and 1/2.18, indicating the existence of two phases in x = 0.5 sample. The average grain sizes of the samples corresponding to x = 0, 0.2, 0.4 and 0.5 were estimated to be 11 ␮m, 14 ␮m, 12 ␮m and 5 ␮m, respectively. It is clear that the smaller grains with lighter color impede the grain growth of the darker grains with the increase of Lu content. With further increasing Lu content to x ≥ 0.6, the size of the lighter grains is gradually close to, equal to, and larger than that of the darker grains (Fig. 5) and the average grain size of both grains is much smaller (<3 ␮m). The lighter grains in Fig. 5 are corresponding to the defective fluorite phase with higher average atomic number (more Lu content) and the darker grains are corresponding to the pyrochlore phase (less Lu content). With the increase of Lu content, much more lighter grains are observed, which illustrates the defective fluorite phase gradually becomes dominant. The composition analysis of x = 0.8 and 1.2 samples were carried out and the EDS layered images and the spot scanning sites are shown in Fig. 6. La and Lu elements are labeled by blue and violet color, respectively. The two samples are both constituted by lanthanum-rich phase and lutetium-rich phase. Table 1 lists the EDS area and

spot scanning results of the two samples. The La/Lu ratio of the area scanning results are 1.47/1 and 1/1.53 respectively, which are very close to the apparent value 1.5/1 and 1/1.5. However, the compositions of the two phases in the two samples are not exactly the same. As shown in Table 1, La/Lu ratios of the two phases for x = 0.8 are 3.10/1 (pyrochlore) and 1/3.88 (defective fluorite), while the ratios are 2.83/1 (pyrochlore) and 1/3.21 (defective fluorite) for x = 1.2. The small variation is consistent with the XRD analysis (the slight shift of the diffraction peaks of the two phases) and the calculated lattice parameters of the two phases (Fig. 3, x = 0.8 and 1.2). The ternary phase diagram of La2 O3 –Lu2 O3 –ZrO2 is not available in literatures. In this study, we can only surmise from the binary phase diagram23–25 that there are many similar phase compositions with the three oxides. Fig. 7 shows the secondary electron images (SEI) of thermaletched surfaces of La2−x Lux Zr2 O7 (x = 1.4–2.0) ceramics. The average grain sizes of the four samples are 82 ␮m, 75 ␮m, 70 ␮m and 60 ␮m, respectively. For x = 1.4 sample (Fig. 7a), there are many scratches resulted from the polishing process, indicating that the hardness of the x = 1.4 sample is very low. In comparison, the hardness of x = 1.6 sample (Fig. 7b) is much better, but still some fragments from the polishing process were inlaid into the grain surface or in the residual pores. The grain boundaries of x = 1.8 and 2.0 samples (Fig. 7c and d) are very clean but residual pores are observed in the grain and grain boundaries, which would deteriorate optical quality of the ceramics. The hardness of x = 2.0 sample is also not high and there are some scratches and pits left after the polishing process (Fig. 7d).

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Fig. 5. BEI of thermal-etched surfaces of La2−x Lux Zr2 O7 ceramics: (a) x = 0.6, (b) x = 0.8, (c) x = 1.0 and (d) x = 1.2.

Fig. 6. EDS layered images of (a) x = 0.8, (b) x = 1.2 ceramics and the spot scanning sites labeled by Spectrum 1 and Spectrum 2. La and Lu elements are labeled by blue and violet color (In print: La and Lu elements are labeled by different grayscale colors). (For interpretation of reference to color in this figure legend, the reader is referred to the web version of this article.) Table 1 The EDS area and spot scanning results (atomic percent) of x = 0.8 and 1.2 samples. Element

O Zr La Lu Hf Total La/Lu

Sample x = 0.8

Sample x = 1.2

Area scanning

Spectrum 1

Spectrum 2

Area scanning

Spectrum 1

Spectrum 2

63.57 17.24 11.07 7.52 0.60 100.00 1.47/1

63.53 17.14 14.22 4.59 0.52 100.00 3.10/1

63.77 18.00 3.56 13.82 0.85 100.00 1/3.88

63.59 17.20 7.30 11.17 0.75 100.00 1/1.53

63.65 17.33 4.29 13.79 0.93 100.00 1/3.21

63.51 17.04 13.98 4.94 0.53 100.00 2.83/1

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Fig. 7. SEI of thermal-etched surfaces of La2−x Lux Zr2 O7 ceramics: (a) x = 1.4, (b) x = 1.6, (c) x = 1.8 and (d) x = 2.0.

Fig. 8 shows the in-line transmittance of La2−x Lux Zr2 O7 (x = 0.2–2.0) transparent ceramics. The x = 0 sample is totally opaque as there are too many residual pores in the grains and on the grain boundaries (Fig. 4a). And the transparency increases with the decrease of residual pores (x = 0.2 and 0.4). In the two-phase region (x = 0.6–1.2), the in-line transmittance continues to increase with the increase of Lu content. Then the in-line transmittance decreases from x = 1.2 to 1.6 and finally increases after x > 1.6. The highest in-line transmittance appears at x = 1.2. Some investigators26,27 considered that the grain size

in transparent ceramics should be minimal, thus reducing the probability that pores would be captured by growing crystals, as well as shortening the diffusion path for removal of intracrystalline pores. But others argued that this would increase the length of grain boundaries and the light scattering on them would increase, particularly for nonisotropic structures.28 In the present work, the two-phase composition is helpful to inhibit the grain growth and the small grain size is beneficial for transparency. Besides, birefringence of the samples might be too small to produce significant scattering. The similar phenomenon occurred in translucent alumina ceramics.29 When the grain size of translucent alumina ceramics reduced to smaller than 1 ␮m by HIP process, the in-line transmittance increased though the number of grain boundaries increased. Apetz and van Bruggen30,31 assumed that the microstructure of a polycrystalline material consisting of birefringent crystals is equivalent to a dispersion of uniform size spheres with diameter G (radius r = G/2), which is the same size of the average grain size. The light scattering of the spheres can be calculated by Mie theory. For small grains (G < 10 ␮m) and small birefringence, the Mie theory leads to an analytical approximation called Rayleigh–Gans–Debye scattering. The real in-line transmittance (RIT) can be obtained by the following equation: 

Fig. 8. In-line transmittance of La2−x Lux Zr2 O7 (x = 0.2−2.0) transparent ceramics (1 mm thick).

RIT = (1 − Rs ) exp

−3π2 rΔn2 d λ20

 (1)

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Fig. 9. Densities of La2−x Lux Zr2 O7 (x = 0−2.0) transparent ceramics.

where r = G/2, G = average grain size of the polycrystalline material, n = average difference in the refractive indices of adjacent grains, d = thickness of the test specimen, λ0 = wavelength of light in vacuum, and Rs = total surface reflection. The above theory is applicable to the x = 0.6–1.2 samples with small grains (G < 3 ␮m) and small birefringence in our study. When n, d, λ0 , Rs are constant, the in-line transmittance is a function of the average grain size and it increases with a decrease of the average grain size. In this work, the average grain size of x = 1.2 sample is the smallest (∼2.5 ␮m), so the in-line transmittance of this sample is the highest (Fig. 8). Except for the residual pores and grain size, secondary phases on grain boundaries also affect the transparency of transparent ceramics. That is, the small amount of secondary phase observed in x = 0.4 sample (Fig. 4c) would cause light scattering at grain boundaries and on the surface, which reduces the transparency. In the same way, impurities on the surface (Fig. 7b, x = 1.6) also affect transparency. Furthermore, surface scattering caused by scratches (Fig. 7, x = 1.4–2.0) and pits (Fig. 7b and d) also reduces the in-line transmittance. For x = 2.0 sample, though the crystal structure is rhombohedral for Lu4 Zr3 O12 , the in-line transmittance is higher than that of x = 1.8 sample, which might be explained by the small birefringence of Lu4 Zr3 O12 . The densities of La2−x Lux Zr2 O7 ceramics measured by the Archimedes method are listed in Fig. 9. As x increases, the density of the ceramics linearly increases from 5.95 g/cm3 to 7.84 g/cm3 , which can be attributed to the higher atomic number of Lu. According to the requirements for scintillator applications, higher X-ray stopping power needs higher density. Transparency is also critical because the visible scintillation photons must be efficiently transported to the photodetector.20 Ceramics with high transparency, density and effective atomic number obtained in this study have the potential to be chosen as host materials for ceramic scintillators. Further research on scintillation properties will be carried out.

4. Conclusions La2−x Lux Zr2 O7 (x = 0–2.0) transparent ceramics were successfully fabricated by vacuum sintering at 1830 ◦ C for 6 h. The crystal structure, microstructure and transparency of the La2−x Lux Zr2 O7 ceramics were all affected by the Lu content. The crystal structure gradually transformed from pyrochlore to defective fluorite phase with the existence of a two-phase region as Lu content increased. Grain growth of pyrochlore phase was inhibited due to the appearance of the defective fluorite phase. Grain sizes of samples (x ≤ 0.4) dominated by the pyrochlore phase were 11−14 ␮m. Samples in the two-phase region (x = 0.6−1.2) had much smaller grain sizes (<3 ␮m), while samples dominated by the defective fluorite phase had larger grain sizes (82−60 ␮m). All the samples (except for x = 0) were transparent and the highest in-line transmittance was reached at x = 1.2 (72.4% @ 1100 nm) with the smallest grain size (∼2.5 ␮m). Ceramics with high transparency, density and effective atomic number obtained in this study are promising candidates for scintillator hosts. Acknowledgement The authors gratefully acknowledge financial supports from National Natural Science Foundation of China (No. 51172258). References 1. Blanchard PE, Liu S, Kennedy BJ, Ling CD, Avdeev M, Aitken JB, et al. Investigating the local structure of lanthanoid hafnates Ln2 Hf2 O7 via diffraction and spectroscopy. J Phys Chem C 2013;117:2266–73. 2. Subramanian MA, Aravamudan G, Subba Rao GV. Oxide pyrochlores – a review. Prog Solid State Chem 1983;15:55–143. 3. Blanchard PE, Clements R, Kennedy BJ, Ling CD, Reynolds E, Avdeev M, et al. Does local disorder occur in the pyrochlore zirconates? Inorg Chem 2012;51:13237–44. 4. Rushton M, Grimes RW, Stanek C, Owens S. Predicted pyrochlore to fluorite disorder temperature for A2 Zr2 O7 compositions. J Mater Res 2004;19:1603–4.

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