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Phase separation in metamict zircon under electron irradiation
Nuclear Instruments and Methods in Physics Research B 211 (2003) 549–555 www.elsevier.com/locate/nimb
Phase separation in metamict zircon under electron irradiation Philippe Carrez, Christophe Forterre, Delfin Braga 1, Hugues Leroux
*
Laboratoire de Structure et Propri et es de l’Etat Solide – UMR CNRS 8008, Universit e des Sciences et Technologies de Lille, F-59655 Villeneuve d’Ascq Cedex, France Received 18 February 2003; received in revised form 27 May 2003
Abstract Metamict (amorphous) zircon ZrSiO4 has been irradiated by an electron beam in situ in a transmission electron microscope. Our observations indicate that the material undergoes solid–solid phase transformations. First a phase separation into SiO2 -rich and ZrO2 -rich domains occurred in the glass owing to a spinodal decomposition. Second, small crystallites of ZrO2 are nucleated and grew within the ZrO2 -rich domains. This study shows that metamict zircon are not stable under ionizing radiation and tends to decompose into SiO2 -rich and ZrO2 -rich domains. 2003 Elsevier B.V. All rights reserved. PACS: 81.10.)h; 61.80.)x; 61.72.)y Keywords: Zircon; Electron irradiation; TEM; Microstructure; Phase transformation
1. Introduction Zircon is one of the minerals proposed for the immobilization of actinides coming from high level nuclear wastes, such as U, Pu and Th [1,2]. Indeed, zircon can incorporate significant amount of UO2 , PuO2 or ThO2 in solid solution with ZrO2 . Zircon undergoes amorphization promoted by a-decay events of radiogenic elements [1,3,4]. During nuclear disintegration, the emission of an a particle is accompanied by a recoil nucleus. Amorphization is caused by the ballistic collisions of the recoil
*
Corresponding author. E-mail address: [email protected] (H. Leroux). 1 Now at: Laboratoire de Physique des Solides – UMR CNRS 8502, Universite Paris-Sud, B^at 510, F-91405 Orsay, France.
nucleus, which cause displacement cascades. In natural zircon, this amorphization process is called metamictization. a particles have a high energy of 4–6 MeV, but their role on the structural evolution is not clearly understood. They dissipates almost all it energy by ionization processes, and it is believed that they cause the formation of large number of isolated defects such as Frenkel pairs along their paths, e.g. [5]. A large number of studies have been devoted to the evolution of zircon under irradiation, in particular for the understanding of the amorphization process. One of the important unsolved (and controversial) question concerns the structure of the metamict state, although some great progresses has been recently performed [6] and reference therein. Is metamict zircon a homogeneous amorphous phase with ZrSiO4 composition or does it undergo phase separation into SiO2 and ZrO2 -rich domains? This
0168-583X/$ - see front matter 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0168-583X(03)01710-5
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question is important because such a phase separation could have some influence on the physical and chemical stability of irradiated zircon. If the role of the nucleus recoil on zircon amorphization has been intensively studied, the role of ionising events occurring along a particles paths is still poorly documented. Ionization events may create point defects, which could influence the evolution of amorphous zircon. In this paper we address the question of the role of an ionizing radiation on the structural evolution of a metamict zircon. To simulate the ionising radiation, the material has been irradiated in situ in a transmission electron microscope (TEM) at 300 keV and room temperature.
2. Experimental procedure 2.1. Sample and sample preparation For this study, we used a fully metamict zircon from Sri Lanka, 570 ± 20 million years old. This sample is the same as the one labelled Ô157Õ by Zhang et al. [6,7] and Capitani et al. [8]. The radiation dose for this sample is given to be 13 · 1018 a-events g1 . Chemical composition and infrared spectroscopic measurements can be found in Zhang et al. [6]. Heat treatments followed by TEM and Raman spectroscopic observations have been performed by Zhang et al. [7] and in Capitani et al. [8]. The TEM sample was prepared from standard petrographic sections. After mechanical thinning down to 30 lm and polishing on both faces, the specimen was ion-thinned by Arþ bombardment until perforation was reached (accelerating voltage of 5 kV, angle of incidence of 15). The sample was then carbon coated to prevent charge effects in the TEM during the electron irradiation or observations. 2.2. Electron irradiation The TEM used for irradiation and observation of microstructural evolution is a Philips CM30 (University of Lille), operating with an acceleration voltage of 300 kV. The beam current was es-
timated by measuring the screen current density (taking into account the back-scattered electron correction factor) through a hole in the sample, before and after the irradiation experiments. The electron fluence rate has been obtained by dividing the beam current by the irradiated area (diameter of the focussed beam) on the sample. For irradiation experiments, the diameter of the beam was typically 1 lm, and the electron flux was in the range of 3 · 1019 –5 · 1020 electrons/cm2 s. For the observation of the microsctrural evolution, the electron beam was defocused in order to minimize irradiation as soon as possible. The experiments were performed at room temperature. Under the experimental conditions we used, large beam heating is not expected, but this effect can not be definitively ruled out since zircon is not a good thermal conductor (see discussion in [9] for detailed comments on this topic). The microstructural evolution as well as the kinetics of the transformation were studied with conventional imaging (bright field mode) and selected area electron diffraction.
3. Results Selected area diffraction of the starting material confirms that it is metamict, as revealed by the diffuse halo characteristic of an amorphous material (Fig. 1(a)). Conventional imaging does not show any clear distinguishable structure in this amorphous phase (Fig. 2(a)). After a relatively short irradiation fluence (1022 electrons/cm2 ), a structure begins to appear in the amorphous phase (Fig. 2(b)). Nano-sized domains are visible and their sizes increase with the fluence (Fig. 2(c)). Using a Fourier transform of the irradiated area on the bright field images (software Scion image of Scion Corporation, Beta 4.0.2 version), we have estimated the domain size evolution with the fluence (Fig. 3). The domain size increases rapidly at the beginning of the experiments and is found to stabilize around 10 nm. Despite the appearance of domains on the images, the diffraction patterns show that the irradiated material is still amorphous. The diffuse halo is still present but its diameter increases with increasing fluence (Fig. 4). It
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Fig. 1. Sequence of electron diffraction patterns recorded during irradiation, showing the structural evolution of the material. (a) Starting metamict zircon. The diffuse halo illustrates the presence of amorphous material. (b,c) The maxima intensity of the diffuse halo is narrowed and its diameter increases. (d) After a fluence of 1023 electrons/cm2 , ZrO2 nano-crystallites are appearing. The two stronger peaks of ZrO2 are clearly present. The diffraction peaks are broadened because the small domain size of the crystallites. (e,f) For fluences >1023 electrons/cm2 , the ZrO2 peaks are clearly visible are progressively narrowed because of grain growth.
Fig. 2. Texture evolution in metamict zircon under electron irradiation. (a) Non-irradiated zircon. (b) Bright field image of the core of an irradiated area. A contrast is appearing under electron irradiation despite the diffraction pattern indicate it is still amorphous (electron fluence ¼ 4 · 1022 electrons/cm2 ). (c) The domains are coarsened with the increasing fluence (electron fluence ¼ 1.5 · 1023 electrons/cm2 ). At this stage ZrO2 nano-crystallites are present. The width of the photographs (a), (b) and (c) is 150 nm.
shows that the structure of the amorphous phase evolved with electron fluence. After a fluence of about 1023 electrons/cm2 , the material crystallises. Small crystallites with random orientation appear as revealed by rings on the diffraction patterns (Fig. 1(d)–(f)). Because of the very small size of the crystallites, the diffraction rings are broad when they appear. Their widths decrease sharply with increasing fluence indicating
a growth process of the initial nuclei. The strongest reflections correspond to d-spacings of 0.296, 0.183 and 0.153 nm, which have been assigned to zirconia ZrO2 . It is difficult to distinguish the cubic from the tetragonal structure since their dhkl distances, with comparable factor structure, overlap. Some amorphous material is still present as revealed by the diffuse halo which superimposes on the ZrO2 rings, although its intensity strongly
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detected. It was observed that the thickness plays a role for the crystallization critical fluence. Indeed, the thinner areas appear to progress a little bit more rapidly than the thick areas showing that the free surfaces play a role for enhanced the reaction. This fact is not surprising since the free surface energy is known to enhance kinetics of thermodynamic processes of materials. In addition, the free surface may act as preferential nucleation site. Finally, it was also observed that the irradiation is companied by a volume reduction as shown by the distortion of the border of the thin foil near the irradiated areas (Fig. 5).
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Domain size (nm)
10 8 6 4 2 0 0
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10
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35
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Fluence (x 1022 electrons/cm2) Fig. 3. Domain size evolution as a function of the fluence. It decreases when the fluence increases. The ZrO2 crystallites are appearing after a fluence of about 1023 electrons/cm2 . The flux of the experiment was 2 · 1020 electrons/cm2 s.
Diffraction maxima distance (nm)
0.36
0.35
0.34
0.33
0.32
0.31 1
2
3
4
5
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7
8
9
Fluence (x 1022 electrons/cm2) Fig. 4. Evolution of the diffuse halo maxima in function of the electron fluence. This curved is stopped at a fluence below 1023 electrons/cm2 , i.e. before the appearance of the first ZrO2 crystallites. The flux was 1.3 · 1020 electrons/cm2 s.
decreases. It is to be mentioned that the maxima intensity of the diffuse halo corresponding to the initial amorphous material progressively moves to the strongest ZrO2 reflection (dhkl ¼ 0:296 nm) distance. Finally, the critical electron fluence for crystallization (about 1023 electrons/cm2 ) was measured for different irradiation experiments within the flux range 3 · 1019 –6 · 1020 electrons/ cm2 s. No influence of the fluence rate has been
4. Discussion This study shows that ionizing electron irradiation induces phase transformation in metamict zircon. Phase transformation under electron irradiation is not restricted to amorphous zircon since it has been observed for a number of oxides, e.g. [9,10]. In our sample, electron irradiation-induced modifications occur in two stages. First the amorphous material evolves as revealed by the evolution of the diffuse halo on the diffraction patterns. The diffuse halo is progressively narrowed and its diameter decreases. At this stage a texture appears on the TEM images. The morphology of the texture and the absence of crystalline phase show that the material undergoes a spinodal decomposition, suggesting that ZrO2 and SiO2 are not miscible in the proportion of 50–50 mol% under our experimental conditions. Such a decomposition is not surprising since the ZrO2 – SiO2 phase diagram displays an liquid immiscibility domain between 2250 and 2450 C for the composition range 60–80 mol% of SiO2 [11]. If we extrapolate the immiscibility curves at low temperature (this hypothesis is thermodynamically valid for a glass which exhibits a comparable structure to a liquid), the 50–50 mol% composition is included in the immiscibility domain below the spinodal line. This region is thus thermodynamically unstable. It will tend to decompose into ZrO2 and SiO2 rich domains. Thermodynamical instability linked with electron irradiation enhance phase separation and decomposition of the
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Fig. 5. Irradiation induces a volume contraction, as revealed by the deflexion of the border of the thin foil near the irradiated area. Experimental conditions are: (a) 4 · 1022 electrons/cm2 , (b) 7.5 · 1022 electrons/cm2 , (c) 1.5 · 1023 electrons/cm2 and (d) 7.5 · 1023 electrons/cm2 . The width of the photographs (a), (b), (c) and (d) is 1.2 lm.
amorphous state. Indeed, during spinodal decomposition, any composition fluctuation becomes highly unstable with an ‘‘up-hill’’ diffusion process. This decomposition requires Zr–Si interdiffusion mainly enhanced by electron irradiation. The phase separation observed indicates that the elements become mobile under electron irradiation. Together with [9] we attribute the structural change to an ionization process induced by the electron irradiation. With the electron energy used in this study, enhanced mobility resulting from elastic interaction is not dominant (see [10,12] for detailed calculation on comparable materials). Ionization process involves bond breaking and subsequent rearrangement and/or mobility of the atoms in a more favourable position or bond orientation. At this stage of spinodal decomposition, no nucleation process is necessary. A composition
wave develops with time. It is well visible on the bright field images because of the high contrast of atomic number (ZrO2 -rich versus SiO2 -rich domains). Coarsening occurs then by ‘‘up-hill’’ diffusion. However, the spinodal decomposition appearance is very sudden and coarsening starts at low fluences. A growth process, similar to an Ostwald ripening is also expected in this case of decomposition. Growth of the domain size calculated for these experiments is somewhat proportional to time1=3 , compatible with ripening kinetics (Fig. 6). The second stage of the electron-induced modification in amorphous zircon is the nucleation of small ZrO2 crystallites. They appear for a fluence 1023 electrons/cm2 . They might occur in the ZrO2 -rich domains of the spinodal decomposition until a critical composition is obtained, while the
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zircon would tend to decompose into its end member oxides.
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10
5. Conclusion 8 6 4 2 0
0
2
4
6
8
10
12
14
Time1/3 (s1/3) Fig. 6. Mean spinodal domain size (in nm) plotted against time1=3 . The domain size evolution is compatible with a ripening kinetic process.
SiO2 -rich regions remain amorphous. Indeed, the diameter of the diffuse halo of the amorphous phase progressively decreases and tends to the strongest ZrO2 reflection at dhkl ¼ 0:296 nm. Such evolution also suggests that the ZrO2 -rich amorphous phase strongly contributes for the diffuse halo on the diffraction pattern during the first stage. The nucleation and growth process is probably also activated by the radiation-enhanced diffusion as a result of ionization processes as well as a thermodynamic driving force for crystallization. It should be mentioned that the formation of ZrO2 crystallites in amorphous zircon, in contrast to some other materials, requires a phase separation in two compounds (from a homogeneous ZrSiO4 material to ZrO2 and SiO2 -rich domains), that considerably slows down the growth process of the new crystallites. Formation of ZrO2 crystallites in metamict zircon has been already observed under annealing [7,8,13,14], and it was firstly reported by [9] under electron irradiation. It should be mentioned also that direct decomposition under irradiation at high temperature is also expected inside the thermal spike of the displacement cascades [15]. The phase separation into the displacement cascades has been also suggested by a molecular dynamic study [16]. Altogether, with various observation methods and various solicitations, these studies indicate that the amorphous
The formation of metamict zircon under ionizing radiation (here electron irradiation) is a two steps process involving first a spinodal decomposition with the formation of amorphous SiO2 and ZrO2 -rich domains at the nano-scale. Second there is formation of ZrO2 nano-crystallites, randomly orientated into the ZrO2 -rich domains. Both stages require Si–Zr interdiffusion, enhanced by the ionising irradiation. In metamict zircon, the a particles induce along their paths some ionization processes that might enhance the elemental diffusion. Metamict zircon should thus tend to decompose into SiO2 and ZrO2 -rich domains. However, this phase decomposition has not yet been clearly observed in metamict zircons ([7] and references therein). The high rate of ionization in the TEM probably causes a high concentration of excited states which lowers the activation barriers to the diffusion processes that must take place for phase separation. This situation might not be encounted in natural zircons for which the lifetime of excited state is short relative to the a decay events. Nevertheless, this study shows that metamict zircon is not stable under high ionizing radiation dose and tends to undergo a phase separation into SiO2 and ZrO2 -rich domains. Acknowledgements The authors thank M. Zhang and E.H.K. Salje (University of Cambridge) as well as the National History Museum (London, UK) for providing the sample used for this study.
References [1] R.C. Ewing, B.C. Chakoumakos, G.R. Lumpkin, T. Murakami, Bull. Mater. Res. Soc. 15 (1987) 58. [2] W.J. Weber, R.C. Ewing, C.R.A. Catlow, T. Diaz de la Rubia, L.W. Hobbs, C. Kinoshita, H. Matzke, A.T. Motta, M. Nastasi, E.K.H. Salje, E.R. Vance, S.J. Zinkle, J. Mater. Sci. 13 (1998) 1434.
P. Carrez et al. / Nucl. Instr. and Meth. in Phys. Res. B 211 (2003) 549–555 [3] B.C. Chakoumakos, T. Murakami, G.R. Lumpkin, R.C. Ewing, Science 236 (1987) 1556. [4] W.J. Weber, R.C. Ewing, L.M. Wang, J. Mater. Res. 9 (1994) 688. [5] S. Rios, E.K.H. Salje, J. Phys. Condens. Matter 11 (1999) 8947. [6] M. Zhang, E.K.H. Salje, I. Farman, A. Graeme-Barber, Ph. Daniel, R.C. Ewing, A. Clark, H. Leroux, J. Phys. Condens. Matter 12 (2000) 1915. [7] M. Zhang, E.K.H. Salje, G.C. Capitani, H. Leroux, A.M. Clark, J. Schl€ uter, R.C. Ewing, J. Phys. Condens. Matter 12 (2000) 3131. [8] G.C. Capitani, H. Leroux, J.C. Doukhan, S. Rios, M. Zhang, E.K.H. Salje, Phys. Chem. Miner. 27 (2000) 545.
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[9] A. Meldrum, L.A. Boatner, R.C. Ewing, J. Mater. Res. 12 (1997) 1816. [10] Ph. Carrez, H. Leroux, P. Cordier, F. Guyot, Philos. Mag. A 81 (2001) 2823. [11] W.C. Butterman, W.R. Foster, Am. Miner. 52 (1967) 880. [12] S.X. Wang, L.M. Wang, R.C. Ewing, J. Nucl. Mater. 278 (2000) 233. [13] A.C. McLaren, J.D. Fitzgerald, I.S. Williams, Geochim. Cosmochim. Acta 60 (1994) 1091. [14] S. Ellsworth, A. Navrotsky, R.C. Ewing, Phys. Chem. Miner. 21 (1994) 140. [15] A. Meldrum, S.J. Zinkle, L.A. Boatner, R.C. Ewing, Nature 395 (1998) 56. [16] J.P. Crocombette, D. Dhaleb, J. Nucl. Mater. 295 (2001) 167.
Phase separation in metamict zircon under electron irradiation Philippe Carrez, Christophe Forterre, Delfin Braga 1, Hugues Leroux
*
Laboratoire de Structure et Propri et es de l’Etat Solide – UMR CNRS 8008, Universit e des Sciences et Technologies de Lille, F-59655 Villeneuve d’Ascq Cedex, France Received 18 February 2003; received in revised form 27 May 2003
Abstract Metamict (amorphous) zircon ZrSiO4 has been irradiated by an electron beam in situ in a transmission electron microscope. Our observations indicate that the material undergoes solid–solid phase transformations. First a phase separation into SiO2 -rich and ZrO2 -rich domains occurred in the glass owing to a spinodal decomposition. Second, small crystallites of ZrO2 are nucleated and grew within the ZrO2 -rich domains. This study shows that metamict zircon are not stable under ionizing radiation and tends to decompose into SiO2 -rich and ZrO2 -rich domains. 2003 Elsevier B.V. All rights reserved. PACS: 81.10.)h; 61.80.)x; 61.72.)y Keywords: Zircon; Electron irradiation; TEM; Microstructure; Phase transformation
1. Introduction Zircon is one of the minerals proposed for the immobilization of actinides coming from high level nuclear wastes, such as U, Pu and Th [1,2]. Indeed, zircon can incorporate significant amount of UO2 , PuO2 or ThO2 in solid solution with ZrO2 . Zircon undergoes amorphization promoted by a-decay events of radiogenic elements [1,3,4]. During nuclear disintegration, the emission of an a particle is accompanied by a recoil nucleus. Amorphization is caused by the ballistic collisions of the recoil
*
Corresponding author. E-mail address: [email protected] (H. Leroux). 1 Now at: Laboratoire de Physique des Solides – UMR CNRS 8502, Universite Paris-Sud, B^at 510, F-91405 Orsay, France.
nucleus, which cause displacement cascades. In natural zircon, this amorphization process is called metamictization. a particles have a high energy of 4–6 MeV, but their role on the structural evolution is not clearly understood. They dissipates almost all it energy by ionization processes, and it is believed that they cause the formation of large number of isolated defects such as Frenkel pairs along their paths, e.g. [5]. A large number of studies have been devoted to the evolution of zircon under irradiation, in particular for the understanding of the amorphization process. One of the important unsolved (and controversial) question concerns the structure of the metamict state, although some great progresses has been recently performed [6] and reference therein. Is metamict zircon a homogeneous amorphous phase with ZrSiO4 composition or does it undergo phase separation into SiO2 and ZrO2 -rich domains? This
0168-583X/$ - see front matter 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0168-583X(03)01710-5
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question is important because such a phase separation could have some influence on the physical and chemical stability of irradiated zircon. If the role of the nucleus recoil on zircon amorphization has been intensively studied, the role of ionising events occurring along a particles paths is still poorly documented. Ionization events may create point defects, which could influence the evolution of amorphous zircon. In this paper we address the question of the role of an ionizing radiation on the structural evolution of a metamict zircon. To simulate the ionising radiation, the material has been irradiated in situ in a transmission electron microscope (TEM) at 300 keV and room temperature.
2. Experimental procedure 2.1. Sample and sample preparation For this study, we used a fully metamict zircon from Sri Lanka, 570 ± 20 million years old. This sample is the same as the one labelled Ô157Õ by Zhang et al. [6,7] and Capitani et al. [8]. The radiation dose for this sample is given to be 13 · 1018 a-events g1 . Chemical composition and infrared spectroscopic measurements can be found in Zhang et al. [6]. Heat treatments followed by TEM and Raman spectroscopic observations have been performed by Zhang et al. [7] and in Capitani et al. [8]. The TEM sample was prepared from standard petrographic sections. After mechanical thinning down to 30 lm and polishing on both faces, the specimen was ion-thinned by Arþ bombardment until perforation was reached (accelerating voltage of 5 kV, angle of incidence of 15). The sample was then carbon coated to prevent charge effects in the TEM during the electron irradiation or observations. 2.2. Electron irradiation The TEM used for irradiation and observation of microstructural evolution is a Philips CM30 (University of Lille), operating with an acceleration voltage of 300 kV. The beam current was es-
timated by measuring the screen current density (taking into account the back-scattered electron correction factor) through a hole in the sample, before and after the irradiation experiments. The electron fluence rate has been obtained by dividing the beam current by the irradiated area (diameter of the focussed beam) on the sample. For irradiation experiments, the diameter of the beam was typically 1 lm, and the electron flux was in the range of 3 · 1019 –5 · 1020 electrons/cm2 s. For the observation of the microsctrural evolution, the electron beam was defocused in order to minimize irradiation as soon as possible. The experiments were performed at room temperature. Under the experimental conditions we used, large beam heating is not expected, but this effect can not be definitively ruled out since zircon is not a good thermal conductor (see discussion in [9] for detailed comments on this topic). The microstructural evolution as well as the kinetics of the transformation were studied with conventional imaging (bright field mode) and selected area electron diffraction.
3. Results Selected area diffraction of the starting material confirms that it is metamict, as revealed by the diffuse halo characteristic of an amorphous material (Fig. 1(a)). Conventional imaging does not show any clear distinguishable structure in this amorphous phase (Fig. 2(a)). After a relatively short irradiation fluence (1022 electrons/cm2 ), a structure begins to appear in the amorphous phase (Fig. 2(b)). Nano-sized domains are visible and their sizes increase with the fluence (Fig. 2(c)). Using a Fourier transform of the irradiated area on the bright field images (software Scion image of Scion Corporation, Beta 4.0.2 version), we have estimated the domain size evolution with the fluence (Fig. 3). The domain size increases rapidly at the beginning of the experiments and is found to stabilize around 10 nm. Despite the appearance of domains on the images, the diffraction patterns show that the irradiated material is still amorphous. The diffuse halo is still present but its diameter increases with increasing fluence (Fig. 4). It
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Fig. 1. Sequence of electron diffraction patterns recorded during irradiation, showing the structural evolution of the material. (a) Starting metamict zircon. The diffuse halo illustrates the presence of amorphous material. (b,c) The maxima intensity of the diffuse halo is narrowed and its diameter increases. (d) After a fluence of 1023 electrons/cm2 , ZrO2 nano-crystallites are appearing. The two stronger peaks of ZrO2 are clearly present. The diffraction peaks are broadened because the small domain size of the crystallites. (e,f) For fluences >1023 electrons/cm2 , the ZrO2 peaks are clearly visible are progressively narrowed because of grain growth.
Fig. 2. Texture evolution in metamict zircon under electron irradiation. (a) Non-irradiated zircon. (b) Bright field image of the core of an irradiated area. A contrast is appearing under electron irradiation despite the diffraction pattern indicate it is still amorphous (electron fluence ¼ 4 · 1022 electrons/cm2 ). (c) The domains are coarsened with the increasing fluence (electron fluence ¼ 1.5 · 1023 electrons/cm2 ). At this stage ZrO2 nano-crystallites are present. The width of the photographs (a), (b) and (c) is 150 nm.
shows that the structure of the amorphous phase evolved with electron fluence. After a fluence of about 1023 electrons/cm2 , the material crystallises. Small crystallites with random orientation appear as revealed by rings on the diffraction patterns (Fig. 1(d)–(f)). Because of the very small size of the crystallites, the diffraction rings are broad when they appear. Their widths decrease sharply with increasing fluence indicating
a growth process of the initial nuclei. The strongest reflections correspond to d-spacings of 0.296, 0.183 and 0.153 nm, which have been assigned to zirconia ZrO2 . It is difficult to distinguish the cubic from the tetragonal structure since their dhkl distances, with comparable factor structure, overlap. Some amorphous material is still present as revealed by the diffuse halo which superimposes on the ZrO2 rings, although its intensity strongly
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detected. It was observed that the thickness plays a role for the crystallization critical fluence. Indeed, the thinner areas appear to progress a little bit more rapidly than the thick areas showing that the free surfaces play a role for enhanced the reaction. This fact is not surprising since the free surface energy is known to enhance kinetics of thermodynamic processes of materials. In addition, the free surface may act as preferential nucleation site. Finally, it was also observed that the irradiation is companied by a volume reduction as shown by the distortion of the border of the thin foil near the irradiated areas (Fig. 5).
12
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10 8 6 4 2 0 0
5
10
15
20
25
30
35
40
Fluence (x 1022 electrons/cm2) Fig. 3. Domain size evolution as a function of the fluence. It decreases when the fluence increases. The ZrO2 crystallites are appearing after a fluence of about 1023 electrons/cm2 . The flux of the experiment was 2 · 1020 electrons/cm2 s.
Diffraction maxima distance (nm)
0.36
0.35
0.34
0.33
0.32
0.31 1
2
3
4
5
6
7
8
9
Fluence (x 1022 electrons/cm2) Fig. 4. Evolution of the diffuse halo maxima in function of the electron fluence. This curved is stopped at a fluence below 1023 electrons/cm2 , i.e. before the appearance of the first ZrO2 crystallites. The flux was 1.3 · 1020 electrons/cm2 s.
decreases. It is to be mentioned that the maxima intensity of the diffuse halo corresponding to the initial amorphous material progressively moves to the strongest ZrO2 reflection (dhkl ¼ 0:296 nm) distance. Finally, the critical electron fluence for crystallization (about 1023 electrons/cm2 ) was measured for different irradiation experiments within the flux range 3 · 1019 –6 · 1020 electrons/ cm2 s. No influence of the fluence rate has been
4. Discussion This study shows that ionizing electron irradiation induces phase transformation in metamict zircon. Phase transformation under electron irradiation is not restricted to amorphous zircon since it has been observed for a number of oxides, e.g. [9,10]. In our sample, electron irradiation-induced modifications occur in two stages. First the amorphous material evolves as revealed by the evolution of the diffuse halo on the diffraction patterns. The diffuse halo is progressively narrowed and its diameter decreases. At this stage a texture appears on the TEM images. The morphology of the texture and the absence of crystalline phase show that the material undergoes a spinodal decomposition, suggesting that ZrO2 and SiO2 are not miscible in the proportion of 50–50 mol% under our experimental conditions. Such a decomposition is not surprising since the ZrO2 – SiO2 phase diagram displays an liquid immiscibility domain between 2250 and 2450 C for the composition range 60–80 mol% of SiO2 [11]. If we extrapolate the immiscibility curves at low temperature (this hypothesis is thermodynamically valid for a glass which exhibits a comparable structure to a liquid), the 50–50 mol% composition is included in the immiscibility domain below the spinodal line. This region is thus thermodynamically unstable. It will tend to decompose into ZrO2 and SiO2 rich domains. Thermodynamical instability linked with electron irradiation enhance phase separation and decomposition of the
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Fig. 5. Irradiation induces a volume contraction, as revealed by the deflexion of the border of the thin foil near the irradiated area. Experimental conditions are: (a) 4 · 1022 electrons/cm2 , (b) 7.5 · 1022 electrons/cm2 , (c) 1.5 · 1023 electrons/cm2 and (d) 7.5 · 1023 electrons/cm2 . The width of the photographs (a), (b), (c) and (d) is 1.2 lm.
amorphous state. Indeed, during spinodal decomposition, any composition fluctuation becomes highly unstable with an ‘‘up-hill’’ diffusion process. This decomposition requires Zr–Si interdiffusion mainly enhanced by electron irradiation. The phase separation observed indicates that the elements become mobile under electron irradiation. Together with [9] we attribute the structural change to an ionization process induced by the electron irradiation. With the electron energy used in this study, enhanced mobility resulting from elastic interaction is not dominant (see [10,12] for detailed calculation on comparable materials). Ionization process involves bond breaking and subsequent rearrangement and/or mobility of the atoms in a more favourable position or bond orientation. At this stage of spinodal decomposition, no nucleation process is necessary. A composition
wave develops with time. It is well visible on the bright field images because of the high contrast of atomic number (ZrO2 -rich versus SiO2 -rich domains). Coarsening occurs then by ‘‘up-hill’’ diffusion. However, the spinodal decomposition appearance is very sudden and coarsening starts at low fluences. A growth process, similar to an Ostwald ripening is also expected in this case of decomposition. Growth of the domain size calculated for these experiments is somewhat proportional to time1=3 , compatible with ripening kinetics (Fig. 6). The second stage of the electron-induced modification in amorphous zircon is the nucleation of small ZrO2 crystallites. They appear for a fluence 1023 electrons/cm2 . They might occur in the ZrO2 -rich domains of the spinodal decomposition until a critical composition is obtained, while the
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zircon would tend to decompose into its end member oxides.
12
Domain size (nm)
10
5. Conclusion 8 6 4 2 0
0
2
4
6
8
10
12
14
Time1/3 (s1/3) Fig. 6. Mean spinodal domain size (in nm) plotted against time1=3 . The domain size evolution is compatible with a ripening kinetic process.
SiO2 -rich regions remain amorphous. Indeed, the diameter of the diffuse halo of the amorphous phase progressively decreases and tends to the strongest ZrO2 reflection at dhkl ¼ 0:296 nm. Such evolution also suggests that the ZrO2 -rich amorphous phase strongly contributes for the diffuse halo on the diffraction pattern during the first stage. The nucleation and growth process is probably also activated by the radiation-enhanced diffusion as a result of ionization processes as well as a thermodynamic driving force for crystallization. It should be mentioned that the formation of ZrO2 crystallites in amorphous zircon, in contrast to some other materials, requires a phase separation in two compounds (from a homogeneous ZrSiO4 material to ZrO2 and SiO2 -rich domains), that considerably slows down the growth process of the new crystallites. Formation of ZrO2 crystallites in metamict zircon has been already observed under annealing [7,8,13,14], and it was firstly reported by [9] under electron irradiation. It should be mentioned also that direct decomposition under irradiation at high temperature is also expected inside the thermal spike of the displacement cascades [15]. The phase separation into the displacement cascades has been also suggested by a molecular dynamic study [16]. Altogether, with various observation methods and various solicitations, these studies indicate that the amorphous
The formation of metamict zircon under ionizing radiation (here electron irradiation) is a two steps process involving first a spinodal decomposition with the formation of amorphous SiO2 and ZrO2 -rich domains at the nano-scale. Second there is formation of ZrO2 nano-crystallites, randomly orientated into the ZrO2 -rich domains. Both stages require Si–Zr interdiffusion, enhanced by the ionising irradiation. In metamict zircon, the a particles induce along their paths some ionization processes that might enhance the elemental diffusion. Metamict zircon should thus tend to decompose into SiO2 and ZrO2 -rich domains. However, this phase decomposition has not yet been clearly observed in metamict zircons ([7] and references therein). The high rate of ionization in the TEM probably causes a high concentration of excited states which lowers the activation barriers to the diffusion processes that must take place for phase separation. This situation might not be encounted in natural zircons for which the lifetime of excited state is short relative to the a decay events. Nevertheless, this study shows that metamict zircon is not stable under high ionizing radiation dose and tends to undergo a phase separation into SiO2 and ZrO2 -rich domains. Acknowledgements The authors thank M. Zhang and E.H.K. Salje (University of Cambridge) as well as the National History Museum (London, UK) for providing the sample used for this study.
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