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Swift heavy ion irradiation of pyrochlore oxides: Electronic energy loss threshold for latent track formation
Nuclear Instruments and Methods in Physics Research B 268 (2010) 2933–2936
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Swift heavy ion irradiation of pyrochlore oxides: Electronic energy loss threshold for latent track formation S. Moll a, G. Sattonnay b,*, L. Thomé a, J. Jagielski c,d, C. Legros b, I. Monnet e a
Centre de Spectrométrie Nucléaire et de Spectrométrie de Masse, CNRS, IN2P3, Université Paris-Sud, Bât. 108, F-91405 Orsay, France LEMHE, ICMMO, Université Paris Sud, Bât. 410, F-91405 Orsay, France c Institute for Electronic Materials Technology, Wolczynska 133, Pl-01-919 Warsaw, Poland d Andrzej Soltan Institute for Nuclear Studies, Pl-05-400 Swierk/Otwock, Poland e CIMAP, CEA, CNRS, ENSICAEN, BP 5133, F-14070 Caen Cedex 5, France b
a r t i c l e
i n f o
Article history: Received 21 September 2009 Received in revised form 16 April 2010 Available online 7 May 2010 Keywords: Ion irradiation Pyrochlores X-ray diffraction
a b s t r a c t Pyrochlore pellets with the Gd2(Ti2xZrx)O7 stoichiometry (x = 0, 1 and 2) were irradiated with swift heavy ions in order to investigate the effects of electronic excitation and to determine the electronic stopping power threshold for track formation. XRD results showed that the electronic excitation induced by 870 MeV Xe and 780 MeV Kr ions leads to: (i) a crystalline–amorphous transition for Gd2Ti2O7 and Gd2TiZrO7, (ii) a phase transition towards an anion-deficient fluorite structure (order–disorder transition) for Gd2Zr2O7. Thus, zirconate pyrochlores present a better radiation resistance under swift heavy ion irradiation than titanate pyrochlores. Moreover results underline the existence of an electronic stopping power threshold around 13–14 keV/nm, below which phase transformations do not occur. Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction Pyrochlores are promising materials for their application as matrices for the transmutation or the immobilization of actinides produced in nuclear power plants [1]. In operating conditions or during long-term storage, nuclear matrices are submitted to severe radiations which induce atomic rearrangements. These structural modifications lead to the alteration of the physico-chemical properties of materials. Although the radiation-induced crystallineto-amorphous transformation in pyrochlores has drawn much attention in the past [1–7], only few studies were devoted to the investigation of the damage induced by swift heavy ions which deposit energy via high electronic excitations. The pyrochlores under consideration in this study typically exhibit the A2B2O7 stoichiometry, where actinides or lanthanides can be incorporated on the eight-coordinated A-site and metals occupy the six-coordinated B-site. The stability of pyrochlores seems to be governed by the ratio of the ionic radii of A and B cations (rA/rB) which extends from 1.46 for Gd2Zr2O7 to 1.78 for Sm2Ti2O7 [1,2]. For rA/rB < 1.46 an anion-deficient fluorite structure (A,B)4O7 is the stable form, while a monoclinic structure is exhibited for rA/rB > 1.78 [1]. Beyond cationic radii ratio considerations to predict the stability of pyrochlores under irradiation, improved amorphization characteristics are to be found in compounds that have a
* Corresponding author. Tel.: +33 1 69 15 70 37. E-mail address: [email protected] (G. Sattonnay). 0168-583X/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2010.05.012
natural tendency to accommodate lattice disorder [8,9]. A good indicator of the radiation tolerance of pyrochlores is thus the energy of order–disorder fluorite defect reaction pair (i.e. both cation antisite and anion Frenkel defect formation) [9], which is notably lower in zirconates than in titanates. This criterion was supported by irradiations performed on gadolinium-based pyrochlores with the Gd2(Ti2xZrx)O7 stoichiometry, since they exhibit a systematic decrease of their amorphization susceptibility with increasing Zr content for both low [1,2,5] and high [10–13] energy ions. The end member Gd2Zr2O7 is transformed into a radiation-resistant anion-deficient fluorite structure upon irradiation at room temperature [1,2,5,10–13]. This paper reports preliminary results of the damage created by swift heavy ions in order to investigate the effects of electronic excitation on the phase transformation in pyrochlore oxides. X-ray diffraction (XRD) was implemented to determine the structural changes induced by irradiation, as well as the damage build-up, in pyrochlores of the Gd2(Ti2xZrx)O7 family for x = 0 (rA/rB = 1.74), x = 1 (rA/rB = 1.59) and x = 2 (rA/rB = 1.46).
2. Experimental Polycrystalline Gd2(Ti2xZrx)O7 pellets, with x = 0, 1 and 2, were prepared by a standard solid state process. Stoichiometric amounts of gadolinium oxide (Gd2O3), first heated at 900 °C for 15 h (Alfa Aesar 99.99%), zirconium oxide (ZrO2, Alfa Aesar 99.5%) and titanium oxide (TiO2, Alfa Aesar 99.99%) were intimately mixed in
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S. Moll et al. / Nuclear Instruments and Methods in Physics Research B 268 (2010) 2933–2936
an acetone slurry using a ball-mill and subsequently dried. After drying, the powders were isostatically pressed into rods at 250 MPa. The homogenized mixtures were then subjected to a three step heating protocol with intermittent grindings to attain a better homogeneity. The compacts were first sintered at 1200 °C for 12 h. The procedures (grinding, milling, pressing) were repeated twice with a second heating at 1400 °C for 72 h and a final sintering in air at 1500 °C for 196 h. Pellets were cut with a diamond saw and the specimens were polished to a 0.5 lm diamond finish. Pyrochlore oxides were irradiated at room temperature with 870 MeV Xe and 780 MeV Kr ions at the GANIL accelerator in Caen at fluences ranging from 1010 up to 1013 cm2. Ion currents were always kept lower than 108 cm2 s1 in order to keep target heating <50 °C. The parameters of irradiations are summarized in Table 1 which clearly shows that Se overwhelms Sn by several orders of magnitude. XRD experiments were performed with a X’pert Pro MRD PANalytical diffractometer, using a CuKa anticathode (kKa1 = 0.1540598 nm, kKa2 = 0.1544426 nm). X-ray patterns were recorded in a standard (h–2h) geometry from 13° to 90° (2h) with a step of 0.025°. The diffraction patterns were analyzed using two different approaches. In the first one, for the amorphizable pyrochlores (Gd2Ti2O7 and Gd2TiZrO7), the amorphous fraction fa was deduced from the net areas of the corresponding XRD lines by using the following equation:
Pn fa ¼ 1
Airradiated i i¼1 Aunirradiated i
ð1Þ
n
where Airradiated and Aunirradiated are the net area of the ith XRD line in i i the pattern recorded on irradiated and unirradiated samples, respectively, and n is the number of lines considered. Pseudo-Voigt profiles were used to fit the diffraction peaks. In the second approach, used for Gd2Zr2O7, the diffraction patterns were analyzed by Rietveld refinement, using GSAS package [15], with a procedure described by Wuensch et al. [16] in order to determine the fraction of anion-deficient fluorite structure induced by irradiation (which coexists with the remaining pyrochlore phase).
the scattering powers of the cations that occupy A and B sites, (ii) the occupancy of cation and anion sites, (iii) the displacement of 48f oxygens from the ideal fluorite structure, and (iv) the temperature factor coefficient [16]. It should be noted that in X-ray diffraction the scattering power of oxygen is relatively small compared to that of cations, so that the influence of cations is predominant for XRD patterns. For cations in the A and B sites, extra conditions of reflection (superstructure reflection) are allowed for hkl: h = 2n + 1 or h,k,l = 4n + 2 or h,k,l = 4n [16,17]. As shown in Fig. 1a the intensity of the superstructure reflection peaks decreases with increasing Zr content. Fig. 1b and c show XRD patterns recorded on samples irradiated with 870 MeV Xe ions at a fluence of 2 1012 cm2 and 1013 cm2, respectively. For Gd2Zr2O7 the diffraction peaks related to the supercell of the pyrochlore structure disappear and only the peaks corresponding to the fluorite structure are observed at the final fluence. This result indicates that irradiation has induced an order–disorder phase transition: Gd2Zr2O7 is transformed into an anion-deficient fluorite structure by the disordering of cations occupying the A and B sites and by the creation of anion vacancies [1–7]. It is worth noting that no amorphization is observed for this compound. Conversely, for Gd2TiZrO7 and Gd2Ti2O7 a progressive vanishing of all diffraction peaks is observed and an additional diffuse scattering occurs at the basis of the (2 2 2) pyrochlore peaks, which indicates the amorphization of the samples. These results are in agreement with previous data [10–13] which showed that a systematic increase of the radiation resistance with increasing Zr content is observed for the Gd2(Ti2xZrx)O7 series. Fig. 2 compares the phase transformation build-up, i.e. the variation, versus the ion fluence, of the fraction (f) of a new phase (either amorphous or fluorite), extracted by the fitting procedure exposed previously, for the three compounds irradiated with 870 MeV Xe ions. The variation of f with the ion fluence can be accounted for in the framework of a single impact model [18] (Poisson’s relationship). In this model, it is implicitly assumed that swift heavy ions create individual tracks and that the overall damage observed at high fluence results from the overlapping of ion tracks. Thus, the variation of f may be written as:
f ¼ fsat ½1 expðr/Þ 3. Results and discussion Fig. 1a shows XRD patterns recorded on virgin pellets. Ordered A2B2O7 pyrochlores belong to the Fd3m space group which is a superstructure of the ideal fluorite structure (Fd3m space group) with twice the lattice constant. A and B cations occupy the 16c and 16d sites, respectively, and oxygens are located at the 48f and 8b positions. The anion sublattice can be completed by adding missing oxygens in the 8a site to form the fluorite structure. Actually two series of diffraction peaks are observed. The first series with a high intensity contains the peaks corresponding to the fluorite subcell. The conditions of reflection are such as h + k, k + l and l + h must be a multiple of four (hkl: h + k = 4n, k + l = 4n and l + h = 4n) [16,17]. The second series with a low intensity (marked with a star on Fig. 1a) corresponds to the supercell of the ordered A2B2O7 pyrochlore structure. The intensity of the various diffraction peaks is strongly correlated with: (i) the difference between
Table 1 Projected range (Rp), nuclear stopping power (Sn) and electronic stopping power (Se) for 870 MeV Xe and 780 MeV Kr ions calculated with the SRIM code [14]. Values of Sn and Se are averaged over the first 500 nm. Ion
Energy (MeV)
Rp (lm)
Sn (keV/nm)
Se (keV/nm)
124
870 780
35.5 51.5
0.02 0.01
29.0 14.6
Xe 78 Kr
ð2Þ
where fsat is the transformed fraction at saturation, r is the section of the cylinder surrounding the ion path in which the transformation occurred (amorphization or pyrochlore-to-fluorite transition), and u is the ion fluence. The radius (R) of an ion track may be deduced from the value found for r according to the equation:
R ¼ ðr=pÞ1=2
ð3Þ
The results show that Gd2Ti2O7 is amorphized at a higher rate than Gd2TiZrO7: total amorphization occurs at 5 1012 cm2 for Gd2Ti2O7 and at 1013 cm2 for Gd2TiZrO7. On the other hand, total transformation of Gd2Zr2O7 into an anion-deficient fluorite structure occurs at 1013 cm2. The kinetic of transformation following irradiation with 780 MeV Kr ions (not shown here) is slower than that obtained with Xe ions, but similar tendencies are observed. These results are in agreement with a previous study of Lang et al. for the Gd2(Ti2xZrx)O7 pyrochlore family irradiated with 1.43 GeV Xe ion, even if complete transformations were not achieved in their case due to the too low fluences used [12,13]. The radius of the ion tracks induced in pyrochlores by 870 MeV Xe and 780 MeV Kr irradiations, deduced from XRD analysis, is given on Fig. 3, as a function of Se. The thermal spike model was used to account for the formation of ion tracks in pyrochlores irradiated with swift heavy ions. In this model, the energy lost in the wake of incident ions is given to the target electrons and is then transferred to the lattice through electron–electron and electron–phonon
S. Moll et al. / Nuclear Instruments and Methods in Physics Research B 268 (2010) 2933–2936
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Fig. 1. XRD patterns recorded on Gd2(Ti2xZrx)O7 pellets before (a) and after irradiation with 870 MeV Xe ions at 2 1012 cm2 (b) and 1013 cm2 (c). The peaks related to the supercell of the pyrochlore structure are noted with a star.
Fig. 3. Variation of the track radius (R) as a function of the electronic energy loss (Se) in Gd2(Ti2xZrx)O7 samples irradiated with swift heavy ions. The lines are fit to the data in the framework of the thermal spike model [19].
as a thermal spike. The material is locally melted and tracks are formed during the cooling phase. The Szenes approach [21] assumes that: (i) a thermal spike is created along the ion trajectory, (ii) the lattice temperature has a radial Gaussian distribution which evolves with time, (iii) the track radius is equal to the maximum melt region. The following equations are derived from the model:
R2 ¼ a20 ln R2 ¼ a20
Fig. 2. Variation of the fraction of amorphous (a) or fluorite (b) phases versus the ion fluence for Gd2(Ti2xZrx)O7 pellets irradiated with 870 MeV Xe ions. Lines are fits to XRD data with Eq. (2).
interactions [19–22]. These processes lead to an increase of the temperature around the ion trajectory, which is often referred to
Se Set
Se 2:7Set
for Se < Set
ð4Þ
for Se > 2:7Set
ð5Þ
where Set is the Se threshold for track formation, which is given by the equation:
Set ¼
pqcðT m T irr Þa20 g
ð6Þ
where gSe is the fraction of the deposited energy transferred to the thermal spike, a0 is a parameter which characterizes the initial
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S. Moll et al. / Nuclear Instruments and Methods in Physics Research B 268 (2010) 2933–2936
width of the temperature distribution in the spike (generally close to 4.5 nm [21]), q and c are respectively the density and the specific heat of the material, and Tm and Tirr are the melting and irradiation temperatures. The variation of the track radii with Se (shown in Fig. 3) was reproduced on the basis of the Szenes model. Best fits to the data provide the following values of the parameters: a0 = 5.8 nm and Set = 13.2 keV/nm for Gd2Ti2O7; a0 = 4.9 nm and Set = 13.2 keV for Gd2TiZrO7; a0 = 4.1 nm and Set = 13.8 keV/nm for Gd2Zr2O7. The Se thresholds for track formation were found to be in the range 13–14 keV/nm for the three pyrochlore compositions, although the transformations induced by irradiation appeared to be different (amorphization for Gd2Ti2O7 and Gd2TiZrO7, order– disorder transition for Gd2Zr2O7). Note that the determination of the values of Set relies on two data points only for each composition (obtained in two irradiation experiments – see Fig. 3); additional experiments are in progress using other ion beams and energies in order to obtain more precise values of Set, and thus to definitely assess whether Set depends on the pyrochlore composition or not. Nevertheless, the observation of tracks in the three compounds implies that the temperature reached in the track is above the melting point for the amorphizable compounds (i.e. 1820 °C for Gd2Ti2O7), and at least above the order–disorder transition temperature (1550 °C) for Gd2Zr2O7 (the melting point for this compound is above 2200 °C). Moreover, the amorphization ability depends also on the recrystallization rate versus the quenching rate within thermal spikes. The former rate could well be higher in zirconates than in titanates, explaining the higher amorphization sensitivity of titanates under high electronic excitation. The variation of the track radius with Se depends on the pyrochlore composition: it increases strongly for Gd2Ti2O7, and the increase is smoother when Ti is substituted by Zr. In a recent study of Lang et al. the track diameters of pyrochlores irradiated with 1.43 GeV Xe ions (a higher energy but a similar Se as 870 MeV Xe ions) were found to be smaller for all pyrochlore compositions than the ones determined in the present work. This result is likely due to an ion velocity effect, which has already been observed in, e.g., cubic zirconia irradiated with swift ions [23]. In this material two electronic excitation regimes were determined with different Se thresholds for track formation. This effect was attributed to the different energies of d electrons causing a different spreading of the electronic excitation deposited along the ion tracks. 4. Conclusion Pyrochlores of the Gd2(Ti2xZrx)O7 series (x = 0, 1 and 2) were irradiated with 870 MeV Xe and 780 MeV Kr ions in order to investigate the damage created by electronic excitation. X-ray diffrac-
tion data show that the structural modifications induced by irradiation are strongly dependent on the sample stoichiometry: Gd2Ti2O7 is readily amorphized, whereas Gd2Zr2O7 is transformed into a radiation-resistant anion-deficient fluorite structure. The Se threshold for track formation lies in the range 13–14 keV/nm for the three compounds. These results have to be seriously considered for the design and elaboration of future matrices for actinide immobilization or transmutation. Acknowledgement This work was partially supported by the ‘‘Groupement National de Recherche” (GNR) MATINEX. References [1] R.C. Ewing, W.J. Weber, J. Lian, J. Appl. Phys. 95 (2004) 5949. [2] S.X. Wang, B.D. Begg, L.M. Wang, R.C. Ewing, W.J. Weber, K.V.G. Kutty, J. Mater. Res. 14 (1999) 4470. [3] S.X. Wang, L.M. Wang, R.C. Ewing, G.S. Was, G.R. Lumpkin, Nucl. Instrum. Meth. B 148 (1999) 704. [4] J. Lian, L.M. Wang, S.X. Wang, J. Chen, L.A. Boatner, R.C. Ewing, Phys. Rev. Lett. 87 (2001) 145901. [5] B.D. Begg, N.J. Hess, D.E. McCready, S. Thevuthasan, W.J. Weber, J. Nucl. Mater. 289 (2001) 188. [6] J. Lian, X.T. Xu, K.V.G. Kutty, J. Chen, L.M. Wang, R.C. Ewing, Phys. Rev. B 66 (2002) 054108. [7] J. Lian, L. Wang, J. Chen, K. Sun, R.C. Ewing, J.M. Farmer, L.A. Boatner, Acta Mater. 51 (2003) 1493. [8] K.E. Sickafus, L. Minervini, R.W. Grimes, J.A. Valdez, M. Ishimaru, F. Li, K.J. McClellan, T. Hartmann, Science 289 (2000) 748. [9] K.E. Sickafus, R.W. Grimes, J.A. Valdez, A. Cleave, M. Tang, M. Ishimaru, S.M. Corish, C.R. Stanek, B.P. Uberuaga, Nat. Mater. 6 (2007) 217. [10] M.K. Patel, V. Vijayakumar, D.K. Avasthi, S. Kailas, J.C. Pivin, V. Grover, B.P. Mandal, A.K. Tyagi, Nucl. Instrum. Meth. B 266 (2008) 2898. [11] G. Sattonnay, S. Moll, M. Hebst-Ghysel, L. Thomé, F. Garrido, J.-M. Costantini, C. Trautmann, Nucl. Instrum. Meth. B 266 (2008) 3043. [12] M. Lang, J. Lian, J. Zhang, F. Zhang, W.J. Weber, C. Trautmann, R.C. Ewing, Phys. Rev. B 79 (2009) 224105. [13] M. Lang, F.X. Zhang, R.C. Ewing, J. Lian, C. Trautmann, Z. Wang, J. Mater. Res. 24 (2009) 1322. [14] J.F. Ziegler, J.P. Biersack, U. Littmark, in: J.F. Ziegler (Ed.), The Stopping and Range of Ions in Solids, vol. 1, Pergamon, New York, 1985. [15] A.C. Larson, R.B. Von Dreele, General Structure Analysis System, LANSCE, Los Alamos National Laboratory, Report LAUR 86-748, 2000. [16] B.J. Wuensch, K.W. Eberman, C. Heremans, E.M. Ku, P. Onnerud, E.M.E. Yeo, S.M. Haile, J.K. Stalick, J.D. Jorgensen, Solid State Ion. 129 (2000) 111. [17] T. Hahn (Ed.), International Tables for Crystallography, fifth ed., vol. A, Kluwer, Dordrecht, 2002, p. 700. [18] J.F. Gibbons, IEEE 60 (1972) 1062. [19] F. Seitz, J.S. Koehler, in: F. Seitz, D. Turnbull (Eds.), Solid State Physics: Advances in Research and Applications, Academic, NY, 1956, p. 305. [20] M. Toulemonde, C. Dufour, E. Paumier, Phys. Rev. B 46 (1992) 14362. [21] G. Szenes, Phys. Rev. B 51 (1995) 8026. [22] H. Trinkaus, A.I. Ryazanov, Phys. Rev. Lett. 74 (1995) 5072. [23] L. Thomé, S. Moll, G. Sattonnay, L. Vincent, F. Garrido, J. Jagielski, J. Appl. Phys. 105 (2009) 023512.
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Swift heavy ion irradiation of pyrochlore oxides: Electronic energy loss threshold for latent track formation S. Moll a, G. Sattonnay b,*, L. Thomé a, J. Jagielski c,d, C. Legros b, I. Monnet e a
Centre de Spectrométrie Nucléaire et de Spectrométrie de Masse, CNRS, IN2P3, Université Paris-Sud, Bât. 108, F-91405 Orsay, France LEMHE, ICMMO, Université Paris Sud, Bât. 410, F-91405 Orsay, France c Institute for Electronic Materials Technology, Wolczynska 133, Pl-01-919 Warsaw, Poland d Andrzej Soltan Institute for Nuclear Studies, Pl-05-400 Swierk/Otwock, Poland e CIMAP, CEA, CNRS, ENSICAEN, BP 5133, F-14070 Caen Cedex 5, France b
a r t i c l e
i n f o
Article history: Received 21 September 2009 Received in revised form 16 April 2010 Available online 7 May 2010 Keywords: Ion irradiation Pyrochlores X-ray diffraction
a b s t r a c t Pyrochlore pellets with the Gd2(Ti2xZrx)O7 stoichiometry (x = 0, 1 and 2) were irradiated with swift heavy ions in order to investigate the effects of electronic excitation and to determine the electronic stopping power threshold for track formation. XRD results showed that the electronic excitation induced by 870 MeV Xe and 780 MeV Kr ions leads to: (i) a crystalline–amorphous transition for Gd2Ti2O7 and Gd2TiZrO7, (ii) a phase transition towards an anion-deficient fluorite structure (order–disorder transition) for Gd2Zr2O7. Thus, zirconate pyrochlores present a better radiation resistance under swift heavy ion irradiation than titanate pyrochlores. Moreover results underline the existence of an electronic stopping power threshold around 13–14 keV/nm, below which phase transformations do not occur. Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction Pyrochlores are promising materials for their application as matrices for the transmutation or the immobilization of actinides produced in nuclear power plants [1]. In operating conditions or during long-term storage, nuclear matrices are submitted to severe radiations which induce atomic rearrangements. These structural modifications lead to the alteration of the physico-chemical properties of materials. Although the radiation-induced crystallineto-amorphous transformation in pyrochlores has drawn much attention in the past [1–7], only few studies were devoted to the investigation of the damage induced by swift heavy ions which deposit energy via high electronic excitations. The pyrochlores under consideration in this study typically exhibit the A2B2O7 stoichiometry, where actinides or lanthanides can be incorporated on the eight-coordinated A-site and metals occupy the six-coordinated B-site. The stability of pyrochlores seems to be governed by the ratio of the ionic radii of A and B cations (rA/rB) which extends from 1.46 for Gd2Zr2O7 to 1.78 for Sm2Ti2O7 [1,2]. For rA/rB < 1.46 an anion-deficient fluorite structure (A,B)4O7 is the stable form, while a monoclinic structure is exhibited for rA/rB > 1.78 [1]. Beyond cationic radii ratio considerations to predict the stability of pyrochlores under irradiation, improved amorphization characteristics are to be found in compounds that have a
* Corresponding author. Tel.: +33 1 69 15 70 37. E-mail address: [email protected] (G. Sattonnay). 0168-583X/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2010.05.012
natural tendency to accommodate lattice disorder [8,9]. A good indicator of the radiation tolerance of pyrochlores is thus the energy of order–disorder fluorite defect reaction pair (i.e. both cation antisite and anion Frenkel defect formation) [9], which is notably lower in zirconates than in titanates. This criterion was supported by irradiations performed on gadolinium-based pyrochlores with the Gd2(Ti2xZrx)O7 stoichiometry, since they exhibit a systematic decrease of their amorphization susceptibility with increasing Zr content for both low [1,2,5] and high [10–13] energy ions. The end member Gd2Zr2O7 is transformed into a radiation-resistant anion-deficient fluorite structure upon irradiation at room temperature [1,2,5,10–13]. This paper reports preliminary results of the damage created by swift heavy ions in order to investigate the effects of electronic excitation on the phase transformation in pyrochlore oxides. X-ray diffraction (XRD) was implemented to determine the structural changes induced by irradiation, as well as the damage build-up, in pyrochlores of the Gd2(Ti2xZrx)O7 family for x = 0 (rA/rB = 1.74), x = 1 (rA/rB = 1.59) and x = 2 (rA/rB = 1.46).
2. Experimental Polycrystalline Gd2(Ti2xZrx)O7 pellets, with x = 0, 1 and 2, were prepared by a standard solid state process. Stoichiometric amounts of gadolinium oxide (Gd2O3), first heated at 900 °C for 15 h (Alfa Aesar 99.99%), zirconium oxide (ZrO2, Alfa Aesar 99.5%) and titanium oxide (TiO2, Alfa Aesar 99.99%) were intimately mixed in
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S. Moll et al. / Nuclear Instruments and Methods in Physics Research B 268 (2010) 2933–2936
an acetone slurry using a ball-mill and subsequently dried. After drying, the powders were isostatically pressed into rods at 250 MPa. The homogenized mixtures were then subjected to a three step heating protocol with intermittent grindings to attain a better homogeneity. The compacts were first sintered at 1200 °C for 12 h. The procedures (grinding, milling, pressing) were repeated twice with a second heating at 1400 °C for 72 h and a final sintering in air at 1500 °C for 196 h. Pellets were cut with a diamond saw and the specimens were polished to a 0.5 lm diamond finish. Pyrochlore oxides were irradiated at room temperature with 870 MeV Xe and 780 MeV Kr ions at the GANIL accelerator in Caen at fluences ranging from 1010 up to 1013 cm2. Ion currents were always kept lower than 108 cm2 s1 in order to keep target heating <50 °C. The parameters of irradiations are summarized in Table 1 which clearly shows that Se overwhelms Sn by several orders of magnitude. XRD experiments were performed with a X’pert Pro MRD PANalytical diffractometer, using a CuKa anticathode (kKa1 = 0.1540598 nm, kKa2 = 0.1544426 nm). X-ray patterns were recorded in a standard (h–2h) geometry from 13° to 90° (2h) with a step of 0.025°. The diffraction patterns were analyzed using two different approaches. In the first one, for the amorphizable pyrochlores (Gd2Ti2O7 and Gd2TiZrO7), the amorphous fraction fa was deduced from the net areas of the corresponding XRD lines by using the following equation:
Pn fa ¼ 1
Airradiated i i¼1 Aunirradiated i
ð1Þ
n
where Airradiated and Aunirradiated are the net area of the ith XRD line in i i the pattern recorded on irradiated and unirradiated samples, respectively, and n is the number of lines considered. Pseudo-Voigt profiles were used to fit the diffraction peaks. In the second approach, used for Gd2Zr2O7, the diffraction patterns were analyzed by Rietveld refinement, using GSAS package [15], with a procedure described by Wuensch et al. [16] in order to determine the fraction of anion-deficient fluorite structure induced by irradiation (which coexists with the remaining pyrochlore phase).
the scattering powers of the cations that occupy A and B sites, (ii) the occupancy of cation and anion sites, (iii) the displacement of 48f oxygens from the ideal fluorite structure, and (iv) the temperature factor coefficient [16]. It should be noted that in X-ray diffraction the scattering power of oxygen is relatively small compared to that of cations, so that the influence of cations is predominant for XRD patterns. For cations in the A and B sites, extra conditions of reflection (superstructure reflection) are allowed for hkl: h = 2n + 1 or h,k,l = 4n + 2 or h,k,l = 4n [16,17]. As shown in Fig. 1a the intensity of the superstructure reflection peaks decreases with increasing Zr content. Fig. 1b and c show XRD patterns recorded on samples irradiated with 870 MeV Xe ions at a fluence of 2 1012 cm2 and 1013 cm2, respectively. For Gd2Zr2O7 the diffraction peaks related to the supercell of the pyrochlore structure disappear and only the peaks corresponding to the fluorite structure are observed at the final fluence. This result indicates that irradiation has induced an order–disorder phase transition: Gd2Zr2O7 is transformed into an anion-deficient fluorite structure by the disordering of cations occupying the A and B sites and by the creation of anion vacancies [1–7]. It is worth noting that no amorphization is observed for this compound. Conversely, for Gd2TiZrO7 and Gd2Ti2O7 a progressive vanishing of all diffraction peaks is observed and an additional diffuse scattering occurs at the basis of the (2 2 2) pyrochlore peaks, which indicates the amorphization of the samples. These results are in agreement with previous data [10–13] which showed that a systematic increase of the radiation resistance with increasing Zr content is observed for the Gd2(Ti2xZrx)O7 series. Fig. 2 compares the phase transformation build-up, i.e. the variation, versus the ion fluence, of the fraction (f) of a new phase (either amorphous or fluorite), extracted by the fitting procedure exposed previously, for the three compounds irradiated with 870 MeV Xe ions. The variation of f with the ion fluence can be accounted for in the framework of a single impact model [18] (Poisson’s relationship). In this model, it is implicitly assumed that swift heavy ions create individual tracks and that the overall damage observed at high fluence results from the overlapping of ion tracks. Thus, the variation of f may be written as:
f ¼ fsat ½1 expðr/Þ 3. Results and discussion Fig. 1a shows XRD patterns recorded on virgin pellets. Ordered A2B2O7 pyrochlores belong to the Fd3m space group which is a superstructure of the ideal fluorite structure (Fd3m space group) with twice the lattice constant. A and B cations occupy the 16c and 16d sites, respectively, and oxygens are located at the 48f and 8b positions. The anion sublattice can be completed by adding missing oxygens in the 8a site to form the fluorite structure. Actually two series of diffraction peaks are observed. The first series with a high intensity contains the peaks corresponding to the fluorite subcell. The conditions of reflection are such as h + k, k + l and l + h must be a multiple of four (hkl: h + k = 4n, k + l = 4n and l + h = 4n) [16,17]. The second series with a low intensity (marked with a star on Fig. 1a) corresponds to the supercell of the ordered A2B2O7 pyrochlore structure. The intensity of the various diffraction peaks is strongly correlated with: (i) the difference between
Table 1 Projected range (Rp), nuclear stopping power (Sn) and electronic stopping power (Se) for 870 MeV Xe and 780 MeV Kr ions calculated with the SRIM code [14]. Values of Sn and Se are averaged over the first 500 nm. Ion
Energy (MeV)
Rp (lm)
Sn (keV/nm)
Se (keV/nm)
124
870 780
35.5 51.5
0.02 0.01
29.0 14.6
Xe 78 Kr
ð2Þ
where fsat is the transformed fraction at saturation, r is the section of the cylinder surrounding the ion path in which the transformation occurred (amorphization or pyrochlore-to-fluorite transition), and u is the ion fluence. The radius (R) of an ion track may be deduced from the value found for r according to the equation:
R ¼ ðr=pÞ1=2
ð3Þ
The results show that Gd2Ti2O7 is amorphized at a higher rate than Gd2TiZrO7: total amorphization occurs at 5 1012 cm2 for Gd2Ti2O7 and at 1013 cm2 for Gd2TiZrO7. On the other hand, total transformation of Gd2Zr2O7 into an anion-deficient fluorite structure occurs at 1013 cm2. The kinetic of transformation following irradiation with 780 MeV Kr ions (not shown here) is slower than that obtained with Xe ions, but similar tendencies are observed. These results are in agreement with a previous study of Lang et al. for the Gd2(Ti2xZrx)O7 pyrochlore family irradiated with 1.43 GeV Xe ion, even if complete transformations were not achieved in their case due to the too low fluences used [12,13]. The radius of the ion tracks induced in pyrochlores by 870 MeV Xe and 780 MeV Kr irradiations, deduced from XRD analysis, is given on Fig. 3, as a function of Se. The thermal spike model was used to account for the formation of ion tracks in pyrochlores irradiated with swift heavy ions. In this model, the energy lost in the wake of incident ions is given to the target electrons and is then transferred to the lattice through electron–electron and electron–phonon
S. Moll et al. / Nuclear Instruments and Methods in Physics Research B 268 (2010) 2933–2936
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Fig. 1. XRD patterns recorded on Gd2(Ti2xZrx)O7 pellets before (a) and after irradiation with 870 MeV Xe ions at 2 1012 cm2 (b) and 1013 cm2 (c). The peaks related to the supercell of the pyrochlore structure are noted with a star.
Fig. 3. Variation of the track radius (R) as a function of the electronic energy loss (Se) in Gd2(Ti2xZrx)O7 samples irradiated with swift heavy ions. The lines are fit to the data in the framework of the thermal spike model [19].
as a thermal spike. The material is locally melted and tracks are formed during the cooling phase. The Szenes approach [21] assumes that: (i) a thermal spike is created along the ion trajectory, (ii) the lattice temperature has a radial Gaussian distribution which evolves with time, (iii) the track radius is equal to the maximum melt region. The following equations are derived from the model:
R2 ¼ a20 ln R2 ¼ a20
Fig. 2. Variation of the fraction of amorphous (a) or fluorite (b) phases versus the ion fluence for Gd2(Ti2xZrx)O7 pellets irradiated with 870 MeV Xe ions. Lines are fits to XRD data with Eq. (2).
interactions [19–22]. These processes lead to an increase of the temperature around the ion trajectory, which is often referred to
Se Set
Se 2:7Set
for Se < Set
ð4Þ
for Se > 2:7Set
ð5Þ
where Set is the Se threshold for track formation, which is given by the equation:
Set ¼
pqcðT m T irr Þa20 g
ð6Þ
where gSe is the fraction of the deposited energy transferred to the thermal spike, a0 is a parameter which characterizes the initial
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width of the temperature distribution in the spike (generally close to 4.5 nm [21]), q and c are respectively the density and the specific heat of the material, and Tm and Tirr are the melting and irradiation temperatures. The variation of the track radii with Se (shown in Fig. 3) was reproduced on the basis of the Szenes model. Best fits to the data provide the following values of the parameters: a0 = 5.8 nm and Set = 13.2 keV/nm for Gd2Ti2O7; a0 = 4.9 nm and Set = 13.2 keV for Gd2TiZrO7; a0 = 4.1 nm and Set = 13.8 keV/nm for Gd2Zr2O7. The Se thresholds for track formation were found to be in the range 13–14 keV/nm for the three pyrochlore compositions, although the transformations induced by irradiation appeared to be different (amorphization for Gd2Ti2O7 and Gd2TiZrO7, order– disorder transition for Gd2Zr2O7). Note that the determination of the values of Set relies on two data points only for each composition (obtained in two irradiation experiments – see Fig. 3); additional experiments are in progress using other ion beams and energies in order to obtain more precise values of Set, and thus to definitely assess whether Set depends on the pyrochlore composition or not. Nevertheless, the observation of tracks in the three compounds implies that the temperature reached in the track is above the melting point for the amorphizable compounds (i.e. 1820 °C for Gd2Ti2O7), and at least above the order–disorder transition temperature (1550 °C) for Gd2Zr2O7 (the melting point for this compound is above 2200 °C). Moreover, the amorphization ability depends also on the recrystallization rate versus the quenching rate within thermal spikes. The former rate could well be higher in zirconates than in titanates, explaining the higher amorphization sensitivity of titanates under high electronic excitation. The variation of the track radius with Se depends on the pyrochlore composition: it increases strongly for Gd2Ti2O7, and the increase is smoother when Ti is substituted by Zr. In a recent study of Lang et al. the track diameters of pyrochlores irradiated with 1.43 GeV Xe ions (a higher energy but a similar Se as 870 MeV Xe ions) were found to be smaller for all pyrochlore compositions than the ones determined in the present work. This result is likely due to an ion velocity effect, which has already been observed in, e.g., cubic zirconia irradiated with swift ions [23]. In this material two electronic excitation regimes were determined with different Se thresholds for track formation. This effect was attributed to the different energies of d electrons causing a different spreading of the electronic excitation deposited along the ion tracks. 4. Conclusion Pyrochlores of the Gd2(Ti2xZrx)O7 series (x = 0, 1 and 2) were irradiated with 870 MeV Xe and 780 MeV Kr ions in order to investigate the damage created by electronic excitation. X-ray diffrac-
tion data show that the structural modifications induced by irradiation are strongly dependent on the sample stoichiometry: Gd2Ti2O7 is readily amorphized, whereas Gd2Zr2O7 is transformed into a radiation-resistant anion-deficient fluorite structure. The Se threshold for track formation lies in the range 13–14 keV/nm for the three compounds. These results have to be seriously considered for the design and elaboration of future matrices for actinide immobilization or transmutation. Acknowledgement This work was partially supported by the ‘‘Groupement National de Recherche” (GNR) MATINEX. References [1] R.C. Ewing, W.J. Weber, J. Lian, J. Appl. Phys. 95 (2004) 5949. [2] S.X. Wang, B.D. Begg, L.M. Wang, R.C. Ewing, W.J. Weber, K.V.G. Kutty, J. Mater. Res. 14 (1999) 4470. [3] S.X. Wang, L.M. Wang, R.C. Ewing, G.S. Was, G.R. Lumpkin, Nucl. Instrum. Meth. B 148 (1999) 704. [4] J. Lian, L.M. Wang, S.X. Wang, J. Chen, L.A. Boatner, R.C. Ewing, Phys. Rev. Lett. 87 (2001) 145901. [5] B.D. Begg, N.J. Hess, D.E. McCready, S. Thevuthasan, W.J. Weber, J. Nucl. Mater. 289 (2001) 188. [6] J. Lian, X.T. Xu, K.V.G. Kutty, J. Chen, L.M. Wang, R.C. Ewing, Phys. Rev. B 66 (2002) 054108. [7] J. Lian, L. Wang, J. Chen, K. Sun, R.C. Ewing, J.M. Farmer, L.A. Boatner, Acta Mater. 51 (2003) 1493. [8] K.E. Sickafus, L. Minervini, R.W. Grimes, J.A. Valdez, M. Ishimaru, F. Li, K.J. McClellan, T. Hartmann, Science 289 (2000) 748. [9] K.E. Sickafus, R.W. Grimes, J.A. Valdez, A. Cleave, M. Tang, M. Ishimaru, S.M. Corish, C.R. Stanek, B.P. Uberuaga, Nat. Mater. 6 (2007) 217. [10] M.K. Patel, V. Vijayakumar, D.K. Avasthi, S. Kailas, J.C. Pivin, V. Grover, B.P. Mandal, A.K. Tyagi, Nucl. Instrum. Meth. B 266 (2008) 2898. [11] G. Sattonnay, S. Moll, M. Hebst-Ghysel, L. Thomé, F. Garrido, J.-M. Costantini, C. Trautmann, Nucl. Instrum. Meth. B 266 (2008) 3043. [12] M. Lang, J. Lian, J. Zhang, F. Zhang, W.J. Weber, C. Trautmann, R.C. Ewing, Phys. Rev. B 79 (2009) 224105. [13] M. Lang, F.X. Zhang, R.C. Ewing, J. Lian, C. Trautmann, Z. Wang, J. Mater. Res. 24 (2009) 1322. [14] J.F. Ziegler, J.P. Biersack, U. Littmark, in: J.F. Ziegler (Ed.), The Stopping and Range of Ions in Solids, vol. 1, Pergamon, New York, 1985. [15] A.C. Larson, R.B. Von Dreele, General Structure Analysis System, LANSCE, Los Alamos National Laboratory, Report LAUR 86-748, 2000. [16] B.J. Wuensch, K.W. Eberman, C. Heremans, E.M. Ku, P. Onnerud, E.M.E. Yeo, S.M. Haile, J.K. Stalick, J.D. Jorgensen, Solid State Ion. 129 (2000) 111. [17] T. Hahn (Ed.), International Tables for Crystallography, fifth ed., vol. A, Kluwer, Dordrecht, 2002, p. 700. [18] J.F. Gibbons, IEEE 60 (1972) 1062. [19] F. Seitz, J.S. Koehler, in: F. Seitz, D. Turnbull (Eds.), Solid State Physics: Advances in Research and Applications, Academic, NY, 1956, p. 305. [20] M. Toulemonde, C. Dufour, E. Paumier, Phys. Rev. B 46 (1992) 14362. [21] G. Szenes, Phys. Rev. B 51 (1995) 8026. [22] H. Trinkaus, A.I. Ryazanov, Phys. Rev. Lett. 74 (1995) 5072. [23] L. Thomé, S. Moll, G. Sattonnay, L. Vincent, F. Garrido, J. Jagielski, J. Appl. Phys. 105 (2009) 023512.