ION IRRADIATION DAMAGE IN ILMENITE AT 100 K

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4 /. REVISED VERSION ~ o N F 9G044F-- ION IRRADIATION DAMAGE IN ILMENITE AT 100 K J.N. Mitchell N. Yu R. Devanathan K.E. Sickafus and M.A. Nastasi Materials Science and Technology Division Los Alamos National
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4 /. REVISED VERSION ~ o N F 9G044F-- ION IRRADIATION DAMAGE IN ILMENITE AT 100 K J.N. Mitchell N. Yu R. Devanathan K.E. Sickafus and M.A. Nastasi Materials Science and Technology Division Los Alamos National Laboratory Los Alamos NM 87545; G.L. Nurd Jr. United States Geological Survey Reston VA ABSTRACT A natural single crystal of ilmenite (FeTiO) was irradiated at 100 K with 200 kev Ar2+. Rutherford backscattering spectroscopy and ion channeling with 2 MeV He' ions were used to monitor damage accumulation in the surface region of the implanted crystal. At an irradiation flu' ~ cm-2considerable near-surface He' ion dechanneling was observed to the ence of l ~ l O A? extent that ion yield from a portion of the aligned crystal spectrum reached the yield level of a random spectrum. This observation suggests that the near-surface region of the crystal was amorphized by the implantation. Cross-sectional transmission electron microscopy and electron diffraction on this sample confirmed the presence of a 150 nm thick amorphous layer. These results are compared to similar investigations on geikielite (MgTiO) and spinel (MgAlO) to explore factors that may influence radiation damage response in oxides. INTRODUCTION Spinel is exceptionally resistant to ion and neutron irradiation and as a result is being considered as an insulating material for fusion reactor applications [l-41.recently Sickafus et al. (51 suggested that several characteristics may enhance radiation resistance in oxides: complexity of composition and the tendency for cation disorder. Clinard et al. [2] were t first to show evidence that compositional complexity enhances radiation resistance and it does so by suppressing the nucleation and growth of dislocation loops and voids. Good examples of the defect characteristics of complex and simple compounds are spinel and MgO. In spinel the formation of a dislocation loop requires the condensation of two or more MgO*AlO anti-schottky septets. Moreover it is hard to condense point defects into loops because they are invariably faulted. Additionally in spinel the h. DISCLAIMER This report was prepared as an account of work sponsored by an agency of the United States Government Neither the United States Government nor any agency thereof nor any of their employees make any warrantyexpress or implied or assumesany legal liability or mponsiiility for the accuracy completeness or usefulness of any information apparatus product or process disdosed or represents that its use wwld not infringe privately owned rights. Reference herein to any specific commercial product process or service by trade name trademark manufacturer or otherwise does not necessarily constitute or imply its endorsement recommendation or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof. formation of anti-site defects occurs at much lower energies than either Frenkel or Schottky defects [6]. Thus the major low energy defect structure is cation disorder. In MgO however it is much simpler to condense MgO molecular units and the lack of stacking faults makes loop nucleation easier. Also these loops grow readily during irradiation leading to a vacancy bias void formation and concomitant swelling [2]. To test the proposed radiation damage resistance criteria we recently began an investigation of radiation damage response in ilmenite-group oxides. We chose this family of oxides because of their relative compositional complexity (two cations) and the tendency of Fe2+and Ti& cations in ilmenite (FeTiO) to disorder at high temperatures. In this paper we describe the results of 200 kev A? irradiations of a natural ilmenite single crystal. Rutherford backscattering spectrometry combined with ion channeling (RBSK) indicates that this material amorphizes at doses less than lx 10'' A?/cm2. Transmission electron microscopy (TEM) and electron diffraction confumed this observation and revealed the presence of a 160 nm thick amorphous surface layer. The low ion irradiation tolerance of ilmenite suggests that chemical composition and crystal structure may be important in determining the radiation resistance of an oxide. BACKGROUND The family of compositions we refer to as the ilmenite-group oxides are related by the fundamental composition A2Ti4+0.In natural crystals the divalent cation A can be Fe (ilmenite) Mg (geikielite) Mn (pyrophanite) or Zn (ecandrewsite) and there is substantial solid solution within this system. Crystals with Co Cd and Ni in the A site have also been synthesized. In nature ilmenite is by far the most common of the rhombohedral titanates. Ilmenite &of interest as a potential substrate material for high T superconducting films [7] as a high-temperature wide band gap semiconductor [8] and as a component of heavy concrete for radiation shielding in fission reactors [e.g. 91. Ilmenite has been studied as a source for oxygen on proposed lunar bases [lo] as a resource for He3+for space fusion energy applications [ 111 and as a radiation-resistant semiconductor for satellites and related space applications [ Rhombohedral oxides such as ilmenite have crystal structures based on the hexagonal closepacking scheme. The cations sit on two-thirds of the available octahedral sites. The ilmenite structure is essentially an ordered version of the a-alumina structure ( R ~ c ). The occupation of Fe2' and Ti4-' instead of A13+doubles the number of crystallographically nonequivalent cation sites reducing the space group symmetry to R5. As shown in Fig. 1 Fe and Ti cations are layered along the [OOOl] direction. This ordering results in displacement of the anions away from the layers with larger cations toward the layers with the smaller cations. The physical properties of ilmenite are closely linked to its solid solution relations with hematite (Fe203). The properties of this solid solution series are the result of cation and magnetic orderdisorder and low temperature immiscibility. The phase diagram (Fig. 2) of the system FeOFeTiO has several important features: (1) a cation ordering transition; (2) a miscibility gap between disordered (hematite) and ordered (ilmenite) phases; and (3) a magnetic order-disorder transition. At room temperature ilmenite with ~ 2 7 %hematite component is a p-type conductor whereas if the hematite component is 27% it is a n-type conductor [14]. The magnetic properties of ilmenite-hematite solid solutions range from paramagnetic to ferrimagnetic to antiferromagnetic [ Except under extremely reducing conditions natural ilmenite tends to have some component of FeO. Ilmenite grains in volcanic rocks cool rapidly and Fe03 remains dissolved in the quenched ilmenite. In rocks cooied over long periods of time the hematite and ilmenite components will exsolve and form composite crystals with the abundance of these phases controlled by bulk chemical composition Lunar ilmenite has no hematite component due to the absence of Fe3+ on the Moon. 1. The crystal used for this study cooled slowly over millions of years and has approximately 20 volume 9% hematite that occurs as micron-scale ovoid exsolution structures. The cation orderdisorder transition temperature for a crystal with this bulk composition is much higher than the temperature during our experiment (100 K). However neutron- and ion-irradiated spinel crystals show greatly increased cation disordering at temperatures lower than capable of producing equiva- 3 1. lent thermally induced disorder [ Thus we anticipate that disordering in ilmenite may be similarly influenced by irradiation. EXPERIMENT The sample used in this study is a natural single crystal of ilmenite collected in the Adirondack Mountains New York. The crystal was oriented using Laue x-ray back reflection cut into wafers perpendicular to the c axis and polished to an optical finish on one side for ion-inahation studies. Rutherford backscattering spectrometry combined with ion channeling (RBS/C) and ion inadiations were conducted at the Ion Beam Materials Laboratory at Los Alamos National Laboratory. Two MeV He+ ion beam RBSK measurements were performed on the samples to verify the orientation of the crystal and to assess the quality of the polished surface. Aligned RBS spectra were obtained while the incident He' beam was aligned along the 0001 axis of the crystal. Minimum backscattering yield (x) defined as the RBS yield ratio of the aligned spectrum to that of the random spectrum was used to quantify the quality of the sample surface. The initial high xmin(-30%) indicated substantial residual damage in the near-surface region due to mechanical polishing. The damaged layer was effectively removed by etching the sample in a 50:50 mixture of hydrofluoric acid and water at roqm temperature for -5 minutes as indicated by the reduction in xminto -6% along the c-axis. Following etching the crystal was irradiated at 100 K on a liquid nitrogen conduction-cooled sample stage. Ion-irradiation experiments were performed using 200 kev A?+ at doses of IxlO'' and 2x1015A?'/cm2 on different portions of the crystal. The samples we& 'tilted by 10 from the c-axis during the irradiations to minimize channeling effects. One section of the crystal was masked from the ion irradiation and used for ion channeling alignment following the irradiations. Unirradiated and irradiated regions of the crystal were analyzed with REWC along the coool axis using a 2 MeV He+ ion beam. Random spectra were collected by rocking the sample about 3 off the 0001 channeling direction and are used for comparison with the aligned spectra. 4 The Monte Carlo code TRIM (Transport of Ions in Matter) [ 191 was used to estimate ion range and damage parameters (Fig. 3). TRIM simulations indicate that the projected range of 200 kev A? ions in ilmenite is 125 nm with a straggle of 42 nm. The peak concentration of Ar ions is 0.1 atomic 9% at a dose of 1x10i5A?+/cm2. At this dose the peak damage level is -1 displacement per atom (dpa) assuming threshold energies of 20 ev for cations and 60 ev for anions. Finally the irradiated sample was prepared in cross section for transmission electron microscopy (TEM) observation to assess changes in microstructures induced by ion irradiation. Two portions of the section irradiated to a dose of lx1015 A?/cm2 were glued face-to-face and then ground and polished to a thickness of -20 microns. The sample was further thinned by ion milling using 3-5 kev Ar ions. The finished sample was examined using a Philips CM30 TEM operating at 300 kv. RESULTS RBS/C spectra of the three portions of the 0001 aligned ilmenite crystal are shown with the random spectrum in Fig. 4. The spectrum from the unirradiated portion of the crystal is characterized by a low x ~ and very small Fe Ti and 0 surface peaks. The presence of hematite exsolu- tion structures may have increased the dechanneling in this spectrum. The spectra acquired from sample regions irradiated to 1 and 2x10i5A8+/cm2are virtually identical with the latter resulting in slightly higher RBS yields. The dechanneling yield from the irradiated layer at fluences of 1 and 2x10i5A?/cm2 coincide with the backscattering yield in a random-orientation spectrum. This is indicative of the formation of an amorphous layer in the irradiated region or that the irradiated zone \. has become polycrystalline. The RI3SIC results do not indicate the presence of defective crystalline material in the irradiated region as has been observed in spinel [2021]. A more direct way to assess the nature of the damage in the irradiated region is by transmission electron microscopy of cross-sectioned samples. Using this technique we observed in the sample portion irradiated with lx10i5ar /cm2a thin (150 nm) homogenous layer (Fig. 5a). Bend contours terminate at the interface between this layer and the substrate indicating an abrupt transition 5 from a crystalline to amorphous material. Selected-area electron diffraction (SAED) patterns of this layer show diffuse rings around the transmitted beam indicating that the layer is amorphous (Fig. 5b). In contrast an SAED pattern of the substrate reveals the rhombohedral symmetry of ilmenite (Fig. 5c). No hematite was observed in the thin regions of the TEM foil but its widespread presence in the bulk sample suggests that the lack of hematite in the foil is probably just by chance. DISCUSSION Matzke [22] reported that ion-irradiated hematite became quasi-amorphous at a flux of 2 ~ 1 0 ~ ions/cm2 although the ion type energy and experimental temperature are not specified. Compari- son between the experiment reported in this paper k d those of Matzke [22] are difficult but if hematite amorphizes more easily than ilmenite the hematite grains present in ow composite sample may act as amorphization nuclei and result in premature amorphization of the bulk crystal. However the absence of hematite in the portion of the crystal that we studied with TEM suggests that ilmenite may have amorphized without the local influence of hematite. A more detailed investigation is needed to explore this potential relationship. It is possible that the ion-irradiation process is not inert and that chemical reactions took place during the experiment [23]. Of particular concern is the possibility that redox reactions during irradiation may assist the precipitation of new phases such as FeO FeO and TiO resulting in a polycrystalline surface layer. R B S K spectra taken from such a layer would be similar to those acquired from an amorphous surface layer. Korenevskii et al. [24] reported that iron in hematite was partially reduce4 to Fe2+during exposure to a fluence of 1.3~10 neutronskm at a temperature of 150 C. Nevertheless the electron diffraction pattern of the surface layer of.the irradiated ilmenite in this experiment is consistent with an amorphous material though we have not examined the valence state of iron in this region. Our parallel investigation of radiation damage in geikielite (MgTiO) indicates that it is consid- erably more radiation resistant than ilmenite under similar cryogenic irradiation conditions [23]. Geikielite and ilmenite are isostructural differing only in composition. Similarly pure fayalite 6 (FeSiO) is less radiation resistant than fayalite-forstente (MgSiO) solid solutions [25]. Thus chemical composition and insusceptibility to redox reactions may also play a role in determining ion radiation resistance. The early stages of damage accumulation need to be examined to better discern the relationship between possible redox reactions and amorpbzation in composite ilmenite-hematite crystals. Also ion-irradiation experiments on pure hematite would be useful to predict the behavior of hematite during irradiation of ilmenite-hematite intergrowths. In general it does appear that ilmenite is easily amorphized by ion irradiation under low and ambient temperature conditions. However we have recently demonstrated that ilmenite is not amorphized by 900 kev electron exposure [26] suggesting that it may be useful in environments where a semiconducting material that is resistant to Light particle bombardment is needed. SUMMARY We performed cryogenic ion-irradiation experiments on single-crystal ilmenite using 200 kev A?+ to assess the radiation tolerance of ilmenite-group minerals. RBS/C and TEM indicate that the crystal amorphized easily compared to cryogenic irradiations of MgTiO and MgAlO. These results suggest that numerous factors may influence the radiation response of an oxide including chemical composition propensity for cation disorder insusceptibility to redox reactions and crystal structure. ACKNOWLEDGMENTS This research was funded by the U.S. Dept. of Energy Office of BasicEnergy Sciences Division of Materials Sciences. REFERENCES [l] L.W. Hobbs and F.W. C h a r d Jr. J. Phys. 41 (1980) C6-232 [2] F.W. C h a r d Jr. G.F. Hurley and L.W. Hobbs J. Nucl. Mater (1982) 655 7 [3] S.J. Zinkle J. Am. Ceram. SOC. 72 (1989) [4] K. Nakai K. Fukumoto and C. Kinoshita J. Nucl. Mater (1992) 630. [5] K.E. Sickafus N. Yu and M. Nastasi Nucl. Instru. Meth. B 116 (1996) 85. [6] S.P. Chen M. Yan J.D. Gale R.W. Grimes R. Devanathan K.E. Sickafus N. Yu M. Nastasi Phil. Mag. Lett. 73 (1996) 51. [7] P.F. McDonald A. Parasiris R.K. Pandey B.L. Gries and W.P. Kirk J. Appl. Phys. 69 (1991) [SI S.S. Sunkara and R.K. Pandey Cer. Trans. 60 (1995) 83. [9] A.L. Brake Nuclear Energy Eng. March (1959) 142 [lo] R.A. Briggs and A. Sacco Jr. Met. Trans. A 24A (1993) [ll] L.J. Wittenberg J.F. Santarius and G.L. Kulcinski Fus. Tech. 10 (1986) 167. [12] R. Wilkins and R.K. Pandey personal communication 1996 [13] G.L. Nord and C.A. Lawson J. Geophys. Res. 97 (1992) [14] Y. Ishikawa J. Phys. SOC. Japan 13 (1958) 37 El51 S. Uyeda J. Geomagn. Geoelectr. 7 (1957) 61. E161 Y. Ishikawa and S. Akimoto J. Phys. SOC. Japan 12 (1957) [17] K.E. Sickafus A.C. Larson N. Yu M. Nastasi G.W. Hollenberg F.A. Garner and R.C. Bradt J. Nucl. Mater. 219 (1995) 128. [18] E.A. Cooper C.D. Hughes W.L. Earl K.E. Sickafus G.W. Hollenberg F.A. Garner and R.C. Bradt Mat. Res. SOC. Symp. Proc. 373 (1995) 413. (191 J.F. Ziegler J.P. Biersack and U. Littmark The Stopping and Range of Ions in Solids (Pergamon New York 1985). \. [20] N. Yu K.E. Sickafus and M. Nastasi Phil. Mag. Lett. 70 (1994) [21] R. Devanathan K.E. Sickafus N. Yu and M. Nastasi Phil. Mag. Lett. 72 (1995) 155. [22] Hj. Matzke Can. J. Phys. 46 (1968) 621. [23] J.N. Mitchell N. Yu K.E. Sickafus M. Nastasi T.N. Taylor K.J. McClellan and G.L. Nord Jr. Mat. Res. SOC. Sym. Proc. 398 (1996) in press. 8 [24] V.V. Korenevskii G.K. Krivokoneva B.J. Pergamenschik E.G. Ryabeva and G.A. Sidorenko Inorg. Mater. 7 (1971) 921. [25] L.M. Wang and R.C. Ewing Mat. Res. SOC.Bull. 17 (1992) 38. [26] R. Devanathan unpublished results (1996). FIGURE CAPTIONS Figure 1. Ball and stick crystal structure model of ilmenite (Rg). Other divalent cations that can substitute for Fe are Mg (gelkielite) h411 (pyrophanite) Zn (ecandrewsite) Ni Cd and Co. Figure 2. Phase diagram of the FeO- FeTiO system. Important features are the magnetic and cation order-disorder boundaries and the tricritical point. The crystal used in this study has approximately 80% ilmenite and 20% hematite. Figure adapted from Nord and Lawson [131. Figure 3. Displacements per atom (dpa) and Ar concentration versus ion implantation depth calculated using TRIM [19]. TRIM simulations indicate that the projected range of 200 kev AJ? ions in ilmenite is 124 nm and that the peak concentration of Ar ions is 0.1 atomic % at a dose of I x 10 A F cm-2. d Figure 4. RBS/C spectra of irradiated and unirradiated portions of the ilmenite crystal used in this study. See text for discussion. Figure 5. Bright-field transmission electron micrograph (a) of a cross section of the irradiated ilmenite crystal (lxlo A?-+ cm-2). The light region near the top of the photomicrograph is glue used to make the cross section. The underlying 160 nm thick layer is amorphous ilmenite as revealed by the corresponding electron diffraction pattern in (b). Crystalline ilmenite underlies the amorphous layer as indicated in (c). 9 Figure 1 Mitchell et al. 0 C Figure 2 Mitchell et al. I I I I I 1 1 I I I Figure 3 Mitchell et al. A a 1.o (d ti 0 Y & a Q).; (d a m 3 0 I. I.. I 0 50 loo 150 I m Depth (nm) Figure 4Mitchell et al. ' 0.6 I ' Energy (MeV) l 1.4 l 2500 q 2000 m Q) cn 1500 E9 f?: Channel Figure 5 Mitchell et al.
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