Nuclear Instruments and Methods in Physics Research B 268, 3243 (2010)

Swift heavy ion irradiation-induced microstructure modification of two delta-phase oxides: Sc₄Zr₃O₁₂ and Lu₄Zr₃O₁₂

M. Tang ^{a,*, P. Kluth ^b, J. Zhang ^a,
M.K. Patel ^c, B.P. Uberuaga ^a,
C.J. Olson Reichhardt ^d, K.E. Sickafus ^a}

^aMaterials Science & Technology Division, Mail-Stop G755, Los Alamos National Laboratory, Los Alamos, NM 87545, USA
^bDepartment of Electronic Materials Engineering, The Australian National University, Canberra, ACT 0200, Australia
^cHigh Pressure Physics Division, Bhabha Atomic Research Centre, Mumbai 400 085, India
^dTheoretical Division, Mail-Stop B268, Los Alamos National Laboratory, Los Alamos, NM 87545, USA

ARTICLE INFO
Article history:
Received 2 October 2009
Received in revised form 28 May 2010
Available online 2 June 2010
Keywords:
Cermaic oxides
Irradiation damage effects
Phase transformation
TEM
XRD

Abstract
Swift gold ions (185 MeV) were used to systematically investigate the radiation damage response of delta phase compounds Sc₄Zr₃O₁₂ and Lu₄Zr₃O₁₂ in the electronic energy loss regime. Ion irradiation-induced microstructural modifications were examined using X-ray diffraction (XRD) and transmission electron microscopy (TEM). XRD investigations indicate a phase transformation from ordered rhombohedral to disordered fluorite (O-D) in both compounds, with the Sc compound transforming at a higher ion fluence compared with the Lu compound. This result is consistent with our previous study on Sc₄Zr₃O₁₂ and Lu₄Zr₃O₁₂ under displacive radiation environment in which the nuclear energy loss is dominant. High resolution TEM revealed that individual ion tracks maintain crystalline structure, while the core region experiences an O-D phase transformation. TEM observations also suggest that for the doses in which the tracks overlap, the O-D phase transformation occurs across the entire ion range.

1. Introduction
2. Experimental procedure
3. Results and discussion
4. Summary
References

1. Introduction

The radiation damage behavior of various complex oxides with structures related to fluorite (with potential application as host materials for nuclear waste or as advanced nuclear fuel forms for the transmutation of minor actinides) has been the focus of many recent studies [1, 2, 3, 4, 5]. The fluorite structure (CaF₂ type, Fm3̅m), consisting of a face-centered cubic array of cations with anions in all of the tetrahedral interstices is adopted by MO₂ dioxides (M represents a metal cation, while O refers to an oxygen anion). Certain oxygen-deficient fluorite structural derivatives (MO_{2 - x} ), such as pyrochlore (M₄O₇) compounds (more specifically A³⁺ ₂B⁴⁺ ₂ O⁷ ); bixbyite (M₂O₃) compounds (like rare earth sesquioxides); and δ-phase (M₇O₁₂) compounds (more specifically A³⁺ ₄B⁴⁺ ₃ O₁₂ ), exhibit amorphization resistance upon irradiation to very high radiation dose (pyrochlores [6], bixbyite [7], and δ-phase [5, 8]).

The purpose of the study presented here is to investigate radiation damage effects in Sc₄Zr₃O₁₂ and Lu₄Zr₃O₁₂ in the electronic energy loss regime. Swift heavy ions (SHI), namely, 185 MeV Au ions were used to simulate radiation damage effects similar to those induced by fission products in nuclear fuel. Both Sc₄Zr₃O₁₂ and Lu₄Zr₃O₁₂ compounds are known to crystallize in the δ-phase structure, which is described as a rhombohedral distortion (space group R3̅ [9, 10]) of the fluorite structure with anionic vacancies on one of the cubic 3-fold axes. We showed previously [5, 8] that these compounds are very resistant to irradiation-induced amorphization, even at cryogenic temperature (100 K). However, these δ-phase compounds do undergo an order-to-disorder (O-D) phase transformation from an ordered δ-phase to a disordered cubic fluorite structure, in a displacive irradiation environment.

2. Experimental procedure

High purity ZrO₂, Lu₂O₃ and Sc₂O₃ powders from Alfa Aesar (99.99% purity) were used to produce sintered polycrystalline Sc₄Zr₃O₁₂ and Lu₄Zr₃O₁₂ pellets. Pellet fabrication was performed using conventional ceramic processing procedures. X-ray diffraction measurements showed the pristine oxide ceramics to be phase pure and possess rhombohedral symmetry. The pellets were polished with alumina lapping films to obtain a mirror finish. In preparation for ion irradiation, all of these samples were final-polished using 40 nm colloidal silica slurry (Syton HT50, DuPont AirProducts NanoMaterials L.L.C, Tempe, AZ) to remove the surface damage created by mechanical polish.

Irradiations were performed at room temperature with 185 MeV Au¹³⁺ ions over a range of fluencies ranging from 1 × 10¹¹ ions/cm² to 1 × 10¹³ ions/cm², at the Heavy-Ion Accelerator Facility of The Australian National University, Canberra, Australia. The ion beam was scanned over an aperture of 3 × 6 mm², upstream of the sample, to ensure homogeneity. The beam current was maintained between 5 nA and 10 nA resulting in power densities below 1 W/cm². The Monte Carlo program SRIM [11] was used to estimate electronic and nuclear stopping in SHI irradiated Sc₄Zr₃O₁₂ and Lu₄Zr₃O₁₂ . A threshold displacement energy of 40 eV was used for all target elements (this is an arbitrary assumption). Fig. 1 shows the results of a SRIM simulation for Sc₄Zr₃O₁₂ . The electronic stopping power, (dE/dx)_electronic, exceeds the nuclear stopping power, (dE/dx)_nuclear, except at the very end-of-range of the ion. Upon entering the solid, the electronic stopping powers is ~30 keV/nm/ion for Sc₄Zr₃O₁₂ and Lu₄Zr₃O₁₂ (not shown here).

Figure 1: Monte Carlo simulation estimates of electronic versus nuclear energy loss for 185 MeVAu ions as function of penetration depth for Sc₄Zr₃O₁₂.

Irradiated samples were analyzed using both X-ray diffraction (XRD) and transmission electron microscopy (TEM). XRD measurements were made using a Bruker AXS D8 Advanced X-ray diffractometer, Cu-K_α radiation, a graphite monochromator, and θ-2θ geometry. The diffractometer was equipped with a Göebel mirror to achieve parallel beam diffraction optics. Irradiated samples were also prepared in cross-sectional and plan-view geometries for TEM examination. The SHI irradiation-induced microstructural evolution was examined using both a Philips CM-30 and a FEI Tecnai F30 electron microscope, each operating at 300 kV. Microdiffraction (μD) was used in this study to obtain diffraction patterns from small sample areas (the electron probe size at the sample was focused to a diameter of 10-20 nm).

3. Results and discussion

Figure 2: X-ray diffraction (XRD) patterns obtained from (a) Sc₄Zr₃O₁₂ and (b) Lu₄Zr₃O₁₂ before and after irradiation with 185 MeV Au ions. XRD indicated that both compounds experience an irradiation-induced order-to-disorder (O-D) phase transformation, from an ordered rhombohedral to a disordered fluorite. Plots are shown as logarithmic in intensity.

Fig. 2a and b are GIXRD patterns obtained from Sc₄Zr₃O₁₂ and Lu₄Zr₃O₁₂ samples before and after 185 MeV Au ion irradiation, respectively. The initial structure of Sc₄Zr₃O₁₂ can be described as an ordered rhombohedral δ-phase. The peaks labeled 'R' in Fig. 2 represent this rhombohedral, δ-phase. Upon ion irradiation to the highest fluence of 1 × 10¹³ Au/cm², one important observation is that there are no apparent broad diffraction features, attributable to an amorphous structure. This observation suggests that no SHI irradiation-induced amorphization occurs in either Sc₄Zr₃O₁₂ or Lu₄Zr₃O₁₂. Interestingly, with increasing ion irradiation fluence the weakest δ-phase R peaks decrease in intensity more than the four most prominent diffraction peaks (at ~31°, 36°, 52°, 61° 2θ). These R peaks are almost completely absent at the highest fluence, 1 × 10¹³ Au/cm² . The four major diffraction maxima peaks are associated with the "parent" fluorite structure (diffraction peaks labeled 'F') in Fig. 2, while the weaker R peaks are due to the special structural arrangement associated with the fluorite derivative, δ-phase structure. The absence of the weaker δ-phase (R) reflections with increasing ion irradiation dose suggests that Sc₄Zr₃O₁₂ and Lu₄Zr₃O₁₂ gradually undergo an O-D transformation, from an ordered δ-phase structure to a disordered fluorite structure. One difference between Sc₄Zr₃O₁₂ and Lu₄Zr₃O₁₂ structural evolution with irradiation dose, is that for Lu₄Zr₃O₁₂ , all R peaks associated with δ-phase disappear leaving only F peaks by a fluence of 5 × 10¹² Au/cm². This effect occurs at a lower ion fluence for Lu₄Zr₃O₁₂ compared to Sc₄Zr₃O₁₂ . To summarize, XRD investigations indicate that SHI irradiation induces an O-D phase transformation from an ordered rhombohedral (R) to a disordered fluorite (F) phase in both Sc₄Zr₃O₁₂ and Lu₄Zr₃O₁₂ , with the Sc compound transforming at a higher ion fluence compared to the Lu compound. This result is consistent with our previous study in which we compared the radiation damage response of Sc₄Zr₃O₁₂ and Lu₄Zr₃O₁₂ under displacive radiation damage conditions (i.e., conditions wherein the nuclear energy loss dominates) [5].

Figure 3: Plan-view high resolution TEM images obtained from Sc₄Zr₃O₁₂ irradiated to a fluence of 1 × 10¹¹ Au/cm² . (a) Individual ion tracks are observed with round shapes of ~3-4 nm in diameter; (b) higher magnification than (a), showing an individual ion track consisting of disordered fluorite core (~3-4 nm). Also shown in (b) are diffractograms obtained by Fast Fourier Transforms (FFT) from both the core and matrix.

Fig. 3 shows plan-view, high resolution TEM images obtained from Sc₄Zr₃O₁₂ irradiated to a fluence of 1 × 10¹¹ Au/cm² . These images were obtained from a sample thinned to reveal the structure at the irradiated sample surface (corresponding to an electronic stopping power of ~30 keV/ion/nm). In Fig. 3, single ion tracks are visible, each with round shapes of ~3-4 nm in diameter (Fig. 3a). Lattice fringes are clearly resolved inside each ion track. This indicates that δ-phase Sc₄Zr₃O₁₂ is not amorphized by the individual track formation process. Also note in Fig. 3 the strong dark contrast apparent in some regions. This is most likely caused by strain contrast due to volume changes within the different phases. Lattice fringes in Fig. 3b reveal that the cores of ion tracks in Sc₄Zr₃O₁₂ possess a different structure compared to the matrix. Fast Fourier Transform (FFT) analysis suggests that the ions producing these tracks induced an O-D phase transformation. We based this conclusion on the partial disappearance of the superlattice reflections associated with the rhombohedral, δ-phase. High resolution TEM observations reveal that ion tracks in Sc₄Zr₃O₁₂ consist of a disordered fluorite structured core surrounded by a δ-phase matrix.

Figure 4: Cross-sectional TEM bright field (BF) images and microdiffraction patterns obtained from (a) Sc₄Zr₃O₁₂ irradiated to a fluence of 1 × 10¹³ Au/cm² ; (b) Lu₄Zr₃O₁₂ irradiated to a fluence of 5 × 10¹² Au/cm² . The decrease in intensity of specific diffraction spots suggests that an O-D phase transformation occurs within the ion range.

Fig. 4a and b show cross-sectional TEM images and microdiffraction (μD) patterns obtained from Sc₄Zr₃O₁₂ irradiated to a fluence of 1 × 10¹³ Au/cm² and Lu₄Zr₃O₁₂ irradiated to a fluence of 5 × 10¹² Au/cm² , respectively. At these high ion fluences, it is possible to assess microstructural change induced by overlapping ion tracks. In Fig. 4, the μD patterns were obtained from different depths within the irradiated regions, progressing from the surface to the ion end-of-ranges, respectively. The $mu;D patterns obtained from the ion range (~12 um) in Fig. 4a and b are consistent with a two-phase structure, where both phases are oriented with an epitaxial relationship with respect to the pristine δ-phase substrate. The strong reflections in μD patterns are consistent with a cubic fluorite structure. The weaker reflections are equivalent to those in the pristine δ-phase substrate pattern (referred to as superlattice reflections, characteristics of pristine delta phase). The diminishing intensities of the superlattice reflections associated with the rhombohedral δ-phase, compared to the stronger, fundamental fluorite reflections, suggests that the irradiated Sc₄Zr₃O₁₂ and Lu₄Zr₃O₁₂ experience O-D phase transformations following Au ion irradiation. The only μD pattern obtained out of the ion range in Fig. 4b is clearly consistent with a single phase, the rhombohedral δ-phase. The TEM observations in Fig. 4 corroborate the XRD results presented in Fig. 2 indicating that Sc₄Zr₃O₁₂ and Lu₄Zr₃O₁₂ remain crystalline, but experience a structural transformation after overlapping of ion tracks, from an ordered δ-phase to a structure indistinguishable from a cubic fluorite. Observations by μD revealed that this O-D phase transformation occurred across the entire ion range (corresponding to electronic stopping power ranging from ~30 keV/ion/nm to <5 keV/ion/nm).

The O-D phase transformation in Sc₄Zr₃O₁₂ and Lu₄Zr₃O₁₂ has been observed previously under displacive irradiation damage conditions, using both heavy ions (300 keV Kr) [5, 8] and light ions (200 keV Ne) [12]. Nuclear energy loss dominates the Kr irradiation process, whereas electronic energy loss plays a greater role in the total stopping power for Ne. As the disordering process under displacive conditions, the Sc δ-phase compound transforms at higher ion fluence compared to the Lu compound. In this study using swift heavy ions, we observed the same O-D phase transformation as in the previous displacive damage irradiations, and once again, we found differences between the irradiation damage response of Sc₄Zr₃O₁₂ and Lu₄Zr₃O₁₂ . The O-D phase transformation in δ-phase rhombohedral compounds is normally associated with high-temperature polymorphic transformation, as revealed in Temperature-Composition (T-C) phase diagrams. Irradiation-induced phase transformations to higher temperature polymorphs have been observed previously in other oxides [13, 14, 15]. By analogy, the different irradiation response in these two materials examined in this study (Sc and Lu) may be anticipated by phase-stability characteristics, as revealed in T-C diagrams. These T-C phase diagrams [16] show that a thermally-induced O-D phase transformation occurs at ~1800 °C in Sc₄Zr₃O₁₂ and at ~1700 °C in Lu₄Zr₃O₁₂ . Our experimental results on irradiation dose dependence of the O-D phase transformation observed in Sc₄Zr₃O₁₂ and Lu₄Zr₃O₁₂ appear to be consistent with the temperature dependence of the O-D phase transformation observed in the phase diagram for these two materials.

Based on our XRD measurements, we find that the molecular density increases in both Sc₄Zr₃O₁₂ and Lu₄Zr₃O₁₂ (or the volume decrease) 0.37% and 0.57%, respectively, upon ion irradiation to the highest fluence (1 × 10¹³ Au/cm² ) used in these experiments. However, the experimental data presented in Lopato et al. [9] indicates that there is virtually no change in volume during the O-D phase transformation in Sc₄Zr3O₁₂ . This is consistent with our SHI irradiation-induced O-D phase transformation. The 0.37% density (or volume change) found in our experiments is not statistically significant. Essentially, we find no change in density or volume upon transformation.

4. Summary

We performed SHI irradiation experiments under room temperature on polycrystalline δ-phase compounds, Sc₄Zr₃O₁₂ and Lu₄Zr₃O₁₂ , using 185 MeV Au¹³⁺ ions. Between a fluence of 5 × 10¹² Au/cm² and 1 × 10¹³ Au/cm² , a crystal structure transformation occurs from an ordered rhombohedral to a disordered fluorite structure in both compounds, where the Sc compound transforms at an higher ion fluence compared with the Lu compound. This transformation seems to be identical to a thermally-induced O-D transformation that occurs at high temperature in these compounds. HRTEM observation reveals that disordered fluorite regions are formed at the core of the ion tracks. The diameters of these disordered regions in Sc₄Zr₃O₁₂ is ~3-4 nm diameter. Electron diffraction patterns also suggest that the O-D phase transformation occurs across the entire ion range in both the Sc and Lu compounds (corresponding to electronic stopping power ranging from ~30 keV/ion/nm to <5 keV/ion/nm). No irradiation-induced amorphization was observed in either Sc₄Zr₃O₁₂ or Lu₄Zr₃O₁₂ .

Acknowledgment

This work was sponsored by the Laboratory Directed Research & Development-Exploratory Research (LDRD-ER) at Los Alamos National Laboratory. We thank the staff at the ANU Heavy-Ion Accelerator Facility for technical assistance. P.K further acknowledges the support of the Australian Research Council.

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