Ion-beam irradiation of lanthanum compounds in the systems La2O3–Al2O3 and La2O3–TiO2

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Abstract

Thin crystals of La2O3, LaAlO3, La2/3TiO3, La2TiO5, and La2Ti2O7 have been irradiated in situ using 1 MeV Kr2+ ions at the Intermediate Voltage Electron Microscope-Tandem User Facility (IVEM-Tandem), Argonne National Laboratory (ANL). We observed that La2O3 remained crystalline to a fluence greater than 3.1×1016 ions cm−2 at a temperature of 50 K. The four binary oxide compounds in the two systems were observed through the crystalline–amorphous transition as a function of ion fluence and temperature. Results from the ion irradiations give critical temperatures for amorphisation (Tc) of 647 K for LaAlO3, 840 K for La2Ti2O7, 865 K for La2/3TiO3, and 1027 K for La2TiO5. The Tc values observed in this study, together with previous data for Al2O3 and TiO2, are discussed with reference to the melting points for the La2O3–Al2O3 and La2O3–TiO2 systems and the different local environments within the four crystal structures. Results suggest that there is an observable inverse correlation between Tc and melting temperature (Tm) in the two systems. More complex relationships exist between Tc and crystal structure, with the stoichiometric perovskite LaAlO3 being the most resistant to amorphisation.

Graphical abstract

La2TiO5 with atypical co-ordination for Ti, TiO5 is found to be different in radiation resistance to La2Ti2O7 and La2/3TiO3. Irradiation of La–Ti–O, and La–Al–O based systems has found that radiation damage resistance is related to the ability of the system to disorder.

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Introduction

The behaviour of oxide compounds when used for the safe storage of nuclear waste undergoing radioactive decay has been studied as a means to improve current, and develop new waste form materials for future nuclear systems [1]. In order to explain how damage affects a structure it is not always possible to undertake experiments using radioactive isotopes, in many cases it is quicker to use ion-beam irradiation to simulate the short-term aspects of the damage formation and recovery processes [2], [3], [4]. While such a technique may not give ‘real world’ results it does allow ‘model’ systems to be studied systematically, whose results can then be used to understand currently relevant materials. One area of discussion in the literature is the effect of structure versus composition [3], as part of this process we have studied a number of compounds in the systems La2O3–Al2O3 and La2O3–TiO2, primarily the binary oxide compounds, LaAlO3, La2/3TiO3, La2Ti2O7 and La2TiO5. In these systems one must consider both the extent of phase stability field and detail of the different crystal structures as a function of composition, especially within the La2O3–TiO2 system as there are a number of binary oxide phases.

Of the three simple oxide compounds defining the two systems, ion irradiation data have been previously reported for Al2O3 in the corundum structure (Rc) [5], [6] and for TiO2 in the rutile (P42/mnm), brookite (Pbca), and anatase (I41/amd) modifications [3]. La2O3 is one of the five known A-type lanthanide oxide compounds (Pm1) with an atomic configuration that can be described as a rhombohedrally distorted CsCl type structure, e.g., La2□O3, in which the La ions occupy a 7-coordinated "monocapped" polyhedron [7].

Within the binary oxide compounds in the two systems, LaAlO3 is a stoichiometric perovskite crystallising in space group Rc at room temperature [8]. This compound transforms to the primitive cubic structure at 820 K, which is within the temperature range accessible for ion irradiation experiments in this study. La2/3TiO3, a cation-deficient perovskite, has previously been studied by X-ray and neutron diffraction [9], [10], [11] and confirmed to adopt Cmmm symmetry, with ordered occupancy of one of the two La3+ crystallographic sites, coordinated by either 10 or 12 oxygen anions. In order to maintain charge balance the ten-coordinated La3+ site has partial occupancy of 33%, and is randomly ordered. According to the latest phase diagram [10], La2/3TiO3 transforms to the tetragonal (P4/mmm) modification above ∼640 K, retaining the layered cation-vacancy ordering but not the octahedral tilting as found in the lower temperature form.

La2Ti2O7 is isostructural with Ca2Nb2O7 with a cation arrangement similar to that of perovskite [12], [13]. La3+ is located within four different irregular polyhedra with co-ordination numbers ranging from 7 to 11, while the Ti-cations remain octahedrally coordinated by oxygen. Two La3+ cations are located within a highly distorted perovskite block while two occupy sites within layers between the perovskite blocks. Adjacent perovskite blocks are offset by a crystallographic shear along the [100] direction, the offset being equivalent to one half of the BO6 octahedra. This arrangement of atoms results in this material being considered a ferroelectric insulator with P21 symmetry and a high Curie temperature, ∼1770 K.

La2TiO5 is orthorhombic and adopts Pnma symmetry with atypical La3+ and Ti4+ co-ordination with oxygen [14], [15], [16]. In La2/3TiO3 and La2Ti2O7 the La3+ co-ordination tends to be large, 7–11, the exact value dependent on the distances used to determine the coordinating anions, while the Ti–O first co-ordination shell is 6 (octahedral). In La2TiO5 the La–O co-ordination is 7 and the Ti–O is 5. Such co-ordination gives rise to polyhedral geometries not often seen with these cations, e.g., TiO5 is an off-centre square-based pyramid. Schematics of each of the structures are shown in Fig. 1.

Section snippets

Experimental

The samples used in this study are those that have been previously prepared and characterised [10], [17], [18]. The compositions of the grains used were checked using TEM-EDX measurements, using method previously published [19], prior to irradiation to minimise any effects due to unexpected compositional changes. In situ ion irradiation experiments were performed using a Hitachi H-9000NAR transmission electron microscope (operated at 300 kV) interfaced with a NEC ion accelerator in the

Results

The results indicate that La2O3 retained crystallinity to a fluence above 3.1×1016 ions cm−2 at 50 K. This result is consistent with the available data reported in the literature for other lanthanide oxides [21], Al2O3 [5], [6], and the rutile form of TiO2 [3] with low levels of chemical impurities. As reported earlier [3], anatase, brookite, and chemically impure rutile specimens can be rendered amorphous at 50 K using 1.0 MeV Kr ions. All four of the intermediate binary oxide compounds in the two

Discussion

When the values for Fc0 are compared (Table 2) it can be seen that La2TiO5, which has the highest Tc, has the lowest Fc0, whereas LaAlO3 with the lowest Tc has the highest Fc0. As the critical fluence at 0 K is inversely related to the amorphisation cross-section, LaAlO3, thermodynamically, should recover more rapidly than La2TiO5 at 0 K.

When the nuclear and electronic stopping powers for these systems are calculated, for 1 MeV Kr using SRIM2008 [23], the values for the La–Ti–O systems are as

Conclusion

The results indicate that changes in crystal structures directly affect the response to radiation damage of materials, e.g. La2TiO5 and La2Ti2O7, while being chemically similar have different responses to radiation damage. The variations can be related to crystal structure, chemical and structural disorder and in these systems an inverse correlation with melting point is found. This work is being expanded to include MD simulations of damage and recovery processes for other ternary oxides, e.g.

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