Structure and radiation response of anion excess bixbyite ${\mathrm{Gd}}_2{\mathrm{Ce}}_2{\mathrm{O}}_7$

The crystal structure analysis of ${\mathrm{Gd}}_2{\mathrm{Ce}}_2{\mathrm{O}}_7$ reveals that it crystallizes in a bixbyite-type symmetry ($I{2}_{1}3$). Analysis of the structure suggests a randomly occupied cation sublattice with infinite correlation length associated with long-range ordered anion sublattice with half of the vacant sites of an ideal bixbyite filled, hence the name anion-excess bixbyite. Ion irradiation experiments and quantitative x-ray diffraction analysis were used to study the separate response of the anion sublattice to swift heavy ion radiation. Analysis of anion and cation correlation lengths as a function of fluence shows that the topological disorder on the anion sublattice grows faster than that on the cation sublattice. The microstructural response at increasing radiation fluences leads to a decrease of the strain after an initial increase, while the variance of the strain increases following the increase of the microdomain wall density. This particular behavior seems responsible for the exceptional radiation resistance of this system that does not display any significant amorphization, even at the highest fluence.

Figure 1

(a) Electronic vs nuclear energy loss for 92 MeV Xe ions in ${\mathrm{Gd}}_2{\mathrm{Ce}}_2{\mathrm{O}}_7$ as a function of target depth. The ordinate on the left shows electronic (black circles) or nuclear (red triangles) energy losses on a logarithmic scale. The scale on the right shows the ratio of electronic-to-nuclear energy loss (green open circles). (b) Penetration depth of Cu-$\mathrm{K}\alpha$ x rays in ${\mathrm{Gd}}_2{\mathrm{Ce}}_2{\mathrm{O}}_7$ (shaded blue) for the Bragg-Brentano reflection geometry as a function of the scattering angle ($2\theta$, bottom scale). 97% of the scattered intensity comes from the corresponding depth in the sample. Xe implantation profile (red line, top scale). The comparison shows all of the x-ray scattered signal comes from the irradiated region where no significant implantation occurs.

Figure 2

Observed (black dots), calculated (red solid line) x-ray diffraction (XRD) pattern for pristine ${\mathrm{Gd}}_2{\mathrm{Ce}}_2{\mathrm{O}}_7$ showing the Rietveld fit with a disordered anion excess bixbyite model ($I{2}_{1}3, a$ = 10.8515(1) Å, $Z$ = 32). The blue solid line indicates the difference between the observed and calculated patterns.

Figure 3

(a) Electron diffraction pattern corresponding to the $\left[111\right]$ zone axis in pristine ${\mathrm{Gd}}_2{\mathrm{Ce}}_2{\mathrm{O}}_7$. (b) Simulated electron diffraction pattern using the initial structural model of Table 1 corresponding to the same $\left[111\right]$ zone axis. Full-size versions of (a) and (b) are provided as Supplemental Material [29].

Figure 4

Polyhedral model of a slab of pristine ${\mathrm{Gd}}_2{\mathrm{Ce}}_2{\mathrm{O}}_7$ obtained by the Rietveld refinement (Table 3). Four reference cells are displayed. Gold is the octahedron, purple is the distorted cube, and blue is the capped octahedron. Black spheres materialize the O vacancies.

Figure 5

Detail of the x-ray diffraction (XRD) patterns obtained from pristine and irradiated ${\mathrm{Gd}}_2{\mathrm{Ce}}_2{\mathrm{O}}_7$ as a function of increasing fluence. The intensities are represented using a logarithmic scale to display the noticeable broadening of the peaks carrying the information about the periodic stacking of O vacancies in the anion excess bixbyite structure. These sensitive reflections are those that do not fold onto the reflections of an face-centered cubic lattice with halved parameters.

Figure 6

(a) Correlation length of the anion sublattice as a function of fluence in ${\mathrm{Gd}}_2{\mathrm{Ce}}_2{\mathrm{O}}_7$. This correlation length in pristine samples is $\sim 34$ nm. (b) Correlation length of cation sublattice as a function of fluence in ${\mathrm{Gd}}_2{\mathrm{Ce}}_2{\mathrm{O}}_7$. This correlation length in pristine is $\sim 116$ nm. (c) Strain and strain variance obtained from Rietveld analysis as a function of the Xe ion fluence. The strain and its variance are near zero for the pristine sample.