Percolating hierarchical defect structures drive phase transformation in Ce1-x Gd x O2-x/2: a total scattering study

Area of Science:

• Materials Science
• Solid-State Chemistry
• Crystallography

Background:

• Understanding structural disorder is crucial for optimizing material properties.
• Ceria-based solid solutions doped with rare earths exhibit complex structural behaviors.
• Existing methods often struggle to capture disorder across multiple length scales.

Purpose of the Study:

• To develop a hierarchical approach for analyzing structural disorder in gadolinium-doped ceria (Ce$_{1-x}$Gd$_x$O$_{2-x/2}$) solid solutions.
• To correlate short-range structural features with the average structure.
• To investigate disorder on the mesoscopic scale.

Main Methods:

• Hierarchical analysis combining real-space (pair distribution function) and reciprocal-space (Rietveld refinement, microstructure probing) techniques.
• X-ray powder diffraction data analysis.
• Electron spin resonance (ESR) investigations.

Main Results:

• Formation of Gd- and Ce-rich droplets on a local scale, exhibiting fluorite (CeO$_{2}$) or C-type (Gd$_{2}$O$_{3}$) structures.
• Emergence of C-type nanodomains embedded in a fluorite matrix for low Gd concentrations (x$_{Gd}$ ≤ 0.25).
• Observation of a structural phase transformation at the site percolation threshold (x$_{Gd}$ ≃ 0.311) with a step-like change in ESR line width due to increased Gd-Gd interactions.

Conclusions:

• A crystallographic rationale for the fluorite-to-C-type phase transformation in these solid solutions is established.
• The hierarchical approach effectively elucidates structural disorder across different length scales.
• This methodology can be broadly applied to analyze disorder in other highly doped materials.