Phase diagram and oxygen-vacancy ordering in the CeO₂-Gd₂O₃ system: a theoretical study

Area of Science:

• Materials Science
• Computational Materials Science
• Solid State Chemistry

Background:

• Cerium oxide (CeO2) based materials are vital for catalysis and solid oxide fuel cells.
• Gadolinium doping in ceria influences oxygen vacancy formation and ordering.
• Understanding the phase behavior of Ce1-xGdxO2-x/2 (CGO) is critical for optimizing its properties.

Purpose of the Study:

• To calculate the phase diagram of Ce1-xGdxO2-x/2 (CGO) across all compositions.
• To investigate the thermodynamic equilibrium and quenched states of CGO.
• To analyze the impact of oxygen vacancy ordering on CGO's structural properties.

Main Methods:

• Combined Density Functional Theory (DFT), cluster expansion, and lattice Monte Carlo simulations.
• Modeling of thermodynamic equilibrium and quenched systems.
• Analysis of lattice parameters and cation relaxation patterns.

Main Results:

• Reliable phase diagram calculation for CGO across the entire composition range (xGd ≤ 1).
• Observed oxygen vacancy ordering onset at 1200-3300 K (for xGd = 0.3-1) and phase separation below 1000 K.
• Confirmed that cation configurations do not hinder vacancy ordering in quenched systems.

Conclusions:

• The computational methodology accurately predicts CGO phase transitions.
• Calculated transition temperatures for vacancy ordering align well with experimental data.
• Vacancy ordering and composition significantly affect CGO lattice parameters and cation relaxation.