Work-biased path-sampling calculations of chemical potentials: Principles and applications to uranium oxide

Area of Science:

• Materials Science
• Computational Chemistry
• Solid State Physics

Background:

• Calculating chemical potentials is crucial for understanding material properties.
• Existing methods can be computationally intensive or limited in scope.
• Accurate simulations of uranium dioxide (UO2) are vital for nuclear fuel applications.

Purpose of the Study:

• To introduce and validate a novel work-biased path-sampling scheme for atomic scale simulations.
• To investigate the relationship between cluster structure stability and oxygen chemical potential in UO2+x.
• To explore the influence of temperature and composition on these properties.

Main Methods:

• Developed a work-biased path-sampling scheme for calculating chemical potentials.
• Applied the scheme to simulate Xe atom insertion in UO2 (test case).
• Investigated oxygen chemical potential and cluster stability in UO2+x using two empirical potentials across a temperature range (800–2000 K).

Main Results:

• Validated the work-biased path-sampling scheme with a Xe insertion test case.
• Observed irregular evolution of oxygen chemical potential in UO2+x, with notable dips linked to specific oxygen cluster formations.
• Identified five stable oxygen clusters at 800 K correlating with significant chemical potential dips.
• Found that four- and five-atom oxygen clusters compete in stability at intermediate temperatures.
• Noted that clusters become unstable at higher temperatures, leading to monotonous chemical potential changes.

Conclusions:

• The work-biased path-sampling scheme provides an effective route for chemical potential calculations.
• Oxygen cluster stability significantly influences chemical potential in UO2+x, exhibiting complex behavior with temperature and composition.
• The findings offer insights into the defect chemistry of uranium dioxide fuels.