Polymorphism and phase transitions in Na₂U₂O₇ from density functional perturbation theory

Area of Science:

• Materials Science
• Solid State Chemistry
• Computational Materials Science

Background:

• Sodium diuranate (Na₂U₂O₇) exhibits complex polymorphism and phase transitions, crucial for understanding its behavior under varying temperatures.
• Previous experimental studies have indicated transitions between α, β, and γ phases, but the precise mechanisms and thermodynamic stability remain incompletely understood.

Purpose of the Study:

• To investigate the polymorphism and phase transitions of sodium diuranate (Na₂U₂O₇) using advanced computational methods.
• To predict the thermal properties and relative stability of different Na₂U₂O₇ polymorphs, particularly the high-temperature γ phase.
• To elucidate the phase transition pathways, including the sluggish α → β transition and the proposed β → γ transition.

Main Methods:

• Density Functional Perturbation Theory (DFPT) was employed to perform phonon calculations for predicting thermal properties.
• Quasi-harmonic approximation was used to calculate standard molar isochoric heat capacities for the α and β polymorphs.
• Gibbs free energy calculations were conducted to assess the thermodynamic stability and transition temperatures between different phases.

Main Results:

• Thermal properties, including heat capacities, were predicted for the α (P2₁/a) and β (C2/m) Na₂U₂O₇ polymorphs.
• The γ-Na₂U₂O₇ (R3̄m) phase's thermal properties were calculated for the first time, providing insights into high-temperature behavior.
• Gibbs free energy calculations indicated near-degeneracy between α and β phases at low temperatures, with β becoming more stable upon heating, consistent with experimental observations of phase mixtures and sluggish transitions.
• A metastable α-Na₂U₂O₇ structure with P2₁ symmetry was identified.
• No direct β → γ solid-solid transition was predicted; instead, a multi-step decomposition-recrystallization process is suggested for the γ-phase formation at high temperatures.

Conclusions:

• The study provides a detailed theoretical understanding of sodium diuranate polymorphism and phase transitions, complementing experimental data.
• The findings clarify the relative stability of Na₂U₂O₇ polymorphs and suggest a complex mechanism for high-temperature phase formation.
• Computational predictions offer valuable guidance for future experimental investigations into sodium diuranate's thermal and phase behavior.