[Paper Review] Prediction of Stable Ground-State Uranium Nitrides at Ambient and High Pressures

Dawei Zhou, Jiahui Yu+2 more|arXiv (Cornell University)|Mar 31, 2018

High-pressure geophysics and materials40 references3 citations

TL;DR

This study predicts new stable ground-state uranium nitrides in the U-N system up to 150 GPa using first-principles swarm structure searches. It identifies two new high-pressure phases—U2N3 and UN2—along with metastable stoichiometries like UN4, UN3, U3N5, and U2N, revealing unique structural motifs such as N2-dimers and zigzag N4 units under pressure.

ABSTRACT

Uranium nitrides have been the subject of intense research owing to their potential applications as advanced nuclear fuels. However, the phase diagram of the U-N system at low temperature and high pressure still remains unclear. In this paper, we explore extensively the phase diagram of the U-N system up to 150 GPa based on first-principles swarm structure searches. The phase diagrams of the experimentally known stoichiometries like U2N3 and UN2 are refined. At zero temperature and pressure, the experimentally observed CaF2-type UN2 is found to transform into another new I41/amd-type UN2, which is related to the dynamical instability originated from Peierls mechanism. Two new stable high-pressure phases of U2N3 and UN2 are identified for the first time. Besides, several new chemical stoichiometries (UN4, UN3, U3N5 and U2N) are found to have stability fields on the U-N phase diagram. The pressure-induced phase transitions for the U-N system are further investigated. The peculiar structural features such as N2-dimers, planar SO3-like N(N)3 units, non-coplarnar zigzag N4 units, and zigzag U chains are found in U-N compounds under pressure. Our results on the structure exploring provide a better understanding of the structural characteristics and physical properties of uranium nitrides under pressure.

Motivation & Objective

• To resolve the unclear phase diagram of the U-N system at low temperatures and high pressures.
• To identify stable ground-state uranium nitrides under ambient and high-pressure conditions.
• To explore novel stoichiometries and structural motifs in U-N compounds beyond known phases.
• To investigate pressure-induced phase transitions and their underlying mechanisms.
• To provide a comprehensive structural and energetic analysis of uranium nitrides for advanced nuclear fuel applications.

Proposed method

• Employing first-principles density functional theory (DFT) calculations with the PBE functional for electronic structure and energy evaluation.
• Applying swarm structure search algorithms to systematically explore the configurational space of U-N compounds.
• Evaluating formation energies and phonon band structures to assess thermodynamic and dynamical stability.
• Analyzing structural features such as N2-dimers, planar SO3-like N(N)3 units, and zigzag N4 units using electronic structure and bonding analysis.
• Mapping the phase diagram of U-N compounds up to 150 GPa by computing free energies and identifying stability fields.
• Using the Peierls mechanism to explain the dynamical instability of the CaF2-type UN2 phase at ambient pressure.

Experimental results

Research questions

• RQ1 What are the stable ground-state phases of uranium nitrides at ambient and high pressures up to 150 GPa?
• RQ2 How do pressure-induced phase transitions affect the structural and electronic properties of U-N compounds?
• RQ3 What novel stoichiometries and structural motifs emerge in the U-N system under high pressure?
• RQ4 Why is the experimentally observed CaF2-type UN2 dynamically unstable at ambient pressure, and what is the true ground state?
• RQ5 What is the role of the Peierls mechanism in driving structural transitions in UN2 under pressure?

Key findings

• The CaF2-type UN2 phase is dynamically unstable at ambient pressure due to a Peierls distortion, leading to a transformation into a new I41/amd-type UN2 phase.
• Two new stable high-pressure phases—U2N3 and UN2—are predicted for the first time, with stability fields extending up to 150 GPa.
• Novel stoichiometries including UN4, UN3, U3N5, and U2N are found to have thermodynamically stable regions on the U-N phase diagram.
• Structural motifs such as N2-dimers, planar SO3-like N(N)3 units, non-coplanar zigzag N4 units, and zigzag U chains are identified in high-pressure U-N compounds.
• The I41/amd-type UN2 phase is energetically more favorable than the CaF2-type phase at ambient pressure, resolving a long-standing discrepancy in experimental observations.
• The phase diagram refinement shows that the I41/amd-type UN2 becomes increasingly stable with increasing pressure, indicating a clear pressure-driven phase transition pathway.