Anomalies in non-stoichiometric uranium dioxide induced by pseudo-phase transition of point defects

TL;DR

This paper reveals a pseudo-phase transition in non-stoichiometric uranium dioxide caused by defect transformations, leading to anomalies in thermodynamic and electronic properties, with potential experimental implications.

Contribution

It demonstrates that pseudo phases in uranium dioxide undergo a transition driven by defect species changes, a phenomenon analogous to conventional phase transitions, analyzed via first-principles calculations.

Findings

Anomalies in thermodynamic properties at low temperatures.

Transition from pseudo phase to smooth crossover with increasing temperature.

Correlation between defect populations and Gibbs free energy explains the transition.

Abstract

A uniform distribution of point defects in an otherwise perfect crystallographic structure usually describes a unique pseudo phase of that state of a non-stoichiometric material. With off-stoichiometric uranium dioxide as a prototype, we show that analogous to a conventional phase transition, these pseudo phases also will transform from one state into another via changing the predominant defect species when external conditions of pressure, temperature, or chemical composition are varied. This exotic transition is numerically observed along shock Hugoniots and isothermal compression curves in UO2 with first-principles calculations. At low temperatures, it leads to anomalies (or quasi-discontinuities) in thermodynamic properties and electronic structures. In particular, the anomaly is pronounced in both shock temperature and the specific heat at constant pressure. With increasing of the temperature, however, it transforms gradually to a smooth cross-over, and becomes less discernible. The underlying physical mechanism and characteristics of this type of transition are encoded in the Gibbs free energy, and are elucidated clearly by analyzing the correlation with the variation of defect populations as a function of pressure and temperature. The opportunities and challenges for a possible experimental observation of this phase change are also discussed.