Pressure-driven insulator-metal transition in cubic phase UO2

Understanding the electronic properties of actinide oxides under pressure poses a great challenge for experimental and theoretical studies. Here, we investigate the electronic structure of cubic phase uranium dioxide at different volumes using a combination of density functional theory and dynamical mean-field theory. The ab initio calculations predict an orbital-selective insulator-metal transition at a moderate pressure of ∼45GPa. At this pressure the uranium’s 5f5/2 state becomes metallic, while the 5f7/2 state remains insulating up to about 60GPa. In the metallic state, we observe a rapid decrease of the 5f occupation and total angular momentum with pressure. Simultaneously, the so-called “Zhang-Rice state”, which is of predominantly 5f5/2 character, quickly disappears after the transition into the metallic phase. Introduction. – Over the past decades, actinide materials including pure elements, hydrides, oxides, carbides, and nitrides have been extensively studied with numerous experimental and theoretical tools, due to their fundamental importance in the nuclear energy industry and military technology [1,2]. Even though great progress has been made, many problems and puzzles still remain. Of particular interest are the electronic structures of these actinide materials under extreme conditions (for example high pressure and high temperature in a reactor environment or a reactor accident) [3,4]. Experimentally, it has been observed that under pressure many actinide materials may undergo a series of structural phase transitions, such as the cubic to orthorhombic phase transitions occurring in some actinide dioxides [5], and the three successive phase transitions in Am [6]. The explanation of these complex phase transitions requires an accurate description of the properties of the 5f electrons over a wide range of pressures. It is well known that the 5f electrons, which play a pivotal role in determining the key physical and chemical properties of the actinide materials, react sensitively to changes in the surrounding environment [1,2]. So, it is natural to expect that the 5f electronic structures of actinide materials will be modified and some exotic effects and phenomena may emerge when an external pressure is applied. Up to now, only a few experiments and calculations have been conducted to explore the high-pressure properties of actinide materials, and most of these efforts were devoted to study their structural phase transitions and phase instabilities [4–7]. As a consequence, the high-pressure electronic structures of actinide materials are poorly understood. Motivated by these facts, we employ a state-ofthe-art first-principles approach to shed new light onto the pressure-driven electronic transitions in uranium dioxide, which is one of the most important nuclear fuels. Under ambient pressure, stoichiometric UO2 is in a fluorite (CaF2) structure. A transition from the cubic phase to orthorhombic cotunnite structure in the range of 42–69GPa [5] was already determined by high-precision X-ray diffraction experiments. The ground state of UO2 is an antiferromagnetic Mott insulator with a sizeable band gap of ∼2.1 eV [8]. To study the electronic structure of UO2, many traditional first-principles approaches have been employed, such as the local density approximation (LDA) (or generalized gradient approximation (GGA)) plus Hubbard U approach [9–13], the hybrid functional method [14], and the self-interaction corrected local spin-density approximation (SIC-LSDA) [15]. However, none of the above methods can provide a satisfactory description of UO2 over a wide range of conditions. For instance, even though the band gap at ambient pressure is 1 ht tp :// do c. re ro .c h Published in "EPL (Europhysics Letters) 119 (5): 57007, 2017" which should be cited to refer to this work.