Improved SMTB-Q model applied to oxygen migration and pressure phase transitions in UO2

The second-moment tight-binding variable-charge (SMTB-Q) interatomic potentials have been implemented in the molecular dynamics (MD) code LAMMPS in order to study the static and dynamical properties of uranium dioxide UO₂. With respect to a previous work on UO₂ the SMTB-Q model has been slightly modified in introducing a splitting energy of the U 5f  orbitals. This improvement results in a better description of the electronic structure of UO₂ namely the gap estimation which is now close to the experimental value (~2 eV). The structural and mechanical properties along with the cohesive energy of bulk UO₂ are in good agreement with the experimental data. The ionic charges on uranium and oxygen are respectively equal to 2.86 and  -1.43, very close to the Bader charges derived from ab initio calculations. The migration energies and the diffusion coefficient calculated respectively for oxygen vacancy (VO) and oxygen interstitials (IO) in under and over stoichiometry compare well with ab initio calculations and experimental data. The oxygen diffusivity is consistent at high temperature when additional Frenkel thermally formed swamps the effect of single IO and VO defects with recent prediction from EAM semi-empirical potentials. Additionally, a study on phase transitions between high pressure polymorphs of UO₂ has been performed and has shown the good transferability of the SMBT-Q potential over different coordination. It is found that the UO₂ phases stability order under tensile and compressive stresses, compared with stable fluorite phase at 0 GPa, are respected.

Atomic scale simulations of pyrochlore oxides with a tight-binding variable-charge model: implications for radiation tolerance

Atomistic simulations with new interatomic potentials derived from a tight-binding variable-charge model were performed in order to investigate the lattice properties and the defect formation energies in Gd2Ti2O7 and Gd2Zr2O7 pyrochlores. The main objective was to determine the role played by the defect stability on the radiation tolerance of these compounds. Calculations show that the titanate has a more covalent character than the zirconate. Moreover, the properties of oxygen Frenkel pairs, cation antisite defects and cation Frenkel pairs were studied. In Gd2Ti2O7 the cation antisite defect and the Ti-Frenkel pair are not stable: they evolve towards more stable defect configurations during the atomic relaxation process. This phenomenon is driven by a decrease of the Ti coordination number down to five which leads to a local atomic reorganization and strong structural distortions around the defects. These kinds of atomic rearrangements are not observed around defects in Gd2Zr2O7. Therefore, the defect stability in A2B2O7 depends on the ability of B atoms to accommodate high coordination number (higher than six seems impossible for Ti). The accumulation of structural distortions around Ti-defects due to this phenomenon could drive the Gd2Ti2O7 amorphization induced by irradiation.

Insulator-metal transition of VO₂ ultrathin films on silicon: evidence for an electronic origin by infrared spectroscopy

We report on the first simultaneous observations of both electronic and structural temperature-induced insulator-to-metal transition (IMT) in VO2 ultrathin films, made possible by the use of broad range transmission infrared spectroscopy. Thanks to these techniques, the infrared phonon structures, as well as the appearance of the free carrier signature, were resolved for the first time. The temperature-resolved spectra allowed the determination of the temperature hysteresis for both the structural (monoclinic-to-rutile) and electronic (insulator-to-metallic) transitions. The combination of these new observations and DFT simulations for the monoclinic structure allows us to verify the direct transition from monoclinic (M1) to rutile and exclude an intermediate structural monoclinic form (M2). The delay in structural modification compared to the primer electronic transition (325 K compared to 304 K) supports the role of free charges as the transition driving force. The shape of the free charge hysteresis suggests that the primer electronic transition occurs first at 304 K, followed by both its propagation to the heart of the layer and the structural transition when T increases. This study outlines further the potential of VO2 ultrathin films integrated on silicon for optoelectronics and microelectronics.

Bulk, surface and point defect properties in UO2 from a tight-binding variable-charge model

A tight-binding variable-charge model (SMTB-Q) has been used to calculate bulk, surface and point defect properties in uranium dioxide. It provides us with a better description of the iono-covalent oxides than classical, purely ionic models. A good agreement is found in the structural properties and cohesive energy between the model and experimental data; the charges calculated on the uranium and oxygen ions are Q(U) = 2.804 and Q(O) =- 1.402 respectively. The stability and relaxation of low index surfaces were evaluated: the (111) surface consistently has the lowest surface energy and the smallest surface relaxation, followed by the (110) surface and the (100) surface, in agreement with previous predictions from semi-empirical potentials and from ab initio calculations. The energy ranking of intrinsic defects is oxygen Frenkel pair < Schottky trio < uranium Frenkel pair, which is consistent with literature. The clustering energy of small vacancy clusters has been also calculated. Additionally, the atomic relaxations and the charge transfer at surfaces and around defects have been investigated. All the results obtained in the present work prove the ability of the SMTB-Q model to describe the bulk properties as well as the surface and defect properties in uranium dioxide. Finally, this model provides us with a new fundamental insight into the role played by the charge transfer in UO2 properties.

Wetting and structural transition induced by segregation at grain boundaries: a monte carlo study

Wetting of the Sigma = 5 (310) <001> symmetrical tilt grain boundary (GB) close to the solubility limit in the Cu(Ag) solid solution has been observed by means of Monte Carlo simulations at T = 600 K. More precisely, a finite thickness film almost pure in Ag, separating the two initial Cu(Ag) grains, can be obtained from a critical intergranular germ induced by the strong segregation of Ag in the GB. As this film is actually a single crystal, this implies a complete rearrangement of the GB core structure. Thus the initial GB is replaced by two Cu(Ag)/Ag(Cu) interfaces. Evidence is presented for the increase of the film thickness when approaching the solubility limit, as expected in wetting phenomena.