Julia Wiktor

DFT + U investigation of charged point defects and clusters in UO2.

We present a physically justified formalism for the calculation of point defects and cluster formation energies in UO2. The accessible ranges of chemical potentials of the two components U and O are calculated using the U-O experimental phase diagram and a constraint on the formation energies of vacancies. We then apply this formalism to the DFT + U investigation of the point defects and cluster defects in this material (including charged ones). The most stable charge states obtained for these defects near stoichiometry are consistent with a strongly ionic system. Calculations predict similarly low formation energies for V(U)(4)(-) and I(O)(2)(-) in hyperstoichiometric UO2. In stoichiometric UO2, V(O)(2)(+) and I(o)(@)(-) have the same formation energy in the middle of the gap and in hypostoichiometric UO2, V[Formula: see text] is the most stable defect.

Predictive Determination of Band Gaps of Inorganic Halide Perovskites

We carry out first-principles calculations of band gaps of cubic inorganic perovskites belonging to the class CsBX3, with B = Pb, Sn and X = Cl, Br, I. We use the quasi-particle self-consistent GW method with efficient vertex corrections to calculate the electronic structure of the studied materials. We demonstrate the importance of including the higher-lying core and semicore shells among the valence states. For a meaningful comparison with experimental values, we account for thermal vibrations and disorder through ab initio molecular dynamics. Additionally, we calculate the spin-orbit coupling at levels of theory of increasing accuracy and show that semilocal density functionals significantly underestimate these corrections. We show that all of these effects need to be properly included in order to obtain reliable predictions for the band gaps of halide perovskites.

Origin of low electron–hole recombination rate in metal halide perovskites

To address the slow recombination of photogenerated charges in tetragonal CH3NH3PbI3, the evolution of extra electrons and holes is simulated through advanced ab initio molecular dynamics. We show that the localization of the charge carriers and their hopping from one polaronic state to another occur on a subpicosecond time scale. The localization, attended by weak bond contractions and elongations in the inorganic sublattice, is induced by thermal vibrations and only moderately perturbed by the disordered field generated by the organic cations. The simultaneous simulation of extra electrons and holes shows that they preferentially localize in spatially distinct regions. As determined by the overlap between the electron and hole wave functions, the probability of radiative bimolecular recombination is lowered by two orders of magnitude compared with that of optical generation. The separate polaronic localization of electrons and holes emerges as the key feature for achieving exceptional photovoltaic properties.

Partial Molar Volumes of Aqua Ions from First Principles

Partial molar volumes of ions in water solution are calculated through pressures obtained from ab initio molecular dynamics simulations. The correct definition of pressure in charged systems subject to periodic boundary conditions requires access to the variation of the electrostatic potential upon a change of volume. We develop a scheme for calculating such a variation in liquid systems by setting up an interface between regions of different density. This also allows us to determine the absolute deformation potentials for the band edges of liquid water. With the properly defined pressures, we obtain partial molar volumes of a series of aqua ions in very good agreement with experimental values.

Note: Assessment of the SCAN+rVV10 functional for the structure of liquid water

The performance of the SCAN+rVV10 functional in modeling the structural properties of liquid water is studied through constant-volume ab initio molecular dynamics simulations with both classical and quantum nuclei. The radial distribution functions are found to be slightly overstructured with respect to experiment, but overall similar to those achieved with the bare SCAN and the rVV10 functionals. From the pressures calculated during the dynamics, it is inferred that the SCAN+rVV10 functional leads to a noticeable overestimation of the density of liquid water.

Mechanism suppressing charge recombination at iodine defects in CH3NH3PbI3 by polaron formation

Metal-halide perovskites exhibit high efficiencies in photovoltaic applications and low recombination rates, despite the high concentrations of intrinsic defects. We here study the hole trapping at the negative iodine interstitial, which corresponds to the dominating recombination center in CH3NH3PbI3. We calculate the free energy profile for the hole trapping at 300 K using the Blue Moon technique based on hybrid functional molecular dynamics. We find that the hole trapping is energetically unfavorable and requires overcoming an energy barrier. This behavior stems from the position of the vertical (−/0) transition level of the iodine defect and the formation of a polaron. Our simulations show that the polaron does not interact with the iodine interstitial and hops through the lattice on a sub-picosecond scale. Our results highlight a mechanism by which the low mononuclear (trap-assisted) recombination rates in CH3NH3PbI3 can be explained.

pH-Dependent Surface Chemistry from First Principles: Application to the BiVO4(010)–Water Interface

We present a theoretical formulation for studying the pH-dependent interfacial coverage of semiconductor-water interfaces through ab initio electronic structure calculations, molecular dynamics simulations, and the thermodynamic integration method. This general methodology allows one to calculate the acidity of the individual adsorption sites on the surface and consequently the pH at the point of zero charge, pHPZC, and the preferential adsorption mode of water molecules, either molecular or dissociative, at the semiconductor-water interface. The proposed method is applied to study the BiVO4(010)-water interface, yields a pHPZC in excellent agreement with the experimental characterization. Furthermore, from the calculated p Ka values of the individual adsorption sites, we construct an ab initio concentration diagram of all adsorbed species at the interface as a function of the pH of the aqueous solution. The diagram clearly illustrates the pH-dependent coverage of the surface and indicates that protons are found to be significantly adsorbed (∼1% of available sites) only in highly acidic conditions. The surface is found to be mostly covered by molecularly adsorbed water molecules in a wide interval of pH values ranging from 2 to 8. Hydroxyl ions are identified as the dominant adsorbed species at pH larger than 8.2.

First-principles calculations of momentum distributions of annihilating electron-positron pairs in defects in UO2

We performed first-principles calculations of the momentum distributions of annihilating electron-positron pairs in vacancies in uranium dioxide. Full atomic relaxation effects (due to both electronic and positronic forces) were taken into account and self-consistent two-component density functional theory schemes were used. We present one-dimensional momentum distributions (Doppler-broadened annihilation radiation line shapes) along with line-shape parameters S and W. We studied the effect of the charge state of the defect on the Doppler spectra. The effect of krypton incorporation in the vacancy was also considered and it was shown that it should be possible to observe the fission gas incorporation in defects in UO2 using positron annihilation spectroscopy. We suggest that the Doppler broadening measurements can be especially useful for studying impurities and dopants in UO2 and of mixed actinide oxides.