A.Y. Lozovoi

Ab initio simulation of charged slabs at constant chemical potential

We present a practical scheme for performing ab initio supercell calculations of charged slabs at constant electron chemical potential μ, rather than at constant number of electrons Ne. To this end, we define the chemical potential relative to a plane (or “reference electrode”) at a finite distance from the slab (the distance should reflect the particular geometry of the situation being modeled). To avoid a net charge in the supercell, and thus make possible a standard supercell calculation, we restore the electroneutrality of the periodically repeated unit by means of a compensating charge, whose contribution to the total energy and potential is subtracted afterwards. The “constant μ” mode enables one to perform supercell calculation on slabs, where the slab is kept at a fixed potential relative to the reference electrode. We expect this to be useful in modeling many experimental situations, especially in electro-chemistry.

Field-evaporation from first-principles

Under the application of a strong electric field, atoms from a metal surface can rupture their bonds and escape, leading to ‘field-evaporation’. We present a first-principles description of this phenomenon, taking as an example the evaporation of Al adatoms from an Al(111) surface. The ‘charged-plane’ method [Lozovoi, A. Y., and Alavi, A., 2003, Phys. Rev. B, 68, 246416.] has been implemented in the context of a localized-basis code (SIESTA). This enables appreciable fields to be stably and efficiently applied to surfaces, represented using slab geometries. We quantify details of the evaporation process as a function of the applied field strength. The field at which the zero-temperature barrier disappears (evaporation field) is predicted and possible scenarios of the evaporation of surface atoms are discussed. Results are compared to the ‘image-hump’ model for this process. The field dependence of the barrier is described by this model surprisingly well, despite the potential energy surface not being satisfactorily reproduced.

Boron in copper: A perfect misfit in the bulk and cohesion enhancer at a grain boundary

Using first principles electronic structure methods, we calculate the effects of boron impurities in bulk copper and at surfaces and grain boundaries. We find that boron segregation to the

Surface energy and the early stages of oxidation of NiAl(110)

\ensuremath{\Sigma}5(310)[001]

Analysis of the Alloying System in Ni-Base Superalloys Based on Ab Initio Study of Impurity Segregation to Ni Grain Boundary

grain boundary should strengthen the boundary up to 1.5 ML coverage

Universal tight binding model for chemical reactions in solution and at surfaces. II. Water

(15.24\phantom{\rule{0.3em}{0ex}}\mathrm{at.}∕{\mathrm{nm}}^{2})

Universal tight binding model for chemical reactions in solution and at surfaces. I. Organic molecules

. The maximal effect is observed at 0.5 ML and corresponds to boron atoms filling exclusively grain boundary interstices. In copper bulk, B causes significant distortion both in interstitial and regular lattice sites, for which boron atoms are either too big or too small. The distortion is compensated to a large extent when the interstitial and substitutional boron combine together to form a strongly bound dumbbell. Our prediction is that bound boron impurities should appear in a sizable proportion if not dominate in most experimental conditions. A large discrepancy between calculated heats of solution and experimental terminal solubility of B in Cu is found, indicating either a significant failure of the density functional approach or, more likely, strongly overestimated solubility limits in the existing B-Cu phase diagram.

Universal tight binding model for chemical reactions in solution and at surfaces. III. Stoichiometric and reduced surfaces of titania and the adsorption of water

Abstract We have studied the (110) surface of NiAl, an ordered alloy of B2 structure, using a plane-wave pseudopotential method. The clean surface and several oxidized surfaces were investigated, with oxygen coverages up to 1.5 ML (1 ML = 1 O-atom per surface metal atom). In order to compare the energies of the oxidized structures, which comprise different numbers of metal and oxygen atoms, one has to take account of the chemical potentials of the Ni, Al and O. To this end, for each system, we have applied simple analytic models to study their surface energy as a function of temperature, alloy stoichiometry (assumed to be near 50–50) and oxygen partial pressure. The calculations predict how, at oxygen pressures just above the threshold for decomposing NiAl, the clean surface should be coated by an alumina layer, with the consequent depletion of Ni near the surface. Varying stoichiometry has the relatively minor effect of shifting the crossovers in oxygen pressure at which different oxidized surfaces become stable.

Point Defects in NiAl Alloys Under Pressure

A new approach to the design of Ni-based polycrystalline superalloys is proposed. It is based on a concept that under given structural conditions, the performance of superalloys is determined by the strength of interatomic bonding both in the bulk and at grain boundaries of material. We characterize the former by the cohesive energy of the bulk alloy, whereas for the latter we employ the work of separation of a representative high angle grain boundary. On the basis of our first principle calculations we suggest Hf and Zr as “minor alloying additions” to Ni-based alloys. Re, on the other hand, appears to be of little importance in polycrystalline alloys.

Constitutional and thermal point defects in B2 NiAl

A revised water model intended for use in condensed phase simulations in the framework of the self consistent polarizable ion tight binding theory is constructed. The model is applied to water monomer, dimer, hexamers, ice, and liquid, where it demonstrates good agreement with theoretical results obtained by more accurate methods, such as DFT and CCSD(T), and with experiment. In particular, the temperature dependence of the self diffusion coefficient in liquid water predicted by the model, closely reproduces experimental curves in the temperature interval between 230 K and 350 K. In addition, and in contrast to standard DFT, the model properly orders the relative densities of liquid water and ice. A notable, but inevitable, shortcoming of the model is underestimation of the static dielectric constant by a factor of two. We demonstrate that the description of inter and intramolecular forces embodied in the tight binding approximation in quantum mechanics leads to a number of valuable insights which can be missing from ab initio quantum chemistry and classical force fields. These include a discussion of the origin of the enhanced molecular electric dipole moment in the condensed phases, and a detailed explanation for the increase of coordination number in liquid water as a function of temperature and compared with ice--leading to insights into the anomalous expansion on freezing. The theory holds out the prospect of an understanding of the currently unexplained density maximum of water near the freezing point.