The structural, electronic, and magnetic properties for nondefective and defective structures and the diffusion of both cations and anions in chromium oxide (α-Cr2O3) are investigated theoretically with the periodic quantum-chemical method. Three different point defects are studied, namely, Cr vacancy, Cr Frenkel defect (composed of an interstitial Cr atom and a Cr vacancy), and O vacancy. All these defects affect the electronic properties of Cr2O3 drastically and are involved in diffusion processes in passive film growth. The calculated defect formation energy shows that the stability of defects falls in the following order: Cr Frenkel defect (E Fr(Cr) = 2.36 eV) > Cr vacancy (E V(Cr) = 4.84 eV) > O vacancy (E V(O) = 5.12 eV). Relaxation occurs only on the first and the second nearest neighbors in each case. Each defect adds an extra localized level inside the band gap. Cr Frenkel defects add donor levels composed of O states; Cr vacancy defects add acceptor levels composed of states from both Cr and O atoms; and O vacancies do not give any level in the gap. Defects influence the magnetic moments on surrounding atoms, especially on the first nearest neighbors. Various diffusion processes of both cations and anions are investigated by calculating the Cr3+ and O2– diffusion among various sites using the climbing-image nudged-elastic-band (cNEB) approach. The activation energy E D (2.57–3.21 eV) obtained for the diffusion of Cr3+ is in good agreement with the experimental E D (2.46 eV). The calculated E D for O2– ranges from 2.21 to 3.65 eV, which is in agreement with experimental data. For each investigated diffusion pathway, frequencies calculated by finite difference methods are used to obtain jump frequencies using transition state theory (TST). Combining the pre-exponential factors with activation energies, the diffusion coefficients are calculated which are compared with experimental values.

The present paper describes the nanoscale modeling of the adsorption and dissociation of water molecules on the titanium Ti (001) surface at different applied potentials and its connection to a previously proposed continuum scale approach to the growth and dissolution of thin barrier films on Ti in water-containing electrolytes. The theoretical study of titanium/water interactions is presented as combination of static and molecular dynamics calculations using density functional theory (DFT). The vibration frequencies of adsorbed H2O put emphasis on its interaction with the titanium surface and provide further information about the bond strength and the changes in the molecular structure. The adsorption and dissociation geometry of water molecules on Ti (001) surface in an electrochemical environment is investigated using the double reference method that allows assessing the elementary steps of water dissociation as depending on the applied potential. The dependence of the free energy of the Ti/water system on potential is obtained and discussed in terms of sequential dissociation of water to adsorbed hydroxyl and oxygen.