Vacancies and Vacancy-Mediated Self Diffusion in Cr₂O₃: A First-Principles Study

Charged and neutral vacancies and vacancy-mediated self-diffusion in α-Cr₂O₃ were investigated using first-principles density functional theory (DFT) and periodic supercell formalism. The vacancy formation energies of charged defects were calculated using the electrostatic finite-size corrections to account for electrostatic interactions between supercells and the corrections for the bandgap underestimation in DFT. Calculations predict that neutral oxygen (O) vacancies are predominant in chromium (Cr)-rich conditions and Cr vacancies with −2 charge state are the dominant defects in O-rich conditions. The charge-transition levels of both O and Cr vacancies are deep within the bandgap, indicating the stability of these defects. Transport calculations indicate that vacancy-mediated diffusion along the basal plane has lower energy barriers for both O and Cr ions. The most favorable vacancy-mediated self-diffusion processes corresponds to the diffusion of Cr ion in Cr³⁺ charge state and O ion in O^2– state, respectively. Our calculations reveal that Cr triple defects composed of Cr in octahedral interstitial sites with two adjacent Cr vacancies along the <i>c</i> axis have a lower formation energy compared with that of charged Cr vacancies. The formation of such triple defects facilitates Cr self-diffusion along the <i>c</i> axis

First-Principles Investigation of Native Interstitial Diffusion in Cr₂O₃

First-principles density functional theory investigation of native interstitials and the associated self-diffusion mechanisms in α-Cr₂O₃ reveals that interstitials are more mobile than vacancies of corresponding species. Cr interstitials occupy the unoccupied Cr sublattice sites that are octahedrally coordinated by 6 O atoms, and O interstitials form a dumbbell configuration orientated along the [221] direction (diagonal) of the corundum lattice. Calculations predict that neutral O interstitials are predominant in O-rich conditions and Cr interstitials in +2 and +1 charge states are the dominant interstitial defects in Cr-rich conditions. Similar to that of the vacancies, the charge transition levels of both O and Cr interstitials are located deep within the band gap. Transport calculations reveal a rich variety of interstitial diffusion mechanisms that are species-, charge-, and orientation-dependent. Cr interstitials diffuse preferably along the diagonal of corundum lattice in a two-step process via an intermediate defect complex comprising a Cr interstitial and an adjacent Cr Frenkel defect in the neighboring Cr bilayer. This mechanism is similar to that of the vacancy-mediated Cr diffusion along the <i>c</i>-axis with intermediate Cr vacancy and Cr Frenkel defect combination. In contrast, O interstitials diffuse via bond switching mechanism. O interstitials in −1 and −2 charge states have very high mobility compared to neutral O interstitials

Direct in Situ TEM Observation of Modification of Oxidation by the Injected Vacancies for Ni–4Al Alloy Using a Microfabricated Nanopost

Vacancy injection and selective oxidation of one species in bimetallic alloy at high temperature is a well-known phenomenon. However, detailed understanding of the behavior of the injected vacancies and consequently their effect on oxidation remains elusive. The current research examines the oxidation of high-purity Ni doped with 4.1 at. % Al using in situ transmission electron microscopy (TEM). Experiments are performed on nanoposts fabricated from solution-annealed bulk material that are essentially single crystal samples. Initial oxidation is observed to occur by multisite oxide nucleation, formation of an oxide shell followed by cavity nucleation and growth at the metal/oxide interface. One of the most interesting in situ TEM observations is the formation of a cavity that leads to the faceting of the metal and subsequent oxidation occurring by an atomic ledge migration mechanism on the faceted metal surface. Further, it is directly observed that metal atoms diffuse through the oxide layer to combine with oxygen at the outer surface of the oxide. The present work indicates that injection of vacancies and formation of cavity will lead to a situation where the oxidation rate is essentially controlled by the low surface energy plane of the metal, rather than by the initial terminating plane at the metal surface exposed to the oxidizing environment

Direct in Situ TEM Observation of Modification of Oxidation by the Injected Vacancies for Ni–4Al Alloy Using a Microfabricated Nanopost

Vacancy injection and selective oxidation of one species in bimetallic alloy at high temperature is a well-known phenomenon. However, detailed understanding of the behavior of the injected vacancies and consequently their effect on oxidation remains elusive. The current research examines the oxidation of high-purity Ni doped with 4.1 at. % Al using in situ transmission electron microscopy (TEM). Experiments are performed on nanoposts fabricated from solution-annealed bulk material that are essentially single crystal samples. Initial oxidation is observed to occur by multisite oxide nucleation, formation of an oxide shell followed by cavity nucleation and growth at the metal/oxide interface. One of the most interesting in situ TEM observations is the formation of a cavity that leads to the faceting of the metal and subsequent oxidation occurring by an atomic ledge migration mechanism on the faceted metal surface. Further, it is directly observed that metal atoms diffuse through the oxide layer to combine with oxygen at the outer surface of the oxide. The present work indicates that injection of vacancies and formation of cavity will lead to a situation where the oxidation rate is essentially controlled by the low surface energy plane of the metal, rather than by the initial terminating plane at the metal surface exposed to the oxidizing environment

Catalyst Composition and Impurity-Dependent Kinetics of Nanowire Heteroepitaxy

The mechanisms and kinetics of axial Ge–Si nanowire heteroepitaxial growth based on the tailoring of the Au catalyst composition <i>via</i> Ga alloying are studied by environmental transmission electron microscopy combined with systematic <i>ex situ</i> CVD calibrations. The morphology of the Ge–Si heterojunction, in particular, the extent of a local, asymmetric increase in nanowire diameter, is found to depend on the Ga composition of the catalyst, on the TMGa precursor exposure temperature, and on the presence of dopants. To rationalize the findings, a general nucleation-based model for nanowire heteroepitaxy is established which is anticipated to be relevant to a wide range of material systems and device-enabling heterostructures