Temperature Dependence of Self-Diffusion in Cr₂O₃ from First Principles

Abstract

Understanding and predicting the dominant diffusion processes in Cr 2 O 3 is essential to its optimization for anticorrosion coatings, spintronics, and other applications. Despite significant theoretical effort in modeling defect mediated diffusion in Cr 2 O 3 the correlation with experimentally measured diffusivities remains poor partly due to the insufficient accuracy of the theoretical approaches. Here an attempt to resolve these discrepancies is made through high accuracy density functional theory simulations coupled with grand canonical formalism of defect thermochemistry. In this approach, point defect formation energies were computed using hybrid exchange correlation functional. This level of theory proved to be essential for achieving the agreement with experimental self-diffusion coefficients. The analysis of the resulting self-diffusion coefficients indicate that chromium has higher mobility at low temperatures and high oxygen partial pressures, in particular at standard temperature and pressure conditions. At high vacuum, high temperature conditions, oxygen diffusion becomes dominant. At Cr/Cr 2 O 3 interfaces O vacancies were found to be more mobile than Cr vacancies at all temperatures. Cr diffuses preferentially along the c-axis at low temperatures but switches to basal plane at higher temperatures. O diffusion is primarily bound to basal plane at all temperatures.

Ab Initio Evaluation of Solid-State Transformation Pathways from Ferrihydrite to Goethite

“…While self-diffusion of iron and/or oxygen in iron oxides such as Hm and Mt at high temperatures (e.g., >1000 K) in controlled atmospheres is well-studied and -known, − extrapolation of diffusion coefficients to ambient conditions, although dubious, nonetheless suggests insignificant ionic mobilities over the same length scale and a requirement of extremely short diffusion distances. Activation energies derived from Arrhenius fits to the temperature dependence tend to be on the order of 3–6 eV. − Beyond phenomenological characterization, such studies tend not to provide definitive insights into the ionic transport mechanism, for example, whether vacancy-mediated or based on interstitials, − due to multiple interdependent processes. Our study thus focuses on evaluating (1) various possible reaction pathways leading to iron ionic transport and (2) the magnitude of the associated energy barriers.…”

A DFT study of defects in paramagnetic Cr2O3

“…The values in the literature are dispersed probably because each author used different finite-size correction methods and different reference atomic chemical potentials. 37,38,[40][41][42]47,63 However, some trends emerge across all these results. Chromium vacancies tend to be lower in energy than oxygen interstitials, themselves having on average a lower energy than oxygen vacancies.…”

“…3 Standard defect formation energies for AFM Cr 2 O 3 in this study and in the literature. 37,38,[40][41][42]47,63 position indicates that charged Cr vacancies will be dominant in the neutralizing intrinsic system.…”

“…The formation of intrinsic point defects in AFM Cr 2 O 3 was investigated in several studies. [36][37][38][39][40][41][42] Some found the existence of some defect-induced discrete states in the band gap. 37,[40][41][42] Intensive states induced near the valence band maximum (VBM) or the conduction band minimum (CBM) reduced the effective band gap, especially for a supercell with Cr defects.…”

“…[36][37][38][39][40][41][42] Some found the existence of some defect-induced discrete states in the band gap. 37,[40][41][42] Intensive states induced near the valence band maximum (VBM) or the conduction band minimum (CBM) reduced the effective band gap, especially for a supercell with Cr defects. There is a lot of scatter in the defect formation energies calculated by different authors.…”