Structural, Magnetic, Electronic, Defect, and Diffusion Properties of Cr₂O₃: A DFT+U Study

Abstract

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.

Conducting mechanism in the epitaxialp-type transparent conducting oxideCr2O3:Mg

“…However, as the oxygen partial pressure in MBE is substantially lower than for atmospheric growth techniques such as spray pyrolysis, we do not observe any brown discoloration that has been attributed to Cr 4+ or Cr 6+ formation [50]. Native p-and n-type defects such as V Cr and Cr i could be responsible for charge carriers, as previously theorized [48]; however, no native conductivity was present in undoped samples. Therefore, we expect this effect to be negligible.…”

“…The solubility of MgO in Cr 2 O 3 is related to the oxygen partial pressure and the growth temperature [47]. It is likely that the redistribution of Mg dopants is the most dominant effect as oxygen vacancies are not predicted to be readily formed [48] and oxygen interstitials are predicted to be electrically inactive. Thus the oxygen pressure dependence in Fig.…”

Density Functional Theory applied to magnetic materials: Mn3O4 at different hybrid functionals

“…Another work discussed that such dependence can be overcome by the replacement of these adjusted functionals for Heyd-ScuseriaErnzerhof (HSE) hybrid approach or advanced variational pseudoself-interaction correction method [43]. Recently, a lot of theoretical studies have been developed using DFTþU methodology mainly for magnetic properties predictions [44][45][46]. Nevertheless, the choice of Hubbard parameters is often arbitrary, although it drastically affects physical properties [47].…”

Anisotropic ion diffusion in α-Cr2O3: an atomistic simulation study

“…The transport of cations and anions specifically through a-Cr 2 O 3 has been experimentally measured [6][7][8][9][10][11][12][13] and simulated, [14][15][16] though to a degree far lower than would be expected given its technological importance. The experimental literature is more extensive, as tracer experiments of oxygen and cation transport have been performed by multiple groups.…”