Ab Initio Calculations of the Transfer and Aggregation of F Centers in CaF₂

Abstract

The F center and R center in CaF 2 crystals have been studied by using density functional theory (DFT) with a hybrid B3PW description of exchange and correlation. Our calculations show that the F-center diffusion barrier is equal to 1.67 eV. During the F-center transfer, the trapped electron is more delocalized than that in the regular F-center case, and the gap between defect level and CB in the α-spin state decreases. The surface F-center investigation shows the trend of F centers to locate near the surface. The association energy calculations of R centers indicate stable aggregations of isolated F centers. During the F-center aggregation, a considerable covalency forms between two neighboring fluorine vacancies with trapped electrons. Three incompletely paired electrons trapped in the R center have an up−down−up spin arrangement and induce three defect levels in the gaps between valence bands (VB) and conduction bands (CB) for both the αand β-spin polarized band structures, respectively. More defect bands lead to more complex electron transitions, which were classified into two F-and four M-like transitions. The DOS calculations clearly reveal the components of defect bands.

Oxygen Reactions in a Non‐Aqueous Li+ Electrolyte

“…[1] Widely investigated in aqueous media, O 2 reduction in non-aqueous solvents, such as CH 3 CN, has been studied for several decades. [2][3][4][5][6][7] Today, O 2 reduction in non-aqueous Li + electrolytes is receiving considerable attention because it is the reaction on which operation of the Li-air (O 2 ) battery depends. The Li-O 2 battery is generating a great deal of interest because theoretically its high energy density could transform energy storage.…”

“…Spectroscopic studies of Li 2 O 2 oxidation demonstrate that LiO 2 is not an intermediate on oxidation, that is, oxidation does not follow the reverse pathway to reduction.In this work CH 3 CN was used as the solvent because it has been used widely for O 2 reduction and shown to be sufficiently stable towards reduced O 2 species for the studies undertaken herein. [4,5] Au was chosen as the electrode because it is an excellent substrate for surface enhanced Raman spectroscopy (SERS). [30] To confirm that the electrode/electrolyte combination used herein is sufficiently stable for our studies, cyclic voltammograms (CVs) were collected in 0.1m nBu 4 NClO 4 -CH 3 CN saturated with O 2 at an Au electrode ( Figure S1 in the Supporting Information).…”

Ab initio hybrid DFT calculations of BaTiO3 bulk and BaO-terminated (001) surface F-centers

“…This effect is common not only for ABO 3 perovskites, but also for other materials, for example, CaF 2 , BaF 2 and SrF 2 . [57][58][59] Our B3PW calculations reveal a considerable increase of the chemical bond covalency between the F -center of BaTiO 3 bulk and two Ti atoms nearest to it (+0.320e) as compared to relevant Ti-O chemical bonding in perfect BaTiO 3 crystal. The chemical bond population between the F -center located on the BaO-terminated (001) surface and its nearest Ti atom (+0.480e) is even larger than for the BaTiO 3 bulk F -center.…”

“…Increase of the optical bandgap for material containing the F -center defect with respect to the perfect material is common, to the best of our knowledge, for all ABO 3 perovskites as well as for another materials, like CaF 2 , BaF 2 and SrF 2 . [57][58][59] For example, according to our recently performed B3PW calculations, the Γ-Γ bandgap for perfect SrZrO 3 bulk is equal to exactly 5.00 eV, but our calculated Γ-Γ bandgap for the SrZrO 3 bulk containing the single F -center defect is increased till 5.07 eV. 24 As follows from our calculations for 3 × 3 × 3 supercell containing the single F -center, the defect level induced by the oxygen vacancy appears only 0.23 eV below the conduction band (CB) bottom in the BaTiO 3 perovskite bulk ( Table 2).…”

“…The general features of the F -center in the BaTiO 3 bulk and on its BaO-terminated (001) surface are similar to electron defects in partly covalent SiO 2 , 62 but they are quite different from F -centers in ionic materials, like, MgO, SrF 2 , CaF 2 and BaF 2 , where approximately 90% of missing electron density is localized inside the F -center. [57][58][59]63…”