Annual Report 2012 - Latvijas Universitātes Cietvielu fizikas institūts

GB1 with oxygen vacancy revealed the electronic states close to the bottom of
conduction band. It was so far possible to calculate this GB for PBE only. We, thus,
emphasize the importance of the choice of exchange-correlation functional and careful
treatment of GB structure relaxation for the first-principle prediction of GB properties.
a)
b)
bulk BaZrO 3
GB1 in BaZrO 3
bulk BaZrO 3
GB1 in BaZrO 3
DOS, arb. units
GB2 in BaZrO 3
PBE0
DOS, arb. units
PBE
-5 -4 -3 -2 -1 0 1 2 3 4 5 6
Energy, eV
-5 -4 -3 -2 -1 0 1 2 3 4 5 6
Energy, eV
Fig. 3. The total density of states calculated for bulk and grain boundary of BaZrO 3 with
PBE0 (CRYSTAL) and PBE (VASP) exchange-correlation functionals.
Ab initio CALCULATIONS OF DEFECTS ON MeF 2 AND SrZrO 3 SURFACES
R.I. Eglitis
The electronic F- and R centers in CaF 2 fluorite crystals have been studied using density
functional theory (DFT) with a hybrid B3PW functional for of exchange and correlation
effects. We estimated the F-center diffusion barrier as 1.7 eV. During the F-center jump,
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 energy preference for the F centers to locate near the surface.
The association energy calculations of R centers indicate energy preference for
aggregation of isolated F centers. During such the F-center aggregation, a considerable
chemical bond covalency arises between two neighboring fluorine vacancies with
trapped electrons. Three incompletely paired electrons trapped at the R center have an
up-down-up spin arrangement and induce three defect levels in the gap between valence
bands (VB) and conduction bands (CB) for both the α- and 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.
OH - radicals in CaF 2 and BaF 2 bulk crystals and on the (111) surfaces have been studied
as well. Three bulk and 20 surface OH - configurations were investigated, and the
energetically most favorable configurations detected. For the (111) CaF 2 surface atomic
layers, the surface hydroxyls lead to a remarkable XY-translation and a dilating effect in
the Z-direction, overcoming the surface shrinking effect in the perfect slab. The
chemical bond population analysis shows a considerable covalency between the oxygen
and hydrogen atoms, and the surface effect strengthens the covalency of surface OH -
impurities. There are two defect levels induced by OH - impurities. The O 2p orbitals
form two superposed occupied O bands, located above the valence bands (VBs), and the
H s orbitals do the major contribution to an empty H band, located below the conduction
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