Ab Initio Calculations of Hydroxyl Impurities in CaF2

The Journal of Physical Chemistry CArticleimpurities in CaF 2 . For the oxygen impurity, the formationenergy equals −0.71 eV. The negative sign indicates the stabilityof oxygen replacing fluorine in CaF 2 . We also investigated theoxygen impurity located near the (111) CaF 2 surface. Accordingto our calculation, the energy of the oxygen located at the surfacehas the smallest value, indicating a trend of the oxygen impuritylocated near the surface. The total energies of the oxygenimpurity at the second, third, and fourth fluorine sublayers arelarger than that for the first fluorine sublayer by 1.14, 1.24, and1.16 eV, respectively. Additionally, for the hydrogen impurity, theformation energy is +0.33 eV. Unlike the oxygen case, theformation of a hydrogen impurity in CaF 2 needs extra energy.Further, according to our calculation regarding a hydrogen atomadded on the CaF 2 surface, the hydrogen atom has a trend ofseparating from the surface.Here, we can summarize that oxygen does the primarycontribution to the formation of OH − impurities in CaF 2 .Isolatedoxygen atoms surrounding CaF 2 adsorb on the surface easily, andthen, the oxygen impurities trap the hydrogen atoms near thesurface, forming the OH − impurities. So, if we can control theconcentration of the oxygen atoms near the surface, the formationof OH − impurities could be avoided in CaF 2 crystal.IV. CONCLUSIONSWe applied the first-principles approach within the hybridDFT-B3PW scheme to calculations on OH − in CaF 2 . Threebulk configurations including Configs OH (111) ,HO (111) , andOH (100) , and 16 surface configurations including 8 (111)-oriented and 8 (111)-unoriented OH − impurities were studied.For the bulk case, Config OH (111) , in which the hydroxylorients along the (111) axis and the oxygen occupies a regularfluorine site, is the energetically most favorable configuration.For the surface case, we found that Config HO (\) 11 , in which theOH − is located at the upper fluorine sublayer of the first surfacelayer, H lies above O obliquely, and the obliquity is around3.2°, is the most stable configuration for the surface OH − -impurity systems. We also performed calculations on the (111)CaF 2 surfaces full covered by OH − . Config HO (\) full has thelowest total energy among the four OH − full-covered configurations,and the deviation angle equals around 2.2°. Thelengths of surface OH − impurities for all the configurations arearound 0.96−0.98 Å, being close to that in the bulk cases(0.95−0.97 Å), as well as in water molecule and Ca(OH) 2 andBa(OH) 2 crystals, indicating that the surface effect on thelength of surface hydroxyls is not remarkable and the OH − as adiatomic group has a steady geometrical structure. Thecalculations on the relaxations of atoms surrounding the surfaceOH − impurity demonstrated that the atomic layers containingsurface OH − impurities have a remarkable XY-translation.Effective charge analysis shows that the OH − charge equalsaround −0.722 e, being much smaller than the fluorine (substitutedby the OH − ) charge (−0.902 e) in perfect CaF 2 crystal,and some charges localized on the neighbor atoms (especiallyfor the nearest calcium atoms) transfer inward to the OH − .Because of the surface effect, the charges of the surface OH −impurities are larger than those of the bulk cases for most of thesurface OH − configurations. Bond population calculationsindicate that the surface effect strengthens the covalency ofOH − impurities located near the surface. The main surfaceeffect on the OH − impurities is on the electronic structuresinstead of the geometrical structures.The studies on band structures and DOS of the OH − -impurity systems demonstrate that there are two defect levels6399induced by OH − impurities. One is two superposed occupied Obands mainly consisting of the O p orbitals, located above theVBs, and the other is an empty H band to which the H sorbitals do the major contribution, almost superposing with theCBs. Because of the surface effect, the O bands move downward,toward the VBs with respect to these bands in the bulk case, andthis leads to narrowing of the VB → O gap and widening of theO → H gap which corresponds to the first optical absorption.Finally, we investigated the formation of OH − impurities inCaF 2 . The calculation on the formation energy of the OH −impurity shows that isolated hydroxyls are favorite to substitutefluorine ions and adsorb on the surface energetically. On theother hand, the formation of OH − impurities in CaF 2 may alsobe due to the aggregation of separated oxygen and hydrogenimpurities. The formation of oxygen impurities is favorable,whereas that of hydrogen impurities is unfavorable energetically.So, we concluded that oxygen does the primary contribution tothe formation of OH − impurities in CaF 2 , and if we can controlthe concentration of the oxygen atoms near the surface, theformation■of OH − impurities could be avoided in CaF 2 crystal.AUTHOR INFORMATIONCorresponding Author*E-mail address: shihongting@gmail.com.NotesThe authors declare no competing financial interest.■ ACKNOWLEDGMENTSH.S. was supported by NSFC Grant No. 11004008.R.I.E. was supported by ESF Grant No. 2009/0202/1DP/1.1.1.2.0/APIA/VIAA/141.■ REFERENCES(1) Letz, M.; Parthier, L. Phys. Rev. B 2006, 74, 064116.(2) Burnett, J. H.; Levine, Z. H.; Shirley, E. L. Phys. Rev. B 2001, 64,241102(R).(3) Letz, M.; Parthier, L.; Gottwald, A.; Richter, M. Phys. Rev. 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