The Journal <strong>of</strong> Physical Chemistry CArticleimpurities <strong>in</strong> CaF 2 . For the oxygen impurity, the formationenergy equals −0.71 eV. The negative sign <strong>in</strong>dicates the stability<strong>of</strong> oxygen replac<strong>in</strong>g fluor<strong>in</strong>e <strong>in</strong> CaF 2 . We also <strong>in</strong>vestigated theoxygen impurity located near the (111) CaF 2 surface. Accord<strong>in</strong>gto our calculation, the energy <strong>of</strong> the oxygen located at the surfacehas the smallest value, <strong>in</strong>dicat<strong>in</strong>g a trend <strong>of</strong> the oxygen impuritylocated near the surface. The total energies <strong>of</strong> the oxygenimpurity at the second, third, and fourth fluor<strong>in</strong>e sublayers arelarger than that for the first fluor<strong>in</strong>e 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 <strong>of</strong> a hydrogen impurity <strong>in</strong> CaF 2 needs extra energy.Further, accord<strong>in</strong>g to our calculation regard<strong>in</strong>g a hydrogen atomadded on the CaF 2 surface, the hydrogen atom has a trend <strong>of</strong>separat<strong>in</strong>g from the surface.Here, we can summarize that oxygen does the primarycontribution to the formation <strong>of</strong> OH − impurities <strong>in</strong> CaF 2 .Isolatedoxygen atoms surround<strong>in</strong>g CaF 2 adsorb on the surface easily, andthen, the oxygen impurities trap the hydrogen atoms near thesurface, form<strong>in</strong>g the OH − impurities. So, if we can control theconcentration <strong>of</strong> the oxygen atoms near the surface, the formation<strong>of</strong> OH − impurities could be avoided <strong>in</strong> CaF 2 crystal.IV. CONCLUSIONSWe applied the first-pr<strong>in</strong>ciples approach with<strong>in</strong> the hybridDFT-B3PW scheme to calculations on OH − <strong>in</strong> CaF 2 . Threebulk configurations <strong>in</strong>clud<strong>in</strong>g Configs OH (111) ,HO (111) , andOH (100) , and 16 surface configurations <strong>in</strong>clud<strong>in</strong>g 8 (111)-oriented and 8 (111)-unoriented OH − impurities were studied.For the bulk case, Config OH (111) , <strong>in</strong> which the hydroxylorients along the (111) axis and the oxygen occupies a regularfluor<strong>in</strong>e site, is the energetically most favorable configuration.For the surface case, we found that Config HO (\) 11 , <strong>in</strong> which theOH − is located at the upper fluor<strong>in</strong>e sublayer <strong>of</strong> 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 <strong>of</strong> surface OH − impurities for all the configurations arearound 0.96−0.98 Å, be<strong>in</strong>g close to that <strong>in</strong> the bulk cases(0.95−0.97 Å), as well as <strong>in</strong> water molecule and Ca(OH) 2 andBa(OH) 2 crystals, <strong>in</strong>dicat<strong>in</strong>g that the surface effect on thelength <strong>of</strong> surface hydroxyls is not remarkable and the OH − as adiatomic group has a steady geometrical structure. Thecalculations on the relaxations <strong>of</strong> atoms surround<strong>in</strong>g the surfaceOH − impurity demonstrated that the atomic layers conta<strong>in</strong><strong>in</strong>gsurface OH − impurities have a remarkable XY-translation.Effective charge analysis shows that the OH − charge equalsaround −0.722 e, be<strong>in</strong>g much smaller than the fluor<strong>in</strong>e (substitutedby the OH − ) charge (−0.902 e) <strong>in</strong> perfect CaF 2 crystal,and some charges localized on the neighbor atoms (especiallyfor the nearest calcium atoms) transfer <strong>in</strong>ward to the OH − .Because <strong>of</strong> the surface effect, the charges <strong>of</strong> the surface OH −impurities are larger than those <strong>of</strong> the bulk cases for most <strong>of</strong> thesurface OH − configurations. Bond population calculations<strong>in</strong>dicate that the surface effect strengthens the covalency <strong>of</strong>OH − impurities located near the surface. The ma<strong>in</strong> surfaceeffect on the OH − impurities is on the electronic structures<strong>in</strong>stead <strong>of</strong> the geometrical structures.The studies on band structures and DOS <strong>of</strong> the OH − -impurity systems demonstrate that there are two defect levels6399<strong>in</strong>duced by OH − impurities. One is two superposed occupied Obands ma<strong>in</strong>ly consist<strong>in</strong>g <strong>of</strong> 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 superpos<strong>in</strong>g with theCBs. Because <strong>of</strong> the surface effect, the O bands move downward,toward the VBs with respect to these bands <strong>in</strong> the bulk case, andthis leads to narrow<strong>in</strong>g <strong>of</strong> the VB → O gap and widen<strong>in</strong>g <strong>of</strong> theO → H gap which corresponds to the first optical absorption.F<strong>in</strong>ally, we <strong>in</strong>vestigated the formation <strong>of</strong> OH − impurities <strong>in</strong>CaF 2 . The calculation on the formation energy <strong>of</strong> the OH −impurity shows that isolated hydroxyls are favorite to substitutefluor<strong>in</strong>e ions and adsorb on the surface energetically. On theother hand, the formation <strong>of</strong> OH − impurities <strong>in</strong> CaF 2 may alsobe due to the aggregation <strong>of</strong> separated oxygen and hydrogenimpurities. The formation <strong>of</strong> oxygen impurities is favorable,whereas that <strong>of</strong> hydrogen impurities is unfavorable energetically.So, we concluded that oxygen does the primary contribution tothe formation <strong>of</strong> OH − impurities <strong>in</strong> CaF 2 , and if we can controlthe concentration <strong>of</strong> the oxygen atoms near the surface, theformation■<strong>of</strong> OH − impurities could be avoided <strong>in</strong> CaF 2 crystal.AUTHOR INFORMATIONCorrespond<strong>in</strong>g Author*E-mail address: shihongt<strong>in</strong>g@gmail.com.NotesThe authors declare no compet<strong>in</strong>g f<strong>in</strong>ancial <strong>in</strong>terest.■ 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.; Lev<strong>in</strong>e, Z. H.; Shirley, E. L. Phys. Rev. B 2001, 64,241102(R).(3) Letz, M.; Parthier, L.; Gottwald, A.; Richter, M. Phys. Rev. B 2003,67, 233101.(4) Shimizu, Y.; M<strong>in</strong>owa, M.; Suganuma, W.; Inoue, Y. Phys. Lett. B2006, 633, 195.(5) Barth, J.; Johnson, R. L.; Cardona, M.; Fuchs, D.; Bradshaw, A. M.Phys. Rev. B 1990, 41, 3291.(6) Bennewitz, R.; Smith, D.; Reichl<strong>in</strong>g, M. Phys. Rev. B 1999, 59,8237.(7) Catti, M.; Dovesi, R.; Pavese, A.; Saunders, V. R. J. Phys.: Condens.Matter 1991, 3, 4151.(8) Ch<strong>in</strong>g, W. Y.; Gan, F. Q.; Huang, M. Z. Phys. Rev. B 1995, 52,1596.(9) de Leeuw, N. H.; Cooper, T. G. J. Mater. Chem. 2003, 13, 93.(10) de Leeuw, N. H.; Purton, J. A.; Parker, S. C.; Watson, G. W.;Kresse, G. Surf. Sci. 2000, 452, 9.(11) Foster, A. S.; Barth, C.; Shluger, A. L.; Niem<strong>in</strong>en, R. M.;Reichl<strong>in</strong>g, M. Phys. Rev. B 2002, 66, 235417.(12) Gan, F. Q.; Xu, Y. N.; Huang, M. Z.; Ch<strong>in</strong>g, W. Y.; Harrison, J. G.Phys. Rev. B 1992, 45, 8248.(13) Jockisch, A.; Schroder, U.; Dewette, F. W.; Kress, W. J. Phys.:Condens. Matter 1993, 5, 5401.(14) Ma, Y. C.; Rohlf<strong>in</strong>g, M. Phys. Rev. B 2008, 77, 115118.(15) Merawa, M.; Llunell, M.; Orlando, R.; Gelize-Duvignau, M.;Dovesi, R. Chem. Phys. Lett. 2003, 368, 7.(16) Puch<strong>in</strong>, V. E.; Puch<strong>in</strong>a, A. V.; Huis<strong>in</strong>ga, M.; Reichl<strong>in</strong>g, M.J. Phys.: Condens. Matter 2001, 13, 2081.(17) Puch<strong>in</strong>a, A. V.; Puch<strong>in</strong>, V. E.; Kotom<strong>in</strong>, E. A.; Reichl<strong>in</strong>g, M.Solid State Commun. 1998, 106, 285.(18) Rubl<strong>of</strong>f, G. W. Phys. Rev. B 1972, 5, 662.dx.doi.org/10.1021/jp211075g | J. Phys. Chem. C 2012, 116, 6392−6400
The Journal <strong>of</strong> Physical Chemistry CArticle(19) Schmalzl, K.; Strauch, D.; Schober, H. Phys. Rev. B 2003, 68,144301.(20) Verstraete, M.; Gonze, X. Phys. Rev. B 2003, 68, 195123.(21) Wu, X.; Q<strong>in</strong>, S.; Wu, Z. Y. Phys. Rev. B 2006, 73, 134103.(22) Shi, H.; Eglitis, R. I.; Borstel, G. Phys. Rev. B 2005, 72, 045109.(23) Shi, H.; Eglitis, R. I.; Borstel, G. J. Phys.: Condens. Matter 2006,18, 8367.(24) Shi, H.; Eglitis, R. I.; Borstel, G. J. Phys.: Condens. Matter 2007,19, 056007.(25) Shi, H.; Eglitis, R. I.; Borstel, G. Comput. Mater. Sci. 2007,39, 430.(26) Jia, R.; Shi, H.; Borstel, G. Phys. Rev. B 2008, 78, 224101.(27) Shi, H.; Luo, L.; Johansson, B.; Ahujia, R. J. Phys.: Condens.Matter 2009, 21, 415501.(28) Shi, H.; Jia, R.; Eglitis, R. I. Phys. Rev. B 2010, 81, 195101.(29) Adler, H.; Kveta, I. S.B. Osterreich Akad. Wiss. Math.-Nat. Klasse.<strong>Ab</strong>t. II 1957, 166, 199.(30) Bont<strong>in</strong>ck, W. Physica 1958, 24, 650.(31) Becke, A. D. J. Chem. Phys. 1993, 98, 5648.(32) Perdew, J. P.; Wang, Y. Phys. Rev. B 1986, 33, 8800.(33) Perdew, J. P.; Wang, Y. Phys. Rev. B 1989, 40, 3399.(34) Perdew, J. P.; Wang, Y. Phys. Rev. B 1992, 45, 13244.(35) Saunders, V. R.; Dovesi, R.; Roetti, C.; Causa, M.; Harrison,N. M.; Orlando, R.; Zicovich-Wilson, C. M. CRYSTAL-2006 UserManual; University <strong>of</strong> Tor<strong>in</strong>o: Tor<strong>in</strong>o, Italy, 2006.(36) Dovesi, R.; Ermondi, C.; Ferrero, E.; Pisani, C.; Roetti, C. Phys.Rev. B 1984, 29, 3591.(37) Monkhorst, H. J.; Pack, J. D. Phys. Rev. B 1976, 13, 5188.(38) Pisani, C. Quantum-Mechanical <strong>Ab</strong>-<strong>in</strong>itio <strong>Calculations</strong> <strong>of</strong> theProperties <strong>of</strong> Crystall<strong>in</strong>e Materials ; Lecture Notes <strong>in</strong> Chemistry 67;Spr<strong>in</strong>ger-Verlag: Berl<strong>in</strong>, Heidelberg, Germany, 1996.6400dx.doi.org/10.1021/jp211075g | J. Phys. Chem. C 2012, 116, 6392−6400