Ab Initio Calculations of Hydroxyl Impurities in CaF2

The Journal of Physical Chemistry CArticleThe calculated atomic displacements of the OH − andsurrounding atoms for Config OH (111) are listed in Table 1.Table 1. Atomic Relaxations (D (% a 0 ), as a Percentage ofthe Lattice Constant, 5.50 Å) and Effective Charges (Q(e))of the OH − and Surrounding Atoms in the CaF 2 96-AtomSupercell for Config OH (111)aatoms (shell) no. D (% a 0 ) Q (e) ΔQ (e)O 1 1.93 −0.997 −H 1 − +0.275 −Ca1 3 1.10 +1.772 −0.031Ca2 1 0.73 +1.799 −0.004F1 3 0.34 −0.913 −0.011F2 3 0.32 −0.915 −0.013F3 6 0.33 −0.901 +0.001F4 3 0.28 −0.903 −0.001F6 3 0.18 −0.900 +0.002a ΔQ(e) is the charge difference between the defective and perfectcrystals (Q Ca = +1.803 e, Q F = −0.902 e in perfect CaF 2 ). The shelllabels have been defined in Figure 1.The three Ca1 atoms are repulsed from the OH − by around1.10% of a 0 , and the one Ca2 moves toward the OH − byaround 0.73% of a 0 . The OH − −Ca1 and OH − −Ca2 distancesboth increase by around 0.25% and 1.20% of a 0 , respectively,implicating a repulsion of the OH − impurity to the neighborcalcium atoms. Finally, the relaxations of neighbor fluorineatoms are slight and less than 0.4% of a 0 .Table 1 also presents the effective charges of the OH − andsurrounding atoms for Config OH (111) . The total charge of theOH − , i.e., the sum of the O and H charges, is −0.722 e, beingmuch smaller than the charge of the substituted fluorine atom(−0.902 e) by 0.180 e. Around 0.097 and 0.072 e localized onthe four nearest Ca and six second-nearest F, respectively,transfer inward to the OH − . For Configs HO (111) and OH (100) ,the effective charges of OH − are −0.713 and −0.737 e, beingclose to the corresponding value of Config OH (111) . We alsocalculated the OH − charges in Ca(OH) 2 and Ba(OH) 2 crystals.The results show that the total effective charges of OH − inCa(OH) 2 and Ba(OH) 2 are equal to −0.781 and −0.825 e,respectively. The OH − charge in Ca(OH) 2 is larger than OH −as impurities in CaF 2 crystal by around 0.4−0.7 e , and thisphenomenon can be explained by the fact that fluorine has astronger oxidative property than that of OH − . It is well-knownthat the hydroxyl has a considerable covalency between theoxygen and hydrogen, which is also demonstrated by our bondpopulation calculations for the OH − −CaF 2 systems. Thepresence of the covalency of OH − in CaF 2 is also clearlyshown in the charge density map (see Figure 2). The covalentbonds between the oxygen and hydrogen atoms are 486, 508,and 536 me for Configs OH (111) , HO (111) , and OH (100) ,respectively. Compared with the covalent bonds in Ca(OH) 2(458 me) and Ba(OH) 2 (434 me) crystals, the covalency of theOH − impurities in CaF 2 is stronger than that in the hydroxidecrystals. Here, we can conclude that OH − as an atomic grouphas a steady geometrical structure instead of electronicproperties in different materials.Alkaline-earth fluorides with defects degrade their opticalquality and exhibit optical absorption. Our calculations on thedefect levels induced between the valence bands (VBs) andconduction bands (CBs) suggest a possible mechanism for theoptical absorption. In the one-electron approximation scheme,Figure 2. Electron density contours in CaF 2 with OH − from the (110)side view, being from 0 to 0.4 e/bohr 3 with a linear increment of 0.01e/bohr 3 .the experimentally observed optical absorption could be due toan electron transition from the ground state to the empty bandinduced by the OH − impurities. Our calculated band structureregarding the OH − -impurity system of Config OH (111) is shownin Figure 3, and the optical band gaps for Configs OH (111) ,Figure 3. Calculated B3PW band structure for the 96-atom supercellmodeling the OH − impurity in CaF 2 for Config OH (111) .HO (111) and OH (100) are collected in Table 2. The optical bandgaps between the VBs and CBs for the CaF 2 96-atom supercellTable 2. Direct Optical Band Gaps (eV) (Γ → Γ) of theOH − -Impurity Systems for Configs OH (111) ,HO (111) , andOH (100)gaps OH (111) HO (111) OH (100)O → H 9.04 8.99 9.04O → CB 9.74 9.78 10.24VB → H 10.47 10.52 10.65VB → CB 11.17 11.32 11.85containing a OH − at the Γ point are 11.17, 11.32, and 11.85 eVfor Configs OH (111) ,HO (111) , and OH (100) , respectively, beingmuch larger than the relevant gap of the perfect CaF 2 crystal(10.96 eV). It is implied that the OH − impurities widen theVB−CB gap, especially for Config OH (100) . For Config OH (111) ,the empty defect level induced by OH − impurities is located0.70 eV below the bottom of the CB at the Γ point (see Figure 3).The occupied defect levels, containing two superposed bands6394dx.doi.org/10.1021/jp211075g | J. Phys. Chem. C 2012, 116, 6392−6400