A complex defect studied through DFT:: The Fe3+-O2- pair in KMgF3

Electron paramagnetic resonance (EPR) and electron nuclear double resonance (ENDOR) spectroscopies performed by different authors in KMgF3, KZnF3, and RbCdF3 have detected a strongly axial Fe3+ center that was tentatively assigned to a FeF5O4- complex. The current work is devoted to exploring the properties of the FeF5O4- center (C-4V symmetry) in KMgF3 by means of density functional calculations. The equilibrium distance between Fe3+ and O2- (1.83 Angstrom) is found to be only similar to10% higher than that computed for the Fe3+ -O2- pair in vacuo (1:63 Angstrom), but similar to25 pm smaller than that corresponding to FeO69- units. These data together with the charge transferred onto the oxygen in the antibonding a(1)* (approximate to3z(2) - r(2)) orbital, reveals the formation of a Fe3+-O2- pair inside the KMgF3 lattice. In comparison with FeF63- the substitution of a F- ion by an O2- one is found to produce an increase of distances between Fe3+ and the four equatorial (R-eq) and apical (R-ax) F- ligands equal to 10 pm and 17 pm, respectively. The current calculations reasonably account for the experimental fingerprint of this center, where the isotropic superhyperfine constant for the equatorial fluorines (A(s)(eq)) is found to be about twice that for the apical fluorine (A(s)(ax)). This huge difference between A(s)(eq) and A(s)(ax) cannot be-explained on the basis of the equilibrium R-eq and R-ax values and the law relating A(s) to the metal-ligand distance, R, for the octahedral FeF63- unit. This salient feature stresses that laws valid for a given kind of complex cannot be transferred to complexes with lower symmetry. Along this line, the current results cast doubts on the conclusions previously reached using the empirical superposition model. (C) 2002 Wiley Periodicals, Inc.