Local relaxation around Fe3+ in fluorides:: Influence on electronic properties

The local relaxation around Fe3+ impurities in different fluoride lattices has been explored by means of density-functional (DF) calculations on clusters including up to third neighbors of Fe3+. For the same purpose the dependence of the isotropic superhyperfine constant, A(s), on the metal-ligand distance, R, has been studied for clusters of different size using the self-consistent charge extended Huckel and multiple-scattering X alpha methods as well. In all cases A(s) is found to be proportional to R(-n)sn(s) lying between 6 and 7.5. Using this result the difference, Delta R-e, between the equilibrium distance for CsCdF3:Fe3+ and KMgF3:Fe3+ would be equal only to about 2.3 pm from the electron-nuclear double resonance (ENDOR) data reported for both systems. This figure, which has to be compared with the value Delta R-0 = 23 pm corresponding to the perfect host lattice, is compatible with the R-e values derived from total-energy calculations. Although the value Delta R-e = 2.3 pm is much smaller than Delta R-e = 7+/-1 pm corresponding to Mn2+ in the same lattices, it is shown to be consistent with the w(A(1g)) frequency for both kind of impurities. From the present results R-e changes caused by a hydrostatic pressure down to 0.05 pm can be detected through A(s) variations measured by ENDOR. Moreover it is pointed out that good information about the actual impurity-ligand distance for transition-metal impurities in insulators can be obtained from DF calculations on clusters. Finally, the 10Dq value and its R dependence are shown to be strongly related to the small 3d-2s(F) hybridization in the antibonding e(g)* level which also determines A(s).