THE DEPENDENCE OF 10DQ UPON THE METAL-LIGAND DISTANCE, R, FOR TRANSITION-METAL COMPLEXES - WHAT IS ITS MICROSCOPIC ORIGIN

Experimental results on 3d O(h) complexes in insulators reveal that 10 Dq alpha R(-n), where R is the metal-ligand distance and n is close to five. This strong dependence determines the Huang-Rhys factor, S(A1g) , associated to the symmetric A1g mode of the first excited state of c omplexes like MnX6(4-) and CrX6(3-) (X = halide) and makes it possible to measure R changes down to approximately 10(-3) angstrom. This work is devoted to understanding, within a molecular orbital framework, th e microscopic origin of such a dependence, which is related to the cor responding one displayed by the transferred spin densities f(sigma), f (s), and f(pi). The analysis is focused on MnF6(4-). As a main result, it is shown that though f(sigma) much-greater-than f(s) the interacti on between d(e(g)) orbitals and 2s orbitals of F- is not only primaril y responsible for the R dependence of 10Dq but also makes a significan t contribution to the 10Dq value itself. The present work thus shows t hat the significant dependence of 10Dq upon R is ultimately related to the strong dependence of f(s) and the isotropic superhyperfine consta nt A(s) upon R displayed by the experimental results of several 3d imp urities. (C) 1994 John Wiley & Sons, Inc.