Transition metal impurities in fluorides: Role of electronic structure of fluorine on spectroscopic properties

This work examines the relation between optical properties of a MF₆^q− complex (M = transition-metal cation) and the chemical bonding paying especial attention to the role played by the electronic structure of fluorine. A main goal of the present study is to understand why if the effective Racah parameters, B and C, as well as the cubic splitting parameter, 10Dq, all depend on the covalency nevertheless the latter one is much more sensitive to a hydrostatic pressure than the former ones. The analysis carried out in this work, together with the results of ab initio calculations on CrF₆³⁻ embedded in the cubic elpasolite K₂NaScF₆, demonstrates that, although the 2s-2p separation for fluorine is 23 eV, 10Dq does not come mainly from the dominant 3d(Cr)-2p(F) covalency but from the tiny admixture of deep 2s(F) levels of fluorine in the antibonding e_g(∼3z² − r², x² − y²) orbital. By contrast, it is pointed out that the reduction of Racah parameters essentially reflects the global covalency in the bonding. The way of measuring the 2p(F) and 2s(F) admixtures into the mainly 3d(Cr) level through Electron Paramagnetic Resonance data for MF₆^q− complexes with unpaired σ electrons in the ground state is also explained in some detail. At the same time the reasons avoiding its measurement from optical spectra are pointed out as well. The present results stress that the microscopic origin of an optical parameter like 10Dq can certainly be very subtle.