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

• Published in: Journal of fluorine chemistry 2011-10, Vol.132 (10), p.747-754
• Main Authors: Trueba, A., Garcia-Fernandez, P., García-Lastra, J.M., Aramburu, J.A., Barriuso, M.T., Moreno, M.
• Format: Article
• Language: English
• Subjects: Admixtures; Bonding; Cations; Covalence; Electronic structure; Fluoride complexes in insulators; Fluorine; Ground state; Measurement of covalency through EPR; Microscopic origin; Orbitals; Role of deep 2 s levels of fluorine; Spectroscopic parameters; Subtle effects of chemical bonding
• ISSN: 0022-1139; 1873-3328
• DOI: 10.1016/j.jfluchem.2011.05.030

[Display omitted] ► Transition metal complexes with fluorine as ligand. ► Chemical bonding and spectroscopic parameters. ► 10Dq depends on the metal ligand distance. Racah parameters are little sensitive. ► 10Dq does not depend on the global covalency but on the tiny 3d–2s hybridization. ► Relation between optical and EPR parameters. This work examines the relation between optical properties of a MF 6 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 CrF 6 3− embedded in the cubic elpasolite K 2NaScF 6, 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 (∼3 z 2 − r 2, x 2 − y 2) 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 MF 6 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.