Dependence of the covalency of the (MnF6)4- complex ion upon the Mn2+-F- distance.

Abstract

The covalency of the (${\mathrm{MnF}}_{6}$${)}^{4\mathrm{\ensuremath{-}}}$ cluster has been examined by means of cluster-in-vacuo and cluster-in-the-lattice Hartree-Fock-Roothaan (HFR) calculations. The lattice potentials of ${\mathrm{RbMnF}}_{3}$ and ${\mathrm{KMgF}}_{3}$ have been considered. We have found that the covalency parameters ${f}_{s}$ and ${f}_{p}$=${f}_{\ensuremath{\sigma}}$-${f}_{\ensuremath{\pi}}$ change as the ${\mathrm{Mn}}^{2+}$-${\mathrm{F}}^{\mathrm{\ensuremath{-}}}$ distance R changes from 1.90 to 2.32 A\r{}, as ${R}^{\mathrm{\ensuremath{-}}8.2}$ and ${R}^{\mathrm{\ensuremath{-}}3.2}$, respectively. This result might be helpful in future determinations of these parameters under high pressure. Several contributions to the covalency of the ${\mathrm{Mn}}^{2+}$-${\mathrm{F}}^{\mathrm{\ensuremath{-}}}$ bond, including the 3d-orbital deformation, the action of the empty 4s and 4p metallic orbitals, and the effects of the crystal lattice, have been analyzed from the HFR wave function. Calculations show that, these three contributions being small, the (${\mathrm{MnF}}_{6}$${)}^{4\mathrm{\ensuremath{-}}}$ complex is highly ionic and its ${A}_{s}$ and ${A}_{p}$ superhyperfine constants behave as local observables, i.e., changes in their observed values from crystal to crystal are mainly determined by changes in the ${\mathrm{Mn}}^{2+}$-${\mathrm{F}}^{\mathrm{\ensuremath{-}}}$ distance.The traditional analysis of the superhyperfine tensor directed to obtain empirical covalency parameters has also been reexamined. Quantitative evaluation of the often-neglected metal-ligand terms and fluoride relaxation accompanying impurity substitution has shown that these two factors play a key role in determining the anisotropic covalency parameters from magnetic-resonance data. From electron-nuclear double-resonance measurements in cubic fluoroperovskites doped with ${\mathrm{Mn}}^{2+}$, we have found ${f}_{p}$=0.60\ifmmode\pm\else\textpm\fi{}0.20 %. The new empirical values of ${f}_{p}$ derived in this work for ${\mathrm{Mn}}^{2+}$:${\mathrm{KZnF}}_{3}$, ${\mathrm{Mn}}^{2+}$:${\mathrm{RbCdF}}_{3}$, and ${\mathrm{Mn}}^{2+}$:${\mathrm{CsCaF}}_{3}$ do not show a definite trend when R changes. More experimental work is needed to determine the nature of this variation.