SPECTRA OF $MnF_{6}^{=}$ IN ENVIRONMENTS OF $O_{h}$ AND $D_{3d}$ SYMMETRY

Abstract

The spectra of $MnF_{6}^{=}$ afford a favorable opportunity for the stuy of the electronic states of a $d^{3}$ complex ion because of their sharpness and resolution and the correlation possible between infrared, ESR, visible, and near ultraviolet measurements. The ESR spectra indicate considerable covalent character of the bonds; the interatomic distances $-1.73$ {\AA} in $MnF_{6}^{=}$ and 1.93 {\AA} in $CrF_{6}^\equiv-$ support this view. The appearance in the optical spectra of sharp absorption and emissoin lines corresponding to the three u vibrations of an octahedral molecule show that $MnF_{6}^{=}$ should be considered an individual molecule and not simply $Mn^{4+}$ surrounded by six fluoride ions which may be shared with other cations. The $^{4}A_{2_{g}}\rightarrow ^{2}E_{g}$ and $^{4}A_{2_{g}}\rightarrow ^{2}T_{1_{g}}$ transitions in the region 16 000 $cm^{-2}$ to 17 000 $cm^{-1}$ can be simply interpreted. The polarized spectra in the case of the hexagonal environmental permit the identification of magnetic dipole transitions. The $0.7 cm^{-1}$ splitting of the ground state in the hexagonal symmery is seen in these spectra. The $^{4}A_{2_{g}}\rightarrow ^{2}T_{2_{g}}$ transition, observed at $4^{\circ} K$ , in the region 20 500 $\rightarrow$ 24 000 $cm^{-2}$ shows, sharp (0.5 to $1.0 cm^{-1}$ half width) magnetic dipole lines and also sharp vibronic lines, approximately $10^{3}$ times as intense. The assignments in this case are questionable. Our favored interpretation is based on a greatly reduced spin-orbit interaction, molecular and lattice vibrations and combinations of these. The effect of the environment seems due less to a distortion of the octahedron than to the changes around the octahedron which affect the vibrational frequencies markedly.