Electron-Transfer States of Pairs of Unlike Transition-Metal Ions in Perovskite Fluorides

Experimental evidence is presented for the observation of electron-transfer bands between unlike transition-metal ions. Two examples are considered. First, the spectra of crystals of composition $\mathrm{K}{\mathrm{Mn}}_x{\mathrm{Ni}}_{1-x}{\mathrm{F}}_3$ and $\mathrm{K}{\mathrm{Zn}}_{1-x-y}{\mathrm{Mn}}_x{\mathrm{Ni}}_y{\mathrm{F}}_3$ show an intense ultraviolet band (greater than 50 000 ${\mathrm{cm}}^{-1\mathrm{}}$) associated with electron transfer from nickel to manganese. Polarization data from Ba${\mathrm{Mn}}_{\mathrm{½}}$ ${\mathrm{Ni}}_{\mathrm{½}}$ ${\mathrm{F}}_4$ crystal spectra support this conclusion. Second, an electron-transfer band has been observed for the pair Cu - F - Mn in KZn${\mathrm{F}}_3$ crystals. The band is centered near 41 500 ${\mathrm{cm}}^{-1\mathrm{}}$, it is very broad with a long vibrational progression and it has an absorption intensity which increases on cooling of the crystal. These features can all be explained by the assignment ${\mathrm{Mn}}^{2+}\left(3\mathrm{}{d}^5\right)$, ${\mathrm{Cu}}^{2+}\left(3\mathrm{}{d}^9\right)\to {\mathrm{Mn}}^{1+}\left(3\mathrm{}{d}^{5}4s\right)$, ${\mathrm{Cu}}^{3+}\left(3\mathrm{}{d}^8\right)$. It is concluded that this state determines the antiferromagnetic coupling in the ground state of the pair. A consideration of the possible electron-transfer states available to the pair when the manganese is in its excited ${}_{}{}^4{}_{}{}^{}A_{1g}^{}{}_{}{}^{}$ state provides an explanation for the observed ferromagnetic coupling of this state with ${\mathrm{Cu}}^{2+}$.