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 KMnₓNi_1-xF₃ and KZn_1-x-yMnₓNiyF₃ show an intense ultraviolet band (greater than 50 000 cm^-1) associated with electron transfer from nickel to manganese. Polarization data from BaMnNi_{{{}}}F₄ crystal spectra support this conclusion. Second, an electron-transfer band has been observed for the pair Cu - F - Mn in KZnF₃ crystals. The band is centered near 41 500 cm^-1, 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 Mn²⁺(3d⁵), Cu²⁺(3d⁹)→Mn¹⁺(3d⁵4s), Cu³⁺(3d⁸). 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 ⁴A1g state provides an explanation for the observed ferromagnetic coupling of this state with Cu²⁺.