Intervalence Electron Transfer and Magnetic Exchange in Reduced Nontronite

Abstract

The effects of chemical reduction of structural Fe³⁺ in nontronite SWa-1 (ferruginous smectite) on intervalence electron transfer (IT) and magnetic exchange were investigated. Visible absorption spectra in the region 800-400 nm of a chemical reduction series of the SWa-1 nontronite revealed an IT band near 730 nm (13,700 cm⁻¹). Both the intensity and position of this band were affected by the extent of Fe reduction. The intensity increased until the Fe²⁺ content approached 40% of the total Fe, then decreased slightly with more Fe²⁺. The position of the band also shifted to lower energy as the extent of reduction increased.

Variable-temperature magnetic susceptibility measurements showed that the magnetic exchange in unaltered nontronite is frustrated antiferromagnetic, but ferromagnetic in reduced samples. Magnetic ordering temperatures are in the range 10–50 K, depending on the extent of reduction. The ferromagnetic component in the magnetization curve increased with increasing Fe²⁺ in the crystal structure. The positive paramagnetic interaction likely is due to electron charge transfer from Fe²⁺ to Fe³⁺ through such structural linkages as Fe²⁺-O-Fe³⁺ (perhaps following a double exchange mechanism), which is consistent with the visible absorption spectra.