Intervalence Electron Transfer and Magnetic Exchange in Reduced Nontronite

Paul R. Lear and Joseph W. Stucki
Department of Agronomy, University of Illinois Urbana, Illinois 61801

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.

Key Words: Intervalence electron transfer • Iron • Magnetic susceptibility • Nontronite • Reduction • UV-visible spectroscopy

Clays and Clay Minerals; October 1987 v. 35; no. 5; p. 373-378; DOI: 10.1346/CCMN.1987.0350507
© 1987, The Clay Minerals Society
Clay Minerals Society (www.clays.org)