TEMPERATURE-DEPENDENT ELECTRON DELOCALIZATION IN (MG,FE)SIO3 PEROVSKITE

The oxidation state and location of Fe in the structure of two Fe-bear ing silicate perovskite samples, with compositions Mg0.95Fe0.05SiO3 an d Mg0.90Fe0.10SiO3, have been studied with Fe-57 Mossbauer spectroscop ic techniques over the temperature range 30-450 K. At low temperatures , the resonant absorption spectra consist of a broad absorption envelo pe due to discrete Fe2+ valencies. The line shape of this envelope is the sum of contributions from different local electronic environments. Superimposed on the Fe2+ absorption envelope is a partially resolved, narrow, quadrupole split doublet due to discrete Fe3+ valencies. The molar Fe3+/Fe(tot) is about 0.12 (+/- 0.02) for both samples. At tempe ratures above 180 and 77 K for the Mg0.95Fe0.05SiO3 and Mg0.90Fe0.10Si O3 compositions, respectively, there is a systematic increase in the r elative intensity of an additional broad absorption pattern with inter mediate hyperfine parameters between those of Fe2+ and Fe3+. This broa d absorption is attributed to thermally activated Fe2+-Fe3+ electron d elocalization. The hyperfine parameters of Fe3+ are consistent with oc tahedral coordination. Fe2+ in the structure is assigned to the distor ted eight- to 12-fold-coordinated polyhedra on the basis of the hyperf ine parameters and bond distances in agreement with the interpretation of the electron delocalization. The electron exchange most likely occ urs between face-sharing Fe3+-bearing octahedra and distorted Fe2+-bea ring polyhedra in the perovskite structure. These crystallographically distinct site have the shortest M-M distances. The activation energy of Fe2+-Fe3+ exchange processes in silicates and oxides is compatible with the result expected from a mechanism associated with electrical c onduction in perovskite. An electron-hopping mechanism may thus be imp ortant in explaining the conductivity profile determined from geomagne tic data.