The effects of chemical composition on electron delocalization and magnetic ordering in ilvaite, Ca[Fe2+, Fe3+][Fe2+]Si2O7O(OH)Ca[Fe2+,Fe3+][Fe2+]Si2O7O(OH)

Abstract

Substitutional impurities are expected to have significant influences on long-range, order–disorder phenomena associated with magnetic ordering and electron delocalization in mixed-valence solids. These influences have been investigated by means of superconducting quantum interference device susceptometry and 57Fe57Fe Mössbauer spectroscopy on more than 40 mineral specimens of the mixed-valence solid ilvaite, Ca[Fe2+, Fe3+][Fe2+]Si2O7O(OH),Ca[Fe2+,Fe3+][Fe2+]Si2O7O(OH), with manganese contents between 0.2 and 10 wt %. At 295 K, the 57Fe57Fe Mössbauer spectra are quite diverse, exhibiting different numbers of lines, varying degrees of resolution and line broadening. The complexity of the spectra correlate well with increasing Mn content. At 160 K, the spectra are remarkably similar and are characterized by three overlapping subspectra. The Isomer shifts are: 1.09, 1.07, and 0.64 mm s−1, respectively, which may be unambiguously assigned to 2 Fe2+2Fe2+ and 1 Fe3+1Fe3+ species. The magnetic ordering transition near 120 K, associated with the ordering of the moments of the Fe2+Fe2+ and Fe3+Fe3+ ions involved in electron delocalization, is not evident in the susceptibility data. Further, the susceptibility versus temperature curves exhibited no qualitative changes with varying Mn content. The magnetic transition associated with the ordering of the magnetic moments of the Fe2+Fe2+ ion, not involved in electron delocalization, is clearly evident in the susceptibility data and exhibits a systematic dependence on Mn content, occurring at 27, 26, 25, and 23 K for Mn contents of 0.23, 0.70, 0.90, and 0.97 wt %. For Mn contents above 1 wt %, this magnetic transition is not observed. © 1999 American Institute of Physics.