Ilvaite: A study of temperature dependent electron delocalization by the mössbauer effect |
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Authors: | D A Nolet R G Burns |
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Institution: | 1. Department of Earth and Planetary Sciences, Massachusetts Institute of Technology, 02139, Cambridge, Massachusetts, USA
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Abstract: | The mixed valence iron silicate ilvaite, CaFe 2 2+ Fe3+Si2O8(OH), displays electron delocalization associated with Fe2+→Fe3+ charge transfer as observed by Mössbauer spectroscopy. Previous studies report the observation of an ‘electron hopping phenomenon’ with resolution of discrete valence states below 320 K. Mössbauer spectra of a suite of naturally occurring ilvaites were recorded over a temperature range, 80 K to 575 K. Five quadrupole doublets were resolved by computer fitting and assigned to Fe2+(A), Fe2+(B), Fe3+(A), and Fe2+(A)→Fe3+(A)‖c and ⊥c. Contrary to prior work, doublets associated with electron delocalization are resolved at 80 K and preclude the use of a Verwey-type order-disorder model. We propose a thermal activation model and discuss its criteria from molecular orbital and mineralogical viewpoints. |
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