The crystal chemistry of ferric iron in Fe_0.05Mg_0.95SiO₃ perovskite as determined by Mössbauer spectroscopy in the temperature range 80–293 K

Abstract 

Mössbauer spectra were recorded at multiple temperatures between 80 and 293 K to study the nature of Fe³⁺ in Fe_0.05Mg_0.95SiO₃ perovskite that had been synthesised in a multianvil press at 1650 °C and 25 GPa at its mimimum stability limit. The Mössbauer data were fitted to a model with quadrupole splitting distributions (Fe²⁺) and Lorentzian lineshapes (Fe³⁺ and Feⁿ⁺). The centre shift data were fitted to a Debye model with the following results: Θ_M (Fe²⁺)=365±52 K and Θ_M (Fe³⁺)=476±96 K. Hyperfine parameter data for Fe³⁺ suggest occupation of the octahedral site only. The average valence seen by the Mössbauer effect in rapid electron exchange that occurs between Fe²⁺ and Fe³⁺ is calculated from the hyperfine parameters to be 2.50±0.07. Correction of area fractions for site-dependent recoil-free fractions gives a value for Fe³⁺/∑Fe of 9.4±1.4%, which is independent of temperature. A perovskite phase of similar composition synthesised in the multianvil press at higher oxygen fugacity gives a value for Fe³⁺/∑Fe of 16±3%, where Fe³⁺ appears to occupy both sites in the perovskite structure.