The effects of 57Fe enrichment and count rates on NRIXS-derived force constants

Abstract

Force constant measurements employing NRIXS (nuclear resonant inelastic X-ray scattering) have seen increasing applications in isotope geochemistry, as it bypasses the need for equilibration experiments. This synchrotron technique measures the strength of chemical bonds, which are represented by force constants <F>. The equilibrium isotope fractionation between any two materials is directly proportional to their difference in <F>. While this method is promising, it is restricted to measuring Mössbauer-sensitive isotopes such as 57Fe. To expedite analyses, samples are typically doped with 57Fe, and it has not yet been tested whether this enrichment affects the measured <F>. In this study, we systematically tested the effect of 57Fe enrichment to determine whether heavy isotope enrichment has a resolvable effect on <F>. We enriched wüstite, fayalite, and bcc-Fe samples with varying 57Fe/ΣFe and measured their <F> with NRIXS. Results show that while high levels of 57Fe enrichment do not affect <F>, too little 57Fe is a concern. We establish a cut-off of 50 cps at the highest non-resonant peak to yield robust NRIXS results and recommend exercising caution while using NRIXS to measure <F> on natural samples. Force constants obtained for bcc-Fe, wüstite (Fe0.94O), and fayalite are 182 ± 6 N/m, 162 ± 7 N/m, 175 ± 8 N/m, respectively.