Comment on “Spin crossover in (Mg,Fe)O: A Mössbauer effect study with an alternative interpretation of x-ray emission spectroscopy data”

Abstract

Electronic spin-pairing transition of iron in ferropericlase-(Mg,Fe)O has been recently studied with x-ray emission and M\"ossbauer spectroscopies under high pressures. While these studies reported a high-spin to low-spin transition of iron to occur at pressures above approximately $50\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$, the width of the observed transition varies significantly. In particular, Kantor et al. [Phys. Rev. B 73, 100101(R) (2006)] reported that the transition in $({\mathrm{Mg}}_{0.8},{\mathrm{Fe}}_{0.2})\mathrm{O}$ occurs over a pressure range of approximately $50\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$ in high-pressure M\"ossbauer measurements. To account for the discrepancy in the transition pressure, Kantor et al. reanalyzed the x-ray emission spectra by Lin et al. [Nature (London) 436, 377 (2005)] using a simple spectral decomposition method and claimed that x-ray emission measurements are also consistent with a spin crossover of iron at high pressures. Here, we show that the proposed fitting method is inadequate to describe the x-ray emission spectrum of the low-spin $\mathrm{Fe}{\mathrm{S}}_{2}$ and would result in an erroneous satellite peak $(K{\ensuremath{\beta}}^{\ensuremath{'}})$ intensity, leading to an artificial high-spin component and, consequently, to invalid conclusions regarding the width of the pressure-induced transition in ferropericlase. Furthermore, we show that an energy decrease of $\ensuremath{\sim}1.6\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ in the $K{\ensuremath{\beta}}_{1,3}$ main peak was observed across the spin transition in ferropericlase, which can be used as an additional line of evidence for the electronic spin-pairing transition of iron in (Mg,Fe)O.