Critical behavior of $\mathrm{Mg_{1–x}Fe_{x}O}$ at the pressure-induced iron spin-state crossover

We present a high-pressure study of Mg$_{1–x}$Fe$_{x}$O (ferropericlase, Fp) single crystals 0.04(1)$\leqslant x \leqslant$ 0.33(1) with a focus on ferrous iron spin-state crossover and the material behavior preceding it. Using Vegard's law and highly accurate high-pressure single-crystal experimental data, we extract the lattice parameter $a^{HS}_{FeO}$ of the “FeO high spin lattice contribution” in the MgO-FeO solid solution. We find that ferropericlases with a wide range of compositions share the same critical parameter $a_C$ (defined as the minimum $a^{HS}_{FeO}$ after which spin crossover starts). Furthermore, we discuss the effect of composition on spin crossover in ferrous iron in ferropericlase in the limits of low and moderate concentrations of Fe$^{2+}$