Spin crossover in ferropericlase: beyond ideal HS-LS ideal solid solution model*

The pressure- and temperature-induced high spin (HS) to low spin (LS) crossover of Fe²⁺ ions in ferropericlase (Fp), Mg_(1-x)Fe_xO, affects mantle properties such as density, elasticity, thermal properties, iron partitioning between Fp and other co-existing phases, etc. It further affects how we interpret lower mantle observations. Here, we present results of thermodynamic properties computed using a free energy model that goes beyond the ideal HS-LS solid solution framework. As in the past, ab initio calculations were performed using the DFT+ U_SC method with structure and electronic configuration dependent U_SC. Results show the influence of iron-iron interactions (elastic or exchange type) on the pressure range of the crossover. Comparison with available experimental data shows considerably improved agreement over that of the ideal solid solution model.