The impact of local ordering on physical properties in Fe-Mg solid solutions

Abstract We investigate the influence of short-range Mg–Fe cation ordering on the elastic and vibrational properties of olivine (Fe x x Mg 1- x ) 2 SiO 4 (0 < x < 1) and ferropericlase (Fe x x Mg 1- x )O (0 < x < 1) solid solutions using atomistic simulations based on empirical force fields. Supercells representing distinct local distributions of Mg and Fe cations around equiatomic composition (Fe:Mg 50:50, x 0.5) were generated and structurally relaxed. Short range order states include cation clustering, random cation ordering and preferred heteroatom paring. In both solid solution series, bulk physical properties, such as elastic moduli and seismic velocities, are primarily controlled by overall composition rather than the specific local cation arrangement. However, vibrational properties such as heat capacity and the phonon dispersion curves reveal a stronger sensitivity to short-range order. This sensitivity is enhanced in structurally simpler ferropericlase, while the increased structural complexity of olivine suppresses these differences.