Revisiting spin-state crossover in (MgFe)O by means of high-resolution x-ray diffraction from a single crystal

by Konstantin Glazyrin (DESY)




(MgFe)O (xFe~0.2) is a second most abundant material in the Earth’s lower mantle. Under compression ferrous iron in this compound undergoes a spin state crossover, a phenomenon redistributing electrons on a single iron site between different orbitals. An insignificant microscopic phenomenon leads to major effects at planetary conditions affecting elastic, electrical properties and even chemistry of the lower mantle (e.g. iron partitioning). This solid state solution system is characterized with an intrinsic disorder of iron, and a picture of the disorder becomes even more complicated in the pressure region of spin state crossover introducing competition of high (HS) and low spin (LS) states of iron. Spin state crossover at high pressure affects bulk properties of the material, but the origin for such changes originates on the nanoscale. The microscopic processes taking place during the crossover were not explored in a great detail.

In this study we use X-ray diffraction at high resolution and investigate changes of single crystal Bragg peak (111) under compression. We report changes in the Bragg spot size which can be correlated with the HS-LS relative abundance, which is the first experimental observation confirming the presence of enhanced disorder correlated with HS/LS competition. In addition, we report the formation of structural defects in a single crystal as a material response to the HS-LS crossover trying to compensate strain of HS/LS competition in a form of static defects, a new process which has not been given attention before.