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

TL;DR

This study uses high-resolution X-ray diffraction on a single crystal to investigate the spin state crossover in (MgFe)O under high pressure, revealing subtle structural changes and defect formation associated with the transition.

Contribution

It introduces a novel high-resolution X-ray diffraction method to analyze spin state crossover and microscopic disorder in (MgFe)O at high pressure.

Findings

Detection of subtle changes in Bragg spot shape correlated with spin state abundance

Observation of structural defect formation during the HS-LS transition

Identification of static defects accumulated during the transformation

Abstract

(MgFe)O is a solid solution with ferrous iron undergoing the high to low spin state (HS-LS) crossover under high pressure. The exact state of the material in the region of the crossover is still a mystery, as domains with different spin states may coexist over a wide pressure range without changing the crystal structure neither from the symmetry nor from the atomic positions point of view. At the conditions of the crossover, (MgFe)O is a special type of microscopic disorder system. We explore the influences of (a) stress-strain relations in a diamond anvil cell, (b) time relaxation processes, and (c) the crossover itself on the characteristic features of a single crystal (111) Bragg spot before, during and after the transformation. Using high resolution X-ray diffraction as a novel method for studies of unconventional processes at the conditions of suppressed diffusion, we detect and discuss subtle changes of the (111) Bragg spot projections which we measure and analyze as a function of pressure. We report changes of the spot shape which can be correlated with the HS-LS relative abundance. In addition, we report the formation of structural defects as an intrinsic material response. These static defects are accumulated during transformation of the material from HS to LS.