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

(MgFe)O is a solid solution with ferrous iron undergoing 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 in terms of symmetry nor in terms of the atomic positions. 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)(hkl) Bragg spot across the transformation. Using high-resolution x-ray diffraction as a method for studies of unconventional processes at the conditions of suppressed diffusion, we detect and discuss subtle changes in the (111)(hkl) Bragg spot projections which we measure and analyze as a function of pressure. We report changes in the spot size which can be correlated with the HS-LS relative abundance. In addition, we report the formation of structural defects in a single crystal as an intrinsic material response to the HS-LS crossover. These static defects are formed and accumulated during the compression within the crossover pressure range.