  (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.
