FeO is a fundamentally important material as a member of the transition metal monoxides, which are important for understanding electronic properties such as insulator-metal transitions in condensed matter. Magnesiowustite (Mg,Fe)O, also known as ferropericlase, is one of the key minerals that make up the Earth's lower mantle. The stoichiometric endmember FeO does not exist in nature, compositions close to the 1:1 elemental ratio are common. For this reason, the high pressure behavior of FeO has been the subject of intensive study for nearly 50 years.

In research recently published in Physics and Chemistry of Minerals, CDAC students Pamela Kaercher, and Jane Kanitpanyacharoen, along with their advisor CDAC Partner Hans-Rudolf Wenk from UC Berkeley, former CDAC Student Lowell Miyagi (University of Utah), and researchers from the Geoscience Research Center in Potsdam, Germany, compressed two powdered samples, (Mg_0.08Fe_0.92)O magnesiowüstite and Fe_0.94O wüstite, non-hydrostatically up to ~37 GPa at ambient temperature in a diamond anvil cell (DAC) in order to observe textural evolution as a function of pressure. Under uniaxial stress in the DAC, {100}c planes aligned perpendicular to the compression direction for both (Mg_0.08Fe_0.88)O and Fe_0.94O (Fig. 1), and texture sharpness increased with pressure. Near 19 GPa and room temperature, cubic FeO transitioned to rhombohedral symmetry. Polycrystal plasticity simulations show that deformation in the cubic phase of the (Mg,Fe)O solid solution is due mostly to slip on {110}<1-10>c as previously found for other B1 structures. In the rhombohedral phase, slip system activity changed slightly with one of the daughters of {111}<1-10>c, {10-11}<-12-10>r (inactive in the cubic phase), becoming active.

Texture near the transition pressure suggests that crystals in the cubic phase with their {100}c planes facing the compression direction are preferentially oriented to transform to the rhombohedral phase first. Orientations which developed and strengthened in the rhombohedral phase remained after decompression back to the cubic phase. Upon decompression, a texture similar to that of the cubic phase before the phase transition was observed, suggesting a perfect memory during this displacive transition [P. Kaercher et al., Phys. Chem. Minerals 39, 613-626 (2012)].