Diffraction intensity contrast maps for Fe0.94O measured at 24.7 GPa. From Yang Ding et al., Appl. Phys.Lett. 100, 041903 (2012).

(PhysOrg.com) -- The transition-metal monoxide FeO is an archetypal example of a Mott insulatora material that should conduct electricity under conventional band theories but becomes an insulator when measured, especially at low temperaturesand a major iron-bearing component of the Earths interior. Understanding the high-pressure behavior of this material is important for both solid-state physics and Earth science. But despite considerable study over the past 30 years, the origin of the well-known high-pressure-induced cubic-rhombohedral ferroic transition in FeO, which is a distortion of the original cubic structure to that of as rhomboid shape, has been not well understood.

Now the first imaging of non-reflection domain wall structures forming in the ferroic transition of the ferrous oxide Fe0.94O has been reported by researchers from the U.S. Department of Energy Office of Sciences Advanced Photon Source (APS) and the High Pressure Synergetic Consortium (HPSynC). The team carried out the study at the 2-ID-D x-ray beamline at the APS, applying a pressure of approximately 250,000 atm in a pressure vessel called a diamond anvil cell, and imaging the materials crystalline structure using the new high-pressure nanodiffraction imaging technique developed by these researchers.

The teams results revealed a non-reflection type of domain wall structure forming due to the so-called cubic-rhombohedral transition, where the crystal structure of the material changes from cubic to rhomboidal. This discovery suggests the cubic-rhomboid transition could be associated with defects in the material and is unlikely to be caused by ferroelasticity, in which a material may develop a spontaneous strain, as predicated by previous research.

The surprising impact of defects on structural stability discovered by this study not only brings with it a new understanding of the origin of the cubic-rhomboid transition, but also underscores the need for a greater understanding of how defects in a material influences electronic and thermoelastic properties at high pressure, which has almost never been taken into consideration in previous high-pressure studies of materials.

In addition, this study demonstrates the power of the new nanodiffraction imaging technique for investigation of pressure-induced phase transitions, which has emerged as a very active area in condensed matter physics, but until now has lacked suitable in situ techniques for probing the nanoscopic origins of the transitions.

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More information: Zhonghou Cai, et al. Nanoscale diffraction imaging of the high-pressure transition in Fe1-xO, Appl. Phys.Lett. 100, 041903 (2012). Zhonghou Cai, et al. Nanoscale diffraction imaging of the high-pressure transition in Fe1-xO,100, 041903 (2012). DOI: 10.1063/1.3679117