High-pressure behavior of Fe₂O₃

High pressure behavior of Fe₂O₃ has been a long-standing subject of research due to its high importance for understanding Earth's interiors. At pressures from 40 to 60 GPa it undergoes a series of transformations, such as structural changes with a large volume discontinuity (~10 %), a drop of the resistivity, a spin crossover of Fe³⁺, and a disappearance of the ordered magnetic state. The crystal structure of the phase(s) observed on compression at ambient temperature above 50 GPa is still under question since only powder X-ray diffraction (XRD) data were available so far. Mössbauer and Raman spectroscopy studies cannot provide definitive structural information. Applying laser heating to Fe₂O₃, compressed up to 70 GPa and above, results in a distinct reconstructive phase transition to the CaIrO₃-type structure, according to powder XRD. Poverty of the available structural data encouraged us to perform a series of high-pressure and high-temperature XRD experiments on single crystals of Fe₂O₃ in diamond anvil cells. We have studied the behavior of Fe₂O₃ at pressures up to 100 GPa and temperatures up to 2500 K. Here we report crystal structures of two novel high-pressure Fe₂O₃ polymorphs, as well as the relations between a spin state of iron atoms and the crystal chemistry of the iron compound. In our compression experiments initially hematite-structured Fe₂O₃ transformed to a new phase at ~54 GPa with 10 % of the volume reduction. This phase has a triclinic distorted perovskite-type structure. The second reconstructive transition occurred at 66-70 GPa with 3 % of the volume discontinuity and resulted in formation of an orthorhombic phase. Laser heating to ~2100¹100 K at pressures above 70 GPa promoted a transition to a Cmcm CaIrO₃-type phase, whose crystal structure was refined by means of single crystal XRD to R₁ ~ 9.7 %. Decompression experiments showed that the Cmcm phase transforms back to hematite at pressures between ~25 and 15 GPa.

Nanocrystalline diamond (NCD): an insight into structure-property relationships

Nanocrystalline diamond (NCD) is a unique material we produce by direct conversion of glassy carbon into diamond at ca. 20 GPa and 2200 K in a multi anvil press. One of precursor materials we use is commercially available in the form of glassy carbon balls with a diameter of 20 to 50 microns. NCD demonstrates superior mechanical properties (e.g. extremely high yield strength under confining pressure) and has been successfully used for ultra-high static pressure generation (above 600 GPa) in a double-stage diamond anvil cell (DAC) (Ref. 1). To elucidate structure-property relationships in this extremely strong and seemingly inscrutable material we have investigated its microstructure using HRTEM and HAADF-STEM, measured its compressibility by means of synchrotron X-ray diffraction in a DAC, and evaluated its hardness in comparison to that of the hardest known materials - single-crystal diamond and nano-polycrystalline diamond (NPD) (Ref. 2). An additional insight into the volume compressibility was obtained due to X-ray phase contrast micro-imaging using a coherent high-energy synchrotron radiation. The established structure-property relationships will be presented and analyzed.