Structural transformation of iron hydroxide oxide nanosolids under high pressure

In this study, β-FeOOH nanosolids were produced by compacting β-FeOOH nanoparticles (average size is 12 nm) synthesized by the method of chemical hydrolysis. The X-ray diffraction measurements show that the structure and crystallite size of β-FeOOH nanosolids do not change under high pressure up to 4.5 GPa, but differential thermal analysis indicates that the temperature of structural transformation increases strongly with increasing pressure, from 203.8 °C under atmosphere pressure to 274 °C under the pressure of 4.5 GPa. After treatment under hot pressure, the structure of the β-FeOOH nanosolids changed significantly. We observed for the first time the transformation from the β-FeOOH phase to the α-FeOOH phase under hot pressure of 3.0 GPa, 200 °C and 4.5 GPa, 350 °C and obtained a new metastable phase of FeOOH (tetragonal sys., a = 0.98968 nm, c = 1.02167 nm) under hot pressure of 4.5 GPa, 200 °C.