Pressure-induced phase transitions in nanocrystalline ReO<SUB>3</SUB>

Pressure-induced phase transitions in the nanocrystals of ReO3 with an average diameter of ~12 nm have been investigated in detail by using synchrotron x-ray diffraction and the results compared with the literature data of bulk samples of ReO3. The study shows that the ambient-pressure cubic I phase (space group Pm3&#x0305;m ) transforms to a monoclinic phase (space group C 2/c), then to a rhombohedral I phase (space group R3&#x0305;c ), and finally to another rhombohedral phase (rhombohedral II, space group R3&#x0305;c ) with increasing pressure over the 0.0-20.3 GPa range. The cubic I to monoclinic transition is associated with the largest volume change (~5%), indicative of a reconstructive transition. The transition pressures are generally lower than those known for bulk ReO3. The cubic II (Im3&#x0305;) or tetragonal (P4/mbm) phases do not occur at lower pressures. The nanocrystals are found to be more compressible than bulk ReO3. On decompression to ambient pressure, the structure does not revert back to the cubic I structure