Pressure-induced phase transitions in nanocrystalline $ReO_3$

Pressure-induced phase transitions in the nanocrystals of $ReO_3$ with an average diameter of \sim 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 $ReO_3$. The study shows that the ambient-pressure cubic I phase (space group Pm \bar{3}m ) transforms to a monoclinic phase (space group C 2/c), then to a rhombohedral I phase (space group R \bar{3}c ), and finally to another rhombohedral phase (rhombohedral II, space group R \bar{3}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 (\sim 5 \%), indicative of a reconstructive transition. The transition pressures are generally lower than those known for bulk $ReO_3$. The cubic II (Im \bar{3} ) or tetragonal (P4/mbm) phases do not occur at lower pressures. The nanocrystals are found to be more compressible than bulk $ReO_3$. On decompression to ambient pressure, the structure does not revert back to the cubic I structure