Structural transformations in cubic Dy2O3 at high pressures

• Dy2O3 undergoes successive C→B→A phase transitions on compression.
• The compressibility for B and A phases of Dy2O3 is strongly anisotropic.
• The irreversible C→B phase transition is due to reconstructive nature.
• A possible phase transition mechanism for Dy2O3 is proposed.
• The C→B transition pressure of Re2O3 increases with the decreasing cation size.

The structural stability of cubic Dy2O3 under high pressure has been investigated using synchrotron radiation in a diamond anvil cell up to 49.0 GPa. The diffraction data reveals the cubic phase undergoes two successive phase transitions on compression. The phase transition from a cubic to a monoclinic structure starts at 7.7 GPa and is complete at 18.8 GPa with a ~7.9% volume collapse. The monoclinic phase further transforms to a hexagonal phase starting at ~10.9 GPa and the hexagonal phase becomes dominant at 26.6 GPa. This high-pressure hexagonal phase with a small amount of retained monoclinic phase is stable up to the highest pressure of 49.0 GPa in this study. After pressure release, Dy2O3 is a monoclinic structure. A third-order Birch–Murnaghan fit yields zero pressure bulk moduli (B0) of 191(4), 179(9) and 231(22) GPa and their pressure derivatives (B0′) of 2.8(7), 4.2(6), 3.5(6) for the cubic, monoclinic and hexagonal phases, respectively. Comparing with other rare-earth sesquioxides, we confirm that the transition pressure from cubic to monoclinic phase, as well as the bulk modulus of the cubic phase, increases with the decreasing of the cation radius of rare-earth sesquioxides.