Pressure driven polymorphic transitions in nanocrystalline Lu₂O₃, Tm₂O₃ and Eu₂O₃

The crystallite size of the materials considerably influences the material properties, including their compressibility and resistance to external forces and the stability of the crystalline structure; a corresponding study for which, so far, has been limited for the important class of nanocrystalline Rare Earth Sesquioxides (REOs). In the present study, we report the crystallographic structural transitions in nanocrystalline Rare Earth Oxides (REOs) under the influence of pressure, investigated via high-energy X-Ray Diffraction (XRD) measurements. The study has been carried out on three of the REOs, namely Lutetium oxide (Lu₂O₃), Thulium oxide (Tm₂O₃) and Europium oxide (Eu₂O₃) up to the pressures of 33, 22 and 11 GPa, respectively. The diffraction data of Lu₂O₃ and Tm₂O₃ suggests the occurrence of irreversible structural transitions from cubic to monoclinic phase, while Eu₂O₃ showed a transition from the cubic to hexagonal phase. The transitions were found to be accompanied by a collapse in the volume and the resulting Pressure-Volume (P-V) graphs are fitted with the 3rd order Birch-Murnaghan (BM) equation of state (EOS) to estimate the bulk moduli and their pressure derivatives. Our study establishes a qualitative relationship between the crystallite size and various material properties such as the lattice parameters, transition pressure, bulk modulus etc., and strengthens the knowledge regarding the behaviour of this technologically important class of materials.