Structural Evolution of Yb2O3 Under High Pressure and Phase Transition Enthalpies

Abstract

The high-pressure structural evolution of cubic Yb2O3 has been studied using in situ synchrotron angle dispersive X-ray diffraction (AD-XRD) in combination with diamond anvil cell techniques up to 44.1 GPa. The XRD measurements revealed an irreversible reconstructive phase transition from cubic to the monoclinic structure at 11.2 GPa and extending up to 28.1 GPa with ~8.1% volume collapse, and a subsequent reversible displacive transition from monoclinic to hexagonal phase starting at 22.7 GPa. The monoclinic phase coexists with the hexagonal phase up to 44.1 GPa. After pressure release, the hexagonal Yb2O3 reverts to the monoclinic structure. The third-order Birch-Murnaghan equation of state fit to the pressure-volume data yields a bulk modulus of 201(4), 187(6) and 200(4) GPa for the cubic, monoclinic and hexagonal phases, respectively, when their first pressure derivatives (B0ʹ) are fixed at 4. Combining the results of this work with previous studies on rare earth sesquioxides (Ln2O3) at high pressures, it can be concluded that the onset transition pressure as well as the bulk modulus of the cubic Ln2O3 phase show an essentially linear increase with decreasing cation radius due to lanthanide contraction. In addition, based on the fundamental thermodynamic equations, the enthalpies of the cubic to monoclinic and monoclinic to hexagonal phase transitions of Yb2O3 at ambient conditions were determined to be 37.0 and 17.4 KJ/mol, respectively, using the onset transition pressures and corresponding volume changes obtained from high-pressure XRD experiments.

Keywords: Rare earth sesquioxides, phase transition, High pressure, X-ray Diffraction, Enthalpy change, synchrotron radiation