High-pressure x-ray diffraction and Raman spectroscopy of phase transitions in Sm_2O_3

摘要

The structural phase transitions in Sm_2O_3 with mixed phases of cubic and monoclinic as starting material were studied by in situ high-pressure synchrotron angle dispersive x-ray diffraction and Raman scattering measurements up to 40.1 GPa and 41.0 GPa, respectively. The x-ray diffraction data indicate that the monoclinic and cubic phases begin to transform to a hexagonal phase at 2.5 and 4.2 GPa, respectively. The hexagonal phase is stable up to at least 40.1 GPa and could not be quenched to ambient conditions. These phase transitions have also been confirmed by Raman spectroscopy. A third-order Birch-Murnaghan fit based on the observed pressure-volume data yields zero pressure bulk moduli B_0= 149(2), 153(7), and 155(5) GPa for cubic, monoclinic, and hexagonal phases, respectively, when their first pressure derivatives (B'_0) were fixed as 4. The pressure coefficients of Raman peaks and the mode Griineisen parameters of different Raman modes were also obtained. Coupled with previous results, we conclude that the transition pressure of medium rare-earth sesquioxides from the cubic and monoclinic to the hexagonal phase increase with the decreasing of the cation radius.