5:15 PM - 6:45 PM
[SIT14-P22] High pressure phase transitions and the equations of state of BaO
Keywords:B1-B2 phase transition, Equations of state, Diamond anvil cell
Terrestrial exoplanets with 1-10 times Earth's mass are called Super-Earths. In deep mantle of them, the perovskite phase of CaSiO3 decomposes into the B2-type CaO and pyrite-type SiO2, and the B2-type CaO is expected to metallize above 400 GPa based on first principles calculation (Tsuchiya & Tsuchiya, 2011). In the case of alkaline earth metal oxides, the B1-B2 phase transition pressure and metallization pressure are expected to decrease from the top to the bottom of the periodic table (MgO→CaO→SrO→BaO), and the B1-B2 phase transition pressures of CaO and SrO have been experimentally confirmed (Yamanaka et al., 2002, Sato & Jeanloz, 1981). Although metallization at high pressure has not been experimentally confirmed, ab-initio calculations predict that barium oxide (BaO) will metallize at 230.1 GPa (0 K), the lowest pressure of all alkaline earth metal oxides (Uludoğan et al., 2001). In high pressure experiments of BaO using a diamond anvil cell (DAC), the B8 and distorted B2(d-B2) phases were observed in addition to the B1 phase, which is stable under ambient pressure (Weir et al., 1986). Theoretical calculations predict a phase transition from d-B2 phase to the B2 phase at 120 GPa, and an equation of state for the B2 phase has been proposed up to 150 GPa, but no equation of state (EoS) has been proposed for the pressure range where metallization is expected (Lavanya et al., 2022).
In this study, we conducted static compression experiments of BaO using DAC to confirm the B2 phase and to determine the EoS for the d-B2 and B2 phase. X-ray diffraction experiments were conducted at BL10XU of SPring-8 using a membrane-type DAC with culet diameters of 100 μm and 300 μm and a symmetric-type DAC with diameters of 300 μm and 450 μm. The pressure was determined by the EoS of gold (Fratanduono et al., 2021), the ruby fluorescence (Shen et al., 2020), and the diamond raman gauge (Akahama & Kawamura, 2004). In each experiment, sample was pressurized to 187.0 GPa, 81.0 GPa, 70.3 GPa, and 34.1 GPa, respectively. The B2 phase was confirmed above 132 GPa, but the sample was transparent even at 187.0 GPa, which means that metallization does not occur in the pressure range of this study. Phase transition pressures were 12 GPa (B1-B8), 18 GPa (B8 to d-B2), and 132 GPa (d-B2 to B2), respectively. We analyze uniaxial stress using the gamma-plot, and confirmed the B2 phase was in a quasi-hydrostatic environment. We fitted the pressure-volume data of B2 phase with the third-order Birch-Murnaghan EoS and obtained K0=86±3 GPa and K'=4.58±0.12 (During fitting, V0 is fixed to the value of previous theoretical calculations, V0=36.2 Å3). According to the EoS determined in this study, the volume to metallization predicted by Uludoğan et al. (2001) is reached at 261 GPa.
In this study, we conducted static compression experiments of BaO using DAC to confirm the B2 phase and to determine the EoS for the d-B2 and B2 phase. X-ray diffraction experiments were conducted at BL10XU of SPring-8 using a membrane-type DAC with culet diameters of 100 μm and 300 μm and a symmetric-type DAC with diameters of 300 μm and 450 μm. The pressure was determined by the EoS of gold (Fratanduono et al., 2021), the ruby fluorescence (Shen et al., 2020), and the diamond raman gauge (Akahama & Kawamura, 2004). In each experiment, sample was pressurized to 187.0 GPa, 81.0 GPa, 70.3 GPa, and 34.1 GPa, respectively. The B2 phase was confirmed above 132 GPa, but the sample was transparent even at 187.0 GPa, which means that metallization does not occur in the pressure range of this study. Phase transition pressures were 12 GPa (B1-B8), 18 GPa (B8 to d-B2), and 132 GPa (d-B2 to B2), respectively. We analyze uniaxial stress using the gamma-plot, and confirmed the B2 phase was in a quasi-hydrostatic environment. We fitted the pressure-volume data of B2 phase with the third-order Birch-Murnaghan EoS and obtained K0=86±3 GPa and K'=4.58±0.12 (During fitting, V0 is fixed to the value of previous theoretical calculations, V0=36.2 Å3). According to the EoS determined in this study, the volume to metallization predicted by Uludoğan et al. (2001) is reached at 261 GPa.