11:15 〜 11:30
[SMP26-03] β-CrOODの水素結合配置の温度圧力依存性
キーワード：中性子回折、含水鉱物、高圧、水素結合
The Earth’s mantle contains a certain amount of high-pressure hydrous minerals. However, behaviors of hydrogen bonding and physical properties of such hydrous minerals at high-PT conditions have not been clarified. We focused on hydrogen-bond geometry and compression behavior of guyanaite (β-CrOOH). β-CrOOH is a distorted rutile-type hydrous mineral and isostructural to δ-AlOOH, one of the most promising hydrous phases for transporting hydrogen to the core–mantle boundary. In this study, X-ray and neutron diffraction experiments on β-CrOOD were carried out at various P-T conditions to clarify the P-T dependence of hydrogen-bond geometry in β-CrOOD. Crystal structure was refined using the Rietveld method.
The space group of β-CrOOD was determined to be P21/c, indicating that the hydrogen atoms are partially disordered at ambient condition. Above 450 K, however, the hydrogen atoms were fully disordered, and the space group was Pnnm. Thermal expansion anomaly was observed at temperature range of 300–500 K, where the phase transition (P21/c – Pnnm) occurred. In high-pressure experiments, changes in compression behavior caused by phase transition from P21/c to Pnnm were observed at about 3 GPa and 4.5 GPa at 300 K and 250 K, respectively. However, no significant change in compression behavior was observed at 450 K. Therefore, dP/dT of the phase boundary would have a negative slope. Assuming that the phase boundary of δ-AlOOH (P21nm–Pnnm) has the same tendency as that of β-CrOOD, δ-AlOOH would have a Pnnm structure in the Earth’s mantle and would have higher seismic velocity than the major minerals in the mantle transition zone.
The space group of β-CrOOD was determined to be P21/c, indicating that the hydrogen atoms are partially disordered at ambient condition. Above 450 K, however, the hydrogen atoms were fully disordered, and the space group was Pnnm. Thermal expansion anomaly was observed at temperature range of 300–500 K, where the phase transition (P21/c – Pnnm) occurred. In high-pressure experiments, changes in compression behavior caused by phase transition from P21/c to Pnnm were observed at about 3 GPa and 4.5 GPa at 300 K and 250 K, respectively. However, no significant change in compression behavior was observed at 450 K. Therefore, dP/dT of the phase boundary would have a negative slope. Assuming that the phase boundary of δ-AlOOH (P21nm–Pnnm) has the same tendency as that of β-CrOOD, δ-AlOOH would have a Pnnm structure in the Earth’s mantle and would have higher seismic velocity than the major minerals in the mantle transition zone.