Phase relation and equation of state of iron-titanium oxyhydroxide with α-PbO₂-type crystal structure (hydrous α-phase) was investigated at pressure from middle upper mantle to mantle transition zone. The experiments were performed using multi-anvil apparatus with in situ synchrotron X-ray diffraction. The metal-diamond sample container was used to maintain a closed system with respect to water while allowing X-ray transmittance. Starting materials were mixture of reagent grade goethite and anatase with Fe:Ti = 1:1 and 1:3. The hydrous α-phase was synthesized at 11–14 GPa and 800–900°C. The X-ray diffraction data were obtained over a wide range of pressures from 5 to 22 GPa and temperatures from room temperature to 930°C. Using pressure-volume-temperature data of the hydrous α-phase with Fe:Ti = 1:1 collected at room temperature to 800°C and at 8.9–20.8 GPa, we determined the isothermal bulk modulus (K_T0 = 183 (1) GPa) and the thermal expansivity (α₀ = 3.29 (1) × 10⁻⁵K⁻¹) at ambient condition. The stability field of the hydrous α-phase and phase relation of FeOOH-TiO₂ system at 900°C was well constrained. It was found that the hydrous α-phase decomposes into baddeleyite-type TiO₂ + ε-FeOOH at pressure of approximately 20–21 GPa, and into ilmenite + rutile at 5–6 GPa. This stability field equivalent to depth of 180–600 km in the subduction zone. Our results suggest that the hydrous α-phase is an important water reservoir in the middle upper mantle to mantle transition zone.

中文翻译：

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深部地幔条件下具有α-PbO2型晶体结构的铁钛羟基氧化物的相关系和状态方程

_{研究了α-PbO 2}型晶体结构（含水α相）铁钛羟基氧化物在中上地幔到地幔过渡带压力下的相关系和状态方程。实验使用具有原位同步加速器 X 射线衍射的多砧装置进行。金属金刚石样品容器用于保持相对于水的封闭系统，同时允许 X 射线透射。起始材料是试剂级针铁矿和锐钛矿的混合物，其中 Fe:Ti = 1:1 和 1:3。含水 α 相在 11-14 GPa 和 800-900°C 下合成。X 射线衍射数据是在 5 至 22 GPa 的广泛压力和室温至 930°C 的温度范围内获得的。使用在室温至 800°C 和 8.9–20.8 GPa 下收集的 Fe:Ti = 1:1 水合 α 相的压力-体积-温度数据，我们确定了等温体积模量 ( K _{T 0}  = 183 (1 ) GPa) 和环境条件下的热膨胀率 ( α ₀  = 3.29 (1) × 10 ^-5 K ^{-1 )。}_{FeOOH-TiO 2}体系在900℃时水合α相的稳定场和相关系得到了很好的约束。研究发现，水合α相在约20-21 GPa的压力下分解为斜锆石型TiO ₂  + ε-FeOOH，在5-6 GPa的压力下分解为钛铁矿+金红石。这个稳定场相当于俯冲带180-600公里的深度。我们的研究结果表明，含水α相是中上地幔到地幔过渡带的重要水储层。