Fe spin transition in the Earth's mantle: insight from X-ray Raman scattering and X-ray absorption spectroscopy

矿物中的Fe2+ （3d6-电子构型）和Fe3+ (3d5)在环境条件下均处于高自旋电子态，并可能在地幔中预期的压力和温度下经历从高自旋（HS）到低自旋（LS）态的转变。自旋态的变化对Fe-Mg矿物的结构、物理和化学性质以及依赖于这些性质的地幔地球动力学过程有相当大的影响。该项目的目的是为HS-LS跃迁过程中的电子变化及其与局部和晶体结构变化的关系提供新的实验见解。x射线拉曼散射（XRS）和x射线吸收光谱（包括共振x射线发射光谱，RXES）的应用将获得新的见解。这两种技术都通过测量Fe M、L和k边，为电子和局部结构提供了有价值的见解。XRS也允许调查，例如O k边和Si， Mg l边。在高达50 GPa的压力下，XRS测量菱铁矿和镁-菱铁矿样品的可行性已被证明。项目期间将完成菱镁-菱铁矿固溶体的研究工作。最后的目标是研究更复杂的桥辉石矿物，这是下地幔中最丰富的阶段之一。新获得的数据将用于解决关于该矿物相自旋跃迁机制的争议。根据实测数据，结合基于dft的计算机模拟，建立结构模型。最后，将评估这些结果对下地幔的影响。

Both, Fe2+ (3d6-electronic configuration) and Fe3+ (3d5) in minerals are in high spin electronic state at ambient conditions and may undergo transitions from the high spin (HS) to the low spin (LS) state at pressures and temperatures expected in the Earth mantle. The change in spin state has considerable impact on structural, physical and chemical properties of Fe-Mg minerals as well as on geodynamic processes in the Earth mantle that depend on these. The aim of this project is to provide new experimental insights to the electronic changes during the HS-LS transition and their relationship to changes in the local and crystal structure. New insights will be gained by application of X-ray Raman scattering (XRS) and X-ray absorption spectroscopy (incl. Resonant X-ray emission spectroscopy, RXES). Both techniques provide valuable insights to the electronic and local structure by measurements of the Fe M, L and K-edges. XRS also allows for investigation of the, e.g. O K-edge and Si, Mg L-edge. Feasibility of XRS measurements at pressures up to 50 GPa has been shown on samples of siderite and magnesio-siderite. During the project, the work on the magnesite-siderite solid solution will be completed. Final goal is the investigation of the more complex mineral bridgmanite, which is one of the most abundant phases of the lower mantle. The newly obtained data will be used to solve the controversy regarding the mechanism of the spin transition in this mineral phase. From the measured data, structural models will be developed by combination with DFT-based computer simulations. Finally, the implications of the results for the lower Earth's mantle will be assessed.