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Abstract：
Silicate perovskite, with the mineral name bridgmanite, is the most abundant mineral in the Earth\'s lower mantle. We investigated crystal structures and equations of state of two perovskite-type Fe3+-rich phases, FeMg0.5Si0.5O3 and Fe0.5Mg0.5Al0.5Si0.5O3, at high pressures, employing single-crystal X-ray diffraction and synchrotron Mössbauer spectroscopy. We solved their crystal structures at high pressures and found that the FeMg0.5Si0.5O3 phase adopts a novel monoclinic double-perovskite structure with the space group of P21/n at pressures above 12 GPa, whereas the Fe0.5Mg0.5Al0.5Si0.5O3 phase adopts an orthorhombic perovskite structure with the space group of Pnma at pressures above 8 GPa. The pressure induces an iron spin transition for Fe3+ in a (Fe0.7,Mg0.3)O6 octahedral site of the FeMg0.5Si0.5O3 phase at pressures higher than 40 GPa. No iron spin transition was observed for the Fe0.5Mg0.5Al0.5Si0.5O3 phase as all Fe3+ ions are located in bicapped prism sites, which have larger volumes than an octahedral site of (Al0.5,Si0.5)O6.
摘要：
硅酸盐钙钛矿,矿物名称为Bridgmanite,是地球下地幔中最丰富的矿物。我们研究了两种钙钛矿型富Fe3相的晶体结构和状态方程，FeMg0.5Si0.5O3和Fe0.5Mg0.5Al0.5Si0.5O3，在高压下，采用单晶X射线衍射和同步加速器穆斯堡尔谱。我们在高压下解决了它们的晶体结构，发现FeMg0.5Si0.5O3相采用了一种新型的单斜双钙钛矿结构，在高于12GPa的压力下具有P21/n的空间群，而Fe0.5Mg0.5Al0.5Si0.5O3相采用正交钙钛矿结构，在8GPa以上的压力下具有Pnma的空间群。在高于40GPa的压力下，压力在FeMg0.5Si0.5O3相的（Fe0.7，Mg0.3）O6八面体位置中引起Fe3的铁自旋转变。Fe0.5Mg0.5Al0.5Si0.5O3相没有观察到铁自旋转变，因为所有的Fe3+离子都位于双棱柱位点，其体积大于（Al0.5，Si0.5）O6的八面体位点。
