Development of simultaneous measurements system of electric conductivity and x-ray diffraction on mantle minerals at high pressure and temperature

本课题设计了一种可同时用于电阻和x射线衍射测量的金刚石砧池。该DAC基本上是修改了杠杆式和弹簧式DAC，并安装了用于电阻测量的两电极模型DAC。用FZ法合成了一系列磁铁矿(Fe_3O_4)-乌尖晶石（Fe_2TiO_4）固溶体单晶。然后，对这些样品进行电阻测量，同时进行x射线衍射测量。所有样品的电阻随压力的增加而逐渐减小，这可能是由于电子跳变和能带重叠机制。然而，在某些临界压力以上，电阻突然不表现出压力依赖性。在Fe_<3-x>Ti_xO_4固溶体中，随着铁含量的增加，临界压力增大。Fe_2TiO_4的临界压力约为6.5GPa， Fe_3O_4的临界压力约为24GPa。在临界压力以上获得的x射线粉末衍射图表明，在临界压力以上，一些单衍射峰分裂成双峰。有证据表明，在临界压力处发生结构转变。根据Fe_2TiO_4在高压下的单晶结构分析实验，结构转变可以归结为由立方对称到四方对称的转变，可以提出Mn_3O_4型结构作为Fe_2TiO_4高压形态的候选结构。本研究项目新设计的系统可以为晶体结构与电阻之间的关系提供一些有趣和重要的信息。

A diamond anvil cell, which can apply to electric resistance and x-ray diffraction measurements simultaneously, was designed in this research project. The DAC is basically modified the lever & spring type DAC and two electrodes model for electric resistance measurements is installed the DAC.A series of single crystals of the magnetite (Fe_3O_4)-ulvospinel (Fe_2TiO_4) solid solution were synthesized by the FZ method. Then, electric resistance measurements were performed on those samples and x-ray diffraction measurements were also performed at the same time.Electric resistances on all samples decrease gradually with increasing pressure probably due to the electron hopping and the band overlapping mechanism. However, the electric resistances suddenly show no pressure-dependency-above some critical pressures. The critical pressure increases with iron content in the solid solution of Fe_<3-x>Ti_xO_4. For example, the critical pressures are about 6.5GPa for Fe_2TiO_4 and about 24GPa for Fe_3O_4. X-ray powder diffraction patterns obtained above the critical pressure showed that some single diffraction peaks split into doublet peaks above the critical pressure. The evidence suggests a structural transition occurs at the critical pressure. According to the single crystal structure analysis experiments on Fe_2TiO_4 at high pressure, the structural transition could be assigned as the transition from cubic to tetragonal symmetry and the Mn_3O_4 type structure can be proposed as a candidate of the high-pressure form of Fe_2TiO_4. The newly designed system in this research project can provide some interesting and important information about the relationship between the crystal structures and electric resistances.