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压砧。DAC基本上由杠杆弹簧式DAC改进而来，并在DAC上安装了双电极电阻测量模型。采用FZ法合成了一系列磁铁矿（Fe_3O_4）-钛尖晶石（Fe_2TiO_4）固溶体单晶。然后，对这些样品进行了电阻测量，同时还进行了x射线衍射测量，发现所有样品的电阻都随着压力的增加而逐渐减小，这可能是由于电子跳跃和能带重叠机制造成的。然而，电阻突然显示没有压力依赖性-超过一些临界压力。临界压力随固溶体Fe_ Ti_xO_4中铁含量的增加而增大<3-x>。例如，Fe_2TiO_4的临界压力约为6.5GPa，Fe_3O_4的临界压力约为24 GPa。X射线粉末衍射图谱表明，在临界压力以上，一些单衍射峰分裂成双峰。有证据表明，在临界压力下发生结构转变。根据高压下Fe_2TiO_4的单晶结构分析实验，将其结构转变归结为立方对称性向四方对称性的转变，Mn_3O_4型结构可作为Fe_2TiO_4高压形式的候选结构。本研究计划中新设计的系统可以提供有关晶体结构和电阻之间关系的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.