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

本研究项目设计了一种可同时用于电阻测量和X射线衍射测量的金刚石压腔。DAC基本改装为杠杆弹簧式DAC，安装了测量电阻的双电极模型。用FZ法合成了一系列磁铁矿(Fe_3O_4)-硫尖晶石(Fe_2TiO_4)固溶体单晶。然后对这些样品进行了电阻测量，同时进行了X射线衍射仪的测量，所有样品的电阻都随着压力的增加而逐渐减小，这可能是由于电子跃迁和能带重叠机制所致。然而，在某些临界压力之上，电阻突然显示出不依赖于压力。Fe_2TiO_4和Fe_3O_4固溶体的临界压力随固溶体中铁含量的增加而增加，如Fe_2TiO_4的临界压力约为6.5 Gpa，Fe_3O_4的临界压力约为24 Gpa。有证据表明，在临界压力下会发生结构性转变。根据Fe2TiO4高压下的单晶结构分析实验，结构转变可以归结为从立方对称到四方对称的转变，Mn3O4型结构可以被认为是Fe2TiO4高压形式的候选者。本研究项目中新设计的体系可以提供一些关于晶体结构与电阻关系的有趣和重要的信息。

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.