Pressure-induced phase transitions of transition metal chalcogenids.

电子关联起重要作用的三维过渡金属化合物的电子态一直是人们感兴趣的课题。这些化合物的一个有趣的性质是绝缘体-金属转变。由于FeS是一种反铁磁半导体，它是一个很好的候选人，研究压力诱导绝缘体-金属转变和感兴趣的凝聚态物理以及行星科学。高压X射线衍射结果表明，FeS在3.5和6.5GPa下发生了两次一级结构相变。最近我们测量了高压下的电阻率。结果表明，FeS在8 GPa的压力下发生了超导-金属-半导体的相变。在这些相变中，不仅电子关联而且电子-声子耦合起着重要作用。在阿贡国家实验室APS的3-ID光束线上，利用一台专用的DAC，在80 meV左右的范围内调谐高度单色化的光束，测量了FeS在高压下的核共振非弹性散射，得到了声子密度与压力的关系。使用Fluorinert作为压力传递介质确保了准流体静力条件。用红宝石刻度得到压力，观测到的核共振非弹性散射谱由弹性散射的中心峰和伴随声子湮灭和产生的非弹性散射的边带组成。几乎所有的振动模式中观察到的能量范围高达60 meV。从这些光谱中提取的部分声子态密度的FeS在每个阶段假设一个谐波晶格模型。压力下FeS声子态密度的变化反映了结构相变。

The electronic states of 3d transition-metal compounds where an electron correlation plays an important role, have been interesting subjects. One of the interesting properties in these compounds is an insulator-metal transition. Since FeS is an antiferromagnetic semiconductor, it is a good candidate to study a pressure induced insulator-metal transition and of interest to condensed matter physics as well as planetary science. As indicated by the high-pressure x-ray diffraction measurements, FeS undergoes two first-order structural phase transitions at 3.5 and 6.5 GPa. Recently we have measured the electrical resistivity under high pressure. It was found that the semiconductor-metal-semiconductor transitions occur in FeS under pressure up to 8 GPa. In these phase transitions, not only the electron correlation but also an electron phonon coupling plays an important role. We have measured inelastic nuclear resonate scattering of FeS under high pressure to obtain the pressure dependence of phonon densities.The nuclear resonant inelastic x-ray scattering were measured by tuning the highly monochromatzed beam in a range of about 80meV on 3-ID beamline at APS, Argonne National Laboratory using a special DAC. The use of Fluorinert as pressure-transmitting medium ensured quasihydrostatic conditions. Pressure was obtained by the ruby calibration.The observed nuclear resonant inelastic scattering spectra consist of a center peak originating from elastic scattering and sidebands resulting from inelastic scattering with annihilation and creation of phonon. Almost all-vibrational modes are observed in a energy range up to 60meV. The partial phonon densities of states of FeS in each phase were extracted from these spectra assuming a harmonic lattice model. The observed changes in the phonon densities of states in FeS under pressure reflect the structural phase transitions.