Pressure-induced phase transitions of transition metal chalcogenids.

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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.