Pressure-induced insulator-metal transition in transition metal chalcogenides studied by nuclear resonant inelastic scattering

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Iron sulfide, FeS, has been attracting attention in geosciences as well as condensed matter physics. Although the pressure vs.temperature phase diagram has been contradictory, FeS undergoes two successive first-order phase transitions at room temperature, from the troilite to an MnP-type structure at 3.5 GPa and then to a monoclinic structure at 6.5 GPa. At ambient conditions, FeS is an antiferromagnetic semiconductor, where an electron correlation plays an important role. Electrical resistivity measurements of FeS under pressure show that the semiconductor-metal and metal-semiconductor transitions occur at 3.5 and 6.5 GPa, respectively, corresponding to the structural phase transitions at room temperature. These phase transitions cause simultaneous changes in the electronic and structural properties. Therefore, we need to investigate the phonon properties of FeS in each phase and around these phase transitions from the condensed matter physical points of view.In this research, we have applied the nuclear resonant inelastic x-ray scattering to extract the partial phonon DOS of FeS under pressure and then calculated the total phonon DOS. It is found that the phonon DOS of FeS is modified by the pressure-induced phase transitions.We derive the pressure dependence of thermodynamic parameters from these phonon DOS. In these thermodynamic parameters, the pressure dependence of a force constant is determined by the Hund's rules of Fe^<2+> ion in FeS. A comparison of the observed and estimated compressibilities makes it clear that there is large pure electronic contribution in the observed compressibility in the metallic state. Consequently the larger observed compressibility in the metallic state is provided by a purely electronic contribution, which is estimated to be about 40% of the observed compressibility. Therefore, this discontinuous change of purely electronic contribution characterizes the phase transition at 3.5 GPa.