Pressure-induced Insulator to Metal Transition of Mixed Valence Compound Ce(O,F)SbS${}_2$

Transport properties of Ce${}_{0.85}$F0.15SbS${}_2$ and undoped CeOSbS${}_2$ under high pressure were investigated experimentally and theoretically. Electrical resistivity measurements of the Ce${}_{0.85}$F0.15SbS${}_2$ single crystals were performed under various high pressures using a diamond anvil cell with boron-doped diamond electrodes. The samples showed the insulator to metal transition by applying high pressure up to 30-40 GPa. On the other hand, the undoped CeOSbS${}_2$ showed almost same transport property with the F-doped sample under high pressure. The valence state analysis using X-ray photoelectron spectroscopy revealed a simple valence state of Ce3+ in Ce${}_{0.85}$F0.15SbS${}_2$ and mixed valence state between Ce3+ and Ce4+ in undoped CeOSbS${}_2$. The valence fluctuation in Ce carried out the comparable transport nature in the both samples. A band calculation suggests that the undoped CeOSbS${}_2$ could be metallic under high pressure of 30 GPa in accordance with the experimental results. A superior thermoelectric property of power factor in CeOSbS${}_2$ was estimated under high pressure around 20 GPa in comparison with that of ambient pressure.