Magnetic-electronic properties of FeS and Fe7S8 studied by Fe-57 Mossbauerand electrical measurements at high pressure and variable temperatures

The effect of pressure on the magnetic and electronic properties of synthet ic FeS and Fe7S8 has been investigated by using Fe-57 Mossbauer and electri cal resistance measurements on polycrystalline samples pressurized in minia ture gem anvil cells up to a pressure of similar to 12 GPa in the temperatu re range 300-5 K. FeS in the low-pressure phases (P less than or similar to 7 GPa) has thermally activated charge carriers and a high-spin electronic Configuration along the room temperature isotherm, whereas the high-pressur e monoclinic phase (P greater than or similar to 7 GPa) adopts a magnetical ly quenched low-spin state and a non-metallic behaviour associated with the filled valence band. The non-metallic behaviour observed in all pressure p hases is explained in terms of electron correlation between Fe:3d electrons . In contrast, Fe7S8 is magnetic-metallic below similar to5 GPa and diamagn etic-metallic above this pressure. The metallic behaviour is ascribed to ho le conduction in the S:3p band, as inferred from the temperature dependence of the Mossbauer data. The collapse of band magnetism at similar to5 GPa i n Fe7S8 may be due to pressure-induced band broadening, leading to a breakd own of the Stoner criterion.