High-pressure electrical resistivity and Mössbauer spectroscopic studies on narrow band Co _0.8Fe _0.2S ₂ nanoparticles up to 8 GPa

Pyrite-structured Co_0.8Fe_0.2S₂ colloid nanoparticles, synthesized using solution method with particle distribution around 3-4 nm determined by transmission electron microscope, were studied under high pressure up to 8 GPa using ⁵⁷Fe Mössbauer spectroscopy and electrical resistivity techniques. Drastic decrease in T_C by magnetic measurements indicated nanosize of the particle. Higher quadrupole splitting (QS) at ambient condition was due to large lattice strain and electric field gradient generated by higher surface to volume ratio of these nanosized particles. Under pressure, Mössbauer parameters-isomer shift and QS-showed an expected trend up to ∼5.6 GPa. Above 6.4 GPa, the QS remained constant up to 8 GPa with decreased value. Even after decompression, the high-pressure phase is retained. The variation of pressure coefficient of electrical resistivity from-0.021/GPa to-0.151/GPa across 6.8 GPa suggested a second-order phase transition. The coincidence of observed value with that of the bulk suggested that the particle size does not impart much influence on transition pressure. This is the first report on nanoparticles of Co_0.8Fe _0.2S₂ under pressure.