Magnetic phase diagram and metal-insulator transition of NiS2-xSex

Magnetic phase diagram of NiS2-xSex has been reexamined by systematic studi es of electrical resistivity, uniform magnetic susceptibility and neutron d iffraction using single crystals grown by a chemical transport method. The electrical resistivity and the uniform magnetic susceptibility exhibit the same feature of temperature dependence over a wide Se concentration. A dist inct first order metal-insulator (M-I) transition accompanied by a volume c hange was observed only in the antiferromagnetic ordered phase for 0.50 les s than or equal to x less than or equal to 0.59. In this region, the M-I tr ansition makes substantial effects to the thermal evolution of staggered mo ments. In the paramagnetic phase, the M-I transition becomes broad; both th e electrical resistivity and the uniform magnetic susceptibility exhibit a broad maximum around the temperatures on the M-I transition-line extrapolat ed to the paramagnetic phase.