High-pressure neutron and X-ray studies on the Mott transition of BaCoS₂

X-ray Rietveld analyses for BaCoS₂ under high pressure and magnetic excitation measurements for BaCo_1-xNi_xS₂ at ambient pressure and for BaCoS₂ under high pressure have been carried out. Atomic distances in a CoS₅ pyramid abruptly change in the narrow region around the Mott metal-insulator transition with increasing external pressure (p). It suggests that the redistribution of electron numbers among the relevant bands occurs together with the p-induced Mott transition. The characteristic energy of the spin fluctuation behaves rather differently in two kinds of metallic phases induced by varying x and p, indicating that the main driving forces of the Mott transitions are different.