Theory of pressure-induced magnetic and metal-insulator transitions

A theory of pressure-induced simultaneous metal-insulator and magnetic phase transitions is presented. It is based on a model consisting of a periodic lattice with (1) an itinerant-electron band, (2) a narrow dispersionless band (localized states), (3) a hybridization term, (4) a very strong short-ranged repulsion between electrons of opposite spin in the localized states, and (5) a moderately strong short-ranged repulsion between electrons in the itinerant and localized bands. The problem is treated in the Hartree-Fock approximation, allowing spontaneous ferromagnetic broken symmetry. Pressure, which mainly changes the energy separation between the two bands, induces transitions from insulating, (ferro)magnetic states, to metallic states with no localized moments. The transition may be continuous or discontinuous, depending on the values of the interaction parameters. A richly structured phase diagram is obtained. For some values of the parameters the model reproduces in a reasonable fashion and for T→0 the complex transition recently found in ${\mathrm{NiI}}_2$ at pressures of about 19 GPa.