Bulk Peculiarities: Metal–Nonmetal Transitions

Abstract

Metal–nonmetal transitions in fluid systems are the subject matter of this chapter. It first deals with the limits of the metallic regime as given by the Ioffe–Regel rule and the Mott criterion for the breakdown of electronic screening. Then various mechanisms of electron localization and types of metal–nonmetal transitions are discussed. This includes phenomena such as localization by disorder (Mott–Anderson transition), polaron and F-centre formation, intra-atomic electron correlation (Mott–Hubbard transition), and percolation transitions. The main part of this chapter deals with electronic phase transitions in expanded fluid alkali metals, liquid alkali metal alloys, and metal–molten salt solutions based on experimental investigations of their electronic transport, magnetic, and structural properties. In fluid alkali metals, strong indications of a highly correlated, nearly antiferromagnetic electron gas exist, which is the signature of a Mott–Hubbard transition. The nature of localized electronic states in alkali metal alloys depends on chemical bonding in the nonmetallic regime. Liquid semiconducting behaviour is found in alkali metal–antimony alloys and the transition to metallic states can be consistently described by crossing of a pseudogap. Some alkali metal–gold alloys exhibit predominantly ionic bonding near M − Au stoichiometry and thus behave very similar to alkali metal–alkali halide melts. The nonmetallic regime of the latter is characterized by polaronic defects leading to a metal–nonmetal transition of the Mott–Anderson type.