The metal-non-metal transition and the static and dynamic structure factor of expanded liquid alkali metals

This paper reviews neutron scattering experiments on the static and dynamic structure factor of expanded liquid alkali metals from the melting point up to their critical point. Near the critical point, the change from a liquid metal to a non-metal takes place, which implies that the interatomic forces must exhibit drastic changes when the critical point is approached. Characteristic changes of the microscopic structure of the expanded liquid alkali metals have been observed. They expand non-uniformly and the dominant effect on the properties of the expanded liquids derives from a reduced mean coordination number and not from a drastic change of the interparticle distance. Measurements of the coherent dynamic structure factor S(Q, omega ) of expanded liquid rubidium reveal that up to about three times the critical density, the collective ion-ion dynamics is still controlled by the 'electron sea' of the expanded liquid metal. At lower densities of about twice the critical density, however, a drastic change of shape of S(Q, omega ) is observed, which might be due to the presence of paired units in the expanded liquid.