Liquid liquid transitions and liquid electrides in the alkali metals

We demonstrate the existence of a new type of liquid in simple metals under high pressure. We show that liquid alkali metals localize electrons on non-nuclear sites as “pseudoanions” under sufficient pressure during a continuous liquid liquid transition. This results in a two-component mixed electride-metallic bonding, in which the electride character increases until around. We have used atomistic simulations with both density functional theory and machine-learned potentials, as well as picosecond acoustic measurements. We show how the transformation is evident in K, Rb, Cs by anomalous diffusivity, thermal expansion, sound speed, coordination number, reflectivity, and heat capacity across a wide range of pressure associated with their respective melting line minima. The abnormalities stem from a significant change in local electronic and ionic structures. Although primarily a pressure-induced phenomenon, there is also a thermal expansion anomaly. We resolve the long-standing mystery of how a liquid can be denser than a close-packed solid, in this case forming a liquid electride.