First-Principles Understanding of the Anomalous Structural Change in an Expanded Liquid Alkali Metal

Based on a model derived from the first-principles Hamiltonian by treating the electron-ion pseudopotential as perturbation, we have studied the local ionic structure of an expanded liquid alkali metal to find that the distance between adjacent ions decreases despite the increase of the mean interionic distance with volume expansion. This anomalous feature of the interionic contraction, which is in quantitatively good accord with experiment for liquid Rb, is attributed to an enhanced attraction working between valence electrons in the low-density electron gas, combined with the existence of the ionic core region from which valence electrons are excluded.