Structural and electronic transition in liquid rubidium

Rubidium, like the other alkali metals, exhibits multiple structural transformations in its solid phase upon application of pressure. Recent experiments and calculations suggest it also goes through a phase transformation in its liquid state, along the 573 K isotherm, slightly above its melting curve. We employ ab initio molecular dynamic simulations to study the structural and electronic characteristics of this liquid-liquid transition. Applying the quasicrystalline model, we find that the short-range order of liquid Rb changes abruptly from bcc-like to β-Sn-like at a pressure significantly lower than the bcc to β-Sn transition in the solid phase at lower temperatures. In contrast to previous claims, the transition in the liquid cannot be related to the Peierls transition, as the liquid does not exhibit formation of a pseudogap at the Fermi level, nor to the transfer of charge from the s state to the d state, which is found to progress monotonically with pressure. Instead, a qualitative change in the shape of the electronic d band is shown to occur at the same pressure range as the structural phase transition, which leads to the conclusion that the sd hybridization must be accompanied by a significant change in the electronic band structure, observed in the solid phase, in order to produce a phase transition.