Electronic bandgap of water in the superionic and plasma phases

Abstract

Water has been proposed to be one of the main compositions of icy giant planets like Neptune and Uranus. Its thermodynamic states and transport properties at extremes are of interest not only to constrain the interior models but also to understand abnormal magnetic fields of planets. The electronic bandgap of water, which significantly influences the ionization ratio and the conductivity, however, is still under debate. In this work, we revisit the shock reflectivity data reported in the literature. By applying a Drude model, the electronic bandgap of water in the superionic and plasma phases is determined to be 4.4 ± 0.2 eV, in contrast to the threshold of 1.25 ± 0.04 eV for free ion generation in the molecular and ionic fluid phases. Interestingly, the bandgap of water does not show a significant tendency of “closure” with the increase in pressure or temperature in the investigated regime, and the bandgap value is consistent with the predicted value of 4–6 eV by the density functional theory assuming a hybrid Heyd-Scuseria-Ernzerhof functional [Millot et al., Nat. Phys. 14, 297–302 (2018)]. The electronic bandgap and the energy threshold determined in this work provide essential parameters for estimating the conductivity along the radius of Neptune and Uranus and will promote our understanding of the origin of the abnormal magnetic fields.