Water is a key constituent of Earth’s crust and mantle. Its electronic properties at high temperature and pressure play an important role in determining chemical reactions occurring in the supercritical conditions of Earth’s interior.
Experimentally, it is not yet possible to measure absorption processes taking place in water and ice in diamond anvil cells. The band gap (a measure of the ability to absorb light) of diamond is smaller than that of water and ice, at least up to 30 GPa, and at high temperatures water becomes corrosive. Hence understanding the electronic properties of water under pressure has remained elusive. It has been common practice to assume that water’s band gap is inversely correlated with its measured refractive index, consistent with observations reported for hundreds of materials.
In an article published in Nature Communications, Pan, Wan, and Galli  used ab initio molecular dynamics simulations and electronic structure calculations to show that both the refractive index and the electronic gap of water and ice increase with increasing pressure (up to 30 GPa), contrary to previous assumptions and contrary to the results of simple, widely used models. Subtle electronic effects, related to the nature of inter-band transitions and band edge localization under pressure, are responsible for this apparently anomalous behavior.
Once again water behaves in a unique fashion, distinct from other molecular fluids such as methane, benzene, and hydrogen, in a regime where hydrogen bonding is progressively changed. Pan and colleagues’ work adds yet another water anomaly to the list of those known at, and close to, ambient conditions.
The optical gap and refractive index established here are important for understanding redox reactions under pressure, and for predicting the oxidation states of rocks and minerals. They also have important implications for the transport of charges due to excitation in supercritical conditions in the presence of strain or electric fields.
Image credit: Peter Allen, UCSB and IME, University of Chicago