Water surprises again: light refraction and absorption under pressure

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, Ding Pan, Quan Wan, and Pritzker School of Molecular Engineering Professor Giulia 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.

Prof. Galli’s research was funded by the Sloan foundation, through the Deep Carbon Observatory (DCO), and by the Computational Materials and Chemical Sciences network sponsored by DOE.

Prof. Galli served as chair of the Extreme Physics and Chemistry Directorate of the DCO from 2010 to 2013.

The article, "The refractive index and electronic gap of water and ice increase with increasing pressure," was published in Nature Communications on May 27, 2014.