Electronic, Structural, and Mechanical Properties of SiO_{2} Glass at High Pressure Inferred from its Refractive Index.

We report the first direct measurements of the refractive index of silica glass up to 145 GPa that allowed quantifying its density, bulk modulus, Lorenz-Lorentz polarizability, and band gap. These properties show two major anomalies at ∼10 and ∼40  GPa. The anomaly at ∼10  GPa signals the onset of the increase in Si coordination, and the anomaly at ∼40  GPa corresponds to a nearly complete vanishing of fourfold Si. More generally, we show that the compressibility and density of noncrystalline solids can be accurately measured in simple optical experiments up to at least 110 GPa.