Experimental evidence of tetrahedral symmetry breaking in SiO 2 glass under pressure.

Bimodal behavior in the translational order of silicon's second shell in SiO 2 liquid at high temperatures and high pressures has been recognized in theoretical studies, and the fraction of the S state with high tetrahedrality is considered as structural origin of the anomalous properties. However, it has not been well identified in experiment. Here we show experimental evidence of a bimodal behavior in the translational order of silicon's second shell in SiO 2 glass under pressure. SiO 2 glass shows tetrahedral symmetry structure with separation between the first and second shells of silicon at low pressures, which corresponds to the S state structure reported in SiO 2 liquid. On the other hand, at high pressures, the silicon's second shell collapses onto the first shell, and more silicon atoms locate in the first shell. These observations indicate breaking of local tetrahedral symmetry in SiO 2 glass under pressure, as well as SiO 2 liquid.