Pressure-Induced Structural Transformations and Electronic Transitions in TeO₂ Glass by Raman Spectroscopy

TeO₂ glass has been studied by Raman spectroscopy up to the record pressure of 70 GPa. The boson peak frequency ω_b exhibits a decrease of the ∂ω_b/∂P slope at 5-6 GPa and saturates above 30 GPa with a practically constant value up to 70 GPa. Experiment and theory indicate that pressures up to 20 GPa induce the transformation of single Te-O-Te bridges to double Te-O₂-Te bridges, leading to a more compact structure, while Raman activity developing at higher pressures around 580 cm^-1 signals the increase of Te coordination from 4- to 6-fold. Natural bond orbital analysis shows that double Te-O₂-Te bridges favor the s → d transition and promote the increase of Te coordination through d²sp³ hybridization. This transition leads to the formation of TeO₆ octahedra, in strict difference with crystalline TeO₂ at the same pressure range, and to the development of a 3D network that freezes the medium range order.