Mechanisms of Pressure-Induced Structural Transformation in Confined Sodium Borate Glasses

In this paper, density functional theory simulations were conducted to investigate the structural adaptation of sodium borates xNa₂O·(100-x)B₂O₃ (x = 25, 33, 50, and 60 mol %) during the compression/decompression between 0 and 10 GPa. The sodium borates are confined between two Fe₂O₃ substrates and undergo the compression by reducing the gap between the two surfaces. The results reveal the borate response to the load through a two-stage transformation: rearrangement at low pressure and polymerization at high pressure. The pressure required to initiate the polymerization depends directly on the portion of fourfold-coordinated (^[4]B) boron in the sodium borates. We found that the polymerization occurs through three different mechanisms to form BO₄ tetrahedra with surface oxygen and nonbridging and bridging oxygen. The electronic structure was analyzed to understand the nature of these mechanisms. The conversions from BO₃ to BO₄ are mostly irreversible as a large number of newly formed BO₄ remain unchanged under the decompression. In addition, the formation of a sodium-rich layer can be observed when the systems were compressed to high pressure. Our simulation provides insight into sodium borate glass responses to extreme condition and the underlying electronic mechanisms that can account for these behaviors.