Structural transformations in sodium silicate liquids under pressure: A molecular dynamics study

We present the results of force-field molecular dynamics simulations of the static and dynamic properties of sodium silicate liquids. We studied the relationship between the structure and properties with varying SiO₂ content and pressure. We found that the silicate liquids have at least three types of characteristic structures before the coordination number of Si changes with increasing pressure; these structures are, ionic, network, and coesitic structured liquids. In the ionic liquid like structure, simple diffusion of large silicate anions is dominant in the liquid and their motion is impeded by compression. In contrast, in the network liquid, a small portion of the network of corner-shared SiO₄ tetrahedra diffuses via bond exchange and flow globally. Bond exchange is activated by pressure in the entangled network liquid because of distortion of the corner-shared SiO₄ tetrahedra network and the tetrahedra themselves. The SiO₂ rich liquids soften as a result from of these distortions. The derivative of bulk moduli of the sodium silicate liquids increases significantly at a certain pressure because of changes in densification mechanism. The changes in those densification mechanisms are coherent with the changes in pressure dependence of transport coefficient.