A numerical investigation of the liquid–vapor coexistence curve of silica

Abstract

In using a modified version of the interionic potential of Tsuneyuki, Tsukada, Aoki, and Matsui [Phys. Rev. Lett. 61, 869 (1988)] for silica we have evaluated by molecular dynamics simulation the liquid–vapor coexistence curve of this model fluid. Although the critical point is located at a very high temperature (T_c=11 976 K, ρ_c=0.58 g/cm³, and P_c∼2 kbar) as expected for an ionic fluid, the overall shape of the coexistence curve is found to be very similar to that of a molecular fluid and especially close to that of water in the low temperature region (i.e., near and below the boiling point). A detailed comparison with available experimental data on silica melts and glasses is also presented. An analysis of the evolution of the silica structure with temperature shows that the tetrahedral SiO₄ units subsist into the simulated melt up to approximately 8000 K and that the network formed by corner‐sharing SiO₄ tetrahedra is disrupted by a considerable extent only above this temperature. It is stressed that similar behavior has been recently reported in another network former liquid (i.e., water). Finally it is shown that the nature of the structural transformations exhibited by the simulated silica under very high pressure (up to the megabar range) may provide some clues to improve our knowledge about the constitution and the electrical conductivity of the earth’s mantle.