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

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, rho(c)=0.58 g/cm(3), and P-c similar to 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 derailed 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. (C) 1996 American Institute of Physics.