COMPUTER-SIMULATION OF CAO-WO3 LIQUID OXIDES

Molecular dynamics simulation was used to develop ionic models of (CaO )(x)(WO3)(1-x) noncrystalline oxides at 2000 K for x = 1.0, 0.8, 0.6, 0.4, 0.2, and 0.0 using Born-Mayer pair potentials. Coulomb interactio ns were evaluated by the Ewald-Ansen method. In each simulation, the s ystem included about 500 ions in a cubic box with periodic boundary co nditions. The density was chosen so that the pressure was nearly zero. The average nearest-neighbor W-O distance was calculated to be simila r or equal to 180 pm, in agreement with diffraction data. The volume c hange upon formation of a mixed oxide from noncrystalline CaO and WO3 at 2000 K attains 2.35 cm(3)/mol at x = 0.8; the energy change passes through a minimum at x = 0.6 (-101 kJ/mol). The O-O coordination numbe r distribution is bimodal for x less than or equal to 0.4, with maxima at z = 4 and 8. The structural results indicate that the W6+ ions are mainly coordinated by O2- octahedra. For x less than or equal to 0.4, complete polymerization of the oxide structure is observed. At x = 0. 8, ring-shaped ''polyanions'' are very few. The calculated electrical conductivity is similar or equal to 190 S/m for the x = 0.8 oxide and rapidly drops with decreasing x. In an applied electric field, the W6 ions are entrained by surrounding oxygen ions and migrate against the field, similar to silicon ions in silicates.