Molecular dynamics simulations of molten CaAl2Si2O8; dependence of structure and properties on pressure

Abstract

Molecular dynamics (MD) simulations were carried out on molten CaAl2Si2O8 at nine pressures from 0.2 to 76 GPa at 4000 K, well above the computed glass transition tem- perature. A simple effective pair-potential with both Coulombic interaction and Born-May- er-Huggins short-range repulsion was used. Simulations of duration 30 ps long were per- formed in the microcanonical ensemble (NEV) utilizing 1300 particles (O1Ca1Si1Al). As pressure increases, profound changes occur in the relative abundance of (4) T, T decreases and (6) T increases). At 5 GPa, the average coordination number (CN) of oxygen about Ca is 8. At 20 GPa, (6) T constitutes more than 50% of the TOn subunit population and the CN of oxygen around Ca is 10. At medium range (3 to 10 A ˚), the most significant change in melt structure as pressure increases is replacement of corner-sharing ring-former TOn polyhedra with edge-sharing polyhedra pairs such as TO5-TO6 and TO6- TO6. The destruction of the ring structure correlates with the maximization of Ar solubility as pressure increases. Ring structure collapse is geometrically captured by variation in the distribution of T-O-T angles with increasing pressure. The low-pressure maximum at 1408 with large variance agrees with X-ray data and follows from the distribution of N-member rings with N ranging from 4 to 8 (or larger). At higher pressure, the T-O-T distribution narrows and shifts toward smaller angles around 1058 consistent with the dominating pres- ence of edge sharing between TO6-TO6 and TO5-TO6 pairs. Tracer diffusivities for O, Ca, Al, and Si increase at low pressure as pressure increases. The diffusivity of oxygen cor- relates with ring collapse and attains a maximum at 5 GPa (Dox 5 6.6 3 10 29 m 2 /s); the ratio of diffusivities for Ca, Al, and Si relative to oxygen are 0.67, 0.62, and 0.51, re- spectively. These data suggest that pentahedrally coordinated T acts as an ''activated com- plex'' for oxygen diffusion. Below 5 GPa, activation volumes ( V*) for all species are large in absolute value and lie in the range 225 to 235 cm