title: A molecular dynamics study of ambient and high pressure phases of silica: structure and enthalpy variation with molar volume.
abstract: Extensive molecular dynamics studies of 13 different silica polymorphs are reported in the isothermal-isobaric ensemble with the Parrinello-Rahman variable shape simulation cell. The van Beest-Kramer-van Santen (BKS) potential is shown to predict lattice parameters for most phases within 2%-3% accuracy, as well as the relative stabilities of different polymorphs in agreement with experiment. Enthalpies of high-density polymorphs - CaCl2-type, α-PbO2-type, and pyrite-type - for which no experimental data are available as yet, are predicted here. Further, the calculated enthalpies exhibit two distinct regimes as a function of molar volume-for low and medium-density polymorphs, it is almost independent of volume, while for high-pressure phases a steep dependence is seen. A detailed analysis indicates that the increased short-range contributions to enthalpy in the high-density phases arise not only from an increased coordination number of silicon but also shorter Si-O bond lengths. Our results indicate that amorphous phases of silica exhibit better optimization of short-range interactions than crystalline phases at the same density while the magnitude of Coulombic contributions is lower in the amorphous phase.
authors.affiliation: Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore 560012, India.
authors.forename: Chitra
authors.initials: C
authors.lastname: Rajappa
authors.affiliation: Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore 560012, India.
authors.forename: S Bhuvaneshwari
authors.initials: SB
authors.lastname: Sringeri
authors.affiliation: Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore 560012, India.
authors.forename: Yashonath
authors.initials: Y
authors.lastname: Subramanian
authors.affiliation: Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore 560012, India.
authors.forename: J
authors.initials: J
authors.lastname: Gopalakrishnan