Figure: Different pressure dependence on viscosities of polymerized and depolymerized melts. (a) Viscosities of polymerized melts (blue symbols and shaded area), generally with higher values at 1 atm, exhibit negative pressure dependence at low pressures. The range of viscosity variation decreases with pressure (blue shaded area). For depolymerized melts, viscosities generally increase with pressure (orange symbols and shaded area). Typical experimental uncertainties are given. (b) Normalized viscosities with respect to their 1 atm values. Pressure turnover is clearly seen for the polymerized melts (blue symbols and shaded area). Approximate values of NBO/T for the polymerized melts are given on the right side of the figure. Normalized viscosities of depolymerized melts (orange symbols, with NBO/T≈2) fall in a very narrow range and are not shaded.
A defining characteristic of silicate melts is the degree of polymerization, which dictates viscosity and affects compressibility. While viscosity of depolymerized melts increases with pressure consistent with free volume theory, isothermal viscosity of polymerized melts decreases with pressure up to ~3 - 5 GPa, above which it turns over to normal (positive) pressure dependence. This viscosity turnover in polymerized liquids corresponds to the tetrahedral packing limit, below which the structure is compressed through tightening of the inter-tetrahedral bond angle, resulting in high compressibility, continual breakup of tetrahedral connectivity, and viscosity decrease with increasing pressure. Above the turnover pressure, Si and Al coordination increases to allow further packing, with increasing viscosity and density. These structural responses prescribe the distribution of melt viscosity and density with depth and may play an important role in magma transport in terrestrial planetary interiors.
Xi, X. at al., 2013, Physical Review Letters 111(15): 155701
Figures: (a) The crystal structure of BiTeI with unit cell lengths, a and c. (b) Pressure dependence of the crystallographic c/a ratio (circles) from XRD and the plasma frequency ωp from IR spectroscopy (squares). The minimum in ωp indicates band gap closing and re-opening, signaling the transition.
Pressure induced the polar semiconductor BiTeI into a “topological insulator” state. This is the first case of a pressure-induced topological insulator and its discovery could help scientists find ideal TIs for future electronics applications.