On the structure of liquid antimony pentafluoride

The liquid structure of antimony pentafluoride at room temperature has been investigated using neutron and high-energy X-ray diffraction and subsequently modelled using Empirical Potential structure refinement. The neutron diffraction measurements show that each antimony centre is surrounded by 6 fluorine atoms; four at a non-bridging distance of 1.86 ± 0.03 Å and two bridging fluorines at a distance of 2.03 ± 0.06 Å. The X-ray data show an additional peak at 3.93 ± 0.03 Å attributed to antimony-antimony contacts. The diffraction data were fit to three models; cis-monomer, isolated tetramer and cis-linked chains. The X-ray data rule out the cis-monomer model but good fits are obtained for the isolated tetramer and cis-linked chain models. It is argued that the liquid is comprised of chains of cis-linked tetrameric building blocks when these data and modelling results are considered in light of NMR measurements. © 2006 Elsevier B.V. All rights reserved

Structural change in molten basalt at deep mantle conditions

Silicate liquids play a key part at all stages of deep Earth evolution, ranging from core and crust formation billions of years ago to present-day volcanic activity. Quantitative models of these processes require knowledge of the structural changes and compression mechanisms that take place in liquid silicates at the high pressures and temperatures in the Earth’s interior. However, obtaining such knowledge has long been impeded by the challenging nature of the experiments. In recent years, structural and density information for silica glass was obtained at record pressures of up to 100 GPa (ref. 1), a major step towards obtaining data on the molten state. Here we report the structure of molten basalt up to 60 GPa by means of in situ X-ray diffraction. The coordination of silicon increases from four under ambient conditions to six at 35 GPa, similar to what has been reported in silica glass1, 2, 3. The compressibility of the melt after the completion of the coordination change is lower than at lower pressure, implying that only a high-order equation of state can accurately describe the density evolution of silicate melts over the pressure range of the whole mantle. The transition pressure coincides with a marked change in the pressure-evolution of nickel partitioning between molten iron and molten silicates, indicating that melt compressibility controls siderophile-element partitionin