Azzurra Zucchini

The high-pressure behavior of bloedite: A synchrotron single-crystal X-ray diffraction study

Abstract High-pressure single-crystal synchrotron X-ray diffraction was carried out on a single crystal of bloedite [Na2Mg(SO4)24H2O] compressed in a diamond-anvil cell. The volume-pressure data, collected up to 11.2 GPa, were fitted by a second- and a third-order Birch-Murnaghan equation of state (EOS), yielding V0 = 495.6(7) Å3 with K0 = 39.9(6) GPa, and V0 = 496.9(7) Å3, with K0 = 36(1) GPa and K′ = 5.1 (4) GPa-1, respectively. The axial moduli were calculated using a Birch-Murnaghan EOS truncated at the second order, fixing K′ equal to 4, for a and b axes and a third-order Birch-Murnaghan EOS for c axis. The results were a0 = 11.08(1) and K0 = 56(3) GPa, b0 = 8.20(2) and K0 = 43(3) GPa, and c0 = 5.528(5), K0 = 40(2) GPa, K′ = 1.7(3) GPa-1. The values of the compressibility for a, b, and c axes are βa = 0.0060(3) GPa-1, βb = 0.0078(5) GPa-1, βc = 0.0083(4) GPa-1 with an anisotropic ratio of βa:βb:βc = 0.72:0.94:1. The evolution of crystal lattice and geometrical parameters indicates no phase transition up to 11 GPa. Sulfate polyhedra are incompressible, whereas the Mg polyhedral bulk modulus is 95 GPa. The sodium polyhedron is the softest part of the whole structure with a bulk modulus of 41 GPa. Pressure decreases significantly the distortion of Na coordination. Up to 10 GPa, the donor-acceptor oxygen distances decrease significantly and the difference between the two water molecules decreases with an increase in the strengths of hydrogen bonds. At the same time, the bond lengths from Na and Mg to O atoms of the water molecules decrease faster than other bonds to these cations suggesting that there is a coupling between the Na-Ow and Mg-Ow bond strengths and the “hydrogen transfer” to acceptor O atoms.

s -Processing from MHD-induced mixing and isotopic abundances in presolar SiC grains

It has been known for decades that s-process elements from Sr to Pb are produced by Asymptotic Giant Branch stars. However only recently, physically-based mixing mechanisms for the formation of \(^{13}\)C have been proposed. Among them, we aim to verify the robustness of the model of a MHD induced \(^{13}\)C-pocket formation. In doing that we present results of nucleosynthesis models for low mass AGB stars, developed from the MHD scenario, compared with the isotopic abundance ratios of s-elements from presolar Mainstream SiC grains.