RESUMO
We investigated freezing of pure glycerol as well as glycerol-water (GW) mixtures with 3:1 and 3:2 volume fractions as a function of pressure in the 0-10 GPa range by ruby fluorescence spectroscopy and neutron scattering. We find that the glass transition pressure increases from 5.5 GPa for pure glycerol to 6.5 GPa for the 3:1 GW mixture, with unusually small pressure gradients above. For higher water concentrations close to 3:2, phase separation occurs above 2 GPa where most of the water is expelled in the form of ice VII. The results suggest that glycerol is able to effectively hydrogen bond not more than ≈2.5 H(2)O molecules per glycerol, which seems to support conclusions from molecular dynamics simulations. The data indicate that these fluids could become important as pressure transmitting media for neutron scattering in the 0-7 GPa range, including at low temperatures.
RESUMO
Oxygen is the only elemental molecule which carries an electronic magnetic moment. As a consequence, the different solid phases encountered on cooling show various degrees of magnetic order, and similar behavior is expected under compression. Here we present neutron diffraction data which reveal the magnetic ordering under high pressure in the delta ("orange") phase, i.e., in the range 6-8 GPa and 20-240 K. We show that delta-O2 contains in total three different magnetic structures, all of them being antiferromagnetic and differing in the stacking sequence of O2 sheets along the c axis. This structural diversity can be explained by the quasi-two-dimensional nature of delta-O2 and the strong orientation dependence of the magnetic exchange interaction between O2 molecules. The results show that delta-O2 is a room temperature antiferromagnet.
