title: The ammonia-hydrogen system under pressure.
abstract: Binary mixtures of hydrogen and ammonia were compressed in diamond anvil cells to 15 GPa at room temperature over a range of compositions. The phase behavior was characterized using optical microscopy, Raman spectroscopy, and synchrotron X-ray diffraction. Below 1.2 GPa we observed two-phase coexistence between liquid ammonia and fluid hydrogen phases with limited solubility of hydrogen within the ammonia-rich phase. Complete immiscibility was observed subsequent to the freezing of ammonia phase III at 1.2 GPa, although hydrogen may become metastably trapped within the disordered face-centered-cubic lattice upon rapid solidification. For all compositions studied, the phase III to phase IV transition of ammonia occurred at ~3.8 GPa and hydrogen solidified at ~5.5 GPa, transition pressures equivalent to those observed for the pure components. A P-x phase diagram for the NH(3)-H(2) system is proposed on the basis of these observations with implications for planetary ices, molecular compound formation, and possible hydrogen storage materials.
authors.affiliation: Geophysical Laboratory, Carnegie Institution of Washington, 5251 Broad Branch Road NW, Washington, D.C. 20015, USA.
authors.forename: Bethany A
authors.initials: BA
authors.lastname: Chidester
authors.forename: Timothy A
authors.initials: TA
authors.lastname: Strobel
mesh_terms: D000641:Ammonia / Q000737:chemistry*; D006859:Hydrogen / Q000737:chemistry*; D010316:Particle Size; D011312:Pressure; D012995:Solubility; D013059:Spectrum Analysis, Raman; D013499:Surface Properties
chemical_list: D000641:Ammonia; D006859:Hydrogen