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Pressure-induced chemical reactions in the N2(H2)2 compound: from the N2 and H2 species to ammonia and back down into hydrazine

Abstract

Theory indicates a very rich high pressure chemistry of hydro-nitrogens with the existence of many NxHy compounds. The stability of these phases under pressure is being investigated by the compression of N2-H2 mixtures of various compositions. A previous study had disclosed a eutectic-type N2-H2 phase diagram with two stoichiometric van der Waals compounds: (N2)6(H2)7 and N2(H2)2. The structure and pressure induced chemistry of the (N2)6(H2)7 compound have been already studied. Here, we determine the structure of the N2(H2)2 compound and characterize by Raman spectroscopy measurements the chemical changes under a pressure cycle up to 60 GPa and back to ambient. A N2(H2)2 single crystal was grown from a 1:2 N2-H2 mixture and its crystalline structure was solved using synchrotron X-ray diffraction. Similarly to the (N2)6(H2)7 solid, N2(H2)2 has a remarkable host-guest structure containing N2 molecules orientationally disordered with spherical, ellipsoidal and planar shapes. Above 50 GPa, N2(H2)2 was found to undergo a chemical reaction. The reaction products were determined to be of the azane family, with NH3 as the main constituent, along with molecular nitrogen. Upon pressure decrease, the reaction products are found to react in such a way that below 10 GPa, hydrazine is the sole azane detected. Observed down to the opening of the diamond anvil cell, the formation of metastable hydrazine instead of the energetically favorable ammonia is puzzling and remains to be elucidated. That could change the current view of Jovian planets atmospheres in which ammonia is assumed the only stable hydronitrogen molecule.

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Publication details

The article was received on 28 Nov 2017, accepted on 08 Jan 2018 and first published on 09 Jan 2018


Article type: Paper
DOI: 10.1039/C7CP07989C
Citation: Phys. Chem. Chem. Phys., 2018, Accepted Manuscript
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    Pressure-induced chemical reactions in the N2(H2)2 compound: from the N2 and H2 species to ammonia and back down into hydrazine

    D. Laniel, V. Svitlyk, G. Weck and P. Loubeyre, Phys. Chem. Chem. Phys., 2018, Accepted Manuscript , DOI: 10.1039/C7CP07989C

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