High P-T transformations of nitrogen to 170GPa

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Abstract

X-ray diffraction and optical spectroscopy techniques are used to characterize stable and metastable transformations of nitrogen compressed up to 170 GPa and heated above 2500 K. X-ray diffraction data show that varepsilon-N2 undergoes two successive structural changes to complex molecular phases zeta at 62 GPa and a newly discovered kappa at 110 GPa. The latter becomes an amorphous narrow gap semiconductor on further compression and if subjected to very high temperatures (approximately 2000 K) crystallizes to the crystalline cubic-gauche-N structure (cg-N) above 150 GPa. The diffraction data show that the transition to cg-N is accompanied by 15% volume reduction.

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Single-bonded cubic form of nitrogen.

Mikhail I. Eremets, Ivan A. Trojan, Dymitro A Dzivenko … (2004)

Nitrogen usually consists of molecules where two atoms are strongly triple-bonded. Here, we report on an allotropic form of nitrogen where all atoms are connected with single covalent bonds, similar to carbon atoms in diamond. The compound was synthesized directly from molecular nitrogen at temperatures above 2,000 K and pressures above 110 GPa using a laser-heated diamond cell. From X-ray and Raman scattering we have identified this as the long-sought-after polymeric nitrogen with the theoretically predicted cubic gauche structure (cg-N). This cubic phase has not been observed previously in any element. The phase is a stiff substance with bulk modulus >or=300 GPa, characteristic of strong covalent solids. The polymeric nitrogen is metastable, and contrasts with previously reported amorphous non-molecular nitrogen, which is most likely a mixture of small clusters of non-molecular phases. The cg-N represents a new class of single-bonded nitrogen materials with unique properties such as energy capacity: more than five times that of the most powerfully energetic materials.

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Polymeric nitrogen

C. Mailhiot, L. Yang, A McMahan (1992)

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Semiconducting non-molecular nitrogen up to 240 GPa and its low-pressure stability

Eugene Gregoryanz, Mikhail I. Eremets, Russell J Hemley … (2001)

The triple bond of diatomic nitrogen has among the greatest binding energies of any molecule. At low temperatures and pressures, nitrogen forms a molecular crystal in which these strong bonds co-exist with weak van der Waals interactions between molecules, producing an insulator with a large band gap. As the pressure is raised on molecular crystals, intermolecular interactions increase and the molecules eventually dissociate to form monoatomic metallic solids, as was first predicted for hydrogen. Theory predicts that, in a pressure range between 50 and 94 GPa, diatomic nitrogen can be transformed into a non-molecular framework or polymeric structure with potential use as a high-energy-density material. Here we show that the non-molecular phase of nitrogen is semiconducting up to at least 240 GPa, at which pressure the energy gap has decreased to 0.4 eV. At 300 K, this transition from insulating to semiconducting behaviour starts at a pressure of approximately 140 GPa, but shifts to much higher pressure with decreasing temperature. The transition also exhibits remarkably large hysteresis with an equilibrium transition estimated to be near 100 GPa. Moreover, we have succeeded in recovering the non-molecular phase of nitrogen at ambient pressure (at temperatures below 100 K), which could be of importance for practical use.

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Author and article information

Journal

Title: The Journal of Chemical Physics

Abbreviated Title: The Journal of Chemical Physics

Publisher: AIP Publishing

ISSN (Print): 0021-9606

ISSN (Electronic): 1089-7690

Publication date Created: May 14 2007

Publication date (Print): May 14 2007

Volume: 126

Issue: 18

Page: 184505

Article

DOI: 10.1063/1.2723069

PubMed ID: 17508809

SO-VID: 3acd50e5-cae1-4311-89e5-61f67f79a117

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High P-T transformations of nitrogen to 170GPa

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Gamma Delta T cells

Most cited references 26

Single-bonded cubic form of nitrogen.

Polymeric nitrogen

Semiconducting non-molecular nitrogen up to 240 GPa and its low-pressure stability

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