Novel high-pressure nitrogen phase formed by compression at low temperature

Mungo Frost, Ross T. Howie, Philip Dalladay-Simpson, Alexander F. Goncharov, and Eugene Gregoryanz

Phys. Rev. B 93, 024113 – Published 29 January 2016

Abstract

Raman spectroscopy and powder x-ray diffraction methods have been used to characterize a novel phase of nitrogen which forms on compression from ambient pressure at low temperatures. The new, $λ$ , phase exhibits an exceptionally wide range of pressure stability from below 1 to 140 GPa, overlapping nine other known phases. On heating, its transformations are different to those observed in other phases, implying that the phase nitrogen adopts depends not only on $P − T$ path, but also on the initial structural configuration, which greatly complicates its phase diagram.

Received 20 November 2015

DOI:https://doi.org/10.1103/PhysRevB.93.024113

Physics Subject Headings (PhySH)

Phase diagrams

Pressure techniques Raman spectroscopy X-ray diffraction

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Mungo Frost¹, Ross T. Howie^1,2, Philip Dalladay-Simpson¹, Alexander F. Goncharov^3,4, and Eugene Gregoryanz^1,4,*

¹School of Physics and Astronomy and Centre for Science at Extreme Conditions, University of Edinburgh, Edinburgh EH9 3FD, United Kingdom
²Centre for High Pressure Science and Technology Advanced Research, Shanghai 201203, China
³Geophysical Laboratory, Carnegie Institution of Washington, Washington, DC 20015, USA
⁴Key Laboratory of Materials Physics, Institute of Solid State Physics, Chinese Academy of Sciences, Hefei, Anhui 230031, China

^*e.gregoryanz@ed.ac.uk

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Issue

Vol. 93, Iss. 2 — 1 January 2016

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Images

Figure 1
Transformation and phase diagram of nitrogen. The colored section and black line show the observed transitions from $λ − N_{2}$ . The black dashed lines show some of the $P − T$ paths which will produce $λ − N_{2}$ at low temperature and the dotted lines show some of the key $P − T$ paths used to investigate its phase transitions and the phase obtained. The gray lines show the phase diagram of nitrogen previously observed under compression at 300 K [1, 3, 14].
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Figure 2
Raman spectra of selected nitrogen phases. Top: Raman spectra of $ζ, θ$ , and $λ$ phases around 70 GPa showing the lower frequency vibrons of $λ$ and $θ$ and the low frequency (presumably) lattice modes. The weaker vibrons are shown in the inset. The $λ$ and $θ$ spectra are from the same sample before and after heating. Bottom: Positions of Raman peaks vs pressure. Circles show $λ − N_{2}$ with lines as a guide for the eye, solid circles are room temperature measurements, and open circles are those from low temperatures. Gray points show $ε$ (diagonal crosses) and $ζ$ (vertical crosses) phases obtained via compression at 300 K.
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Figure 3
X-ray powder diffraction of $λ − N_{2}$ using 0.424 18 Å radiation. Left: The integrated powder diffraction pattern (black) and LeBail fit (blue) with residuals taken at 84 GPa. Inset left: The same at 34 GPa. Inset right: The unintegrated pattern at 84 GPa. Right: Volume per nitrogen atom vs pressure. Blue circles are $λ − N_{2}$ , and uncertainties are significantly smaller than the plotting points (about $0.5 %$ ). The fitted Vinet equation of state is shown by the blue line. Diagonal crosses show data for $ε − N_{2}$ and vertical crosses show data for $ζ − N_{2}$ . Inset: The integrated powder diffraction data at various pressures.
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Figure 4
The structure of $λ − N_{2}$ using the atomic positions from theory [17]. Bonds are shown between nearest and next nearest neighbors to illustrate the layered structure. Top: A single layer of $λ − N_{2}$ showing the symmetric intermolecular bonds. Bottom: Viewed down the $b$ axis parallel to the sheets.
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Figure 5
Raman spectra of $λ − N_{2}$ being heated at 50 GPa showing the dramatic changes on conversion to the $ε − N_{2}$ phase starting at 519 K and completed by 533 K.
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Figure 6
Raman spectra of $λ − N_{2}$ being heated at 70 GPa. The transition to $θ − N_{2}$ is characterized by the emergence of the low frequency mode (marked with red arrows) at $229 {cm}^{- 1}$ starting just below 800 K and becoming more intense as the transition goes to completion. The highest frequency lattice mode (marked with blue arrows) is also lost into the strongest one in this transition.
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