High-pressure phases of solid nitrogen by Raman and infrared spectroscopy

Abstract

Raman and infrared spectra of solid nitrogen have been collected between 25 K and room temperature up to 41 GPa. A careful analysis of the spectral band transformations occurring across the high pressure transitions among the δ, δloc, ε, and ζ phases allowed to define the phase diagram in the whole P-T region investigated. In particular, the transition between the ε and ζ phases has been observed in the range 30–230 K and the corresponding phase-boundary drawn. A significant metastability region (spanning about 10 GPa in pressure) hinders the transformation between the ε and ζ phases when pressure is varied at low temperature. Group theory arguments suggest a centrosymmetric structure for the ζ phase and the number of Raman and infrared ν1 and ν2 components can be reproduced both with cubic and tetragonal structures. An appreciable coupling among neighboring molecules is observed, at room temperature, only in the ε phase where the relative orientations of the molecules are fixed.

Ab initiorandom structure searching

“…The phase diagram of nitrogen has been much studied, with a number of apparently stable and metastable molecular phases having been reported [52,53,54], although their structures are mostly unknown. We found a new class of molecular structures which we predicted to be more stable than previously suggested ones over a wide range of pressures [55], see figure 11.…”

Sodium pentazolate: A nitrogen rich high energy density material

“…Their high energy content is due to the large amount of energy released upon decomposition of single-and/or double-bonded nitrogen in the condensed phase into triple-bonded gas-phase diatomic N 2 molecules. Solid phases of nitrogen at low pressure and temperature are composed of weakly interacting N 2 molecules, leading to a plethora of stable molecular crystal phases [5,6]. It was predicted in 1992 that at 50 GPa the ϵ-N 2 molecular crystal transforms into the single-bonded cubic-gauche (cg) polymeric phase of nitrogen [7], which was eventually synthesized by Eremets et al [8] .…”

Solid Nitrogen at Extreme Conditions of High Pressure and Temperature

“…The δ phase (Fig. 2) is proposed to have a disordered cubic structure (space group Pm3n) [22] with two different types of molecules exhibiting sphere -and disk-like disorder and giving rise to two classes of A portion of the phase diagram of nitrogen at moderate pressure and temperature conditions [9,11,12,25]. Dashed gray line and thin solid line at high temperature correspond to the melting curve from Refs.…”

“…1) is complex at moderate pressures and temperatures [9,10] and until recently has been little studied over a wider range. The melting curve of N 2 has been only measured to about 22 GPa [11][12][13].…”

“…Higher P-T data are available from shock-wave CARS study [14], which refer to a liquid state. Solid nitrogen has a wealth of molecular phases that differ in the types of orientational ordering and crystal structures formed [9,10,12,13,[15][16][17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33]. The ordering of the low pressuretemperature α and γ phases is controlled by quadrupole-quadrupole interactions, whereas at higher pressures a class of molecular structures (δ, δ loc , ε, ζ) stabilized by additional anisotropic intermolecular interactions is found [10].…”