We present the discovery of a novel nitrogen phase synthesized using laser-heated diamond anvil cells at pressures between 120-180 GPa well above the stability field of cubic gauche (cg)-N. This new phase is characterized by its singly bonded, layered polymeric (LP) structure similar to the predicted Pba2 and two colossal Raman bands (at ∼1000 and 1300  cm^{-1} at 150 GPa), arising from two groups of highly polarized nitrogen atoms in the bulk and surface of the layer, respectively. The present result also provides a new constraint for the nitrogen phase diagram, highlighting an unusual symmetry-lowering 3D cg-N to 2D LP-N transition and thereby the enhanced electrostatic contribution to the stabilization of this densely packed LP-N (ρ=4.85  g/cm^{3} at 120 GPa).

Dense nitrogen exhibits fascinating molecular and extended polymorphs as well as an anomalous melt maximum at high temperatures. However, the exact solid-liquid phase boundary is still the subject of debate, as both creating and probing hot dense nitrogen, solid and fluid alike, poses unique experimental challenges. Raman studies of nitrogen were performed to investigate the melting curve and solid-solid phase transitions in the pressure-temperature range of 25 to 103 GPa and 300 to 2000 K. The solid-liquid phase boundary has been probed with time-resolved Raman spectroscopy on ramp heated nitrogen in diamond anvil cell (DAC), showing a melting maximum at 73 GPa and 1690 K. The solid-solid phase boundaries have been measured with spatially resolved micro-confocal Raman spectroscopy on resistively heated DAC, probing the δ-ɛ phase line to 47 GPa and 914 K. At higher pressures the θ-phase was produced upon a repeated thermal heating of the ζ-phase, yet no evidence was found for the ι-phase. Hence, the present results signify the path dependence of dense nitrogen phases and provide new constraints for the phase diagram.

We have studied the pressure-induced physical and chemical transformations of tetracyanoethylene (TCNE or C6N4) in diamond anvil cells using micro-Raman spectroscopy, laser-heating, emission spectroscopy, and synchrotron x-ray diffraction. The results indicate that TCNE in a quasi-hydrostatic condition undergoes a shear-induced phase transition at 10 GPa and then a chemical change to two-dimensional (2D) C=N polymers above 14 GPa. These phase and chemical transformations depend strongly on the state of stress in the sample and occur sluggishly in non-hydrostatic conditions over a large pressure range between 7 and 14 GPa. The x-ray diffraction data indicate that the phase transition occurs isostructurally within the monoclinic structure (P21∕c) without any apparent volume discontinuity and the C=N polymer is highly disordered but remains stable to 60 GPa-the maximum pressure studied. On the other hand, laser-heating of the C=N polymer above 25 GPa further converts to a theoretically predicted 3D C-N network structure, evident from an emergence of new Raman νs(C-N) at 1404 cm(-1) at 25 GPa and the visual appearance of translucent solid. The C-N product is, however, unstable upon pressure unloading below 10 GPa, resulting in a grayish powder that can be considered as nano-diamonds with high-nitrogen content at ambient pressure. The C-N product shows a strong emission line centered at 640 nm at 30 GPa, which linearly shifts toward shorter wavelength at the rate of -1.38 nm∕GPa. We conjecture that the observed red shift upon unloading pressure is due to increase of defects in the C-N product and thereby weakening of C-N bonds.

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Abstract. Solidification and crystal growth processes of hydrogen (H 2 ) have been studied under dynamic compression using dynamic-DAC (dDAC) in conjunction with time resolved Raman spectroscopy and high-speed microphotography. Liquid H 2 was compressed at a compression rate of 43 GPa/s across the liquid/solid phase boundary, causing a discontinuous vibron shift at the onset of solidification. The real time sample images, on the other hand, showed that H 2 solidifies into a characteristic, grain boundary free crystal, formed initially from the outside (or the edge of liquid) then grew into the central area within 11 ms. Interestingly, the time scale associated with the glassy solid formation is in good agreement with that of the discontinuous Raman frequency shift. The rate of crystal growth was measured to be 0.3 cm/s. IntroductionCrystallization is a first order phase transition whereby nucleation is controlled by two factors; the difference in bulk free energy of the liquid and solid, and the interfacial free energy of the crystallite [1]. The interplay between the thermodynamics and kinetics of a system directs growth rates and polymorphism generating complexity in understanding crystallization process, which remains challenging even for simple systems [1,2]. In efforts to better understand the process of solidification in simple molecular systems a wealth of research has come from computational work of crystallization in super-cooled liquids [3][4][5][6][7][8]. Despite the significance, the experimental work studying the crystallization process of low-Z elemental molecules is lacking, as they are gaseous at ambient pressures [9]. Hydrogen, in particular, is of intense interest with predictions of conductivity in the condensed state [10] and superfluidity in the deeply cooled liquid [11]. Still, there is an incomplete understanding of the solidification and crystal growth process in hydrogen, especially at room temperature under high compression. This is, in part, due to a lacking diagnostic technique capable of probing the rapid process of solidification at time scales less than 1 s.In this paper we provide insight into the crystallization process of by novel spectroscopic and visual means in a highly compressed environment. To study the dynamic phenomena of rapid events such as solidification and melting in-situ the use of the dynamic diamond anvil cell (dDAC) [12] was employed in conjunction with time-resolved Raman spectroscopy (TRS). The dDAC has been used extensively to study pressure-induced crystal growth kinetics and morphology changes in water [13,14]. More interestingly, dDAC allows for the study of formation of metastable structures in supercompressed materials [15]. It has been well demonstrated that subjecting materials to sufficiently fast shock pressurization or thermal quenching may force a material of a particular phase out of its region