No 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.

An experimental setup for Fourier transform infrared (FTIR) studies in condensed matter at high pressure and low temperatures is described. We have adapted a close-cycle cryostat (T=20–300 K) to the sample compartment, which is used as a cryo chamber, of a FTIR spectrometer (frequency range 10–15 000 cm−1). A Cassegrain-type beam condenser is assembled to measure infrared absorptions of samples contained in a membrane diamond anvil cell (P up to 100 GPa). The tuning of the pressure and the cell alignment is performed from outside the evacuated instrument. An additional light path allows visual observation and in situ pressure calibration. The advantages of this system, demonstrated by its application to CH4 and Ar–(H2)2 crystals, are high radiation throughput, long time stability, visual observation of the sample, remote measurement and variation of the local pressure, and remote alignment of the cell with the IR beam.

The pressure induced reactivity of carbon monoxide was investigated in a wide temperature range (100-400 K) completely avoiding any irradiation of the sample with visible or higher frequency light. FTIR spectroscopy was employed to monitor the reaction and infrared sensors for measuring the pressure. With this approach we have been able to separate the effects of the three variables (P, T and hnu) that establish the conditions for the occurrence of the chemical reaction. A new instability boundary, not affected by the photoactivation of the reaction, is provided. The reaction has been studied in three different crystal phases (epsilon, delta, and beta), but the small differences in the reaction products are ascribable to the temperature changes rather than to the crystalline arrangement. For T<300 K the analysis of the IR spectra reveals the formation of an extended amorphous material formed, according to the vibrational assignment and to the kinetic data, by polycarbonyl linear chains containing a large amount of anhydride groups. For T>or=300 K the formation of carbon dioxide and epoxy rings, and the simultaneous decrease of carbonyl species, let suppose a decarboxylation of the extended solid product. Once exposed to the atmosphere, the reaction product readily and irreversibly reacts with water giving rise to carboxylic groups.