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.
Group theory analysis and synchrotron x-ray diffraction measurements show that the recently proposed crystal structure of ζ-nitrogen is inconsistent with the available experimental data for that phase.
Elementary magnon, libron and vibron excitations as well as combined two-libron excitations of solid
α
oxygen have been investigated by means of Raman scattering at several isobars in
the pressure range up to 1.25 GPa. We deduced the band frequency, bandwidth
and relative band intensity of all modes as a function of temperature and
pressure. On the basis of these results we can exclude the possibility of all
second-order phase transitions in the low temperature, low pressure range of
oxygen stated in the literature. The disappearance of the sublattice magnetization
σ
could be estimated from the frequency of the higher energy magnon mode at the critical
pressure which is in the vicinity of the phase transition from the antiferromagnetic ordered
δ phase to the
non-magnetic ε
phase. The change of several spectroscopic features under increasing pressure
clearly indicates that anharmonic contributions in the libron potential are altered.
Elementary excitations ͑magnon, libron, vibron, and their combinations͒ of solid and liquid oxygen samples of high optical quality have been investigated by high resolution Raman spectroscopy in the temperature range 10-90 K. From spectra we deduced band frequency, bandwidth, and band shape of all modes as a function of temperature. In particular we registered in a very narrow temperature range ⌬TϽ0.5 K at the ␣- phase transition libron spectra of the ␣ phase as well as of the  phase; from the coexistence of both phases we can unambiguously follow that this phase transition is of first order. We deduced also from Raman spectra hints about the magnetic interaction, like vibron-magnon mode coupling ͑unexpected broad vibron band in ␣-O 2 ), or two libron excitations. A joint analysis of the vibron frequencies of isotopomers in the 16 O 2 sample allows us to describe the key characteristics of the fundamental energy zone ͑environmental and resonance frequency shifts͒ in both low temperature phases. The Raman-active phonon sideband of the internal vibrations in ␣-and -O 2 is clearly shifted towards higher frequencies compared to the ir-active one; i.e., both kinds of phonon sidebands therefore possess a different physical origin. We determined the integrated intensity of the magnon Raman band as a function of temperature which is proportional to the magnetic order parameter and which can be modeled by the spin-spin correlation function ͗S i S j ͘.
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