Raman Investigation of the N₂−O₂ Binary System as a Function of Pressure and Temperature

Abstract

We produced mixtures of N2-O2 with different concentrations and performed low-temperature Raman studies at ambient and high pressures. From spectra in vibron and phonon regions, we determined band frequency, bandwidth, and band intensity as a function of temperature, pressure, and concentration. We determined the vibron Raman cross-sections and deduced the true concentrations of mixtures from vibron Raman band intensities. These concentrations were different from those determined from partial gas pressure of the initial gaseous mixtures. From fingerprints in Raman spectra, such as jumps in band frequencies or additional band splitting, we were able to prove phase transitions and propose a preliminary T-x phase diagram. We compared this diagram with two reported in the literature from structural analysis. Comparing all three variants of the T-x phase diagrams we found several discrepancies and inconsistencies, which we associate with different solid sample production techniques. Since we could prove that our samples were in thermodynamic equilibrium, we are convinced that we improved the known phase diagram substantially. From Raman band intensities of the O2 vibrations in different phases of N2 and O2, we were able to determine quantitatively the solubility of O2 in N2. Preliminary Raman studies of 2% and 7% O2 in N2 at high pressure and low temperatures showed that a larger amount of O2 can be dissolved in N2 than at ambient pressure. At the critical pressure (p approximately 15 GPa) we found from Raman spectra that O2 is demixed from 7% O2 in N2 to form epsilon-O2. This was previously called a "new phase" in the literature and not understood up to now. Finally, from band frequencies we determined the environmental shift of oxygen molecules in the mixture which is related to the intermolecular potential U(N2-O2) between different types of molecules.

From optical spectra to phase diagrams—the binary mixture N2–CO

“…In Raman studies the real concentration is easily found from relative intensities in Raman spectra and known Raman cross sections for vibrations (see Fig. 2 in [8]). In FTIR studies about N 2 -O 2 mixtures [9], which are IR inactive, we determined the actual concentration by means of chromatography and mass-spectrometry; after measurements the sample was evaporated into a test volume and was then analyzed.…”

Fourier transform infrared studies of the N2–O2 binary system

“…To avoid partial demixing in the gas-liquid region (see Fig. 2 [1]) and via contact between vapors over the sample and the gas mixing system we followed here another strategy. The cell temperature was set at T = 11 K, and the cell was filled with the immediately freezing gaseous mixture (couple of seconds), so that there was no chance for a concentration change.…”

“…Mixtures of these components were much less studied on the contrary, because of several reasons: due to the complexity in results after pure systems, due to missing theoretical modeling, due to problems in producing samples with sufficiently good crystal quality. In a recent paper [1] we could prove that discrepancies in published phase diagrams T-x% of N 2 -O 2 [2,3] are most likely due to thermodynamic instable samples. The general aim of this paper [1] was to deduce from fingerprints in vibron and phonon spectra lines of phase transitions, to prove reproducibility and reliability of our statements by cooling/heating cycles, to determine quantitatively from relative Raman vibron band intensities the solubility of N 2 in O 2 or vice versa.…”

“…The general aim of this paper [1] was to deduce from fingerprints in vibron and phonon spectra lines of phase transitions, to prove reproducibility and reliability of our statements by cooling/heating cycles, to determine quantitatively from relative Raman vibron band intensities the solubility of N 2 in O 2 or vice versa. In paper [1] we reported only about results from Raman spectra.…”

“…as it is expected from group theory. Also the second statement in [5] about the tuning of the magnon intensity in a-O 2 by doping with N 2 impurities (10% to 50%) is also more than questionable; because it is agreed now [1][2][3] that only less or about 1% of N 2 can be solved in a * -O 2 . Therefore only the magnon absorption of pure O 2 and the one with £ 1% N 2 should be compared.…”