Vibrational Dynamics of Isotopically Dilute Nitrogen to 104 GPa

Raman studies to 104 GPa of a ¹⁴N₂ host with dilute ¹⁵N₂ and ¹⁴N¹⁵N isotopic species indicate an increase in vibrational coupling with pressure. The lowest frequency branch of the isotopic species ¹⁵N₂ reaches a plateau around 100 GPa suggesting the onset of bond weakening for molecules located on particular sites. An unusual intensity increase is observed for the isotopic ν₁ guest modes. The phase transitions above 20 GPa appear to involve an increase in the number of inequivalent sites, with possibly subtle changes in the orientational behavior of particular molecules in the pressure range 25 to 50 GPa not readily revealed in recent x-ray diffraction studies.