Anomalous behavior of the vibrational spectrum of the high-pressure δ phase of nitrogen: A second-order transition

In a careful Raman spectroscopic study on the high pressure δ phase of nitrogen we have observed an anomalous break in the slope of the vibrational Raman shift as a function of temperature. We interpret this as a second-order phase transition due to an orientational localization of the molecules, a phenomenon which has not previously been detected experimentally. Our results explain the change above 12 GPa in the experimental compressibility at room temperature measured by Olijnyk. The interpretation is consistent with the results of computer simulations.