Hydrogen phase IV revisited via synchrotron infrared measurements in H2and D2up to 290 GPa at 296 K

Abstract

The phase transformations under pressure at 296 K in solid H${}_{2}$ and D${}_{2}$ are carefully investigated here up to 290 GPa by combining infrared (IR) and Raman spectroscopy measurements. Two phase transitions, previously detected by Raman studies, are clearly measured at the same pressures for both isotopes. The transition I-III is observed at 215 GPa characterized by a vibron Raman frequency discontinuity and the appearance of a strong IR vibron. At 237 GPa, the III-IV transition is observed associated to a large vibron Raman frequency discontinuity and the appearance of two IR and two Raman vibron modes. There is no sign of free charge carriers in the long-wavelength infrared absorption region, even up to 290 GPa. Hydrogen is thus not entering a metal nor a semimetal state in phase IV. The Raman and IR vibron spectra in phase IV seems to support the alternating layers structural models. However, the comparison for the IR and Raman vibron frequencies between present data and calculations indicates that a graphenelike sheet order is overestimating the coupling between H${}_{2}$ molecules.