Megabar pressure triple point in solid deuterium

Abstract
Solid orthodeuterium and mixed ortho-para crystals have been studied by infrared and Raman spectroscopy to a pressure of 186 GPa. We observe two clear phase transition lines and a triple point where they meet; no critical point is observed as reported for hydrogen. We believe that previous reports of a critical point in hydrogen are erroneous. Symmetry arguments are used to identify and narrow down the possible structures for the various phases.