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.
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Cui et al. (1994) studied this question.
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