Entropy differences between H₂ and D₂ and the phase diagram of hydrogen isotopes

A unique feature of quasi-isentropic dynamic compression experiments investigating the transition from the molecular state to the monatomic metallic fluid state in hydrogen isotopes is the manipulation and control of the entropy of the sample. This entropy, once initially set, is maintained nearly constant during each experiment. Furthermore, entropy can be determined reliably because it is set by the amplitude of the well-diagnosed initial shock in a pressure-temperature regime (P < 5 GPa, T < 300 K) where equation of state models are well calibrated. Reliable entropy measures can therefore be assigned to the optical signatures of absorption (band gap closure) and reflectivity increase (metallization) observed during the experiments. Of particular interest is the entropy difference observed between deuterium and hydrogen for the two types of optical signature. We analyze the entropy data extracted from recent NIF experiments and find results that may suggest new insights into the nature of the hydrogen phase diagram.