Raman studies on hot dense hydrogen
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The study of hydrogen and the understanding of its response to extended pressure and temperatures is of great importance due to its significant universal abundance. Hydrogen is currently not well understood in these extended regimes due to it being inherently difficult to work with experimentally as well as having a poor response to a wide range of diagnostics. Consequently, there have been long standing predicted phenomena which still remain experimentally elusive: (1) melting driven by large zero point oscillations [Brovman 72] and (2) adopting a purely atomic state at higher pressures [Wigner 35]. Coupling high-pressure, high-temperature techniques with in-situ optical diagnostics, the stability of the solid phases of hydrogen were evaluated over an extended pressure-temperature regime. The first ever H2 solid-solid transitions above 300 K are reported and the evolution of the I-IV phase line after the III-IV-I triple point is constrained. A transition which could be attributed to melting is observed at 480 K and 225 GPa, the lowest known melting temperature for any material under these conditions. A new triple point between phase I-IV-Liquid is identified, the third known triple point in the phase diagram and the first on the melting curve. The possible continuations of the melting line are discussed, ultimately revising the melting transition at 300 K and at 0 K to much higher pressures than previously thought [Bonev 04]. The contributing work also marks a new high pressure achievement, obtaining some of the highest ever recorded static pressures in the laboratory. Hydrogen and its heavier isotopes hydrogen deuteride and deuterium were compressed to pressures of 384 GPa, 388 GPa and 380 GPa respectively. These experimental data are indicative that above 325 GPa H2 and HD adopt a new solid phase, phase V. Analysis of the spectra over the IV-V transition is suggestive that under compression the molecular bonding in the G-layers of the Pc structure lengthen and symmetrise, evolving into the Ibam structure. It is speculated that this phase could be a precursor to the elusive, purely atomic I41/amd structure predicted to be stable at higher pressures (>400 GPa) [McMahon 11, Azadi 14].