It has been theorized that at high pressure the increased energy of the zero-point oscillations in hydrogen would destabilize the lattice and form a ground fluid state at 0 K (ref. 1). Theory has also suggested that this fluid state, representing a new state of matter, might have unusual properties governed by quantum effects, such as superfluidity or superconductivity2, 3. Here, by combining Raman spectroscopy and in situ high-temperature, high-pressure techniques, we demonstrate that above 200 GPa a new phase transition occurs as temperature is increased, for example 480 K at 255 GPa. If the transformation is interpreted as melting, it would be the lowest melting temperature of any material at these high pressures. We also find a new triple point between phases I and IV and the new phase, and demonstrate that hydrogen retains its molecular character around this point. These data may require a significant revision of the phase diagram of hydrogen above 200 GPa.
Figure 1: Representative Raman spectra, position and FWHM of the vibrational band in hydrogen.
a, Raman spectra on heating in phase I (dark green) and the HPHT phase (light blue) at pressures between 130 and 155 GPa. b,c, Position (b) and FWHM (c) of the vibrational mode ν1 at 130–155 GPa as a function of temperature through the phase I ↔ HPHT phase transformation, which is marked by the vertical dashed line. The inset to b shows a magnified view of the vibrational mode through the transformation. d, Representative Raman spectra of hydrogen on heating in phases III (orange) and I (dark green) at pressures between 220 and 225 GPa. e,f, Position (e) and FWHM (f) of the vibrational mode ν1 at 220–225 GPa as a function of temperature through the phase I ↔ phase III transformation, which is marked by the vertical dashed line. The broad peak between 1,333 and ~1,700 cm−1 in a and d is due to the stressed diamond. The observed DE frequency is indicated.
Figure 2: Representative Raman spectra of hydrogen on heating at different pressures.
a, Raman spectra on heating in phases IV and I and HPHT phase at 245 GPa. The observed DE frequency is indicated on room and highest temperature spectra. b–e, Positions (b,d) and FWHMs (c,e) of the vibrational modes ν1 and ν2 at 245 (b,c) and 250 GPa (d,e) in phase IV. Note that the positions and FWHMs at 250 GPa were measured with temperature decreasing. The error bars are from the fits to the data. The vertical dashed lines indicate the IV ↔ I ↔ HPHT phase transitions.
