Compression of ${\mathrm{D}}_2$ to 460 GPa and Isotopic Effects in the Path to Metal Hydrogen

How are nuclear quantum fluctuations affecting the properties of dense hydrogen approaching metallization? We report here Raman spectroscopy and synchrotron infrared absorption measurements on deuterium up to 460 GPa at 80 K. By comparing to a previous similar study on hydrogen, isotopic effects on the electronic and vibrational properties in phase III are disclosed. Also, evidence of a probable transition to metal deuterium is observed, shifted by about 35 GPa compared to that in hydrogen. Advanced calculations, quantifying a reduction of the band gap caused by nuclear quantum fluctuations, are compared to the present data.

Figure 1

Selected experimental data (a). Photographs of the deuterium sample under white light illumination above 400 GPa at 80 K. (b) $T_{2g}$ phonon Raman spectra of the diamond anvil tip, with a step shape pointing the wave number used to calculate pressure (as red dot) associated with the diamond tip–${\mathrm{D}}_2$ interface. (c) Infrared transmission spectra. Intrinsic absorption features due to deuterium are indicated by the red star pointing to the vibron peak and the red triangle pointing to the zeroing at high wave numbers due to the band gap decrease.

Figure 2

Wave number versus pressure of the vibron and of the phonon active modes in phase III at 80 K. (a) Infrared measurements, as illustrated by the spectra at 384 GPa (inset). The linear fit of the vibron wave number versus pressure is given by ${\nu }_{\text{vibron}}[{\mathrm{cm}}^{-1}]=3374.4-1.20P[\mathrm{GPa}]$ and that of the phonon wave number by ${\nu }_{\text{phonon}}[{\mathrm{cm}}^{-1}]=945.2–1.62P[\mathrm{GPa}]$. The scaled hydrogen phonon and vibron curves, as red-dashed lines, are obtained by a scaling of previous hydrogen data [11]. (b) Raman measurements, as illustrated by the spectra at 384 GPa (inset). The linear fit of the vibron wave number versus pressure is given by: ${\nu }_{\text{vibron}}[{\mathrm{cm}}^{-1}]=3145.2–1.94P[\mathrm{GPa}]$. Different symbols indicate different runs.

Figure 3

(a) Comparison of the IR absorbance spectrum measured in ${\mathrm{H}}_2$ and in ${\mathrm{D}}_2$ about 415 GPa at 80 K. The red square indicates the position of the direct band gap using the criteria of a $30000{\mathrm{cm}}^{-1}$ absorbance. (b) IR transmittance spectra using a pressure color scale. A different offset in applied for each pressure for better visualization of the onset of the zeroing energy, marked by a rectangle.

Figure 4

(a) Relative shift in the pressure evolution at 80 K of the direct band gap in hydrogen [11] and in deuterium. An abrupt evolution to below 0.1 eV is observed for hydrogen and for deuterium, respectively, at 425 and 460 GPa. (b) The band gap value of hydrogen is compared to various calculations taking into account NQF [20, 23, 24]. In the inset, the reduction of the direct band gap due to NQF is compared to experimental estimate.