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Extreme pressure releases protons from highly compressed hydrogen bond networks.
Hydrogen bond networks play a crucial role in biomolecules and molecular materials such as ices. How these networks react to pressure directs their properties at extreme conditions. We have studied one of the simplest hydrogen bond formers, hydrogen chloride, from crystallization to metallization, covering a pressure range of more than 2.5 million atmospheres. Following hydrogen bond symmetrization, we identify a previously unknown phase by the appearance of new Raman modes and changes to x-ray diffraction patterns that contradict previous predictions. On further compression, a broad Raman band supersedes the well-defined excitations of phase V, despite retaining a crystalline chlorine substructure. We propose that this mode has its origin in proton (H+) mobility and disorder. Above 100 GPa, the optical bandgap closes linearly with extrapolated metallization at 240(10) GPa. Our findings suggest that proton dynamics can drive changes in these networks even at very high densities.
(A) Raman frequencies of HCl up to 200 GPa, and the top indicates the symmetric and antisymmetric H─Cl stretch, with their disappearance associated with the symmetrization of the hydrogen bond network. Square markers denote the HCl excitations that are found to be in an energy regime typical of librational movement (31). The filled symbols correspond to previous studies (31, 32). (B) Pressure dependence of the frequency of the broad disordered mode (DM) highlighted in (D), and dashed line indicates the approximate point of inflection. (C) Raman spectra of HCl phases III, IV, and V/VI up to 205 GPa. Stars indicate the excitations due to trace Cl2 impurities (47). (D) Selected Raman spectra above 89(10) GPa show that a broad band emerges, which increases in intensity while red shifting with pressure. The dashed black line represents a log-normal excitation profile frequently used in the characterization of the boson peak in disordered systems (64).
(A) Powder diffraction patterns of HCl with increasing pressure (λ = 0.4131 Å). Tick marks show the calculated peak positions, and indices are given for major reflections. Line colors denote phases II, IV, and V following Fig. 1. (B) Representative Le Bail refinement of HCl at 102(10) GPa. Experimental data are indicated with points, calculated profile is shown in black, and difference is shown as the lower red trace. Refined unit cell dimensions are a = 2.728(3) and c = 3.925(8) Å. (C) Crystal structures of phase IV (top) and phase V (bottom); new disordered H positions are shown in gray. Thin black lines outline unit cells, and thick green lines indicate the approximate fcc unit cell of the chlorine lattice.
(A) Average proton positions (blue) for phase V at 150 GPa and different temperatures compared to initial geometries (balls and sticks) from DFT MD simulations. From 750 K (at 95 GPa) and above, protons migrate to additional sites lying on Cl⋯Cl interatomic contacts. (B) Proton mean squared displacement MSDH(t) at 95 GPa (red) and 150 GPa (blue) and different temperatures (500 to 1000 K). Linear fits are indicated with black dashed lines. (C) Derived proton diffusion coefficients DH for HCl with increasing temperature.
(A) Left: Optical bandgap measurements, estimated from the absorption edge (see fig. S7), using various broadband light sources as a function of pressure. The dashed black line is a linear fit that indicates that the optical bandgap will close at around 240(10) GPa. Right: Sequential microphotographs of the HCl sample chamber from 10 to 256 GPa; the sample appears darker and redder with pressure as shorter wavelength light becomes attenuated. SC, supercontinuum. (B) Absorbance spectrum over the shortwave IR range. A large increase in absorbance is found between 164 and 198 GPa below 1.1 eV, indicative of free-charge carriers populating the conduction band. No detectable transmission was observed above 210 GPa down to 1700 nm, suggesting that the bandgap is less than 0.7 eV. OD, optical density; LED, light-emitting diode.
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