Quantum hydrogen-bond symmetrization in the superconducting hydrogen sulfide system
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Errea, I., Calandra, M., Pickard, C. J., Nelson, J., Needs, R. J., Li, Y., Liu, H., et al. (2016). Quantum hydrogen-bond symmetrization in the superconducting hydrogen sulfide system. Nature, 532 81-84. https://doi.org/10.1038/nature17175
Hydrogen compounds are peculiar in that the quantum nature of the proton can crucially affect their structural and physical properties. A remarkable example is the high-pressure phases [1, 2] of H₂O, where quantum proton fluctuations favor symmetrization of the H bond and lower by 30 GPa the boundary between asymmetric and symmetric structures . Here we show that an analogous quantum symmetrization occurs in the recently discovered  sulfur hydride superconductor with a record superconducting critical temperature T_c = 203 K at 155 GPa. Superconductivity occurs via formation of a structure of stoichiometry H₃S with S atoms arranged on a body-centered-cubic (bcc) lattice [5–9]. If the H atoms are treated as classical particles, then for P ≳ 175 GPa they are predicted to sit midway between two S atoms in a structure with Im¯3m symmetry. At lower pressures the H atoms move to an off-center position forming a short H−S covalent bond and a longer H· · · S hydrogen bond in a structure with R3m symmetry [5–9]. X-ray diffraction experiments confirmed the H₃S stoichiometry and the S lattice sites, but were unable to discriminate between the two phases . Our present ab initio density-functional theory (DFT) calculations show that quantum nuclear motion lowers the symmetrization pressure by 72 GPa for H₃S and by 60 GPa for D₃S. Consequently, we predict that the Im¯3m phase dominates the pressure range within which a high T_c was measured. The observed pressure-dependence of Tc is closely reproduced in our calculations for the Im¯3m phase, but not for the R3m phase. Thus, the quantum nature of the proton fundamentally changes the superconducting phase diagram of H₃S.
We acknowledge financial support from the Spanish Ministry of Economy and Competitiveness (FIS2013- 48286-C2-2-P), French Agence Nationale de la Recherche (Grant No. ANR-13-IS10-0003- 392 01), EPSRC (UK) (Grant No. EP/J017639/1), Cambridge Commonwealth Trust, National Natural Science Foundation of China (Grants No. 11204111, 11404148, and 11274136), and 2012 Changjiang Scholars Program of China. Work at Carnegie was supported by EFree, an Energy Frontier Research Center funded by the DOE, Office of Science, Basic Energy Sciences under Award No. DE-SC-0001057. Computer facilities were provided by the PRACE project AESFT and the Donostia International Physics Center (DIPC).
External DOI: https://doi.org/10.1038/nature17175
This record's URL: https://www.repository.cam.ac.uk/handle/1810/253595