Quantum hydrogen-bond symmetrization in the superconducting hydrogen sulfide system

Abstract

The quantum nature of the proton can crucially affect the structural and physical properties of hydrogen compounds. For example, in the high-pressure phases of H2O, quantum proton fluctuations lead to symmetrization of the hydrogen bond and reduce the boundary between asymmetric and symmetric structures in the phase diagram by 30 gigapascals (ref. 3). Here we show that an analogous quantum symmetrization occurs in the recently discovered sulfur hydride superconductor with a superconducting transition temperature Tc of 203 kelvin at 155 gigapascals--the highest Tc reported for any superconductor so far. Superconductivity occurs via the formation of a compound with chemical formula H3S (sulfur trihydride) with sulfur atoms arranged on a body-centred cubic lattice. If the hydrogen atoms are treated as classical particles, then for pressures greater than about 175 gigapascals they are predicted to sit exactly halfway between two sulfur atoms in a structure with Im3m symmetry. At lower pressures, the hydrogen atoms move to an off-centre position, forming a short H-S covalent bond and a longer H···S hydrogen bond in a structure with R3m symmetry. X-ray diffraction experiments confirm the H3S stoichiometry and the sulfur lattice sites, but were unable to discriminate between the two phases. Ab initio density-functional-theory calculations show that quantum nuclear motion lowers the symmetrization pressure by 72 gigapascals for H3S and by 60 gigapascals for D3S. Consequently, we predict that the Im3m phase dominates the pressure range within which the high Tc was measured. The observed pressure dependence of Tc is accurately reproduced in our calculations for the phase, but not for the R3m phase. Therefore, the quantum nature of the proton fundamentally changes the superconducting phase diagram of H3S.

Superconductivity in Hydrides Doped with Main Group Elements Under Pressure

“…Theoretical studies, which have shown that the superconductivity observed in Ref. [38] arises primarily from an H 3 S phase [40][41][42][43][44][45][46][47][48][49][50][51][52][53][54][55] whose structure has been recently confirmed experimentally [56], will not be described within this short review. We will, however, discuss the results of first principles calculations carried out for the hydrides of phosphorus [57][58][59].…”

Perspective: Role of structure prediction in materials discovery and design

“…113 As mentioned above, the cubic Im3m structure of H 3 S responsible for the very high T c in H 3 S is stabilized down to considerably lower pressures when anharmonic quantum nuclear zero-point motion is included. 19 Both lithium at low pressures and H 3 S-Im3m at high pressures are close to structural instabilities. Matthias emphasised the importance of structural symmetry in determining superconducting T c 's, 114 and empirical observations suggest that cubic symmetry is particularly efficacious.…”

“…Very recently, Errea et al 19 have demonstrated that the quantum hydrogen-bond symmetrization that occurs on formation of H 3 S-Im3m is responsible for the very high T c . They have also shown that anharmonic vibrations stabilise this structure down to lower pressures than would be obtained within the harmonic approximation.…”

“…The number of papers published on the theory and modelling of high T c superconductivity in hydrogen sulfide has increased rapidly since 2014, see for example Refs. [15][16][17][18][19][20][21][22][23][24][25][26][27].…”

Sulfur hydride and superconductivity theory

“…2 Their finding appears to vindicate the BCS theory because Tian Cui and his team had used the theory 3 to predict the record-breaking high T c before the experiment by Eremets and colleagues, and Ion Errea and coworkers later verified Cui and coworkers' results theoretically. 4 Both groups used the McMillan formula (derived from a generalized version of BCS theory), which relates T c to the electron-phonon coupling strength and the Coulomb pseudopotential, a measure of the Coulomb repulsion between electrons.…”