Hydrogen sulfide (H2S) is a prototype molecular system and a sister molecule of water (H2O). The phase diagram of solid H2S at high pressures remains largely unexplored arising from the challenges in dealing with the pressure-induced weakening of S-H bond and larger atomic core difference between H and S. Metallization is yet achieved for H2O, but it was observed for H2S above 96 GPa. However, the metallic structure of H2S remains elusive, greatly impeding the understanding of its metallicity and the potential superconductivity. We have performed an extensive structural study on solid H2S at pressure ranges of 10-200 GPa through an unbiased structure prediction method based on particle swarm optimization algorithm. Besides the findings of candidate structures for nonmetallic phases IV and V, we are able to establish stable metallic structures violating an earlier proposal of elemental decomposition into sulfur and hydrogen [R. Rousseau, M. Boero, M. Bernasconi, M. Parrinello, and K. Terakura, Phys. Rev. Lett. 85, 1254 (2000)]. Our study unravels a superconductive potential of metallic H2S with an estimated maximal transition temperature of ∼80 K at 160 GPa, higher than those predicted for most archetypal hydrogen-containing compounds (e.g., SiH4, GeH4, etc.).

We use first principles calculations to study structural, vibrational and superconducting properties of H2S at pressures P ≥ 200 GPa. The inclusion of zero point energy leads to two different possible dissociations of H2S, namely 3H2S → 2H3S + S and 5H2S → 3H3S + HS2, where both H3S and HS2 are metallic. For H3S, we perform non-perturbative calculations of anharmonic effects within the self-consistent harmonic approximation and show that the harmonic approximation strongly overestimates the electron-phonon interaction (λ ≈ 2.64 at 200 GPa) and Tc. Anharmonicity hardens H-S bond-stretching modes and softens H-S bond-bending modes. As a result, the electronphonon coupling is suppressed by 30% (λ ≈ 1.84 at 200 GPa). Moreover, while at the harmonic level Tc decreases with increasing pressure, the inclusion of anharmonicity leads to a Tc that is almost independent of pressure. High pressure hydrogen sulfide is a strongly anharmonic superconductor.Cuprates [1] have for many years held the world record for the highest superconducting critical temperature (T c = 133 K) [2]. However, despite almost 30 years of intensive research, the physical mechanism responsible for such a high T c is still elusive, although the general consensus is that it is highly non-conventional. The discovery by Drozdov et al.[3] of T c = 190 K in a diamond anvil cell loaded with hydrogen sulfide (H 2 S) and compressed to about 200 GPa breaks the cuprates record and overturns the conventional wisdom that such a high T c cannot be obtained via phonon-mediated pairing.The claim that hydrogen at high pressure could be superconducting is not new [4] and it was recently supported by first principles calculations based on the harmonic approximation applied to dense hydrogen [5][6][7][8] and several hydrides [9][10][11][12][13][14][15]. More recently, two theoretical papers predicted the occurrence of high T c superconductivity in high-pressure sulfur-hydrides [16,17]. However, as shown in Refs. [18,19], anharmonicity can be crucial in these systems. For example, in PdH, the electron-phonon coupling λ parameter is found to be 1.55 at the harmonic level, while a proper inclusion of anharmonic effects leads to λ = 0.40 [18], in better agreement with experiments. Thus, in hydrogen-based compounds, the phonon spectra are strongly affected by anharmonic effects.Several first principles calculations [16,17,20,26] suggested that decomposition of the H 2 S sample occurs within the diamond-anvil cell at high pressures. The high-T c superconducting material is therefore very unlikely to be H 2 S, while H 3 S is the obvious candidate for the H-rich decomposition product.Here we study the structural, vibrational and superconducting properties of H 2 S above 200 GPa, where the highest T c occurs. We show that the inclusion of zero point motion in the convex hull at 200 and 250 GPa stabilizes two metallic structures, H 3 S and HS 2 . Finally, we show that, contrary to suggestions in previous work [16,20], the harmonic approximation does not explain the measured T c ...

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.

The search for high-temperature superconductors has been focused on compounds containing a large fraction of hydrogen, such as SiH4(H2)2, CaH6 and KH6. Through a systematic investigation of yttrium hydrides at different hydrogen contents using an structure prediction method based on the particle swarm optimization algorithm, we have predicted two new yttrium hydrides (YH4 andYH6), which are stable above 110 GPa. Three types of hydrogen species with increased H contents were found, monatomic H in YH3, monatomic H+molecular “H2” in YH4 and hexagonal “H6” unit in YH6. Interestingly, H atoms in YH6 form sodalite-like cage sublattice with centered Y atom. Electron-phonon calculations revealed the superconductive potential of YH4 and YH6 with estimated transition temperatures (Tc) of 84–95 K and 251–264 K at 120 GPa, respectively. These values are higher than the predicted maximal Tc of 40 K in YH3.

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