Crystal Structure of the Superconducting Phase of Sulfur Hydride

Abstract

A superconducting critical temperature above 200 K has recently been discovered in H₂S (or D₂S) under high hydrostatic pressure1, 2. These measurements were interpreted in terms of a decomposition of these materials into elemental sulfur and a hydrogen-rich hydride that is responsible for the superconductivity, although direct experimental evidence for this mechanism has so far been lacking. Here we report the crystal structure of the superconducting phase of hydrogen sulfide (and deuterium sulfide) in the normal and superconducting states obtained by means of synchrotron X-ray diffraction measurements, combined with electrical resistance measurements at both room and low temperatures. We find that the superconducting phase is mostly in good agreement with theoretically predicted body-centered cubic (bcc) structure for H₃S (Ref.3). The presence of elemental sulfur is also manifest in the X-ray diffraction patterns, thus proving the decomposition mechanism of H₂S to H₃S + S under pressure4-6.

Figure 1. XRD of the sulfur hydride and sulfur deuteride samples.

a, Unrolled powder diffraction image of sulfur hydride at 150 GPa at room temperature recorded on the imaging plate. b, c, The integrated XRD patterns obtained with extraction of the background. The patterns of bcc H₃S and β-Po elemental sulfur at 150 GPa and 170 GPa calculated according to Ref., lie below the obtained patterns. The marks of star indicate the peaks which do not belong to the sample as follows from the scan over the sample (Fig. SI1): these peaks remain unchanged while the sample peaks change with radius of the sample both in position and intensity. Open circle indicates a reflection from high-pressure phase IV of elemental sulfur (incommensurately modulated body centered monoclinic structure). d, XRD patterns of sulfur deuteride at 173 GPa at 300 K and 13 K. The peaks marked by star (⋆) which do not the reflection from sample. The results of analyses are shown in table SI1 in supplementary information.

Figure 2. Pressure dependence of XRD in sulfur hydride and sulfur deuteride samples.

a, b, XRD patterns taken at room temperature and different pressures. Upper (red) and lower (green) ticks indicates the peak position of the predicted bcc structure of H₃S and β-Po elemental sulfur, respectively. The peaks marked with star do not belong to the sample as follows from Fig. SI1. On decreasing pressure in sulfur hydride, it is considerable that the phase transition of elemental sulfur is observed - the peak from β-Po sulfur gradually disappears and that from phase IV (open circle) is enhanced. c, Pressure dependence of the atomic volume of sulfur hydride and deuteride. The experimental data were obtained with increasing of pressure and are fitted with first-order Birch EoS (the black solid line). The volumes of hexagonal (R3m) and bcc (Im-3m) phases obtained from the theoretical work are shown with solid squares (■) and triangle (▲), respectively, and the broken line. The estimated standard deviations are smaller than the size of the symbols.

Figure 3. Pressure dependence of SC transition in sulfur hydride and sulfur deuteride.

a, b, Temperature dependence of resistance in sulfur hydride (on decease of pressure) and sulfur deuteride (on increase of pressure). c, Pressure dependence of critical temperature of superconductivity T_c of sulfur hydride (black points) and sulfur deuteride (red points). Open circles and squares are taken from Ref.. The points marked with solid symbols are from the present work: the circles represent data on decreasing of pressure, the squares and triangles – at increasing of pressure. Broken lines (black for sulfur hydride and red for sulfur deuteride) indicate the phase boundary between R3m and Im-3m structural phase. The error bars indicate the difference between T_c onset and the zero-resistance temperature at each pressures (Fig. 3a and 3b).