Physical Science Research Frontiers 2016 40 Formation process of superconducting phase of sulfur hydride with T c ~203 K At the end of 2014, a joint theoretical and experimental investigation broke the record for the superconducting critical temperature T c by more than 30 K, reaching 203 K for hydrogen sulfide under a high pressure of above 100 GPa,which was the first increase in 20 years [1]. This material has two superconducting phases. One phase, called the “low- T c phase”, is obtained by compressing over 100 GPa at a low temperature of around 200 K, and exhibits T c ~150 K at 200 GPa. The second phase, called the “high- T c phase”, exhibiting T c ~203 K, appears upon annealing the low- T c phase at room temperature. The material shows “conventional” superconductivity, which can be explained by Bardeen-Cooper-Schrieffer (BCS) theory, because of its strong isotope effect in the superconductivity of H 2 S and D 2 S. On the basis of this theory, Ashcroft proposed that metallic hydrogen and hydrogen-rich compounds will become high- T c superconductors because hydrogen is the lightest element and thus has the highest phonon frequency ω ph for a given value of T c among the elements in the periodic table [2]. For metallization, an extremely high pressure will be required (a pressure above 400 GPa was predicted for pure hydrogen in recent theoretical work). However, hydrogen-rich compounds are expected to require a lower pressure. On the basis of this proposal, some hydride superconductors have been searched theoretically, but only silane (SiH 4 ) with T c ~17 K has been observed experimentally so far. H 2 S has been theoretically predicted to have a relatively high T c of ~80 K above 100 GPa compared with other hydrides [3]. This value is consistent with the experimentally observed superconductivity in the low T c phase. However, T c of ~200 K does not follow from this prediction. Around the same time as Ref. 1, a theoretical work proposed that high-pressure-synthesized H 3 S will show superconductivity in its hexagonal and cubic structures under pressure [4]. In particular, the cubic structure was predicted to yield a high T c of ~200 K by contribution from metallic hydrogen realized in this phase. Other stoichiometries of H x S y were further studied theoretically by different groups in numerous works. Most of the results supported the hypothesis that the high T c is related not to H 2 S but to H 3 S in the cubic structure; H 2 S is unstable at high pressures and should decompose to sulfur and higher hydrides, probably to H 3 S (3H 2 S → 2H 3 S+S). Thus, it was suggested that molecular H 2 S decomposes to sulfur hydride with a higher content of hydrogen upon compression and annealing. In order to elucidate the mechanism of the superconductivity and search for synthesize materials having a higher T c towards achieving a room- temperature superconductor, the crystal structure of the superconducting phase and its phase diagram of sulfur hydride must be revealed. To investigate the formation process of the superconducting phases, we performed in situ X-ray diffraction (XRD), at the high-pressure beamline BL10XU , SPring-8 ( Fig. 1 ) using a diamond anvil cell (DAC). At this beamline, simultaneous XRD, electrical resistance and Raman spectroscopy measurements can be performed at low temperatures in the upstream experimental hutch. For the generation of an extremely high pressure of over 100 GPa, a small sample chamber with a diameter of less than 50 m m is needed. For such a small chamber, a focused X-ray beam with a diameter of about 2 m m can be used in the downstream hutch. The DAC was used for the generation of high pressures. Hydrogen sulfide loaded in a small hole in a rhenium gasket was set on a pressure cell with electrodes (vapor- deposited gold or platinum on a diamond surface) for electrical resistance measurement. Pressurization, XRD, resistivity and Raman scattering measurements with the DAC can be performed at the same time in a Gifford- McMahon type refrigerator in the beamline. The first-order Raman spectra of the diamond anvil was measured using the on-line Raman spectroscopy system in the beamline for pressure determination. H 2 S is a gas under ambient conditions and is liquefied by cooling to around 200 K. After loading liquid H 2 S into the sample chamber of the DAC, we pressurized H 2 S to over 100 GPa in the cryostat while maintaining a low temperature of ~200 K, and then the cell temperature was increased to room temperature. From the in situ XRD measurements, we observed that the intensity of the diffraction peaks from the sample suddenly decreases upon cooling to 200 K and the peaks disappeared at pressures of around 20 GPa at 200 K. This also occurred Fig. 1. Measurement system in the upstream-side chamber of BL10XU. X-ray diffraction, electrical resistance and Raman scattering measurements at low temperatures were performed at the same time. Imaging Plate/Flat Panel Raman Spectrometer Cryostat DAC X-ray Research Frontiers 2016 41 during compression to 150 GPa at 200 K and during heating to 256 K at around 150 GPa. A diffraction peak corresponding to the body-centered cubic (bcc) structure appeared from 260 K and its intensity was enhanced with increasing temperature up to room temperature. By using the X-ray scanning method with a focused synchrotron X-ray beam, the peaks of the obtained XRD profile of the high T c phase were separated into the theoretically predicted cubic H 3 S and the b -Po sulfur phase, which is the high-pressure phase of pure sulfur ( Fig. 2). Raman scattering measurements elucidated that there were no hydrogen molecules in the sample chamber. Therefore, it is considered that H 2 S lost its long-range periodic order as a result of low-temperature compression and dissociated into H 3 S and elemental sulfur. It was also confirmed that the cubic H 3 S is the high T c phase from the electrical resistance measurements. However, the XRD measurements were only able to determine the positions of sulfur atoms because of the relatively weak scattering of X-rays by hydrogen atoms. As shown in Fig. 3 , the theoretically predicted structures of the high T c phase have the same bcc symmetry of the positions of sulfur atoms in both structures and the only difference is in the positions of the hydrogen atoms. However, electrical resistance measurements might allow us to distinguish these phases from the change in the pressure dependence of T c around 150 GPa as shown by the dashed line in Fig. 3 [5]. In this article, the recent progress in the crystal structural investigation of the sulfur hydride system has been reported. The discovery of this high- T c superconductor has accelerated technical advances and both experimental and theoretical research on superconductivity at pressure of over 100 GPa. It is expected that the novel experimental process (compression at low temperatures) for obtaining the high- T c phase of sulfur hydride clarified in recent experiments will enable the high-pressure synthesis and further investigation of not only other hydrides but also new materials. Fig. 3. Pressure dependence of T c for high- T c phases. Open circles and squares are from Ref. 1, and closed circles and squares are from Ref. 5. The dashed line indicates the phase boundary between the hexagonal and cubic structures shown at the top of this figure. Fig. 2. XRD profiles of the sulfur hydride and pure sulfur along the line indicated in the image (150 GPa). The scanning was performed using an X-ray beam with a diameter of 2 m m from No. 1 to No. 10 shown in the photograph of the sample chamber. The red and green lines in the plots on the left indicate the reflections from cubic H 3 S and b -Po sulfur, respectively. Open circles indicate the reflection possibly from the high-pressure phase IV of elemental sulfur. The reflections indicated by the asterisks (*) do not belong to the sample. The center graph indicates the position dependence of the intensity of the main peaks of H 3 S and b -Po sulfur. References [1] A.P. Drozdov et al .: Nature 525 (2015) 73. [2] Y. Li et al .: J. Chem. Phys. 140 (2014) 174712. [3] N.W. Ashcroft: Phys. Rev. Lett. 21 (1968)1748; Phys. Rev. Lett. 92 (2004) 187002. [4] D. Duan et al. : Sci. Rep. 4 (2014) 6968. [5] M. Einaga, M. Sakata, T. Ishikawa, K. Shimizu, M.I. Eremets, A.P. Drozdov, Ivan A. Troyan, N. Hirao and Y. Ohishi: Nat. Phys. 12 (2016) 835. Mari Einaga KYOKUGEN, Osaka University Email: einaga@hpr.stec.es.osaka-u.ac.jp 30 μ m Electrodes Sample 2 (degrees) θ Intensity (arb. units ) 8 10 0 100 12 14 16 Intensity (arb. units) = 0.41377 Å * * * * * Sulfur Hydride S(X10) H 3 S λ Center 10 1 10 1 100 100 150 200 250 Hexagonal Cubic Sulfur hydride Sulfur deuteride 300 50 0 150 200 250 Temperature (K) Pressure (GPa) a a b b c c
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