Metallization and Superconductivity in the Hydrogen-Rich Ionic Salt

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The Journal of Physical Chemistry

Metallization and superconductivity in the hydrogen-rich ionic salt BaReH9 Takaki Muramatsu1, Wilson K. Wanene2, Maddury Somayazulu1, Eugene Vinitsky1, Dhanesh Chandra2, Timothy A. Strobel1, Viktor V. Struzhkin1, and Russell J. Hemley1 1

Geophysical Laboratory, Carnegie Institution of Washington, Washington DC 20015 USA

2

Department of Metallurgical & Materials Engineering, University of Nevada, Reno, Nevada 89557 USA

BaReH9 is an exceedingly high hydrogen content metal hydride that is predicted to exhibit interesting behavior under pressure. The high-pressure electronic properties of this material were investigated using diamond-anvil cell electrical conductivity techniques to megabar (100 GPa) pressures. The measurements show that BeReH9 transforms to a metal and then superconductor above 100 GPa with a maximum Tc near 7 K. The occurrence of superconductivity is confirmed by the suppression of the resistance drop on application of magnetic fields. The transition to the metallic phase is sluggish, but is accelerated by laser irradiation. Raman scattering and x-ray diffraction measurements, used to supplement the electrical measurements, indicate that the Ba-Re sublattice is largely preserved on compression at the conditions explored, but there is a possibility that hydrogen atoms are gradually disordered under pressure. This is suggested from sharpening of peaks of Raman spectroscopy and x-ray diffraction by heat treatment as well as temperature dependence of resistance under pressure. The data suggest that the transition to the superconducting state is first order. The possibility that the transition is associated with the breakdown of BeReH9 is discussed.

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I. Introduction Compressed metallic hydrogen has received attention over the years in part due to its predicted high superconducting transition temperature Tc1-3 and the possibility of novel quantum states associated with combined superconductivity and superfluidity.4 Recent experimental and theoretical studies constrain the pressures required for achieving this unusual behavior to be above 400 GPa, above the range of current experimental methods for probing these predicted states.5-7 Alternatively, hydrogen-rich compounds that are predicted to exhibit analogous physical behavior have been considered.8-11 In these compounds, the hydrogen may be considered to be chemically precompressed at ambient pressure, thereby possibly lowering the applied pressure required to reach condensed matter regimes where this interesting electronic behavior may occur.12 Here we report that the very hydrogen-rich, insulating, ionic compound, BaReH9, transforms to a metallic compound under pressure and superconductivity emerges near 100 GPa with a Tc of 7 K. The observation of superconductivity is confirmed with in situ magnetic measurements. An unusual hysteresis and kinetic sluggishness of the insulator-tometal transition was also observed.

A number of candidate hydrogen-containing compounds have been studied under pressure to address this question. Of the Group 4 materials metallization of methane (CH4) requires higher pressure than that of hydrogen89 and non-decomposed silane (SiH4) also appears to remain nonmetallic to 150 GPa.10-11 Transition-metal and rare-earth hydrides in general consist of interstitial hydrogen atoms occupying octahedral or tetrahedral sites of the closepacked metal structures.13 Therefore, the maximum ratio of hydrogen to metal atom is constrained to 1:1 for transition metal hydrides and 1:3 for rare earth metal hydrides.14 In hydrides with ionic bonding between hydrogen and either alkali or alkali earth metal, the metal hydrogen ratio is constrained to either 1:1 or 1:2 for alkali and alkali earth, respectively, except at very high P-T conditions for some systems.15-17 In some metal hydrides showing superconductivity, the metal-hydrogen ratios are below 1:1 and they are solid solutions of hydrogen and the parent metals [e.g., PdHx (x 10 × 109 Ω) at room temperature. The resistance then falls into the measurable range at 10 – 20 GPa (R < 10 × 109 Ω). At higher pressure, the resistance gradually decreases with pressure, and we estimate that R < 10 × 106 Ω at about 60 GPa. The resistance changes are consistent with the visual observation of changes in sample reflectivity in incident light. Above 80 GPa in run 1, low power red laser irradiations were used to determine the pressure from ruby fluorescence. The radiation-induced resistance decrement in metallic behavior (dR/dT > 0) was then observed on cooling (Fig. 3). In the pressure range from 102 GPa to 147 GPa, resistance drops at low temperature were observed, indicative of a superconducting transition (Fig. 3 inset).

In order to compare the influence of laser irradiation on the sample in run 1, in the next run, sample exposure to the laser was minimized (t < 10 sec.) at each pressure when pressure was determined. In this run, the quasi-four-electrode technique was used. Clear metallic temperature dependence of resistivity (dR/dT >0) was not observed up to 139 GPa, whereas overall resistance monotonically decreases with pressure (Fig. 4a). The sample was left for an extended period of time (77 days) at room temperature over which time the electrical resistance was periodically measured. The resistance steadily decreased (Fig. 4b). A resistance drop consistent with the onset of superconductivity emerged at about 3 K at that pressure (Fig. 4d). Extrapolation of the data gives zero resistance at 2.2 K at 139 GPa. Figure 4e shows the resistance drop at 139 GPa in the presence of a magnetic field. We observe that the transition is suppressed above 5 T within the measured temperature range (T > 1.8 K), again consistent with the appearance of superconductivity. The maximum Tc is approximately 7 K. We also measured resistivity on decompression after reaching the superconducting state (Fig. 4c). The resistance increased abruptly on lowering the pressure, and the superconducting transition disappeared between 113

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GPa and 98 GPa; thus semiconducting behavior is recovered with large pressure hysteresis. The diamond anvils collapsed on further decompression below 71 GPa.

Run 3 was also conducted using the quasi-four-electrode technique and limited red laser irradiation to sample to minimize sample damage, possible disproportionation, and further distinguish between possible laser-induced versus pressure-induced transformations. Resistance measurements were performed as a function of temperature and pressure to 122 GPa. Overall, the P-T behavior was similar to that observed in the other runs (Fig. 5a). At 122 GPa, one of the diamond anvils cracked whereupon the pressure jumped down to 91 GPa. The sample at this pressure was found to be superconducting, as demonstrated by the suppression of the resistance drop by application of a magnetic field (Fig. 5d). Notably, two inflection points were apparent in these data. The resistance of the sample kept at this pressure systematically decreased over a period of 28 days (Fig. 5c). In cooling from room temperature, the resistance increased suddenly as a result of a crack on one of the diamond anvils, and on warming the diamond shattered at about 270 K (Fig. 5d).

To further understand the nature of the transitions, the material was examined by Raman scattering and x-ray diffraction in separate runs. For the Raman experiments, a red laser was used at low power in order to avoid photo-induced damage. The Raman triplet of Re-H modes (wagging, scissor, and twisting modes) around 1000 cm-1 was observed along with the stronger Re-H stretching mode (~2000 cm-1). The triplet weakens and broadens with pressure and became indistinct above 25 GPa while the intense Re-H stretching mode survived to much higher pressures (Fig. 6). The x-ray diffraction measurements, which probe the Ba and Re sublattice, were performed to 120 GPa. The overall pattern remained largely unchanged, though there is a weakening and broadening of the peaks (Fig. 7). Taken together, these observations are consistent with the preservation of the underlying Ba-Re sublattice with distortions and/or slight disordering of [ReH9]2- units on pure room-temperature compression of the material.

IV. Discussion The measurements show that BeReH9 transforms to a metal and then superconductor above 100 GPa with a maximum Tc near 7 K. The Tc is relatively low compared to other rich

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hydrogen systems and is in temperature range of some conventional superconductors. This fact reminds us of the importance of considering the contributions of metal ions to the electronic and vibrations states in addition to the metal-hydrogen ratio. Figure 8 shows our suggested ‘insulatormetal/superconductor’ P-T phase diagram of BaReH9. Based on the compression cycle of run 1 and decompression cycle of run 2, we bracket the insulator-metal transformation between 60 GPa and 80 GPa. We conclude that in these runs the superconducting state emerges at about 100 GPa and persists to at least 150 GPa. On the other hand, the transition behavior is complex: the transitions appear to be P-T path dependent, there are long-time kinetic effects, and the changes can be accelerated by laser irradiation. We discuss these factors below, beginning with the constraints on the pressure-induced changes in structure observed on room-temperature compression.

The Raman and x-ray diffraction data provide constraints on the nature of the transitions in the absence of laser-induced changes. As mentioned above, BaReH9 forms a hexagonal NiAstype structure (P63/mmc) at ambient pressure.20-21 Hydrogen positions of BaReH9 cannot be determined by standard x-ray diffraction techniques because of low x-ray scattering cross section of hydrogen. The proposed face-capped trigonal prismatic structure (point group symmetry D3h) of [ReH9]2- is derived from the related compound K2ReH9, for which the hydrogen positions were determined by neutron powder diffraction.19 Vibrational modes of [ReH9]2- observed by Raman and infrared spectra are reasonably consistent with density functional theory calculations based on this structure.25 At the lowest pressures, the bending modes and the stretching modes are intense and well resolved. The triplet and stronger singlet bands observed under pressure are plausibly assigned to H-Re-H wagging (925 cm-1), scissor (983cm-1), twisting (1034 cm-1) modes and Re-H stretching (1992 cm-1) mode respectively by the extrapolation to ambient pressure. With increasing pressure, both sets of modes broaden and weaken considerably (cf., Fig. 6). Above the 70 GPa, the Raman modes were difficult to measure, in part due to strong visible absorption by the sample.

The behavior of the Raman-active modes suggests that compression of the material along the P-T paths followed in this study leads to a frustrated structure in which the ReH9 prisms are highly distorted. The thermal annealing results in either restoring the structural integrity or

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transforming it to a new phase which is superconducting. This distortion of ReH9 may be responsible for the temperature dependences of resistance measured in runs 2 and run 3, and thermal annealing may be necessary to observe a clear metallic resistance behavior of resistance (dR/dT > 0) as observed in run 1. The principal peaks in the x-ray diffraction pattern persist to the highest pressures, implying that the skeletal NiAs structure remains largely unchanged (Fig. 7). However, the formation of a new metal hydride phase with a similar heavy atom sublattice cannot be ruled out. The significant weakening of Raman scattering peaks above about 40 GPa and lack of hydrogen information by x-ray diffractions hinder further detailed analysis on the structure of high pressure metal phase. We note there was no evidence of the loss of hydrogen in the material based on Raman measurements. Nevertheless, should such phases were to form, they may also be of interest as new hydrogen-rich materials.

Though reversibility of the electrical resistance excludes the possibility that changes over time arise from phase separation or elemental decomposition to form Ba or Re. On the other hand, the observation that metallization can be facilitated by laser irradiation raises the question of photo-induced breakdown of BaReH9 to form elemental Ba and Re, possible intermetallic phases, or new hydride phases (e.g., with loss of hydrogen).23 On the other hand, the Raman measurements reveal no discernible features in the region of the hydrogen vibron, despite long accumulation times (10 min.) at different positions on the sample at the pressure. Thus, there is no evidence for the formation of H2, either within a new phase as predicted23 or from decomposition as bulk hydrogen.6-7 We point out that high-pressure superconductivity has been reported for Ba and Re.26-28 However, we are not aware of any observations of Tc’s in the range found here for BaReH9 samples. The expected upper magnetic critical field (Hc2) exceeds 5 T based on extrapolation of the onset temperature to 0 K. The high Hc2 value of BaReH9 is inconsistent with the possibility of superconductivity arising from precipitation of Ba or Re metals (i.e., partial dissociation of the material) undetected by our x-ray diffraction measurements. We note that the two inflections observed in the resistance curves for run 3 are close the transition temperatures observed run 1 and run 2. However, the effect may be due to pressure gradients or coexistence of two high-pressure phases.

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Overall, the results provide evidence for interesting transition kinetics and possible metastable behavior of BaReH9 on compression and decompression. The observation of hysteresis and laser irradiation effects on phase behavior are relevant to the potential recoverability of novel high-pressure phases that have been reported in previous studies. Hydrogen is reported to become electrically conducting at 260 - 270 GPa and 295 K but with hysteresis associated with laser irradiation that facilitated the transition.29 It has been speculated that metallic conductivity of fluid molecular hydrogen at shock pressures of 140 GPa could exhibit hysteresis leading the recovery of a conducting form at lower pressures.30 Recently, the attainment of reported high Tc in compressed H2S, another hydrogen-rich material, was also found to be strongly dependent on P-T path.31 This observation has been the subject of a growing number of studies.32-36 Taken together, these results point to the importance for detailed characterization of hydrogen-rich materials along P-T-t paths in order to further understand the intriguing electronic properties of these materials and their possible recovery as to ambient conditions as kinetically or even thermodynamically stable materials.

V. Conclusions We document that BeReH9 transforms to a conducting metallic phase and then superconductor on compression to pressures above 100 GPa. a large pressure hysteresis is observed, and the transition is both P-T path dependent and accelerated by laser irradiation. The observations indicate the important role of long-time kinetics and metastability of the phase (or phases) formed on room temperature compression of the material. The occurrence of superconductivity is confirmed by the suppression of the resistance drop on application of magnetic fields. The observation of metallization and superconductivity in this very hydrogenrich salt may provide insight into light on the nature of dense hydrogen in its metallic state and predicted superconductivity. Further study of this and related novel hydrogen-rich compounds may lead to eventual recovery of new superconducting phases at ambient pressures that could serve as useful electronic materials.

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Acknowledgements This research was supported by EFree, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences under Award DESC0001057. The synthesis and characterization of BaReH9 was undertaken as a part of the research funded by DOE-BES (DE-FG02-06ER46280). The infrastructure and facilities used are supported by U.S. National Science Foundation (DMR-1106132) and the U.S. Department of Energy/National Nuclear Security Administration (DE-NA-00006, CDAC). Portions of this work were performed at HPCAT (Sector 16), Advanced Photon Source (APS), Argonne National Laboratory. HPCAT operations are supported by DOE-NNSA under Award No. DE-NA0001974 and DOE-BES under Award No. DE-FG02-99ER45775, with partial instrumentation funding by NSF. The Advanced Photon Source is a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357.

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25. Parker, S. F.; Refson, K.; Williams, K. P. J.; Braden, D. A.; Hudson, B. S.; Yvon, K., Spectroscopic and ab initio characterization of the [ReH9]2- ion. Inorg. Chem. 2006, 45, 10951-10957. 26. Chu, C. W.; Smith, T. F.; Gardner, W. E., Superconductivity of rhenium and some rheniumosmium alloys at high pressure. Phys. Rev. Lett. 1968, 20, 198-201. 27. Dunn, K. J.; Bundy, F. P., Pressure-induced superconductivity in strontium and barium. Phys. Rev. B 1982, 25, 194-197. 28. Takahama, K.; Matsuoka, T.; Shimizu, K. In Pressure effect on superconductivity of rhenium, SHOCK 13 Meeting of the American Physical Society, Portland, OR, July 20-25, 2013. 29. Eremets, M. I.; Troyan, I. A., Conductive dense hydrogen. Nature Mater. 2011, 10, 927-931. 30. Weir, S. T.; Mitchell, A. C.; Nellis, W. J., Metallization of fluid molecular hydrogen at 140 GPa (1.4 Mbar). Phys. Rev. Lett. 1996, 76, 1861-1863. 31. Drozdov, A. P.; Eremets, M. I.; Troyan, I. A., Conventional superconductivity at 190 K at high pressures. arXiv:1412.0460, submitted. 32. Duan, D.; Huang, X.; Tian, F.; Li, D.; Yu, H.; Liu, Y.; Ma, Y.; Liu, B.; Cui, T., Pressure induced decomposition of solid hydrogen sulfide. Phys. Rev. B 2015, 91, 180502. 33. Duraiski, A. P.; Szczesniak, R.; Li, Y., Non-BCS thermodynamic properties of H2S superconductor. Physica C 2015, 515. 34. Hirsch, J. E.; Marsiglio, F., Hole superconductivity in H2S and other sulfides under high pressure. Physica C 2015, 511, 45-49. 35. Bernstein, N.; Hellberg, C. S.; Johannes, M. D.; Mazin, I. I.; Mehl, M. J., What superconducts in sulfur hydrides under pressure and why. Phys. Rev. B 2015, 91, 060511. 36. Errea, I.; Calandra, M.; Pickard, C. J.; Nelson, J.; Needs, R. J.; Li, Y.; Liu, H.; Zhang, Y.; Ma, Y.; Mauri, F., High-pressure hydrogen sulfide from first principles: A strongly anharmonic phonon-mediated superconducor. Phys. Rev. Lett. 2015, 114, 157004.

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Figure Captions:

Figure 1. Hexagonal crystal structure of BaReH9 at ambient pressure. Propsed hydrogen positions (6h and 12k in space group P63/mmc) are indicated. Z = 2, Ba = green, Re = blue, H = red and pink

Figure 2. Sample photos in boron nitride gasket with platinum electrodes at 21 GPa in run 2. Left and right photos were taken by both transmission and reflection illumination lights and only transmission light respectively. The culet size is 100 µm in diameter and darker circle area is BaReH9 sample with the chamber size of about 60 µm in diameter.

Figure 3. Temperature dependence of electrical resistance of BaReH9 as a function of pressure to 147 GPa (run 1). At 59 GPa and 62 GPa, the resistance showed semiconducting behavior. At 80 GPa, the overall resistance significantly reduced from that at 62 GPa after long-term red laser irradiation and temperature dependence showed metallic behavior. At 102 GPa, an abrupt resistance drop was observed at about 7 K in temperature scans (see Fig. 4). Similar electrical resistance drops were also observed 5.8 K at 116 GPa and 4.5 K at 125 GPa. The 22 Ω loss of resistance (56 %) out of 39.2 Ω at 102 GPa indicates that the resistance drop was caused by the superconducting transition. The observed resistance drops cannot be explained by the Re gasket or solder and cable used in the experiment, since the resistance from the components is less than 2 Ω. In our He flow cryostat system, the lowest temperature is limited around 3 K so that residual electrical resistances were not observed; however, the resistance expected to decrease further at lower temperatures. The extra resistances of cables and electrodes using two electrodes technique (i.e., Pt electrodes, Cu cables and connectors) were less than 2 Ω at room temperature. The inset shows low temperature resistance in the metallic phase. Overall resistance decreases with pressure from 80 GPa to 147 GPa. At 80 GPa no resistance drop was observed in the range of the measurements. On the other hand, at 102 GPa and 116 GPa clear drops in resistance were observed at 7 K and 5.8 K, respectively. At 127 GPa the temperature showing resistance drop decreased to 4.5 K. The offset in resistance values as a function of temperature at 137 GPa and 147 GPa is within the limits of our experimental set-up. In both cases the resistances begin to dropped at the lowest temperatures. Figure 4. Effect of pressure, temperature, and magnetic field on the superconducting transition in BaReH9. (a) Temperature dependence of resistance of run 2 at 79 GPa to 139 GPa. (b) Time lapse of the temperature dependence of resistance at 139 GPa (run 2). (c) Decompression process of from 139 GPa to 71 GPa (run 2). (d) Resitatnce drop at 139 GPa in run 2. The estimated percentage of resistance drop is 90% based on linear extrapolation to 0 K. (e) Superconducting transition observed at 139 GPa at several magnetic fields (run 2).

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Figure 5. (a)Compression process of temperature dependence of electrical resistance in run 3. During the measurement at 122 GPa, a diamond anvil cracked and the pressure jumped down to 91 GPa. (d) The temperature dependence of electrical resistance at 74 GPa obtained on decompression from 91 GPa. In cooling from room temperature, the resistance increased because of the diamond crack and warming process diamond shattered at about 270 K. (c) The temperature dependence of electrical resistivity at 91 GPa in run 3. The resistance drop caused by superconductivity was observed. (d) Superconducting transition of run 3 as a function of magnetic fields at 91 GPa. Two inflection points are recognized in this transition and this may be due to pressure gradation on sample or coexistence of two high pressure phases.

Figure 6. Representative Rama spectra collected using the 660 nm excitation. To minimize damage due to laser irradiation, the spectra were collected at very low power levels (typically < 10 mW). Intense Re-H stretching mode at low pressure diminishes with pressure. Above about 50 GPa it is indistinguishable from background. Figure 7. Synchrotron powder x-ray diffraction patterns obtained at different pressures. No pressure medium was used. The pressure was determined using the diffraction from the gasket (tungsten). In the figure however, we have intentionally clipped the stronger diffraction lines from the gasket. (110) and (103) peaks are merged into the tungsten peak at 110 GPa. Other minor peaks disappeared due to weakening diffractions under high pressure. Remaining peaks (101) (102) (202) (300) at 110 GPa are still refined by hexagonal NiAs structure of the ambient pressure phase. Figure 8. Proposed ‘insulator-metal/superconductor’ P-T phase diagram of BaReH9. The insulator-tometal transition occurs between 62 and 80 GPa based on compression (run 1) and decompression (run 2).

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Figure 1. Hexagonal crystal structure of BaReH9 at ambient pressure; Ba = green, Re = blue, H = red and pink. Proposed hydrogen positions (6h and 12k in space group P63/mmc, Z = 2) are indicated. 156x174mm (300 x 300 DPI)



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Figure 2. Photomicrographs of a BaReH9 sample in a diamond-anvil cell for megabar electrical resistance measurements using a quasi-four-electrode technique. Left and right images were taken in combined transmitted and reflected light and transmitted light only, respectively. The image shows the c-BN gasket (orange color) with Pt electrodes (black/grey). The anvil culet size is 100 µm in diameter and darker circle area is the BaReH9 sample with the chamber size of about 60 µm in diameter. The sample is from at 21 GPa from run 2. 260x122mm (300 x 300 DPI)


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Figure 3. Temperature dependence of electrical resistance of BaReH9 as a function of pressure to 147 GPa in run 1. At 59 GPa and 62 GPa, the resistance showed semiconducting behavior. At 80 GPa, the overall resistance significantly reduced from that at 62 GPa after long-term red laser irradiation and temperature dependence showed metallic behavior. At 102 GPa, an abrupt resistance drop was observed at about 7 K in temperature scans. Similar electrical resistance drops were also observed 5.8 K at 116 GPa and 4.5 K at 125 GPa. The 22 Ω loss in resistance (56%) out of a total 39.2 Ω at 102 GPa is attributed to the onset of superconductivity. The observed resistance drops cannot be explained by the Re gasket or solder and cable used in the experiment, since the resistance from those components is less than 2 Ω. In our He flow cryostat system, the lowest temperature is limited around 3 K so that residual electrical resistances were not observed; however, the resistance expected to decrease further at lower temperatures. The extra resistance of cables and electrodes using the two electrodes technique (i.e., Pt electrodes, Cu cables and connectors) was less than 2 Ω at room temperature. The inset shows low temperature resistance in the metallic phase. The overall resistance decreases with pressure from 80 GPa to 147 GPa. At 80 GPa no resistance drop was observed in the range of the measurements. On the other hand, at 102 GPa and 116 GPa clear drops in resistance were observed at 7 K and 5.8 K, respectively. At 127 GPa the temperature showing resistance drop decreased to 4.5 K. The offset in resistance values as a function of temperature at 137 GPa and 147 GPa is within the limits of detectability. In both cases the resistance begins to drop at the lowest temperatures. 248x190mm (300 x 300 DPI)



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Figure 4. Effects of pressure, temperature, and magnetic field on the superconducting transition in BaReH9 in run 2. (a) Temperature dependence of resistance at 79 GPa to 139 GPa. (b) Time lapse of the temperature dependence of resistance at 139 GPa. (c) Decompression process of from 139 GPa to 71 GPa. (d) Resistance measured down to the lowest temperature and linear extrapolation of the data to 0 K at 139 GPa. (e) Effect of magnetic field on the resistance at 139 GPa. 261x223mm (300 x 300 DPI)


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Figure 5. Effects of pressure, temperature, and magnetic field on the superconducting transition in BaReH9 in run 3. (a) P-T dependence of electrical resistance in run 3. During the measurement at 122 GPa, a diamond anvil cracked and the pressure dropped 91 GPa. (b) Electrical resistance at 74 GPa obtained on decompression from 91 GPa. (c) Drop in electrical resistivity over an extended time at 91 GPa. (d) Superconducting transition as a function of magnetic fields at 91 GPa showing two apparent inflection points. 275x222mm (300 x 300 DPI)



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Figure 6. Representative Rama spectra collected using the 660 nm excitation. To minimize damage due to laser irradiation, the spectra were collected at very low power levels (typically < 10 mW). 185x227mm (300 x 300 DPI)


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Figure 7. Synchrotron powder x-ray diffraction patterns obtained at different pressures. No pressure medium was used. The pressure was determined using the diffraction from the W gasket. The stronger diffraction lines from the gasket are clipped and shaded. 194x242mm (300 x 300 DPI)



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Figure 8. Proposed ‘insulator-metal/superconductor’ P-T phase diagram of BaReH9. 187x177mm (300 x 300 DPI)


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Metallization and Superconductivity in the Hydrogen-Rich Ionic Salt

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