Rh-hydrogen system under high pressure and low temperature Bing Li1,4, *, Yang Ding1, Duck Young Kim2, Rajeev Ahuja3, Viktor Struzhkin2, WengeYang1, Guangtian Zou4, Ho-Kwang (Dave) Mao1,2,*. 1HPSynC, Carnegie Institution of Washington, 9700 South Cass Avenue, Argonne, Illinois 60439, USA 2Geophysical Laboratory, Carnegie Institution of Washington, Washington, D.C. 20015, USA 3Condensed Matter Theory Group, Department of Physics and Astronomy, Uppsala University, Box 530 SE-751 21, Uppsala, Sweden 4 State Key Laboratory of Superhard Materials, Jilin University, Changchun 130012, China (*: Email address: email@example.com and firstname.lastname@example.org) Abstract Heavy metal hydrides are of great scientific interest for they can become potential high Tc superconductors and hydrogen storage materials. Rhodium metal, as a member of platinum group metals, is the second one after palladium found to form hydride. At ambient condition, rhodium metal adopts FCC structure and it forms a hydride, namely RhH, under high hydrogen pressure around 3.8 GPa. RhH remains in FCC structure but expands a lot from its Rhodium metal matrix. To further explore rhodium-hydrogen system, we performed some in-situ studies on this system to higher pressures and low temperatures range. We discovered that RhH transforms to RhH2 at around 8 GPa, which still remains in an FCC structure. Compared with RhH, RhH2 has much larger volume expansion during the phase transition (~ 1.5 times larger than the first one). Low temperature transport measurement results showed that RhH and RhH2 both showed metal behaviors. Moreover, our thermodynamic stability study of RhH2 showed that it can be quenched up to liquid nitrogen temperature at ambient pressure. These results promise the practical applications using this higher hydrogen storage capability of RhH2 at ambient pressure in the futurecontaining very high amount of hydrogen per volume (163 kg H2/m3). Results 1 Methods Room temperature high pressure experiment Diamond anvil cell (DAC) is used to generate pressure (Fig 1). Sample preparation is done in Geophysical Lab using the gas loading system; powder rhodium was loaded with gaseous hydrogen into a gasket hole to form the rhodium-hydrogen system (Fig 2). X-ray experiments were done inSector 16-IDB and IDD (HPCAT) at Advanced Photon Source, Argonne National Lab (Fig 3, 4).Low temperature was achieved in a liquid flow He At room temperature, we performed high pressure experiments on rhodium-hydrogen system (Fig 6-7 XRD experiment; Fig 8 Absorption spectra). Our results confirmed previous high pressure work that at about 4 GPa rhodium metal starts to react with hydrogen and forms a rhodium hydride (RhH). Furthermore, at higher pressure, a new high pressure phase is formed above ~8 GPa. type cryostat, a gear box was used to adjust pressure during low temperature experiments. 4-probe electrical resistance measurement were done in GL (Fig 5) Fig. 1 Diamond anvil cell Fig. 7 Evolution of XRD with increasing (left) and decreasing (right) pressure at room temperature Fig. 8 Evolution of absorption spectra with increasing pressure at room temperature (from top to bottom) Fig. 6 XRD images Fig. 2 sample con-figuration in DAC Fig. 5 Four-probe setup in GL Fig. 3 X-ray absorption setup at 16-IDD Fig. 4 XRD setup at 16-IDB Results 2 Results 3 Low temperature high pressure experiment From the X-ray diffraction experiment combined with theoretical calculations, we assigned the new high pressure phase RhH2. It is interesting that all these Rh, RhH and RhH2 are fcc structures (Fig 9 left), the hydrogen atoms first occupy octahedral sites in RhH, and then go into the tetrahedral site in RhH2 phase. Fig 9 (right) shows the unit cell volume changes according to pressure, there are two abrupt volume expansion during the phase transitions, the second expansion is about 1.5 times larger than the first one. The dash lines are from theoretical calculations, which are well agreed with experiment data. At room temperature upon releasing pressure, RhH2 transforms back to RhH at about 4 GPa and then back to rhodium at around 3 GPa, suggesting the phase transition is reversible with some hysteresis. We also performed low-temperature high-pressure experiments on the new high pressure phase RhH2. Fig 10(a) shows the results when pressure was released from 19 GPa to 1 bar. The x ray diffraction patterns did not change dramatically, which indicates the new high pressure phase — RhH2 is quenchable to ambient pressure at 6.1 Kelvin. When the system was warmed up to about 100K (Fig 10b), RhH2 partially transformed to RhH forming a RhH-RhH2 mixture phase between 100~150 Kelvin. Keeping warming up, the sample turned back to rhodium metal above 199K (the peak marked with * comes from gasket). So with temperature goes up, RhH2 lost hydrogen step by step, finally turned back to Rh metal. We also performed transport measurement on this system, Fig 11 shows the temperature dependence of the electrical resistance of Rh, RhH and RhH2 below 100K. The curves indicate the metallic behavior for RhH and RhH2, and no superconductivities were found above 5K in these phases. 100um Fig. 9 Crystal structures of Rh, RhH and RhH2 (left). Volume changes of rhodium-hydrogen system depending on pressure at room temperature. The second volume expansion is about 1.5 times larger than the first one. Fig. 11 T-dependence of the electrical resistance of Rh, RhH and RhH2measured at temperatures below 50 K. Fig. 10 XRD of RhH2 at 6Kelvin when releasing pressure (a) and XRD at ambient pressure when warming up (b). Conclusions • 1.We discovered a new high pressure phase of rhodium hydride — RhH2 — above 8 GPa at room temperature. The phase transitions are reversible with large hysteresis at room temperature. The RhH2 contains very high amount of hydrogen per volume (163 kg H2/m3) and moderate amount by mass (1.9 mass%), and is the only dihydride in the platinum group metals. • 2.This new RhH2 phase can be quenched to ambient pressure up to 100 K. Upon warming up to room temperature, RhH2 losses hydrogen in two steps, firstly to RhH1 and finally to rhodium metal. The results indicate the practical applicationwith muchhigher hydrogen storagecapabilityof RhH2at ambient pressure. • 3.Low temperature resistance measurement results show that both RhH and RhH2 are metallic phases. References  J.-M. Joubert International jonual of hydrogen energy 35 (2010) 2104 .  MarekTkacz J. Chem. Phys., Vol. 108, No. 5, 1 February 1998  V.E. Antonov, T.E. Antonova, V.K. Fedotov, and B.A. Gnesin, Journal of Alloys and Compounds 446–447 (2007) 508.  V.E. Antovov, I.T. Belash, O.V. Zharikov. and A. V. Palichenko, phys. stat. sol. (b) 142 (1987) K155. Acknowledgement HPSynC is supported as part of EFree, an Energy Frontier Research Center funded by the U.S. Department of Energy (DOE), Office of Science, Office of Basic Energy Sciences (BES) under Grant Number DE-SC0001057.