Structural Studies on the Cu–H System under Compression

Abstract Hydrogen chemistry at extreme pressures is currently subject to extensive research due to the observed and predicted enhanced physical properties when hydrogen is incorporated in numerous binary systems. Despite the high reactivity of hydrogen, the noble metals (Cu, Ag, and Au) display an outstanding resilience to hydride formation, with no reports of a stable compound with a hydrogen molar ratio ≥ 1 at room temperature. Here, through extreme compression and in situ laser heating of pure copper in a hydrogen atmosphere, we explore the affinity of these elements to adopt binary compounds. We report on the phase behavior and stabilities in the Cu–H system, analyzed via synchrotron X-ray diffraction, up to pressures of 50 GPa. We confirm the existence of the previously reported γ0-CuH0.15, γ1-CuH0.5, and e-Cu2H phases. Most notably, we report the highest hydrogen-content noble-metal hydride stable at room temperature to date: γ2-CuH0.65, which was synthesized through laser heating. This study furthers our understanding of hydrogen-transition metal chemistry and may find applicability in future hydrogen-storage applications.


Dear Editor,
We are submitting our study entitled "Structural studies on the Cu-H system under compression" for your consideration. In this letter, we present a structural characterization of copper hydride formation using X-ray diffraction, laser heating and Diamond Anvil Cell (DAC) techniques.
Hydrogen chemistry is currently of intense interest, with hydrogen-bearing systems (hydrides) displaying remarkable properties, such as the H2S system with reports of high crtitical temperatures for superconductivity. Hydrides could also find commercial application, proving to be volume-efficient tools for hydrogen-storage, with pressure acting as a mechanism to readily trap or release hydrogen gas. Despite this extensive research activity and also hydrogen's reactivity there are still binary-systems which have not been explored, with the noble metals (group 11) making a notable gap in the periodic table.
In this work, with the use of high-pressures, in-situ laser heating and sychrotron x-ray diffraction we explore the Cu-H system up to pressures of 50 GPa and temperatures in excess of 1000 K. We report the previously identified γ 0 -CuH 0.15 , γ 1 -CuH 0.5 and ε-Cu 2 H phases. Crucially, however we we identify a new γ-phase, γ 2 -CuH 0.65 , synthesised via laser heating. This discovery is the highest reported hydrogen content for a group-11 system and strongly suggestive that a fully stoichiometric might be stable at higher pressures. This work will motivate the re-examination and thorough exploration of many other metalhydrogen systems and perhaps encourage the realisation of the other elusive group- 11  Currently, hydrogen chemistry at extreme pressures is subject to extensive research due to the observed and predicted enhanced physical properties when it is incorporated in numerous binary systems. Despite hydrogen's high reactivity, the noble metals (Cu, Ag, and Au) display an outstanding resilience to hydride formation, with no reports of a stable compound with a hydrogen molar ratio ≥ 1 at room temperature. Here, through extreme compression and in-situ laser heating of pure copper in a hydrogen atmosphere, we explore their affinity to adopt binary compounds. We report on the phase behaviour and stabilities in the Cu-H system, analysed via synchrotron X-ray diffraction, up to pressures of 50 GPa. We confirm the existence of the previously reported γ 0 -CuH 0.15 , γ 1 -CuH 0.5 and -Cu 2 H phases. Most notably, we report the highest hydrogen content noble-metal hydride stable at room temperature to date, γ 2 -CuH 0.65 , sythesised through laser heating. This study furthers our understanding of hydrogen-transition metal chemistry and could find applicability in future hydrogen-storage applications.

I. INTRODUCTION
Hydrogen-bearing systems are currently of intense interest, due to their desirable physical properties and as possible hydrogen storage materials. Physical properties such as superconductivity at high temperatures have been reported in the hydrogen-sulfur system 1 , whilst their storage capabilities are best exemplified by the hydrogen amassed in methanehydrogen, CH 4 (H 2 ) 4 2 , and more recently in the hydrogen-iodane system, HI(H 2 ) 13 3 . Despite these prospects and intensive research activity in this field, there remain many systems which have not been explored with the noble metals making a notable hydride-gap in the periodic table.
The noble metals, Cu, Ag and Au are relatively inert under ambient conditions, as seen by their reluctance to form oxides under ambient conditions, an attractive quality for their usage in ancient coinage and electronics. High-pressure has become an indispensable tool in modifying chemical affinities and thereby creating exotic materials [4][5][6][7] . Typically for hydride formation in d-metals, the barrier for molecular dissociation is driven down by pressure, resulting in atomic hydrogen permeating freely through the metallic lattice, tending to reside at interstitial sites. The presence of atomic hydrogen in the metal can lead to changes in the crystalline structure ranging from simple lattice expansion to reconstructive phase transitions and changes in space-group symmetry 8 .
Despite numerous attempts using complex synthesis techniques such as high pressures coupled with resistive and laser heating 9,10 , the definitive formation of Au-H and Ag-H has remained elusive, likely due to their large reduction potentials. On the other hand, Cu, with a reduction potential approximately half that of Ag, has been known to form a binary system with hydrogen for over 100 years, making it the first metal hydride to be discovered 11 .
The Cu-H system is found to have an extensive chemistry with some very unusual chemical pathways, such as sonofication 12  Compressing this phase above 10 GPa leads to further hydrogen entering the lattice forming the second γ 1 -CuH 0.5 phase 9 . In both phases the lattice expansion is a consequence of hydrogen filling the octahedral interstitial sites. A subsequent study, using synchrotron Xray diffraction, identified an -Cu 2 H phase 10 , distinctly different from the previously reported Wurzite-CuH and γ phases, synthesised around 18.6 GPa. This phase adopts an anti-CdI 2 type structure and its stoichiometry was constrained from significantly reduced volumes when compared to the formerly identified phases. As Cu-H has the highest chemical affinity with hydrogen of all the group-11 metals, it is imperative that we endeavour to fully describe its chemistry and perhaps reveal higher hydrogen stoichiometries, as reported for other dmetal systems 15,16 .
Here, with the use of high-pressure, in-situ laser heating and synchrotron X-ray diffraction we explore the Cu-H system up to pressures of 50 GPa and temperatures in excess of 1000 K.
We report the previously identified high-pressure phases, γ 0 -CuH 0.15 , γ 1 -CuH 0.5 9 and Cu 2 H 10 , and constrain their pressure evolution up to 50 GPa respectively. Crucially we identify a new γ phase synthesized by laser heating Cu in a dense hydrogen atmosphere. Unusually this phase decomposes into γ 1 -CuH rather than the -Cu 2 H phase previously found to be stable at these pressures. This work illustrates the complexities that may be found in the noble-metal hydride phase diagrams when explored with the latest experimental techniques.

II. EXPERIMENTAL METHODS
The diamond-anvil cell (DAC) is the workhorse in most static high-pressure experiments.
Its compact and simple design has proven powerful and adaptable finding itself as a principal driving force in high-pressure science for the last 30 years. The concept is simple, by cou- High-purity Cu-grains (Alfa Aesar 99.9 %), approximately 10 µm in size, were placed so that they were centered on a diamond anvil. The DAC was calibrated such that it hermetically sealed in a 2 kbar hydrogen atmosphere (research grade 99.9995%). The loading procedure resulted in significant excess of hydrogen, to promote hydride formation, whilst also providing hydrostatic conditions for synthesised samples.
The Cu-sample was heated in-situ from both sides uniaxially by directly coupling to IR lasers. Powder X-ray diffraction data were collected at the GSECARS beamline at APS, USA . The diffraction from 0.3344Å wavelength X-rays was recorded using a Pilatus 1M image-plate detector, after which it was integrated using DIOPTAS 19 software to a twodimensional data set. The data were subsequently indexed and further underwent Le Bail refinement using Jana2006 20 .

III. RESULTS AND DISCUSSION
Although now a mature field in its own right, modern high-pressure chemistry dates back Cu-H system, in doing so reporting a noble metal-hydride material with the highest reported hydrogen stoichiometry at ambient temperature.
The use of synchrotron radiation has greatly expedited the exploration of high-pressure systems, with their brilliance and tight focus finding great applicability with sample sizes on the order of tens of microns. In this study, harnessing synchrotron generated light, we have ascertained structural insight in the Cu-H system using conventional powder X-ray diffraction, as these studies only require a fraction of accumulation time when compared with conventional in-house sources 9 we present a thorough mapping of phases in the Cu-H system. Through the high-quality spectra obtained, which can be readily seen in Figures 1 and   3, we have identified the appearance of four phases in the Cu-H system: γ 0 -CuH 0.15 , Cu 2 H, γ 1 -CuH 0.5 and γ 2 -CuH 0.65 , the latter two requiring high-temperatures (discussed later). As discussed previously, the Cu-H system has been well characterised at ambient temperatures up to 50 GPa in previous studies 9,10 and our data up to 25 GPa at room temperature are in agreement, observing the direct reaction between Cu and its surrounding H 2 atmosphere.
The reaction product is readily seen by the appearance of diffraction peaks, Figure 1 Therefore, despite revealing CuH 0.65 with a marked 30% molar increase in hydrogen content, the mechanical properties remain largely unchanged. However, although there is not a marked change in their mechanical properties, these materials' electronic properties can be altered profoundly. As reported for the Pd-H system, the absorption of hydrogen to form the monohydride PdH, results in an enhanced critical temperature for superconductivity when compared with elemental Pd (greater than 3 orders of magnitude) 24 . The same has also been proposed for other transition metal hydrides, such as PtH 25,26 . Studying the impact on the electrical propoerties of nobel metal-hydrides through the incorporation of hydrogen would be of particular interest, as these materials find widespread industrial and commercial application due to their desirable conductive properties.
From the highest pressure point of 50 GPa samples were decompressed to determine the stability range of the newly formed hydride phases. γ 2 -CuH 0.65 was found to be stable to 32 GPa (Figure 3) where interestingly it decomposed to γ 1 -CuH 0.5 , rather than the -Cu 2 H phase which is the favoured phase under room temperature compression 10  moderate pressures used here. This method is applicable to any metal-hydrogen system and may be of interest in the study of high-performance alloys under extreme conditions.

IV. CONCLUSIONS
In summary, using high-pressure powder X-ray diffraction, we have explored the Cu-H 2 system up to pressures of 50 GPa. In agreement with previous studies we have verified the synthesis of -Cu 2 H, γ 0 -CuH 0.15 and γ 1 -CuH 0.5 compounds, further constraining their structural evolution with pressure. We find that a second, more hydrogen-rich, γ 2 -CuH 0.65 phase is formed after laser heating above 30 GPa, and is the highest hydrogen content system known to exist for the noble metals stable at room temperatures. This study and its incorporated understanding will reinvigorate interest in H-rich hydrogen bearing materials and the realisation of the other noble metal hydrides, Au-H and Ag-H. GSECARS beamline, we would like to thank Eran Greenberg and Vitali Prakapenka for their assistance during the course of the data collection.