High pressure synthesis and stability of cobalt hydrides

Abstract

In situ high-pressure high-temperature X-ray powder diffraction studies of the cobalt-hydrogen system reveal the direct synthesis of both the binary cobalt hydride (CoH) and a novel cobalt dihydride (CoH). We observe the formation of fcc CoH at pressures of 4 GPa, which persists to pressures of 45 GPa. At this pressure, we see the emergence with time of a further expanded fcc lattice, which we identify as CoH, where the hydrogen atoms occupy the tetrahedral vacancies. We have explored alternative synthesis routes of CoH and can lower the synthesis pressure to 35 GPa by the application of high temperature. CoH is stable to at least 55 GPa and decomposes into CoH below 10 GPa, releasing molecular hydrogen before further decomposing completely into its constituent elements below 3 GPa. As a first-row transition metal, cobalt has a relatively lower mass than other hydride-forming transition metals, and as a result, CoH has a high hydrogen content of 3.3 wt. % and a volumetric hydrogen density of 214 g/l.

Synthesis ofNi2H3at high temperatures and pressures

“…This observation is in good agreement with previous measurements [16,19,33]. Neutron diffraction has shown the H atoms to occupy the larger octahedral vacancies in this phase, in a similar fashion to CoH and other transition-metal hydrides [10,17,34,35]. In the absence of heating, this structure remains stable on compression to at least 52 GPa, which is in good agreement with theoretical predictions [20].…”

“…With the current consensus that reaching a metallic state of hydrogen remains outwith today's capabilities of diamond-anvil cell (DAC) experiments, research interests have shifted to hydrogen-dominant metallic alloys, which have also been predicted to exhibit exotic properties, such as high-temperature superconductivity [4][5][6][7][8][9]. To date the formation of hydrogen-bearing species has been theoretically explored for the majority of elements in the periodic table and recently, polyhydrides of first-row transition metals have been synthesized at high pressures [10][11][12]. However, there remains a gulf between the number of systems explored at pressures above 50 GPa and the extensive predictions in the literature.…”

“…In situ laser heating under compressed hydrogen has been successful in synthesizing new transition-metal compounds, particularly polyhydrides of Fe, Cr, and Co [10][11][12]36,37] and was applied here to explore the possibility of synthesizing nickel polyhydride species. Samples of γ 2 -NiH+H 2 were compressed to 34, 40, and 52 GPa and laser heated at each pressure.…”

Synthesis of Superconducting Cobalt Trihydride

“…12 Through a combination of high pressure and/or high temperature, TMH with more unusual stochiometries become energetically competitive. [13][14][15][16][17][18][19][20][21][22] Iron exhibits several polyhydride species, of which FeH 5 (I4/mmm), synthesised at 120 GPa and temperatures >1500 K, is the most hydrogen-rich and possesses a unique layered structure. 23 Potential superconductivity in FeH 5 has been a subject of debate, with two theoretical works predicting remarkably high T c values ranging between 46 -51 K, and conversely, another computational study finding no superconductivity.…”

Praseodymium polyhydrides synthesized at high temperatures and pressures

“…Laser heating of metals in a high-pressure hydrogen environment has been a successful synthetic tool to overcome kinetic barriers and promote the formation of metal hydrides with unexpected stoichiometries. [4,5,[30][31][32][33][34][35]. In this work, PrH 4+x in a H 2 medium was laser heated using a 1064 nm Nd:YAG laser at pressures above 85 GPa.…”