New superhydride discovered
Research team uncover quantum stabilized manganese superhydride at extreme pressures.
Over the past 10 years, the application of high-pressure physics has proved an effective synthesis route to produce hydrogen-rich materials that would otherwise be unattainable. Amongst them, so-called metal superhydrides: hydrogen-rich compounds containing five or more hydrogen atoms per metal atom, many of which exhibit fascinating properties such as high-temperature superconductivity. While superhydrides can form with a number of rare earth metal species, they are a rarity amongst the group IV-XII transition metals - in fact, only a single example, iron hydride (FeH5), has been reported to date.
In work published in Angewandte Chemie, a research team based in the School’s Institute for Condensed Matter and Complex Systems report the synthesis of the second known transition metal superhydride, MnH7, at pressures above 118 GPa - around 1.2 million atmospheres. Using a tiny sample just 10 micrometers in size (smaller than the width of a human hair), and held between the tips of two opposing diamonds, the team was able to measure various structural and spectroscopic properties. These measurements were accompanied by density functional theory calculations - a method used to obtain the binding energies of materials at an atomistic level. These calculations were useful for determining the number and location of hydrogen atoms within the sample.
Surprisingly, the crystal structure of MnH7 turned out to be completely different from the phase predicted by theory to be the most stable. This result can be explained by the rules of quantum mechanics. Even at absolute zero temperature, nuclei are not fixed. Instead, they remain spread in a range of positions and retain a specific energy, the so-called zero-point energy. This energy, the lowest possible energy state of the quantum nuclear system, depends on the crystal structure of the compound, and is more important for light elements, such as hydrogen.
In MnH7, the role of zero-point energy is so important that, once included in the calculations, the previously most stable structure (which features hydrogen molecules) was superseded by the experimentally determined structure (which has hydrogen atoms) as the most stable phase. This suggests that the new superhydride MnH7 forms because of the quantum nature of the proton within the hydrogen atoms’ nucleus.
The first author of this study is Postdoctoral Research Associate, Dr Mikhail Kuzovnikov. The experimental work was led by Dr Ross Howie and supported by ERC (European Research Council) Starting Grant 'MetElOne', whilst the theoretical work was led by Professor Andreas Hermann.
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