A key challenge in the implementation of a hydrogen economy is storage. An alternative to hydrogen gas is the use of metal hydrides, which form a framework into which the hydrogen atoms sit.
In this study, published in Inorganic Chemistry, ISIS postdoc Zhongsheng Wei used the Pearl instrument at ISIS to study the in situ formation of molybdenum hydride. Using the Paris–Edinburgh press and the support of the sample environment team, the team used neutron diffraction to study the structure of the material as it formed, at pressures of over 6 GPa. The deuterated form was studied to avoid the incoherent scattering from hydrogen atoms.
“Neutron scattering is particularly good for studying metal deuterides as the technique is highly sensitive to light elements such as deuterium, allowing for precise structural analysis even in the presence of heavier elements," explains Zhongsheng. “This enabled us to see the locations and occupancies of the deuterium atoms in the structure."
The large sample volume that the Paris-Edinburgh press can hold means that it is possible to follow the synthesis of a more industrially relevant volume and measure the bulk structure and not just the structure at the surface. Above ~4 GPa, their diffraction data revealed the formation of a hexagonal deuteride phase with an over-stoichiometric deuterium content, calculating a composition of MoD1.15.
“We were able to unambiguously determine the D:Mo ratio as well as the locations of the nonstoichiometric D atoms," she adds.
The refined atomic positions and interatomic distances provide insight into the interactions between deuterium and molybdenum and the origin of non-stoichiometry. Their work provides a deeper understanding into the practicalities of similar reactions for potential industrial applications.
“Measuring these sample volumes and being able to study the bulk and not just the surface of a very small volume is very new and important," adds Craig Bull, Pearl instrument scientist.
“These findings provide insights into the synthesis and structural properties of hydrogen-rich transition metal hydrides and demonstrate the unique information that neutron diffraction can offer for related studies," he adds.
Zhongsheng's postdoctoral fellowship is supported by a grant from the UKRI International Science Partnerships Fund (ISPF) for partnership development between ISIS, Diamond and the Paul Scherrer Institute.
The full paper can be found at DOI: 10.1021/acs.inorgchem.5c04811
