The team used a combination of three different neutron techniques alongside computational modelling to make their discoveries.
The two hydrate phases are precursors in the production of molybdenum oxide hybrid materials. These hybrid materials are potentially useful for catalysis and energy storage, as their structural diversity allows them to be tailored to specific applications. Despite the remarkable structural diversity and metastable character of hydrated MoO3 phases, many of them have only recently been recognised as naturally-occurring minerals persisting in geological environments.
The researchers were interested in the monohydrate (MoO3 · H2O) and dihydrate (MoO3 · 2H2O) phases of MoO3. While techniques such as X-ray crystallography and Raman spectroscopy have been used to explore the crystal structures of molybdenum oxides, neither is sensitive to hydrogen. As a result, noone had yet examined the local environment and nuclear dynamics of the water trapped between layers in the MoO3 crystal lattice of the monohydrate. In contrast, neutrons interact strongly with hydrogen, enabling the researchers to study the water within the lattice.
Computational modelling guided structural studies using high-resolution neutron powder diffraction (NPD) on the WISH instrument at ISIS. The WISH data enabled the researchers to confirm the structure of the dihydrate phase previously established. The value of their combined approach was demonstrated in a study of the monohydrate. The NPD results revealed smeared proton positions, suggesting some level of disorder amongst the water molecules and making it difficult to pinpoint the structure from the experimental data alone. However, when they compared the NPD results with potential structures generated through the modelling studies, the researchers identified one structure that fit the NPD data particularly well.
To understand the nuclear dynamics of hydrogen, the team turned to inelastic neutron scattering (INS) on the TOSCA instrument at ISIS and to neutron Compton scattering (NCS) using high-energy, or epithermal, neutrons on the VESUVIO instrument, alongside modelling.
Together, these two techniques enabled the team to understand the role of temperature and confinement within the crystal lattice on the nuclear dynamics of the hydrogen. The computational modelling allowed the researchers to separate different elements of nuclear dynamics, shedding light on the spectra from the INS and NCS experiments and on which elements respond to the geometry of confinement and which resist temperature and geometry effects.
Related publication: https://doi.org/10.1063/4.0000785
