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When hydrogen glows red
A team of German and Dutch scientists synthesized a series of substances, which show bright luminescence under UV light. Therefore the amount of hydrogen in the structure determines the wavelength – and hence the color – of the emitted light. These compounds could be used as illuminants in LEDs or for chemical hydrogen storage.
Synthesizing the hydrides
In theory the synthesis of hydrides, compounds where hydrogen acts as negative anion, is straightforward. The scientists mixed the correct amounts of first and second main group metal fluorides with europium and heated the mixture in a hydrogen atmosphere at a high pressure. Consequently, compounds with the general formula MCaHxF3−x (M = Rb, Cs) arise.
But there is a catch: “The compounds decompose upon contact with oxygen or moisture. Because of that, all chemicals must be thoroughly dried in specialized vacuum ovens and all manipulations must be carried under an argon atmosphere”, says Dr. Nathalie Kunkel, visiting scientist at the Chair of Inorganic Chemistry with Focus on Novel Materials of Prof. Dr. Thomas Fässler at the Technical University Munich.
Why does hydrogen influence the color of the emitted light?
Photographic images of the solid solution series under daylight (top) and the corresponding emission under 376 nm UV irradiation (bottom). © Alexander Mutschke
Luminescence happens when electrons are pushed into higher energy levels through the external administration of energy and a subsequent release of energy in form of visible light exiting this so-called excited state.
In the synthesized hydrides, the europium is octahedrally surrounded by hydrogen and fluorine atoms – the respective quantity is carefully adjusted during synthesis. Fluorine and hydrogen have very divergent electronegativities, a measurement of their ability to attract electrons. In each sample, a different number of fluorine atoms is substituted with hydrogen, and so the differences in electronegativity and polarizability impact the electrons of europium in a distinct manner. The excited state in a hydrogen rich environment can be reached with less energy, thus less energy is emitted during the transition back into the ground state. The more fluorine is substituted with hydrogen, the more redshifted the emitted light.
Neutrons see hydrogen
To get an idea of how the hydrogen content and the wavelength of emitted light interact with each other, the exact amount of hydrogen must be measured. Under normal circumstances, x-ray fluorescence is used for elucidating the structure of solid-state matter, but unfortunately hydrogen is practically invisible to x-rays. Only with neutron diffraction can the hydrogen content be measured precisely, for which Dr. Markus Hölzel used the SPODI instrument, a high-resolution neutron powder diffractometer at MLZ.
Use in the design of new light sources
Many potential applications are opening up for this class of compounds, ranging from hydrogen technology to optics. For example, in order to use hydrogen as a fuel for vehicles, it is important to store as much hydrogen as possible in as little space as possible. Since solids have a higher density than gases, one obvious thing is to try chemically binding hydrogen as a solid hydride in order to store it.
There are also possible fields of application in optics: For example, LEDs require illuminants that have sharp emission lines. Knowledge of how structure, composition, and properties interact here could be helpful in the design of new light sources.
Alexander Mutschke, Thomas Wylezich, Atul D. Sontakke, Andries Meijerink, Markus Hoelzel, and Nathalie Kunkel. MCaHxF3−x (M = Rb, Cs): Synthesis, Structure, and Bright, Site-Sensitive Tunable Eu2+. Luminescence Adv. Optical Mater. 2002052 (2021), DOI: 10.1002/adom.202002052