La(II) in metal hydrides — crystal chemistry and photoluminescence

• Applicant: Professor Dr. Holger Kohlmann
• Subject Area: Solid State and Surface Chemistry, Material Synthesis ; Physical Chemistry of Solids and Surfaces, Material Characterisation
• Term: since 2018
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 419433503

Lanthanides are often used as activators for light-emitting diodes (LEDs). When d-states are involved in the process (e.g. Ce(III), Eu(II)), the wavelength of emission can be tuned due to the sensitivity of these electronic states to the chemical environment. In the first funding period the project investigated Eu(II) luminescence in metal hydrides and aimed at identifying influences of the hydridic host on the luminescence properties and at searching for new mixed-anionic hydrides with improved stability as host materials. Most of the investigated hydridic matrices are suitable for Eu(II): KCaH3-xFx:Eu, MHX (M = Ca, Sr, Ba; X = Cl, Br, I):Eu, rare earth hydride oxide halides and hydride silicates. The M-H distance is the most important factor influencing the emission wavelength, the nephelauxetic effect of the halogen atoms is the second most important. The rare earth hydride oxides exhibit rich structural chemistry and in some cases surprisingly good chemical and thermal stability; however, their suitability as host compounds for Eu(II) luminescence is limited by the preference of Eu(III) over Eu(II). The yellow colored LiEu2SiO4H is another example of a chemically and thermally quite stable metal hydride. Recently, the photoluminescence of La(II) was reported for the first time. It occurs in LaMg6Ga6S16, and it is believed that the octahedral environment for La(II) together with the chemical softness of the sulfide ions is a prerequisite for this. For various reasons, metal hydrides are excellent candidates for the stabilization of La(II): strongly reducing reaction conditions, LaH2 as a simple starting material with formal La(II), existing octahedral voids, chemically soft hydride anions. Based on these criteria, the following compounds are examined for possible substitution with La(II): MgH2, alkali and alkaline earth Mg hydrides, Mg alanates, M2BN2H1-yXy (M = Ca, Sr; X = F, Cl). The solid-state reactions are carried out (i) diffusion-controlled at high temperature (HT), (ii) by ball milling (BM), (iii) under high hydrostatic pressure (HP). Since the search for La(II)-based hydrides may seem like looking for a needle in a haystack, quantum mechanical calculations will be used for a rational preselection. This will lead to a prioritization of synthesis strategies and chemical systems to increase effectiveness and chances of success. Doping with Ce(III), Eu(II) and Sm(II) will be used as alternatives if La(II) cannot be stabilized (risk mitigation). The characterization includes X-ray and neutron diffraction to determine the crystal structure, XPS spectroscopy to determine the oxidation state of La and luminescence spectroscopy. The aim of this research project is the stabilization of La(II) in solid metal hydrides and the investigation of their solid-state photoluminescence. We hope to contribute to the fascinating phenomenon of La(II)-based photoluminescence in the solid state.

• DFG Programme: Research Grants