Crystal Chemical Parameters for the Eu(II) Luminescence in Solid-State Metal Hydrides

Abstract

Metal hydrides are interesting hosts for activator ions, as they provide opportunities for the tuning of emission wavelengths in luminescent materials. The strong nephelauxetic effect of the hydride ion lowers the barycenter of the d levels and thus also the energy difference from the electronic ground state. General rules on this influence are lacking, however, due to the multitude of variables. Alkaline earth metal hydride halogenides MHX (M = Ca, Sr, Ba; X = Cl, Br, I) are ideally suited for examining the influence of crystal chemical parameters on luminescence properties because they crystallize isotypically in the tetragonal PbFCl type, a relatively simple crystal structure. MHX and Eu(II)-activated samples M_0.99Eu_0.01HX were synthesized by solid-state reactions and structurally characterized by X-ray and neutron powder diffraction, providing evidence of hydrogen positions. Refined crystal structure data for all nine compounds allow for elucidation of detailed crystal chemical systematics. From BaHCl to CaHI, the coordination of the cations changes from nine to eight, the crystal structure acquires a more pronounced layer character, and the bonding properties show a decreasing ionicity. These changes are also reflected in the luminescence spectra. For a given M, increasing the size of X has almost no influence on M–H distances but increases the emission wavelength considerably. The increasing nephelauxetic effect of halogen X is the most likely cause. Increasing the size of M for a constant X moderately increases M–H distances but strongly decreases the emission wavelength. Eu(II) luminescence in metal hydrides thus seems to be dominated by the M–H distances. The nephelauxetic effect of the halogen atoms is a secondary effect. Combining these effects with partial substitution of fluorine for hydrogen or one M for another offers ample opportunities for fine-tuning the luminescence properties of Eu(II) in metal hydride host compounds.