Crystal Chemical\nParameters for the Eu(II) Luminescence\nin Solid-State Metal Hydrides

Metal hydrides are interesting hosts for activator ions,\nas they\nprovide opportunities for the tuning of emission wavelengths in luminescent\nmaterials. The strong nephelauxetic effect of the hydride ion lowers\nthe barycenter of the d levels and thus also the energy difference\nfrom the electronic ground state. General rules on this influence\nare lacking, however, due to the multitude of variables. Alkaline\nearth metal hydride halogenides <i>M</i>H<i>X</i> (<i>M</i> = Ca, Sr, Ba; <i>X</i> = Cl, Br, I)\nare ideally suited for examining the influence of crystal chemical\nparameters on luminescence properties because they crystallize isotypically\nin the tetragonal PbFCl type, a relatively simple crystal structure. <i>M</i>H<i>X</i> and Eu(II)-activated samples <i>M</i><sub>0.99</sub>Eu<sub>0.01</sub>H<i>X</i> were\nsynthesized by solid-state reactions and structurally characterized\nby X-ray and neutron powder diffraction, providing evidence of hydrogen\npositions. Refined crystal structure data for all nine compounds allow\nfor elucidation of detailed crystal chemical systematics. From BaHCl\nto CaHI, the coordination of the cations changes from nine to eight,\nthe crystal structure acquires a more pronounced layer character,\nand the bonding properties show a decreasing ionicity. These changes\nare also reflected in the luminescence spectra. For a given <i>M</i>, increasing the size of <i>X</i> has almost\nno influence on <i>M</i>–H distances but increases\nthe emission wavelength considerably. The increasing nephelauxetic\neffect of halogen <i>X</i> is the most likely cause. Increasing\nthe size of <i>M</i> for a constant <i>X</i> moderately\nincreases <i>M</i>–H distances but strongly decreases\nthe emission wavelength. Eu(II) luminescence in metal hydrides thus\nseems to be dominated by the <i>M</i>–H distances.\nThe nephelauxetic effect of the halogen atoms is a secondary effect.\nCombining these effects with partial substitution of fluorine for\nhydrogen or one <i>M</i> for another offers ample opportunities\nfor fine-tuning the luminescence properties of Eu(II) in metal hydride\nhost compounds.