Bright yellow and green Eu(II) luminescence and vibronic fine structures in LiSrH3, LiBaH3 and their corresponding deuterides
The luminescence of Eu2+ in hydride and deuteride perovskite hosts LiMH3 and LiMD3 (M = Sr, Ba) is reported. Bright yellow (M = Sr) and green (M = Ba) emission is observed and assigned to 4f65d–4f7 emission from Eu2+ in the highly symmetric 12-coordinated M2+ site (mm). The long wavelength of the emission is explained by the strong covalence and crystal field splitting in europium's coordination by hydride anions. A well-resolved vibrational structure in the emission and excitation spectra of Eu2+ in the Sr-compounds allows for an accurate determination of the energy of the lowest 4f65d state and vibrational frequencies, for both the hydride and deuteride. The isotope effect on the energy of the fd states is small (∼70 cm−1), as expected. Surprisingly, also the vibrational energies observed in the vibronic progression are similar for the d–f emission spectra in LiSrH3 and LiSrD3. This is explained by strong coupling of the d–f emission with low energy acoustic phonons which, contrary to optical phonons, are not strongly affected by replacing H by D. The present results provide insight into the long wavelength Eu2+ emission in hydride coordination and the influence of isotope replacement on the luminescence.