Bright yellow and green Eu(ii) luminescence and vibronic fine structures in LiSrH3, LiBaH3 and their corresponding deuterides

Abstract

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.