Electron–Phonon Coupling in Luminescent Europium-Doped Hydride Perovskites Studied by Luminescence Spectroscopy, Inelastic Neutron Scattering, and First-Principles Calculations

We present a case study on the vibrational coupling of lattice phonons to the electronic 4f<sup>7</sup> (<sup>8</sup>S<sub>7/2</sub>)–4f<sup>6</sup>5d<sup>1</sup> (e<sub>g</sub>) transition of divalent europium in the hydrides and deuterides LiMH<sub>3</sub> and LiMD<sub>3</sub> (M = Sr and Ba). For low doping concentrations, these compounds show extraordinarily well-resolved vibronic fine structures at low temperatures. Besides luminescence emission spectroscopy of the europium-doped compounds, we carried out inelastic neutron scattering (INS) experiments of the europium-free compounds. The phonons coupling to the electronic transition are identified, and a good agreement between the vibronic and the INS data is found. The frequencies of the low-energy acoustic modes do not significantly change upon replacing hydride by deuteride, whereas a decrease by a factor of approximately 2 can be observed for the higher energy optic modes. Furthermore, we compare these experimental results to density functional calculations performed with the Vienna Ab initio Simulation Package. Knowledge of the phonons of a host material is of great importance because phonons have a large influence on the optical properties, such as line widths or luminescence quenching. Hydride-containing host lattices are an ideal model system because <sup>1</sup>H can easily be replaced by <sup>2</sup>D so that isotope effects can be investigated.