Anomalous europium luminescence in LaF3

TL;DR

This study investigates the unusual luminescence properties of divalent europium in LaF3, revealing anomalous emission and its relation to absorption bands and dielectric relaxation.

Contribution

It provides new insights into the optical spectra and dielectric behavior of Eu$^{2+}$ in LaF3, highlighting the origin of anomalous luminescence and associated electronic transitions.

Findings

Eu$^{2+}$ exhibits anomalous broadband emission at 600 nm below 80 K.

Absorption bands at 330 and 380 nm are linked to transitions involving fluorine vacancies.

Dielectric relaxation is due to Eu$^{2+}$-anion vacancy dipole rotation.

Abstract

Optical spectra (absorption, emission, excitation, decay) and dielectric relaxation were measured for divalent europium (and partially for ytterbium) in lanthanum fluoride crystals. Absorption of Eu$^{2+}$ contains not only asymmetric weakly structured band at 245 nm but also long-wavelength bands at 330, 380 nm. Broadband Eu$^{2+}$ emission at 600 nm appeared below 80 K, having decay time 2.2 $\mu$s at 7.5 K. Emission at 600 nm is attributed to so-called anomalous luminescence. Bulk conductivity is directly proportional to absorption coefficient of Eu$^{2+}$ bands. Dielectric relaxation peak of LaF$_3$-EuF$_3$ is attributed to rotation of dipoles Eu$^{2+}$-anion vacancy. The long-wavelength absorption bands at 330, 380 nm are assigned to transitions from 4f$^7$ Eu$^{2+}$ ground state to states of neighbouring fluorine vacancy.