Green Luminescence of Divalent Europium in the Hydride Chloride EuHCl

Abstract

Luminescence properties of divalent europium in the mixed-anion hydride chloride EuHCl were studied for the first time. Olive-green single crystals of EuHCl (PbFCl-type structure: tetragonal, P4/nmm, a = 406.58(3) pm, c = 693.12(5) pm, c/a = 1.705, Z = 2) resulted from the reaction of elemental europium (Eu), sodium hydride (NaH) and sodium chloride (NaCl), while powder samples were prepared from the binary components europium dihydride (EuH2) and dichloride (EuCl2). Low temperature X-ray powder diffraction proved the absence of phase transitions for 12(2) K T 295(2) K. Bright green emission was observed under UV-excitation and assigned to the 4f(6)5d(1)-4f(7) transition of divalent europium. Temperature-dependent luminescence absorption and emission, as well as lifetime measurements were carried out on single crystal and powder samples. Surprisingly, only limited concentration quenching was found. Additionally, two emission bands (485 and 510 nm) are observed, whose intensity ratio depends strongly on temperature. In order to explain this behavior for a single Eu2+ site, we suggest either a dynamical Jahn-Teller effect in the excited 5d(1) state or emission from both a 4f(6)5d(1) state and a trapped exciton state.