(INVITED)Synthesis and luminescence properties of M(AlCl4)2:Eu2+ (M = Ca, Sr, Ba) a promising new Class of Eu2+ containing phosphors

In this work, the luminescence properties of Eu2+ doped in the host lattices M(AlCl4)2 (M = Ca, Sr, Ba) have been studied for the first time. The host compounds crystallize in a tetragonal (Ca), an orthorhombic (Sr) and a monoclinic (Ba) crystal structure where the alkaline earth site is 8-fold coordinated by Cl− ions in all cases. Therefore, these materials are ideal candidates to investigate structure related luminescence properties of Eu2+ ions. The emission spectra of the Eu2+ doped compounds are characterized by broad 4f65d1 → 4f7 transition bands located in the UV range between 388 nm and 400 nm (25773 - 25000 cm−1). Their maxima are blue shifted compared to the binary alkaline earth chloride hosts which can be explained by the influence of the [AlCl4]- ion on the crystal field splitting in addition to increasing metal-ligand distances. This shifts the lowest excited 5d level of Eu2+ to much higher energies. Furthermore, Huang-Rhys factors S and coupled phonon energies ħω were determined by temperature dependent emission measurements. In addition, radiative decay times at 10 K were determined. •Promising materials for divalent lanthanide doping.•Detailed Eu2+ spectra allow for an accurate determination of characteristic energies.•Systematic optical properties as contribution to the structure-property relationships.•[AlCl4]- anion leads to a blue-shift of the Eu2+ luminescence.