Luminescence Properties of M(AlCl4)2:Yb2+ (M = Ca, Sr, Ba): Ideal Materials for the Investigation of Structure–Luminescence Relationships

In this work, the chloride system M(AlCl₄)₂ (M = Ca, Sr, Ba) doped with Yb²⁺ is investigated in greater detail. The influence of the [AlCl₄]^- ion on the position of the emission band of Yb²⁺ is investigated and the emission spectra are recorded. The emission spectra of the Yb²⁺-doped materials are characterized by broad 4f¹³5d¹ (HS) ↔ 4f¹⁴ transitions with maxima in the range between 416 nm (Ca) and 421 nm (Ba) (24,061-23,738 cm^-1), whereas the Ba compound features an additional 4f¹³5d¹ (LS) ↔ 4f¹⁴ emission band at 397 nm (25,203 cm^-1) at lower temperatures. The unusual reverse order of the positions of the emission maxima (Ba < Sr < Ca) most probably indicates an influence of the local symmetry on the location of the emission bands. Temperature-dependent emission and decay time measurements reveal strong thermal quenching processes, which gives additional insight into the mechanistic properties of the luminescence. An investigation of the decay times for all transitions reveals generally shorter values compared to values found in the literature, which was likely caused by the strong thermal quenching.