Excitons and rare-earth ions in CsCdBr3

CsCdBr3 has a quasi-linear crystal structure. It consists of covalently bound [CdBr]64- chains separated by chains of Cs+ ions. The trivalent rare-earth (RE) ions substitute for divalent Cd ions forming predominantly pair centers of the type RE3+-(Cd vacancy)-RE3+. A minority of RE ions forms 'single-ion' centers with more distant charge compensation. The electronic structure around the band gap is determined by the [CdBr]64- octahedra. The lowest excitonic states of the lattice are charge-transfer states of these octahedra. At low temperatures they form self-trapped excitons which become mobile around 80 K. In addition we find defect-localized excitons at the RE pairs and single ions with slightly modified spectra. There is a strong energy transfer between the RE ions and the defect-localized excitons in both directions with transition times below 10-8 s. For the cooperative fluorescence transition 1D2x1G4→3H4x3H4 in Pr3+: CsCdBr3 a frequency-modulated vibronic sideband spectrum was found with up to four repetitions of the frequency of the localized optical phonon mode at the ion pair