Excitons and rare-earth ions in CsCdBr3

CsCdBr3 has a quasi-linear crystal structure. It consists of covalently bound [CdBr](6)(4-) 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](6)(4-) 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 D-1(2) x (1)G(4) --> H-3(4) x H-3(4) 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. (C) 2004 Elsevier B.V. All rights reserved.