Comparison of the spectroscopic behaviour of single crystals of lanthanide halides (X = Cl, Br)

Abstract

Halides belong to large band gap matrices that have the perspective of wide applications when doped by optical active ions. This paper presents the results of spectroscopic studies of single crystals of halides (Cl, Br) of Ce, Pr and Nd. Their spectroscopic behaviour: electron–phonon coupling, ion pair interactions and the effect of covalency, is compared. Absorption, emission and emission excitation spectra of single crystals of LnCl3·yH2O (Ln = Nd, Pr, Ce; y=6, 7) were recorded at room temperatures and low temperatures down to 4.2K. The intensities of the electronic lines and the Judd–Ofelt parameters were calculated (Nd, Pr) and compared to those of LnBr3·yH2O presented earlier by us. The relationship between the hypersensitivity and covalency was discussed. With increasing soft character of the halides (Br− > Cl−), the covalent character of Ln–ligand bond increases and the hypersensitive bands become more intense. The Judd–Ofelt intensity analysis resulted in a set of τλ parameters evaluated with quite low standard deviations. The temperature dependences of the intensities have been found and the vibronic coupling in the f–f transitions were analysed. At the low temperature (4.2K), strong vibronic components occur in the electronic lines of the Nd(III) and Pr(III) ions, mainly with the Ln–X vibrations. The modes, which are in resonance with the splitting of the ground state multiplet, mediate in the cooperative transitions.Vibrational studies of the compounds under test were performed at the ambient temperature using IR and Raman spectroscopy. The assignment of the bands was done on the basis of the factor group analysis. The spectral features below 300cm−1 point at the differences between the spectra of the bromides and chlorides of Nd and Pr. Although the spectral features within the FIR region are complex, the bands of the praseodymium monocrystals originated by halogen bridges are clearly visible.