The influence of chlorine ion on the spectroscopic properties of Nd3+- and Eu3+-doped fluorochloride glass

The influence of local coordination ions on the optical properties of Nd3+ and Eu3+ in fluorochloride glass has been investigated by using steady-state and time-resolved laser excited site selective spectroscopy. From the thermal behaviour of lifetimes of the F-4(3/2) state, a T-3 dependence for the nonradiative Nd-Nd relaxation process has been found in the 10-90 K temperature range for Nd3+ concentration higher than 0.5 mol% which is in agreement with a two-site nonresonant process. In spite of the high content of fluorine in this glass, the presence of chlorine anions causes a significant effect on the spectral behaviour and relative quantum efficiency of Nd3+ emission as compared with those of pure fluoride glasses. The excitation wavelength dependence of the lifetimes of the F-4(3/2) state points to the existence of some site segregation for Nd3+ in this fluorochloride glass. This result is also confirmed by fluorescence line-narrowing experiments performed with Eu3+ in the same glass. The behaviour of both the line-narrowed fluorescence of the D-5(0) --> F-7(2) hypersensitive transition and the linewidth of the time-resolved resonant line-narrowed D-5(0) --> F-7(0) emission band as a function of the selective F-7(0) --> D-5(0) excitation wavelength indicates that homogeneous Linewidths are dominated by relaxation time broadening caused by rapid phonon relaxation processes. These results together with the crystal field strength calculated as a function of excitation energy along the F-7(0) --> D-5(0) absorption band might be interpreted as indicating the existence of a crossover between the behaviour of fluorinelike and chlorinelike coordination anions. A discussion on the origin of this behaviour is given.