Relaxation of the $\text{}^{4}$S$\text{}_{3}\text{}_{/}\text{}_{2}$ Level of Er$\text{}^{3+}$ in Cs$\text{}_{2}$NaErCl$\text{}_{6}$

Octahedral (O$\text{}_{h}$) symmetry of the RE$\text{}^{3+}$ sites in the Cs$\text{}_{2}$NaRECl$\text{}_{6}$ crystals removes all of the electric dipole intensity from the 4f-4f transitions. Thus the decay of luminescence originating in the excited states of RE ions is governed by phonon assisted (vibronic) transitions, multiphonon relaxation and phonon assisted energy transfer (cross relaxation and energy migration to impurities). Contribution of these processes to the decay of the $\text{}^{4}$S$\text{}_{3}\text{}_{/}\text{}_{2}$ state of erbium in Cs$\text{}_{2}$NaErCl$\text{}_{6}$ has been studied in the 4.2-300 K temperature region. It has been concluded that at low temperatures the $\text{}^{4}$S$\text{}_{3}\text{}_{/}\text{}_{2}$ state decays exclusively by vibronic transitions. At about 100 K the cross relaxation via the $\text{}^{4}$I$\text{}_{9}\text{}_{/}\text{}_{2}$ state becomes operative. Efficiency of this process grows strongly with increasing temperature, reducing the $\text{}^{4}$S$\text{}_{3}\text{}_{/}\text{}_{2}$ lifetime by almost three orders of magnitude. No evidence has been found that multiphonon relaxations contribute to the decay of the $\text{}^{4}$S$\text{}_{3}\text{}_{/}\text{}_{2}$ state.