Energy Transfer to Trace Impurities in Cs$\text{}_{2}$NaY$\text{}_{1-x}$Pr$\text{}_{x}$Cl$\text{}_{6}$: Consequences for Interpretation of Emission Decay Curves

We report the $\text{}^{3}$P$\text{}_{0}$ → $\text{}^{3}$H$\text{}_{4}$ and $\text{}^{3}$P$\text{}_{0}$ → $\text{}^{3}$H$\text{}_{4}$ luminescence decay curves of Cs$\text{}_{2}$NaY$\text{}_{1-x}$Pr$\text{}_{x}$Cl$\text{}_{6}$ at 20 K as a function of excitation wave number in the region of the transitions to the $\text{}^{3}$P$\text{}_{0}$ state near 20602 cm$\text{}^{-1}$, the $\text{}^{3}$P$\text{}_{1}$ state near 21200 cm$\text{}^{-1}$ and the six components of the $\text{}^{1}$I$\text{}_{6}$ state distributed over the region 21164 to about 22000 cm$\text{}^{-1}$. For x = 0.001 and excitation into the absorption maxima, the decay curves are independent of excitation wave number and of the emission transition monitored, and are exactly exponential. Excitation at regions of weak absorption between the main absorption bands produces markedly different decay curves characterised by a prominent very fast relaxation at short times and a long exponential tail. The fast decay is strongly non-exponential. On increasing the temperature, this fast process becomes less prominent, and it is not detectable in the 298 K curves. For x>=0.05 the fast process is not present at any temperature. We propose that this fast process is due to emission from PrCl$\text{}_{6}^{3-}$ ions perturbed by a nearby water molecule.