Eu2+as an optical probe to follow precipitation processes in the system NaCl-SrCl2

Abstract

The optical properties (absorption and luminescence) of a small concentration of divalent europium (6 mol ppm) incorporated into single crystals of NaCl doped with a higher concentration of divalent strontium (860 mol ppm) have been systematically investigated as a function of different annealing treatments in order to follow strontium precipitation in the NaCl matrix. The results indicate that the ${\mathrm{Eu}}^{2+}$ ions become incorporated into the Sr precipitates. The annealing of quenched samples in the range of temperatures (100-300\ifmmode^\circ\else\textdegree\fi{}C) produces the growth of three emission bands peaking at 450, 415, and 407 nm. The former two bands appear to be associated with metastable strontium precipitates from which grows the stable dihalide phase Sr${\mathrm{Cl}}_{2}$, responsible for the emission at 407 nm. From the analysis of the excitation spectra of these emission bands it is inferred that the structures of the metastable precipitates are quite similar to that of Sr${\mathrm{Cl}}_{2}$. On the other hand, the aging at room temperature for long times produces another two Sr-second-phase precipitates responsible for the emission bands peaking at 410 and 448 nm, the structures of these precipitates being different to that of strontium chloride.