Optical study of defect clustering in NaCl:Sm2+

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Abstract
In this paper, we report a detailed spectroscopic investigation of the NaCl:Sm2+ system. In sharp contrast to the homologous KCl:Sm2+ and KBr:Sm2+ systems, whose narrow−line fluorescence spectra mainly arise from Sm2+−cation−vacancy pairs, the NaCl:Sm2+ system gives rise to spectroscopic properties that are dominated by cluster formation. In addition to the usual narrow 5D0 → 7FJ fluorescence lines originating from single Sm2+−cation−vacancy pairs a host of f → f and d → f lines attributable to small and large Sm2+−defect clusters is observed. The site origins of the fluorescence lines are determined by Zeeman anisotropy fluorescence, lifetime, and polarization measurements. The cluster origins of many intense lines have been ascertained through a high−temperature annealing−quenching procedure. The prominent role played by defect clustering in NaCl:Sm2+ is qualitatively explained in terms of the sizable difference in the ionic radii of the Sm2+ and the Na+ ions.