Abstract
The spatial distribution of europium-dihalide-type precipitates in KCl : KBr :Eu2+ single crystals annealed at 200°C for long periods of time was determined by epifluorescence optical microscopy. Laue-type patterns and optical spectroscopy were used to test the degree of long-range ordering of the specimens and to monitor the precipitation during annealing, respectively. Precipitates smaller than about 0.08 µm were found (2.84 × 1012 precipitates cm−3) all across the host, whereas larger precipitates of about 0.17 µm were found (2.9 × 104 precipitates cm−1) along certain linear structural singularities of the matrix. These singularities, identified as crystal dislocations, were observed to terminate always either at a triple node of singularities or at the crystal surface, forming a three-dimensional network of crystal singularities (2.0 × 106 singularities cm−2), identified as the Frank network. The presence of europium exhaustive matrix zones accompanying the observed europium-decorated paths indicates that impurity segregation processes, occurring during annealing, favour the precipitation phenomena.
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