Morphology and orientated growth of second-phase precipitates in a Eu2+-doped equimolar KCl:KBr solid solution: an epifluorescence microscopy study by using the doping ion as a fluorochrome

Abstract

The shape and orientation of second-phase precipitates in a Eu ²⁺ -doped equimolar KCl:KBr solid solution are reported in this paper as they were unveiled by epifluorescence microscopy. To make this, microscopy images of different optical cross sections of some precipitate fields and, also, of some representative precipitates in these fields, were recorded by using the Eu ²⁺ ion itself as a fluorochrome. From these images, the corresponding precipitate fields and individual precipitates were electronically reconstructed into the host lattice space. Previously, the KCl:KBr:Eu ²⁺ system was characterized by absorption and fluorescence optical spectrophotometry, to tailor properly the fluorescence mirror unit, as well as by powder and single-plate X-ray diffraction, to correlate the host lattice orientation with those of the observed precipitates. These are shaped as plates, with broad faces parallel to host lattice {100}, {110} or {120}planes (the {100}, {110} and {120} precipitates, respectively), and as rods, aligned with a host lattice ˂100> direction (the ˂100> precipitates). The {100}, {110}, {120}-precipitate broad faces are in the shapes of 72.6° rhomboids, rectangles and 59.5° rhomboids, with a side lying along host lattice <310>, <110> and <421> directions, respectively, and with another side lying along a <100> direction. A typical precipitate field and the spatial reconstructions of typical {100}, {110}, {120} and ˂100> precipitates, as well as their corresponding electronic 3D-geometrical models, are described in detail. It is discussed that four different europium precipitation states are responsible for the precipitation and that the precipitate lattices are spatially coherent with the host lattice. The shapes and spatial orientations, in relation to the matrix lattice, of europium second-phase precipitates in an equi-molar KCl:KBr solid solution host are studied for the fi rst time ever. This was performed by epifl uorescence optical microscopy by using the doping divalent europium ion as a fl uorochrome.