| Abstrakt: |
The structural nature and geometry, as well as the lattice‐relative orientation, of an arrangement of crystal defects in a highly textured Eu2+‐doped composite of two alkali‐halide solid solutions was studied by epifluorescence microscopy (EFM) using the doping ion as a fluorochrome. A three‐dimensional reconstruction and a skeleton type model, as built from a sequence of EFM images of different optical cross‐sections of this arrangement, are presented. Structurally, this arrangement is a quadruple node (QN) of triple junctions of grain boundaries. The QN core geometry is that of a tetragonal tristetrahedron (TTTH), centred at the QN site, whose tetrahedron vertices and edges are on the QN triple junctions and grain boundaries, respectively, whereas the tristetrahedron tetragonal axis is nearly parallel to the lattice [001]‐axis. The measured values of the angles between triple junctions and between the grain boundaries forming them are reported. The distinct chemical compositions of the composite solid solutions are discussed to be responsible, in last instance, for the tristetrahedron departure from a cubic configuration. Collaterally, certain families of translationally periodic almost‐parallel (TPAP)‐wall‐like regions which consist of TPAP‐columns of TPAP‐spindle‐like singularities, as well as certain zigzag arrays of columns of this like, existing into the QN grains, are reported to be observed. Three‐dimensional reconstructions of typical individuals of these families and arrays as well as of their constituent parts are presented and geometrically analysed. These families and arrays are discussed to be families of tilt subboundaries, whose constituent dislocations are decorated by cylindrical second‐phase europium di‐halide precipitates, and regularly faceted tilt subboundaries, respectively. Crystal growing and sample preparation, composite structural characterisation by powder and single‐slab X‐ray diffraction (PXRD and SSXRD, respectively), microscopy and fluorescence‐cube unit optics, image processing, electronic three‐dimensional reconstruction and measuring methodologies, are all described in detail. LAY DESCRIPTION: A europium decorated quadruple node of triple junctions of grain boundaries in a spatially coherent cubic‐lattice composite of I(K, Rb) and (Cl, B)(K, Rb) solid solutions was studied by epifluorescence microscopy. The quadruple node core geometry is that of a tristetrahedron which, centred at the quadruple node site, has its legs as lying along the triple junctions, and its faces, as collapsing in pairs onto the grain boundaries. This tristetrahedron is not cubic but tetragonal in symmetry and, aside from this, its tetragonal axis deviates from a lattice ˂001>‐direction. This departure from a cube is proposed to be associated with the existence of micro‐strains at one side and the other side of the grain interfaces during the quadruple node genesis. These micro‐strains are discussed to be due, in last instance, to size differences among the distinct exchange ion species, in each one of the composite solid solutions. The existence of micro‐strains in the quadruple node grains during the quadruple node formation is put in evidence by the observed presence of wall‐like zigzag arrays of translationally periodic parallel columns of, on their own, translationally periodic rod‐like structural singularities. These arrays, columns and singularities are discussed to be regularly faceted segments of tilt subboundaries between crystal domains in the quadruple node grains, subboundary tilt dislocations and second‐phase europium‐dichloride precipitates cylindrical in shape, respectively. Tilt angles of about a half of a second of arc were measured for the observed grain crystal domain subboundaries, in accordance with X‐ray diffraction experiments. That small angles and the observation that families of translationally periodic parallel crystal domain subboundaries exist within the quadruple node grains suggest that epifluorescence microscopy might be suitable to study the quadruple node grains mosaic structure. [ABSTRACT FROM AUTHOR] |
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