Epifluorescence microscopy study of a quadruple node of triple junctions of grain boundaries in a Eu²⁺ -doped Kcl:Kbr...

A typical quadruple node (QN) of triple junctions (TJs) of grain boundaries (GBs) in a Eu²⁺ -doped KCl_0.52 Br_0.48 solid solution is investigated from the geometrical point of view by epifluorescence microscopy using the doping ion as a fluorochrome. The excitation and fluorescence optical properties of the fluorochrome were previously characterised by spectrophotometry whereas the structural nature of the studied material as well as its Bravais lattice type, unit cell size and long-range translational order degree was determined by X-ray diffraction. A three-dimensional reconstruction was built from the microscopy images of different optical cross-sections of the studied arrangement of crystal defects. In the close vicinity of the QN, the studied arrangement of crystal defects adopts the geometry of a collapsed tristetrahedron which, centred at the QN, has its legs along the TJs and, hence, has its faces as collapsed in pairs into the GBs. The angles defined by different TJ couples as well as the dihedral angles defined by the different GB couples meeting in every TJ were measured at the QN site. All, the image recording and stacking as well as the measuring procedures are carefully described. The measured TJ angles (97°, 117°, 95°, 117°, 99° and 130° ± 2°) depart from the characteristic angle (109.47°) of a tetrahedron whereas the measured GB angles (101°, 119°, 140°; 125°, 127°, 108°; 133°, 109°, 119°; 129°, 99° and 132° ± 2°) depart from the angular argument (120°) of a 3-fold symmetry rotation indicating that, in the close neighbourhood of the QN, the studied arrangement of crystal defects is structurally unstable. Such an instability is associated with an observed mismatch in orientation (by angles of 20°, 15°, 33° and 30° ± 2°) between the TJs and some <111> zone axis matrix lattice crystallographic directions ([1¯1¯1¯], [111¯], [11¯1] and [1¯11]), respectively). Local variations in anionic composition, existing within the solid solution matrix, are discussed to be responsible for this mismatching and, therefore, for the observed structural instability.