Geometry and spatial orientation of a quadruple node of triple junctions in a europium‐doped KI crystal as determined by epifluorescence microscopy

The geometry and spatial orientation of a typical arrangement of four triple junctions and six grain boundaries sharing a common quadruple node in a Eu²⁺‐doped KI crystal are investigated by epifluorescence microscopy using the proper doping ion as a fluorochrome. To achieve this, an electronic three‐dimensional reconstruction of the studied arrangement of crystal defects was built from microscopy images of different optical cross‐sections of this arrangement. Previously, the doping ions were induced, by subjecting the crystal to a long annealing treatment, to form europium precipitates into the crystal grain boundaries. The optical properties of these precipitates were characterized by fluorescence spectrophotometry and used to tailor properly the microscope fluorescence mirror unit, whereas the single‐crystal character of the microscope samples was tested by X‐ray diffraction. By inspecting the reconstruction under handling, the dihedral angles between the grain boundaries that meet at a common triple junction as well as the angles between the triple junctions sharing the quadruple node were successfully measured at the quadruple node site. The measuring procedures are carefully described. The resulting values (132º, 109º, 119º, 125º, 111º, 124º, 124º, 111º, 125º, 129º, 109º and 122º ± 2º) for the dihedral angles depart for some few degrees from the characteristic angle (120º) of a 3‐fold symmetry rotation, whereas the resulting values (104º, 111º, 117º, 103º, 100º and 121º ± 2º) for the triple junction angles are not far from the characteristic angle (109.47º) between the legs of a tetrahedron. These results, indicating that in the close neighbourhood of the quadruple node the studied arrangement of crystal defects deviates from a state of full structural stability, allow this arrangement to be fairly modelled in such a neighbourhood by a distorted tetrahedron. The angles between the studied triple junctions and the host lattice directions [11], [11], [11] and [] were also measured at the quadruple node site, and the resulting values (8º, 7º, 6º and 8º ± 2º, respectively) indicate that a symmetry mismatching exists between the tetrahedral model of the studied Eu²⁺‐decorated arrangement of crystal defects and the KI matrix cubic crystal lattice. This symmetry mismatching is discussed to be responsible for the observed deviation from structural stability.