Quadruple node of triple junctions of grain boundaries in a Eu2+-doped solid solution of the ions K+, Rb+, Cl- and Br-: an epifluorescence microscopy study using the doping ion as a fluorochrome.

Abstract

Epifluorescence microscopy was used to unveil the geometry and relative spatial orientation of an arrangement of crystal defects, consisting of a quadruple node of triple junctions of grain boundaries, in a Eu2+-doped solid solution of K+, Rb+, Cl- and Br-. The doping ion was utilized as a fluorochrome. Microscopy images of different optical cross-sections of the arrangement of crystal defects under study were recorded and used to build an electronic three-dimensional reconstruction of this arrangement. The geometry is that of an irregular tristetrahedron, centred at the quadruple node, so that the tristetrahedron legs, deviating from the lattice <111>-zone axis directions, lie along the triple junctions, whereas the tristetrahedron faces collapse onto the grain boundaries. The deviation angles as well as the angles defined by different triple junction pairs and different grain boundary pairs were measured. The orientational deviation is larger than that in a KI single crystal but lower than that in a solid solution of K+, Cl- and Br-, meaning that the structural equilibrium depends on the matrix lattice structural character. Uncompensated local nanostrains, due to the ion substitution, are associated with the observed decrement in structural stability, in relation to the KI case, whereas compensated local nanostrains, provoked by the simultaneous substitution of cations and anions, are argued to be responsible for the observed increment in structural stability, in relation with the K(Cl, Br) case. The optical characterization of the fluorochrome, the recording and processing of the microscopy images, the building of the electronic 3D reconstruction, the angular measure methodology and the geometrical modelling are all carefully described.