Divalent europium as a paramagnetic probe to study strontium precipitates in monocrystalline sodium chloride

Abstract

Precipitation of divalent strontium ions in NaCl has been investigated using Eu2+ as a paramagnetic probe. The results presented in this paper show that the annealing of the quenched samples at 200 °C produces the formation of the precipitated dihalide phase SrCl2 in the NaCl matrix. The EPR data established that the Eu2+ ions are located inside this precipitated structure and that they are substituting the host cation Sr2+ ions. However, the observed EPR spectrum shows a significant difference with respect to that reported for the same impurity ion in SrCl2 single crystals, i.e., the value for the cubic spin-Hamiltonian parameter b40 is larger and its magnitude seems to decrease as the particle size of the precipitate increases. On the other hand, annealing of the quenched samples at room temperature produces another precipitated phase. In this case, the EPR data reveal that the Eu2+ ions also occupy a cubic symmetry site in this precipitated structure. However, the value for the b40 parameter describing this cubic site is considerably smaller than the one measured for Eu2+ in SrCl2. Finally, an EPR spectrum, observed in the fast quenched samples, possessing axial symmetry and probably due to Sr2+–Eu2+-vacancy complexes is presented. Values for the spin-Hamiltonian parameters describing this new spectrum are also reported.