Dissolution of a secondary europium phase in monocrystalline sodium chloride

A detailed study of the thermal resolution of a segregated phase of divalent europium in the sodium chloride lattice has been performed using optical-absorption and electron-paramagnetic-resonance techniques. The dissolution of the segregated phase consists mainly of a two-stage process; in the first stage (290-500 K) the isolated I-V dipoles and the segregated phase inclusions are present. In the second stage (500-700 K) the thermal decomposition of the segregated phase takes place. In order to determine the structure of the secondary europium phase, x-ray-diffraction and electron-microscopy analyses were performed on samples with different doping levels and subjected to various annealing treatments. In all cases, the segregated phase was found to be the stable dihalide phase Eu${\mathrm{Cl}}_2$ which crystallizes in a rombohedral C23 structure. No evidence of a metastable phase, such as the so-called Suzuki phase, was found in our crystals. From the analysis of the increase in the concentration of I-V dipoles as a function of the annealing temperature, dimers were found to be the products of solution of the aggregates. It was also determined that the solubility of associated europium in NaCl is characterized by the energy of solution 0.46±0.02 eV. Two different optical-absorption spectra for the ${\mathrm{Eu}}^{2+}$ ions in NaCl were observed. One of them (I) corresponds to the case in which the impurity is dispersed in the lattice forming I-V dipoles. It consists of two broad bands in the uv range peaking at 243 and 348 nm. The other one (II) is associated with the stable dihalide phase Eu${\mathrm{Cl}}_2$ in the sodium chloride lattice, and it also consists of two broad bands in the uv range but with peaking at 261 and 349 nm. For both spectra I and II, values for the half width and for the oscillator strength of each of the observed bands are reported. Measurements of the Vickers microhardness were also performed, and the results established that those crystals in which the impurity is dispersed in the lattice forming I-V dipoles are harder than those in which the stable dihalide phase Eu${\mathrm{Cl}}_2$ is present.