Optical absorption and luminescence investigations of the precipitated phases ofEu2+in NaCl and KCl single crystals

A detailed study of the optical absorption and luminescence spectra associated with the precipitated phases of divalent europium in NaCl and KCl single crystals is reported. For the system NaCl-Eu${\mathrm{Cl}}_2$, three fluorescence bands peaking at 410, 439, and 485 nm were observed in crystals which were stored for six years at room temperature, without any previous heat treatment. The corresponding optical-absorption spectrum of these crystals consists of two broad bands peaking at 260 and 349 nm with a cubic crystalline field splitting 10Dq=9809 ${\mathrm{cm}}^{-1\mathrm{}}$. After quenching the crystals from 600°C, only one emission band peaking at 427 nm was observed and the corresponding optical-absorption spectrum consists also of two broad bands but now peaking at 240 and 347 nm with a 10Dq splitting of 12 849 ${\mathrm{cm}}^{-1\mathrm{}}$. The emission band at 410 nm in the well-aged crystals was associated with the stable dihalide phase Eu${\mathrm{Cl}}_2$, while those peaking at 439 and 485 nm were ascribed to the metastable precipitated Eu${\mathrm{Cl}}_2$-like plate zones parallel to {111} and {310} planes of the matrix lattice, respectively. For the system KCl-Eu${\mathrm{Cl}}_2$, only one emission band peaking at 427 nm was observed in crystals stored at room temperature for five years, without any previous heat treatment. The corresponding optical-absorption spectrum of these well-aged crystals consists of two broad bands peaking at 244 and 349 nm with a 10Dq value of 12 331 ${\mathrm{cm}}^{-1\mathrm{}}$. After quenching the samples, an emission band at 419 nm was observed and the corresponding optical-absorption spectrum was very similar to that observed before the quenching, although its 10Dq splitting was smaller (11 997 ${\mathrm{cm}}^{-1\mathrm{}}$). In this case, the band peaking at 427 nm, observed in the well-aged crystals, has been tentatively associated with the metastable Suzuki phase of divalent europium in the potassium chloride host. The change in the absorption, excitation, and emission spectra associated with the different precipitated phases of ${\mathrm{Eu}}^{2+}$ in NaCl and KCl as a function of the annealing temperature is also reported.