Electron Paramagnetic Resonance and Spectroscopic Study of LaCl3:Eu2+and Associated Color Centers

Abstract

The electron-paramagnetic-resonance (EPR) spectrum of La${\mathrm{Cl}}_3$: ${\mathrm{Eu}}^{2+}$ was observed to obtain the ground-state splitting of ${\mathrm{Eu}}^{2+}$ in sites of $C_{3h}$ symmetry. A good fit to the observed energy levels in the ground manifold of ${\mathrm{Eu}}^{2+}$ was provided by the parameters ${b_2}^0=427.20\mathrm{}\times {10}^{-4\mathrm{}}$ ${\mathrm{cm}}^{-1\mathrm{}}$, ${b_4}^0=5.48\mathrm{}\times {10}^{-4\mathrm{}}$ ${\mathrm{cm}}^{-1\mathrm{}}$, ${b_6}^0=-0.045\mathrm{}\times {10}^{-4\mathrm{}}$ ${\mathrm{cm}}^{-1\mathrm{}}$, ${b_6}^6=2.97\mathrm{}\times {10}^{-4\mathrm{}}$ ${\mathrm{cm}}^{-1\mathrm{}}$, $g_{\mathrm{II}}=1.9924\mathrm{}$, and $g_{\perp }=1.9927\mathrm{}$. The shift in ${b_2}^0$ from room temperature to 4.2°K was less than 5%. EPR spectra which could be attributed to europium ions in at least one other inequivalent site were observed but not analyzed in detail. The color centers produced in La${\mathrm{Cl}}_3$: ${\mathrm{Eu}}^{2+}$ by ultraviolet radiation appeared to be due to electrons removed by the radiation from ${\mathrm{Eu}}^{2+}$ ions, which were then trapped at certain crystal defects. The ${\mathrm{Eu}}^{2+}$ donors were shown to reside in the normal $C_{3h}$ rare-earth sites, and the polarization of the color-center absorption spectrum indicated that the color-center defects had relatively high symmetry. If the color-center defects were due to anion vacancies such as one finds for certain color centers in alkali halide crystals, then the defect was probably a cluster of more than one chlorine vacancy.