Eu2+→Mn2+energy transfer in NaCl

Crystals of NaCl containing different concentrations of ${\mathrm{Eu}}^{2+}$ and ${\mathrm{Mn}}^{2+}$ ions (the Eu concentration always being less than that of Mn) were investigated by optical and EPR spectroscopy. The results show that ${\mathrm{Eu}}^{2+}$-${\mathrm{Mn}}^{2+}$ pairs form preferentially in this material and that highly efficient energy transfer occurs from the ${\mathrm{Eu}}^{2+}$ ions to the near-neighbor ${\mathrm{Mn}}^{2+}$ ions. Over 99% of the ${\mathrm{Eu}}^{2+}$ ions are paired, and these pairs are not significantly affected by the state of aggregation of the Mn ions, although in the various samples evidence for manganese ions in their dipolar state, as well as precipitated into the Suzuki phase, exists. The excitation spectra of the orange manganese emission show, in addition to the ${\mathrm{Eu}}^{2+}$ 4$f^7$→4$f^6$5d transitions, ${\mathrm{Mn}}^{2+}$ transitions associated with the Suzuki phase and with ${\mathrm{Mn}}^{2+}$-cation vacancy dipoles. The peak positions of these ${\mathrm{Mn}}^{2+}$-crystal-field-sensitive transitions were fitted theoretically with expressions including Racah-Tress and seniority corrections. This procedure allowed the determination of the relevant crystal-field parameters.