Fluorescence of Trivalent-Europium-Doped Yttrium Oxysulfide

Abstract

The fluorescence spectrum of trivalent europium in the site of $C_{\text{3υ}}$ symmetry of polycrystalline Y₂O₂S:Eu³⁺ is investigated in the region 4000–9000 Å. Of the total of 363 transitions between the crystal components of the $\text{J\hspace{0.17em}=\hspace{0.17em}0}$ to $\text{J\hspace{0.17em}=\hspace{0.17em}3}$ states of ${}^{5}D$ and those of the $\mathrm{J}\text{\hspace{0.17em}=\hspace{0.17em}0}$ to $\text{J\hspace{0.17em}=\hspace{0.17em}6}$ states of ${}^{\text{7F}}$ , almost two thirds are observed. Transitions originating in the ${}^5{D}_3$ state of Eu³⁺ are particularly intense at 77°K. Some lines originating in unidentified levels, higher than ${}^5{D}_3$ in energy, are also present. Thirty‐nine of the 44 crystal components of ${}^5{D}_{\text{0–3}}{,}^7{F}_{\text{0–6}}$ are identified. Y₂O₂S:Eu³⁺ is perhaps an optimal case of identification of degeneracies of the crystal components without benefit of the additional information afforded by polarization studies or by the Zeeman effect. From a consideration of all possible assignments of symmetry species for the crystal levels of ${}^7{F}_{\text{1–4}}$ in a systematic fashion, a small number of self‐consistent sets of assignments and crystal‐field parameters is obtained. Intensity data reduce the number of possibilities to four. For the most likely one of these, the deviation between observed and calculated crystal levels is 9 cm⁻¹. The crystal‐field parameters are $B_0^2{\text{\hspace{0.17em}=\hspace{0.17em}124, B}}_0^4{\text{\hspace{0.17em}=\hspace{0.17em}1344, B}}_3^4{\text{\hspace{0.17em}=\hspace{0.17em}823, B}}_0^6{\text{\hspace{0.17em}=\hspace{0.17em}286, B}}_3^6\text{\hspace{0.17em}=\hspace{0.17em}−\hspace{0.17em}362}$ , and $B_6^6\text{\hspace{0.17em}=\hspace{0.17em}554}{\mathrm{cm}}^{\text{−1}}$ . The effect of $J$ mixing is considerable. A fit to the barycenters of ${}^7{F}_{\text{0–4}}$ and ${}^5{D}_{\text{0–2}}$ yields values of the spin–orbit and electrostatic parameters $\text{η\hspace{0.17em}=\hspace{0.17em}1326}{\mathrm{cm}}^{\text{−1}}$ and $F_2\text{\hspace{0.17em}=\hspace{0.17em}395.8}{\mathrm{cm}}^{\text{−1}}$ . A mechanism involving lattice vibrations is proposed which may explain the dramatic increase in intensities of transitions originating in ${}^5{D}_3$ when the temperature is lowered from 300° to 77°K.