The fluorescence and excitation spectra of ${\mathrm{Fe}}^{3+}$ in the ordered and disordered phases of the spinel Li${\mathrm{Al}}_5$${\mathrm{O}}_8$ have been measured at 77 and 4.2 K. At these temperatures, a strong zero-phonon line is observed in the ordered-phase fluorescence accompanied by relatively well-defined vibronic sidebands. These sidebands are separated by a common frequency interval of 199 ${\mathrm{cm}}^{-1\mathrm{}}$. Their origin is attributed to local modes of an [Fe${\mathrm{O}}_4$] center. Several large splittings are observed. A splitting of approximately 600 ${\mathrm{cm}}^{-1\mathrm{}}$ is seen in the ${}_{}{}^4{}_{}{}^{}T_1^{}{}_{}{}^{}\left({}_{}{}^4{}_{}{}^{}G\right)$ and ${}_{}{}^4{}_{}{}^{}T_2^{}{}_{}{}^{}\left({}_{}{}^4{}_{}{}^{}G\right)$ levels at the tetrahedral sites. A splitting of 16 ${\mathrm{cm}}^{-1\mathrm{}}$ seen in the zero-phonon line is believed due to spin-orbit effects. Good agreement is obtained between calculated and observed level positions. Our earlier suggestion that the fluorescence of the ordered phase is due to ${\mathrm{Fe}}^{3+}$ at tetrahedral ($A$) sites while the fluorescence of the disordered phase is due to ${\mathrm{Fe}}^{3+}$ at octahedral ($B$) sites is strengthened. Finally, we present a comparison of certain basic features of Li${\mathrm{Al}}_5$${\mathrm{O}}_8$: ${\mathrm{Fe}}^{3+}$ obtained from optical measurements with those obtained from Mössbauer-effect measurements. The latter will be published separately.

• Received 17 June 1971

DOI:https://doi.org/10.1103/PhysRevB.5.3377