Site-selective excitation, crystal-field analysis, and energy transfer in europium-doped monoclinic gadolinium sesquioxide. A test of the electrostatic model

The fluorescence spectrum of Eu³⁺ in monoclinic Gd₂{O}₃$ is analyzed at 300, 77, and 4.2 K. There are three distinct ${C}ₛ$ crystallographic sites for the rare earth in the monoclinic structure. Through selective excitation of the three $⁵D₀$ levels with a dye laser and time-resolution equipment, three different fluorescence spectra are obtained. To assign the spectra to definite crystallographic sites an electrostatic calculation of crystal-field parameters is undertaken for each site using the latest available structural data. The calculation involves both contributions from point charges and induced dipoles. The result is corrected for the shielding effect of $5s$ and $5p$ electrons and corrections to free-ion radial integrals are derived from experimental determination for other lanthanides in solids. Good agreement is found between simulated and experimental spectra, which allows an assignment of each of the three spectra to each of the three sites. Energy transfer from site to site is measured but is not interpreted at the moment.