4f-5d Transition of Tb³⁺ and Ce³⁺ in MPO₄(M=Sc, Y and Lu)

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Sharp no-phonon lines associated with phonon sidebands have been observed in 4f-5d excitation spectra for Tb³⁺ and Ce³⁺ as well as in emission spectra of Ce³⁺, in Sc, Y and Lu orthophosphates at low temperatures. The spectra manifest themselves as if the 5d electron did not see the tetragonal field component. Energy level schemes of the 4f⁷5d(Tb³⁺) and 5d(Ce³⁺) configurations are discussed. The 4f-5d transition matrix elements are calculated for Ce³⁺. The phonon sidebands are believed to reflect the vibrations of the pure lattices.