Luminescence and vibronic structure in Ca1-xSrxF2:Eu2+mixed crystals

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Abstract
The optical properties and vibronic structures in the luminescence were studied for Ca1-xSrxF2 mixed crystals doped with Eu2+ ions. The peak energy, linewidth, lifetime and non-radiative activation energy showed a non-linear variation with increasing x. The x-dependence of the zero-phonon line position and its linewidth revealed electron-phonon coupling in the weak range (Huang-Rhys factor S<3). This originates from local lattice distortion of the mixed crystal. The calculated results of vibronic structures successfully explained the spectral lineshape of the sideband by superposing the excited state phonon modes onto their ground-state multiple-phonon modes.