About the nature of luminescent bands in undoped and Eu²⁺ doped SrAl2O4 phosphors

In this paper we report the luminescence properties of nanosized undoped and Eu²⁺ (1%) doped SrAl₂O₄ phosphors. The samples were prepared by combustion method at 600 °C followed by annealing of the resultant combustion ash at 1000 °C in a reductive (Ar + H₂) atmosphere. Photo luminescence (PL) and cathodoluminescence (CL) analyses were applied to characterize the phosphors. The qualitative energy level scheme of doped crystal SrAl₂O₄:Eu²⁺ and nature of defects in undoped phosphor is proposed and discussed. Within a simplified model of a single vibration and linear vibronic coupling the shape-function of the vibrationally assisted band (Pekarian) is analyzed with the emphasis on the vibrational structure. We propose an approximate approach to pass from the discrete Pekarian distribution to the structureless crystal field spectra taking into account phonon dispersion. This approach is expected to be useful for the description of the shape of the bands when the electronic levels are close to the conduction band of the host crystal.