A clear effect of charge compensation through Na+ co-doping on the luminescence spectra and decay kinetics of Nd3+-doped CaAl4O7

We present a detailed analysis of luminescence behavior of singly Nd3+ doped and Nd3+, Na+ co-doped calcium aluminates powders: Ca1−xNdxAl4O7 and Ca1−2xNdxNaxAl4O7 (x=0.001–0.1). Relatively intense Nd3+ luminescence in IR region corresponding to typical 4F3/2→4IJ (J=9/2–13/2) transitions with maximum located at about 1079 nm was obtained in all samples on direct excitation into f–f levels. The effect of dopant concentration and charge compensation by co-doping with Na+ ions on morphology and optical properties were studied. The results show that both, the Nd3+ concentration and the alkali metal co-doping affected the optical properties but had no influence on the powders morphology. The studies of luminescence spectra (298 and 77 K) in a function of dopant concentration showed an increasing distortion of the local symmetry of Nd3+with raising activator content due to certain defects created in the crystal lattice. On the other hand Na+ addition led to significant narrowing of absorption and luminescence bands and also a reduction of the number of their components, showing smaller disturbance of Nd3+ ions local symmetries. Consequently, charge compensated by Na+ co-doping materials showed significantly enhanced Nd3+ luminescence. The decrease of emission intensity and luminescence lifetimes with increase of activator concentration was attributed mainly to phonon-assisted cross-relaxation processes between Nd3+ ions. Analysis with Inokuti–Hirayama model indicated dipole–dipole mechanism of ion-ion interaction. Na+ addition led to much smaller concentration quenching due to smaller clustering of dopant ions in CaAl4O7 lattice.

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