High-resolution optical spectroscopy of YAG:Nd: A test for structural and distribution models

Abstract
Results are presented on the multisite structure of Nd3+ in YAG, obtained from a comparative study of melt-grown high-temperature (HT) and flux-grown (F) crystals by using optical spectroscopy in connection with x-ray data. Absorption and emission data by pumping into 4 G5/2, 4 F9/2, and 4 G7/2 Nd3+ multiplets are presented; very good resolution is obtained for pumping into 4 F9/2. The optical spectra show three types of centers: centers present in all samples, centers present mainly in the HT crystals (and very weak in F samples), and minor or irregular centers. Besides the main center (Nd3+ in isolated dodecahedral c sites) all samples contain three satellites, assigned to Nd3+(c)-Nd3+(c) pairs in the first, second, and third coordination spheres. A model is proposed to explain the pair lifetimes and their dependence on Nd concentration by assuming a superexchange interaction for the nearest-neighbor pair and electric dipole coupling for the other pairs. Three satellites of equal intensities, observed in the HT samples (and very weak in the F crystals), are connected with the crystal-field perturbations at the c sites produced by an excess of Y3+ ions in octahedral a sites, as predicted by the nonstoichiometry data. This result is consistent with the very weak intensity and with the systematic equivalence of the {222} forbidden reflections in our x-ray data, obtained only in the HT crystals. Arguments are given that these satellites are involved in cross-relaxation processes similar to those of the main center. The spectral features of a center that dominates the low-temperature 4 F3/2 emission under pumping at 532 nm are also analyzed.

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