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

Abstract

Results are presented on the multisite structure of ${\mathrm{Nd}}^{3+}$ 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}$${\mathit{G}}_{5/2}$, $^{4}$${\mathit{F}}_{9/2}$, and $^{4}$${\mathit{G}}_{7/2}$ ${\mathrm{Nd}}^{3+}$ multiplets are presented; very good resolution is obtained for pumping into $^{4}$${\mathit{F}}_{9/2}$. The optical spectra show three types of centers: centers present in all samples, centers present mainly in the HT crystals (and very weak in F samples), and minor or irregular centers. Besides the main center (${\mathrm{Nd}}^{3+}$ in isolated dodecahedral c sites) all samples contain three satellites, assigned to ${\mathrm{Nd}}^{3+}$(c)-${\mathrm{Nd}}^{3+}$(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 ${\mathrm{Y}}^{3+}$ 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}$${\mathit{F}}_{3/2}$ emission under pumping at 532 nm are also analyzed.