Neodymium ion environment in germanate crystals and glasses

Absorption and fluorescence spectra are presented for neodymium-doped alkali germanate and bismuth germanate glasses and crystals. Narrower spectral components and a shift in intensity distribution toward shorter wavelengths for the composite bands corresponding to the principal neodymium absorption and fluorescence transitions are observed as the radius of the glass modifier ion is increased (Li⁺Na⁺K⁺). These changes in component band width and intensity distribution become more pronounced as the concentration of large radius modifier ion is increased, and result in effectively narrower widths for the fluorescent bands involved in laser emission. Optical spectra for neodymium in a sodium germanate single crystal show broad bands characteristic of those of glasses. Electron microprobe scans reveal non-uniform distribution and clustering of neodymium in alkali germanate crystals. Narrow, temperature sensitive bands similar to those of neodymium in other crystals are found for single crystal and polycrystalline neodymium-doped bismuth germanate, while the glass exhibits broad bands. Substitutional replacement of Bi³⁺ with Nd³⁺ in the bismuth germanate crystal lattice is indicated from the optical spectra but the environment of neodymium in crystalline sodium germanate appears to have only short-range order. The difficulty encountered in introducing neodymium into substitutional positions in alkali germanate crystals may result from the charge difference between neodymium and the alkali cations as well as the oxygen: germanium ratio of these compounds, which favors glass network formation rather than crystallization.