Spectral-Structural Regularities of Rare-Earth Phosphates

Abstract

The results of spectral and structural studies of thulium and lutecium ortophosphates doped with rare-earth Ln³⁺ ions (Ln=Ce, Nd, Eu, Tb, Er) are presented. The compounds have been synthesized by chemical transport method. Luminescence and absorption spectra in visible and IR-spectrum ranges are obtained at 77 and 300K. Studies of characteristic optical transitions of Ln³⁺ ions enabled to identify tneir Stark sublevels and calculate crystalline field parameters. Vibrational spectroscopy data are used for the analysis of luminescence features of rare-earth ions in the orthophos-phate matrix. Raman and middle IR-absorption spectra are investigated at room temperature. IR Fourier transform spectrograms are obtained at 5K. By X-ray diffraction iso-morphic substitution of the lattice cations with other Ln ions (up to 5–10 mol%) has been found to result in the formation of solid solutions based on tetragonal modification of TmPO₄ type. With further increase of the active ion concentration, the material transforms into the two-phase mixture. The combination of the spectroscopic methods enables to analyse in detail the variations of crystal lattice and phosphorus-oxygen-ligand crystalline field parameters as transition from single-phase solid solutions to two-phase mixtures of rare-earth orthophosphates.