Structural, vibrational and optical properties of Eu3+-doped Gd3NbO7 niobates – The mechanism of their structural phase transition

We report the structural, phonon and luminescence properties of Eu3+-doped Gd3-xEuxNbO7 niobates prepared by conventional solid state reaction. Although a few different crystal structures of pure Gd3NbO7 have been reported so far, their properties still remain controversial and the mechanism of their structural phase transition is not well understood. The temperature-dependent IR and Raman spectra show a structural phase transition occurring at 340 K. The detailed analysis of vibrational spectra and correlation diagrams as well as theoretical factor group considerations allowed us to propose the detailed assignment of the observed bands, define the symmetry of low and high-temperature phases and explain the mechanism of their structural transformation from the Pmcn to the Cmcm centrosymmetric orthorhombic space group. Powder X-Ray diffraction patterns confirm our analysis. The temperature-dependent emission, room-temperature excitation spectra and luminescence decay times are reported to achieve more information on structural and optical properties of the studied niobates.