Comparison of structural and spectroscopic properties of Ho3+-doped niobate compounds

Ho³⁺-doped YNbO₄ and Y₃NbO₇ phosphors were synthesised via a co-precipitation synthesis route. Structural characterisation and spectroscopic properties were performed as a function of the Ho³⁺ concentration. The Rietveld type-refinement confirms the stabilisation of the pure monoclinic Fergusonite YNbO₄ phase at the highest temperatures, but the quadratic phase is stabilised for the lowest temperature, whereas a pure cubic fluorite Y₃NbO₇ phase was observed for all of the heat treatments. Raman and infrared spectroscopies were used to compare the cation-oxygen vibrational modes. Emission spectra and up-conversion processes were obtained under excitation at 445 and 975 nm respectively. For a similar Ho³⁺ content, the energy transfers observed show differences between the two crystalline compounds that are related to a stronger interaction between the holmium cations in the fluorite host lattice.