Synthesis and Crystal Structure of Lanthanide Orthoniobates Studied by Vibrational Spectroscopy

RENbO 4 (RE = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu) materials were synthesized by solid-state reactions and then systematically investigated by vibrational spectroscopy. Single-phase materials were obtained at optimized temperature and time conditions, which were probed by X-ray diffraction and Raman scattering. Vibrational spectroscopy was employed to investigate the crystal structure of the lanthanide orthoniobate series by using different excitation lines in the visible region and also infrared techniques. All RENbO 4 ceramics exhibited a monoclinic structure, space group C 2/ c = C 2 h 6 (No. 15), with Z = 4. Raman spectroscopy evidenced 18 modes whose symmetries could be discerned by using polarized scattering, in perfect agreement with group theoretical calculations. The wavenumbers of the active-bands presented generally a decreasing tendency for increasing RE ionic radii, as a consequence of the lanthanide-induced lattice expansion. Fourier-transform infrared spectroscopy was employed to study the optical polar phonons of a typical lanthanide orthoniobate material. The results showed 15 active modes, as predicted by the group-theory for these compounds. Kramers−Krönig analysis was used to corroborate the results obtained after fitting and to establish a complete set of RENbO 4 active phonons, not yet reported in the literature.