STRUCTURAL CHEMISTRY AND RAMAN-SPECTRA OF NIOBIUM OXIDES

A series of niobium oxide reference compounds were investigated by Raman spectroscopy in order to determine the relationship between niobium oxide structures and their corresponding Raman spectra. The assignments of the Raman bands were based on the known niobium oxide structures. The Raman studies indicate that the Raman frequencies strongly depend on the niobium oxide structures. For the slightly distorted octahedral NbO6 structures (KNbO3, NaNbO3, and LiNbO3), the major Raman frequencies appear in the 500-700-cm-1 region. For the highly distorted octahedral NbO6 structures (K8Nb6O19, AlNbO4, and Nb(HC2O4)5), the major Raman frequencies shift from the 500-700- to the 850-1000-cm-1 region. The distortions in the niobium oxide compounds are caused by the corner- or edge-shared NbO6 octahedra. Both slightly distorted and highly distorted octahedral NbO6 sites coexist in the KCa2Nan-3NbnO3n+1, n = 3-5, layered compounds. Most of the niobium oxide compounds possess an octahedrally coordinated NbO6 structure with different extents of distortion, and only a few rare-earth ANbO4 (A = Y, Yb, Sm, and La) compounds possess a tetrahedrally coordinated NbO4 structure. For the tetrahedral NbO4 structure of YbNbO4 the major Raman frequency appears at approximately 813 cm-1. In situ Raman studies assisted in the discrimination between bulk and surface functionalities in the niobium oxide reference compounds possessing high surface areas (Nb2O5.nH2O and HCa2Nb3O10).