Determination of niobium-oxygen bond distances and bond orders by Raman spectroscopy

An empirical correlation is established for relating Raman stretching frequencies of niobium-oxygen (Nb-O) bonds to their respective bond distances in niobium oxide compounds. The correlation is an exponential least-squares fit between measured Raman frequencies and reported crystallographic bond lengths. Niobium-oxygen bond strengths (in valence units) are also related to Raman stretching frequencies by incorporating the present result with that of a previously derived bond strength/bond length relation. The Nb-O correlation established in the present study is expected to offer invaluable insight into the structures of niobate species in chemical systems which are not amenable to analysis by diffraction or other spectroscopic techniques. In the present study, applications of the correlation are illustrated by predicting Raman stretching frequencies for perfect NbO4 and NbO6 structures, determining the Nb-O bond lengths in bismuth niobate, BiNbO4, as well as the bond length and bond strength of the terminal Nb=O bond for dehydrated surface niobate species in the Nb2Os/A1203 system.