Rare earth niobates (RE3NbO7) are materials that present interesting magnetic [[1], [2], [3]], dielectric [4,5], photocatalytic [6,7], electric or ionic conductivity [[8], [9], [10], [11], [12]] and optical properties [13,14]. Such properties could be related to the crystal structures of RE3NbO7 that exists in three different crystalline forms depending on the ionic radii of the rare earth atoms [3,[15], [16], [17], [18], [19]]. Orthorombic (Pnma) and cubic fluorite pyrochlore (Fdm) structures are found for the largest cations, while the smallest radii RE (Dy, Ho, Er, Tm, Eu, Yb …) crystallize into a defect cubic fluorite structure (Fmm) [15,17]. Rare earth and niobium atoms are statistically distributed on cationic sites, as well as oxygen vacancies on anionic sites. This relative disordered structure leads to peculiar optical properties with a significant broadening of the 4f-4f transitions observed in an Eu-doped Y3NbO7 compound...