XPS characterization of SmNbO4 and SmTaO4 precursors prepared by sol-gel method

Samarium niobate SmNbO4 (SNO) and tantalate SmTaO4 (STO) precursors were prepared by sol-gel method and annealing at 1000 degrees C. The different polymorphs, orthorhombic and monoclinic SmNbO4 or tetragonal SmTa7O19 and monoclinic SmTaO4 were identified. Structural properties of precursors were investigated by the X-ray photoelectron spectroscopy (XPS). Core-level XPS studies of sol-gel SNO and STO were performed for the first time. The binding energy differences Delta(OeNb) = B-E (O 1s) - B-E (Nb 3d(5/2)) and Delta(OeTa)= B-E (O 1s) - B-E (Ta 4f(7/2)) were used as suitable parameter to characterize average Nb-O bonding in SNO (323.1 eV) and Ta-O bonding in STO (503.9 eV). The Sm 3d(5/2) peak consists of strong features characteristic for Sm3+ (1083.0 eV) with (4f(5)) electron configuration and Sm2+ (1072 eV) with (4f(6)) electron configuration. The molar concentration of Sm3+ in SNO and STO was higher (21.0 and 24.7 at.%) than Sm2+ in SNO (1.0 at.%) and STO (0.3 at.%). The XPS demonstrated two valence states of Sm in precursors for application as nanophosphors for lighting and display technologies.