Aqueous Synthesis and Structural Comparison of Rare Earth Niobates and Tantalates: (La,K,◻)2Nb2O7−x(OH)2 and Ln2Ta2O7(OH)2(◻ = vacancy; Ln = La−Sm)

Rare-earth niobates and tantalates are functional materials that are exploited as photocatalysts, host lattices for phosphors, and ion conductors. These phases are extremely challenging to synthesize by methods other than solid-state processing, which limits expansion of this useful class of materials. Hydrothermal processing in particular is hampered by the incompatibility of base-soluble tantalate or niobate with acid-soluble rare-earth oxides. Furthermore, an added challenge with tantalates is they are especially inert and insoluble. We present here a general hydrothermal process that has produced a range of rare-earth niobate/tantalate materials; including new phases, (La,K,◻)2Nb2O7−x(OH)2 (1) and Ln2Ta2O7(OH)2 (2) (◻ = vacancy, Ln = La−Sm—excluding radioactive promethium). The structures of 1 and the La-analogue of 2 were determined from powder X-ray diffraction data collected at the APS 11-BM line and corroborated by compositional analyses, infrared spectroscopy, 139La and 1H MAS NMR, and thermogravimetric analyses. The synthesis and characterization studies reveal that the tantalate (2) is compositionally pure with no vacancies or dopants, while the niobate (1) formed under identical conditions has both vacancies and potassium dopants. We attribute these features to the greater flexibility of Nb5+ in oxide lattices to accommodate distorted and lower coordination geometries, whereas Ta5+ is found predominantly in octahedral environments. Other differences in aqueous niobate and tantalate chemistry are noted by the different phases that form as a function of the Ln3+ radius.