Chloride Flux Growth of La₂TiO₅ Crystals and Nontopotactic Solid-State Transformation to LaTiO₂N Crystals by Nitridation Using NH₃

Abstract

Oxynitride perovskites and related phases have received considerable attention due to their potential application for visible-light-responsive photocatalyst and nontoxic inorganic pigments. The changes in bonding and structure by a partial replacement of O 2– by N 3– give rise to interesting dielectric behavior. Here, we report on the fabrication of highly crystalline La 2 TiO 5 crystals by chloride flux growth method and their subsequent nitridation to form the LaTiO 2 N crystals using NH 3 gas. The flux-grown La 2 TiO 5 crystals had a columnar structure grown in the ⟨001⟩ direction. Using the NaCl flux, larger columnar La 2 TiO 5 crystals were grown compared to those grown using the KCl flux. With increasing solute concentration, the aspect ratio of columnar La 2 TiO 5 crystals decreased significantly. The columnar La 2 TiO 5 crystals with smooth surface were readily converted by nitridation at 950 °C for 45 h followed by acid treatment into the LaTiO 2 N crystals with a highly porous structure that formed from the strong segregation of nanocrystals, leading to the largest specific surface area (16.5–18.4 m 2 ·g –1 ). For the La 2 TiO 5 crystals grown using the chloride fluxes, the wavelength of the absorption edges was approximately 320 nm ( E g = 3.87 eV), whereas the absorption edges exhibited by the LaTiO 2 N crystals obtained by nitridation were approximately 600 nm ( E g = 2.06 eV). Particularly, the LaTiO 2 N crystals prepared in this study by nitriding the precursor La 2 TiO 5 crystals did not show a noticeable absorption in the near-infrared region above 600 nm, which is generally attributable to some reduced Ti 3+ species and nitrogen deficiency, even after a long nitridation process. The fabricated LaTiO 2 N crystals with low defect density will be advantageous for various applications that specially require higher specific surface area.