Tuning of Optical Properties in La1-xBaxTaON2 Oxynitride through Composition and Particle Size Controls

La1-xBaxTa (O,N)3 oxynitrides were synthesized by ammonolysis of oxide precursors, prepared by a complex polymerization method. The absorption edge of the solid-solutions did not show a blueshift with an increase of x, in spite of the increase of O/N ratio, but rather the edge was redshifted. The substitution of La3+ by Ba2+ was thought to cause the bandgap to be redshifted due to the improved symmetry in (O,N)-Ta-(O,N) linkages in the perovskite structure. Addition of a NaCl flux during the ammonolysis of oxide precursors suppressed agglomeration of primary particles to inhibit the deterioration in the reflectivity after the longer wavelength region of the absorption edge.