Structural analysis and characterization of layer perovskite oxynitrides made from Dion-Jacobson oxide precursors

A three-layer oxynitride Ruddlesden-Popper phase Rb_{1+ x}Ca₂Nb₃O_10-xN _x·yH₂O (x=0.7-0.8, y=0.4-0.6) was synthesized by ammonialysis at 800 °C from the Dion-Jacobson phase RbCa₂Nb ₃O₁₀ in the presence of Rb₂CO₃. Incorporation of nitrogen into the layer perovskite structure was confirmed by XPS, combustion analysis, and MAS NMR. The water content was determined by thermal gravimetric analysis and the rubidium content by ICP-MS. A similar layered perovskite interconversion occurred in the two-layer Dion-Jacobson oxide RbLaNb₂O₇ to yield Rb_{1+ x}LaNb₂O_7-xN _x·yH₂O (x=0.7-0.8, y=0.5-1.0). Both compounds were air- and moisture-sensitive, with rapid loss of nitrogen by oxidation and hydrolysis reactions. The structure of the three-layer oxynitride Rb _1.7Ca₂Nb₃O_9.3N _0.7·0.5H₂O was solved in space group P4/mmm with a=3.887(3) and c=18.65(1)Å, by Rietveld refinement of X-ray powder diffraction data. The two-layer oxynitride structure Rb_1.8LaNb ₂O_6.3N_0.7·1.0H₂O was also determined in space group P4/mmm with a=3.934(2) and c=14.697(2)Å. GSAS refinement of synchrotron X-ray powder diffraction data showed that the water molecules were intercalated between a double layer of Rb+ ions in both the two- and three-layer Ruddlesden-Popper structures. Optical band gaps were measured by diffuse reflectance UV-vis for both materials. An indirect band gap of 2.51 eV and a direct band gap of 2.99 eV were found for the three-layer compound, while an indirect band gap of 2.29 eV and a direct band gap of 2.84 eV were measured for the two-layer compound. Photocatalytic activity tests of the three-layer compound under 380 nm pass filtered light with AgNO₃ as a sacrificial electron acceptor gave a quantum yield of 0.025% for oxygen evolution.