Preparation and neutron diffraction study of Dion-Jacobson type oxynitrides LiLaTa2O7−3xN2x (x = 0.09, 0.29)

Highlights: ► Dion-Jacobson oxynitrides LiLaTa2O7−3xN2x□x (x < 0.3) by ammonolysis of LiLaTa2O7. ► Enhanced thermal stability and band gap narrowing by the oxynitride modification. ► Locations of N and □ within LiLaTa2O7−3xN2x by neutron diffraction and 7Li MAS NMR. -- Abstract: Ammonolytic heating of Dion-Jacobson type layered perovskite LiLaTa2O7 produced oxynitride derivatives LiLaTa2O7−3xN2x□xvia the substitution, 3O2− → 2N3− + □. The nitridation level increased with raising the temperature and/or increasing the time of reaction, reaching the limit at x ≈ 0.3 by the ammonolysis at 650 °C for 48 h. Upon the nitridation, tetragonal pristine lattice underwent a axis expansion together with c axis contraction. Neutron Rietveld refinement of LiLaTa2O7 and LiLaTa2O7−3xN2x (x = 0.09, 0.29) revealed that the Li-centered tetrahedra are gradually compressed along the 2-fold rotation axis, with the increase of x. Solid state 7Li nuclear magnetic resonance spectroscopy suggested that the anion vacancy resides mostly on the interlayer surface of perovskite block. As measured from the diffuse-reflectance spectroscopy, the nitridation induced marked reduction in the band gap magnitude, from 4.11 eV (x = 0) to 3.35 eV (x = 0.09) to 2.76 eV (x = 0.29).