Intercalation Route to Complex Perovskites AM_0.2Ta_0.8O_2.8N_0.2 (A = Sr, Ba; M = Li, Na): Neutron Diffraction and Nuclear Magnetic Resonance Study

Oxynitride-type complex perovskites, AM_0.2Ta_0.8O_2.8N_0.2 (A = Sr, Ba; M = Li, Na), were synthesized by the ammonolytic heating of a layered perovskite, A₅Ta₄O₁₅, with 0.5M₂CO₃. A Rietveld refinement of the synchrotron X-ray and neutron powder diffraction patterns confirmed the complete structural transformation from a hexagonal layered-perovskite to a three-dimensional perovskite type, as well as the stabilization of alkali cations on the octahedral sites rather than on the dodecahedral sites in the latter. In all four compounds, M⁺ and Ta⁵⁺ were disordered completely despite a charge difference as much as 4. The crystal symmetry of the average structure depended on the size of the dodecahedral cation: simple cubic for BaM_0.2Ta_0.8O_2.8N_0.2 and body-centered tetragonal for SrM_0.2Ta_0.8O_2.8N_0.2. This trend coincides with the symmetry transition from BaTaO₂N (Pm3̅m) to SrTaO₂N (I4/mcm). In both SrM_0.2Ta_0.8O_2.8N_0.2, nitrogen atoms preferentially occupied the c-axial 4a site of the tetragonal cell. Solid state magic angle spinning nuclear magnetic resonance spectroscopy showed that SrNa_0.2Ta_0.8O_2.8N_0.2 and BaNa_0.2Ta_0.8O_2.8N_0.2 exhibited marked downfield shifts of ²³Na, manifesting an octahedral coordination. On the other hand, the ⁷Li NMR of SrLi_0.2Ta_0.8O_2.8N_0.2 and BaLi_0.2Ta_0.8O_2.8N_0.2 indicated a highly symmetrical coordination environment of Li.