Intercalation Route to Complex Perovskites AM0.2Ta0.8O2.8N0.2 (A = Sr, Ba; M = Li, Na): Neutron Diffraction and Nuclear Magnetic Resonance Study

2015-01-07T00:00:00Z (GMT) by Young-Il Kim Younkee Paik Maxim Avdeev
Oxynitride-type complex perovskites, AM0.2Ta0.8O2.8N0.2 (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 BaM0.2Ta0.8O2.8N0.2 and body-centered tetragonal for SrM0.2Ta0.8O2.8N0.2. This trend coincides with the symmetry transition from BaTaO2N (Pmm) to SrTaO2N (I4/mcm). In both SrM0.2Ta0.8O2.8N0.2, nitrogen atoms preferentially occupied the c-axial 4a site of the tetragonal cell. Solid state magic angle spinning nuclear magnetic resonance spectroscopy showed that SrNa0.2Ta0.8O2.8N0.2 and BaNa0.2Ta0.8O2.8N0.2 exhibited marked downfield shifts of 23Na, manifesting an octahedral coordination. On the other hand, the 7Li NMR of SrLi0.2Ta0.8O2.8N0.2 and BaLi0.2Ta0.8O2.8N0.2 indicated a highly symmetrical coordination environment of Li.