Intercalation Route to Complex Perovskites AM<sub>0.2</sub>Ta<sub>0.8</sub>O<sub>2.8</sub>N<sub>0.2</sub> (A = Sr,
Ba; M = Li, Na): Neutron Diffraction and Nuclear Magnetic Resonance
Study
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Abstract
Oxynitride-type complex perovskites,
AM<sub>0.2</sub>Ta<sub>0.8</sub>O<sub>2.8</sub>N<sub>0.2</sub> (A
= Sr, Ba; M = Li, Na), were synthesized
by the ammonolytic heating of a layered perovskite, A<sub>5</sub>Ta<sub>4</sub>O<sub>15</sub>, with 0.5M<sub>2</sub>CO<sub>3</sub>. 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<sup>+</sup> and Ta<sup>5+</sup> 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<sub>0.2</sub>Ta<sub>0.8</sub>O<sub>2.8</sub>N<sub>0.2</sub> and body-centered tetragonal for SrM<sub>0.2</sub>Ta<sub>0.8</sub>O<sub>2.8</sub>N<sub>0.2</sub>. This trend coincides with the symmetry
transition from BaTaO<sub>2</sub>N (<i>Pm</i>3̅<i>m</i>) to SrTaO<sub>2</sub>N (<i>I</i>4/<i>mcm</i>). In both SrM<sub>0.2</sub>Ta<sub>0.8</sub>O<sub>2.8</sub>N<sub>0.2</sub>, nitrogen atoms preferentially occupied the <i>c</i>-axial 4a site of the tetragonal cell. Solid state magic angle spinning
nuclear magnetic resonance spectroscopy showed that SrNa<sub>0.2</sub>Ta<sub>0.8</sub>O<sub>2.8</sub>N<sub>0.2</sub> and BaNa<sub>0.2</sub>Ta<sub>0.8</sub>O<sub>2.8</sub>N<sub>0.2</sub> exhibited marked downfield
shifts of <sup>23</sup>Na, manifesting an octahedral coordination.
On the other hand, the <sup>7</sup>Li NMR of SrLi<sub>0.2</sub>Ta<sub>0.8</sub>O<sub>2.8</sub>N<sub>0.2</sub> and BaLi<sub>0.2</sub>Ta<sub>0.8</sub>O<sub>2.8</sub>N<sub>0.2</sub> indicated a highly symmetrical
coordination environment of Li
