On the origin of high ionic conductivity in Na-doped SrSiO(3)

Understanding the local structure and ion dynamics is at the heart of ion conductor research. This paper reports on high-resolution solid-state (29)Si, (23)Na, and (17)O NMR investigation of the structure, chemical composition, and ion dynamics of a newly discovered fast ion conductor, Na-doped SrSiO(3), which exhibited a much higher ionic conductivity than most of current oxide ion conductors. Quantitative analyses reveal that with a small dose (<10 mol%) of Na, the doped Na integrates into the SrSiO(3) structure to form Na(x)Sr(1–x)SiO(3–0.5x), and with >10 mol% Na doping, phase separation occurs, leading to the formation of an amorphous phase β-Na(2)Si(2)O(5) and a crystalline Sr-rich phase. Variable-temperature (23)Na and (17)O magic-angle-spinning NMR up to 618 °C have shown significant changes in Na ion dynamics at high temperatures but little oxide ion motion, suggesting that Na ions are responsible for the observed high ionic conductivity. In addition, β-Na(2)Si(2)O(5) starts to crystallize at temperatures higher than 480 °C with prolonged heating, resulting in reduction in Na(+) motion, and thus degradation of ionic conductivity. This study has contributed critical evidence to the understanding of ionic conduction in Na-doped SrSiO(3) and demonstrated that multinuclear high-resolution and high-temperature solid-state NMR is a uniquely useful tool for investigating ion conductors at their operating conditions.