On the origin of high ionic conductivity in Na-doped SrSiO.

Understanding the local structure and ion dynamics is at the heart of ion conductor research. This paper reports on high-resolution solid-state Si, Na, and O NMR investigation of the structure, chemical composition, and ion dynamics of a newly discovered fast ion conductor, Na-doped SrSiO, 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 structure to form Na Sr SiO , and with >10 mol% Na doping, phase separation occurs, leading to the formation of an amorphous phase β-NaSiO and a crystalline Sr-rich phase. Variable-temperature Na and 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-NaSiO 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 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.