On the origin of high ionic conductivity in Na-doped SrSiO3

Understanding the local structure and ion dynamics is at the heart of ion conductor research. This paper reports on high-resolution solid-state Si-29, Na-23, and O-17 NMR investigation of the structure, chemical composition, and ion dynamics of a newly discovered fast ion conductor, Na-doped SrSiO3, 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 SrSiO3 structure to form NaxSr1-xSiO3-0.5x, and with >10 mol% Na doping, phase separation occurs, leading to the formation of an amorphous phase beta-Na2Si2O5 and a crystalline Sr-rich phase. Variable-temperature Na-23 and O-17 magic-angle-spinning NMR up to 618 degrees 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, beta-Na2Si2O5 starts to crystallize at temperatures higher than 480 degrees 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 SrSiO3 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.