Solid-State 29Si NMR and Neutron-Diffraction Studies of Sr0.7K0.3SiO2.85 Oxide Ion Conductors

K/Na-doped SrSiO3-based oxide ion conductors were recently reported as promising candidates for low-temperature solid-oxide fuel cells. Sr0.7K0.3SiO2.85, close to the solid-solution limit of Sr1–xKxSiO3–0.5x, was characterized by solid-state 29Si NMR spectroscopy and neutron powder diffraction (NPD). Differing with the average structure containing the vacancies stabilized within the isolated Si3O9 tetrahedral rings derived from the NPD study, the 29Si NMR data provides new insight into the local defect structure in Sr0.7K0.3SiO2.85. The Q1-linked tetrahedral Si signal in the 29Si NMR data suggests that the Si3O9 tetrahedral rings in the K-doped SrSiO3 materials were broken, forming Si3O8 chains. The Si3O8 chains can be stabilized by either bonding with the oxygen atoms of the absorbed lattice water molecules, leading to the Q1-linked tetrahedral Si, or sharing oxygen atoms with neighboring Si3O9 units, which is consistent with the Q3-linked tetrahedral Si signal detected in the 29Si NMR spectra.