Probing the local structures and protonic conduction pathways in scandium substituted BaZrO3 by multinuclear solid-state NMR spectroscopy

Abstract

A comprehensive multinuclear solid-state NMR study of scandium-substituted BaZrO₃ is reported. Static low field and MQMAS very high field ⁴⁵Sc NMR data revealed the presence of both 5- and 6-coordinated scandium atoms, 5-coordinated scandium arising from Sc nearby an oxygen vacancy. ¹⁷O NMR spectra showed the presence of up to three different chemical oxygen environments assigned to Zr–O–Zr, Zr–O–Sc and Sc–O–Sc. From the ratios of these different oxygen sites, the distribution of the scandium cations was close to random but indicated that the maximum scandium incorporation was lower than expected, consistent with the observation of Sc₂O₃ impurities at substitution levels of 30% Sc for Zr. ¹H and ⁴⁵Sc NMR data on the hydrated materials revealed the presence of scandium next to protonic defects. Finally, variable temperature ¹H NMR showed the presence of at least two different proton environments in between which proton transfer occurs at ambient temperatures (300 K).