Hydroxyl Defects and Oxide Vacancies within Ringwooditetoward Understanding the Defect Chemistry of Spinel-Type Oxides

The high-pressure mineral ringwoodite (γ-Mg₂SiO₄) is a major component of the Earth’s mantle and is able to incorporate large amounts of water in the form of hydroxyl defects. Understanding its underlying defect chemistry does not only have implications for geological transport processes, but will also allow for extending this knowledge to defect mechanisms within spinel-type oxide materials in general, which are commonly used for heterogeneous catalysts, battery electrodes, and ionic conductors. Here, we present a comprehensive study combining X-ray diffraction, electron-microprobe analysis, Fourier-transform infrared, and ¹H MAS NMR spectroscopy on a suit of nanoliter crystals with water contents between 0.05 and 2 wt %. We observed that both oxide vacancies and hydroxyl defects are formed and stabilized by Mg and Si vacancies. The latter are the main sources for the water uptake at very low and large water contents, while Mg vacancies dominate the moderate region in between. Intriguingly, the Si defects originate from the inherent Mg/Si cation inversion, typical to spinel-type materials at higher temperatures. By demonstrating that MAS NMR spectroscopy on nanoliter crystals is feasible, we additionally offer a promising avenue for analyzing synthetically challenging materials in the future.