Dynamic nuclear polarization (DNP) significantly enhances the sensitivity of nuclear magnetic resonance (NMR), increasing its applications and the quality of NMR spectroscopy as a characterization tool for materials. Efficient spin diffusion among the nuclear spins is considered to be essential for spreading the hyperpolarization throughout the sample, enabling large DNP enhancements. This scenario mostly limits the polarization enhancement of low-sensitivity nuclei in inorganic materials to the surface sites when the polarization source is an exogenous radical. In metal-ion-based DNP, the polarization agents are distributed in the bulk sample and act as a source of both relaxation and polarization enhancement. We have found that as long as the polarization agent is the main source of relaxation, the enhancement does not depend on the distance between the nucleus and dopant. As a consequence, the requirement of efficient spin diffusion is lifted, and the entire sample can be directly polarized. We exploit this finding to measure high-quality NMR spectra of 17 O in the electrode material Li 4 Ti 5 O 12 doped with Fe(III) despite its low abundance and long relaxation time.

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Comprehending the oxygen vacancy distribution in oxide ion conductors requires structural insights over various length scales: from the local coordination preferences to the possible formation of agglomerates comprising a large number of vacancies. In Y-doped ceria, 89 Y NMR enables differentiation of yttrium sites by quantification of the oxygen vacancies in their first coordination sphere. Because of the extremely low sensitivity of 89 Y, longer-range information was so far not available from NMR. Herein, we utilize metal ion-based dynamic nuclear polarization, where polarization from Gd(III) dopants provides large sensitivity enhancements homogeneously throughout the bulk of the sample. This enables following 89 Y– 89 Y homonuclear dipolar correlations and probing the local distribution of yttrium sites, which show no evidence of the formation of oxygen vacancy rich regions. The presented approach can provide valuable structural insights for designing oxide ion conductors.

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