Oxygen Vacancy Distribution in Yttrium-Doped Ceria from ⁸⁹Y–⁸⁹Y Correlations via Dynamic Nuclear Polarization Solid-State NMR

Abstract

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.

Endogenous 17O Dynamic Nuclear Polarization of Gd-Doped CeO2 from 100 to 370 K

“…The large DNP enhancement also enables 17 O NMR spectra to be acquired with high sensitivity at natural abundance (Figure S6), as also recently reported in (Y,Gd)-CeO 2 . In this case, on the other hand, the sensitivity is lower for 0.01% Gd-CeO 2 than for 0.1% Gd-CeO 2 due to the extremely slow buildup of the former with T B of almost an hour (the experimental sensitivity is then lower for 0.3%, most likely due to significant broadening and quenching).…”

“…The large DNP enhancement also enables 17 O NMR spectra to be acquired with high sensitivity at natural abundance (Figure S6), as also recently reported in (Y,Gd)-CeO 2 . 27 In this case, on the other hand, the sensitivity is lower for 0.01% Gd-CeO 2 than for 0.1% Gd-CeO 2 due to the extremely slow buildup of the former with T B of almost an hour (the experimental sensitivity is then lower for 0.3%, most likely due to significant broadening and quenching). This is because the rate of hyperpolarization falls off with the distance from the source as 1/r 6 , and at natural abundance (0.037%), the low concentration of 17 O results in weak homonuclear dipolar coupling and inefficient spin diffusion to relay the polarization, so nuclei far from an appropriate source are hyperpolarized only extremely slowly.…”

“…3,42,43 Molecular Gd complexes have been developed as alternative polarization sources for frozen solutions, 42−44 and DNP with endogenous Gd 3+ ions was recently used to power a 17 O radiofrequency maser 26 and probe 89 Y− 89 Y correlations in Y-substituted CeO 2 doped with Gd. 27 Modest enhancements up to a factor of 4 have also been shown by endogenous Gd 3+ DNP in Li 4 Ti 5 O 12 22 and oxide glasses. 25 Here, we report the EPR and 17 O DNP properties of 17 Oenriched Gd-CeO 2 samples as a function of Gd concentration between 0.01 and 1%, with further comparisons to naturalabundance samples.…”

“…In metals, these are conduction electrons, − whereas for insulating materials, the radical sources are typically metal ion dopants. This strategy was used extensively for DNP at low fields with static solids − but has recently undergone a renaissance with contemporary high-field magic-angle spinning (MAS) systems, where endogenous DNP has been demonstrated using high-spin metal ions, − as well as radical defects generated by gamma irradiation or electrical discharge …”

“…Gd 3+ can be readily substituted for Ce 4+ in CeO 2 , with concomitant oxygen vacancies to balance the charge. Gd-doped CeO 2 is one of the best performing oxide-ion conductors in the intermediate temperature regime (400–800 °C) and finds widespread commercial use in solid–oxide fuel cells and oxygen sensors. , For DNP, Gd 3+ is of interest because its f 7 configuration has no orbital angular momentum ( L = 0) so that the central transition between the m S = ±1/2 states has a narrow EPR linewidth with a g -factor close to 2. ,, Molecular Gd complexes have been developed as alternative polarization sources for frozen solutions, − and DNP with endogenous Gd 3+ ions was recently used to power a 17 O radiofrequency maser and probe 89 Y– 89 Y correlations in Y-substituted CeO 2 doped with Gd . Modest enhancements up to a factor of 4 have also been shown by endogenous Gd 3+ DNP in Li 4 Ti 5 O 12 and oxide glasses …”