Dynamic Nuclear Polarization of Metal-Doped Oxide Glasses: A Test of the Generality of Paramagnetic Metal Polarizing Agents

Abstract

Nuclear magnetic resonance (NMR) spectroscopy can provide unique, atomic-level, insights into the structure and dynamics of materials, but applications are impeded by its intrinsically low sensitivity. Dynamic nuclear polarization (DNP) is poised to overcome this limitation, and indeed has revolutionized the study of surfaces; however, the current approaches are ill-suited for bulk solids. One potential pathway towards the hyperpolarization of bulk solids by DNP is through the inclusion of paramagnetic metal ions that can serve as polarizing agents. In this work, we compared the relative performance of two such dopants, Mn 2+ and Gd 3+ , in three series of oxide glasses having chemical environments representative of those found in other crystalline and amorphous solids. In our studies, Gd 3+ outperformed Mn 2+ , consistently providing more than one order of magnitude greater time savings. We attributed this difference mainly to its lack of hyperfine interaction.

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

“…The lower enhancement observed for 89 Y in (Y,Gd)-CeO 2 could be due to the relatively high concentration of Y dopants (10–40%) and concomitant oxygen vacancies, which break the cubic symmetry of the metal site, introducing second-rank ZFS and broadening of the EPR resonance. The much lower enhancement of previous endogenous Gd 3+ DNP experiments, ε < 4, , is ascribed to the broad EPR resonances caused by ZFS or dipolar coupling. The higher enhancement of 0.01% Gd-CeO 2 than endogenous DNP , of natural-abundance Mn 2+ - or Fe 3+ -doped Li 4 Ti 5 O 12 is ascribed to the narrow EPR resonance and long 17 O T 1 in Gd-CeO 2 .…”

“…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. An exceptionally high signal enhancement by a factor of 650 is observed at 100 K for 0.01% Gd-CeO 2 , which remains as high as 150 at 370 K. This establishes the feasibility of endogenous DNP of solids at room temperature and higher temperatures, where materials are typically operational, rather than the cryogenic temperatures normally required for efficient DNP.…”

“…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-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 …”

Off-the-Shelf Gd(NO3)3 as an Efficient High-Spin Metal Ion Polarizing Agent for Magic Angle Spinning Dynamic Nuclear Polarization

“…Gd 3+ is of particular interest, as it offers large 1 H NMR signal enhancements ϵ of ∼19 for [Gd­(dota)­(H 2 O)] − and ∼37 for [Gd­(tpcatcn)] complexes at 9.4 T . High-spin metal ions also allow for endogenous hyperpolarization where intrinsic ions are contained in the samples as dopants (in inorganic solids) such as Mn 2+ -doped Li 4 Ti 5 O 12 or (in oxide glasses) such as Gd 3+ -doped lithium silicate, lithium borate, and zinc phosphate for bulk signal enhancements, in Gd 3+ -doped CeO 2 nanocomposite thin films on SrTiO 3 for detailed study of material interfaces, and in spin-labels as chelator tags …”