Coherent Pulsed Dynamic Nuclear Polarization with Endogenous Metal Ions

Abstract Dynamic nuclear polarization (DNP) can overcome the intrinsically low sensitivity of nuclear magnetic resonance (NMR) and enable structural characterization of a wide range of materials. In inorganic solids, DNP can be achieved endogenously by introducing paramagnetic metal ions as polarizing agents directly into the host lattice. However, DNP enhancements obtained with metal ions are typically an order of magnitude lower than those achieved with organic radicals because of the intrinsic properties of high-spin metal-ion electron systems. Here, we present the first experimental demonstration of coherent pulsed endogenous metal-ion DNP using frequency-swept chirped microwave excitation at the W-band (94 GHz). Experiments on Mn(II)- and Gd(III)-doped Li4Ti5O12 (LTO) show that the integrated solid effect (ISE) improves DNP efficiency by up to an order of magnitude compared with the conventional single-frequency solid effect (SE). Furthermore, ISE outperforms the adiabatic solid effect (ASE) for these systems with electron paramagnetic resonance (EPR) spectra that are much broader than the nuclear Larmor frequency, in contrast to the behavior observed for narrow-line organic radicals. Frequency-swept excitation is highly robust and maintains near-optimal performance over wide microwave frequency and magnetic field ranges. We further demonstrate that adiabatic microwave pulses can selectively excite the satellite transitions of high-spin electron systems, increasing the population difference of the central transition and thereby providing an additional mechanism for enhancing the DNP efficiency. These results establish coherent frequency-swept pulsed DNP as an effective strategy for overcoming the limitations of endogenous metal-ion DNP and pave the way for highly sensitive NMR studies of inorganic materials.