In dynamic nuclear polarization nuclear magnetic resonance
(DNP-NMR)
experiments, the large Boltzmann polarization of unpaired electrons
is transferred to surrounding nuclei, leading to a significant increase
in the sensitivity of the NMR signal. In order to obtain large polarization
gains in the bulk of inorganic samples, paramagnetic metal ions are
introduced as minor dopants acting as polarizing agents. While this
approach has been shown to be very efficient in crystalline inorganic
oxides, significantly lower enhancements have been reported when applying
this approach to oxide glasses. In order to rationalize the origin
of the difference in the efficiency of DNP in amorphous and crystalline
inorganic matrices, we performed a detailed comparison in terms of
their magnetic resonance properties. To diminish differences in the
DNP performance arising from distinct nuclear interactions, glass
and crystal systems of similar compositions were chosen, Li2OCaO·2SiO2 and Li2CaSiO4, respectively.
Using Gd(III) as polarizing agent, DNP provided signal enhancements
in the range of 100 for the crystalline sample, while only up to around
factor 5 in the glass, for both 6Li and 29Si
nuclei. We find that the drop in enhancement in glasses can be attributed
to three main factors: shorter nuclear and electron relaxation times
as well as the dielectric properties of glass and crystal. The amorphous
nature of the glass sample is responsible for a high dielectric loss,
leading to efficient microwave absorption and consequently lower effective
microwave power and an increase in sample temperature which leads
to further reduction of the electron relaxation time. These results
help rationalize the observed sensitivity enhancements and provide
guidance in identifying materials that could benefit from the DNP
approach.