Dynamic nuclear polarisation of H in Gd-doped In(OH).

Dynamic nuclear polarisation (DNP) of solids doped with high-spin metal ions, such as Gd, is a useful strategy to enhance the nuclear magnetic resonance (NMR) sensitivity for these samples. Spin diffusion can relay polarisation throughout a sample, which is most effective for dense H networks, while the efficiency of DNP using Gd depends on the symmetry of the metal site. Here, we investigate cubic In(OH) as a high-symmetry, proton-containing material for endogenous Gd DNP. A H enhancement of up to 9 is demonstrated and harnessed to measure the O spectrum at natural abundance. The enhancement is interpreted in terms of clustering of the Gd dopants and the local reduction in symmetry of the metal site induced by proton disorder, as demonstrated by quadrupolar In NMR. This is the first example of H DNP using Gd dopants in an inorganic solid.