Dynamic nuclear polarisation of ¹H in Gd-doped In(OH)₃

Dynamic nuclear polarisation (DNP) of solids doped with high-spin metal ions, such as Gd³⁺, 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 ¹H networks, while the efficiency of DNP using Gd³⁺ depends on the symmetry of the metal site. Here, we investigate cubic In(OH)₃ as a high-symmetry, proton-containing material for endogenous Gd DNP. A ¹H enhancement of up to 9 is demonstrated and harnessed to measure the ¹⁷O spectrum at natural abundance. The enhancement is interpreted in terms of clustering of the Gd³⁺ dopants and the local reduction in symmetry of the metal site induced by proton disorder, as demonstrated by quadrupolar ¹¹⁵In NMR. This is the first example of ¹H DNP using Gd³⁺ dopants in an inorganic solid.