In this work a solid state NMR methodology based on the “NMR visibility” and the concept of NMR blind sphere was established. This method can be used for the study of doping homogeneity of paramagnetic ions, and the resulting “NMR homogeneity” was shown to correlate to the functional material performance. The method has been successfully applied in the model sample series and NMR blind sphere radii for paramagnetic dopants could be obtained.
First, in section 3.1, the correlation between NMR signal and the dopant distribution was established, via the development of the “NMR visibility” and the NMR visibility function f(x) curve, which is the visibility f as a function of the doping level x. Such “NMR visibility” was defined as the molar peak area of paramagnetic doped sample normalized by that of the diamagnetic host. The NMR visibility model was tested on Sr1‑xEuxH2 sample series and the formula of the NMR visibility function for homogeneous sample series was developed to be f(x)=exp(-ar0^3x), where the r0 is the NMR blind sphere radius and a is a number density parameter related to the host. The visibility curve calculated from 1H MAS NMR experimental data was consistent with the visibility function f(x) as well as the results calculated by a home-written Fortran90 program based on a random distribution model.
Subsequently, in section 3.2, the method was tested in different model compounds series including hopeite (Zn1‑xMnx)3(PO4)2·4H2O with the paramagnetic dopant Mn2+ and NMR nuclei 1H and 31P, Sr1‑xEuxGa2S4 with the paramagnetic dopant Eu2+ and NMR nucleus 71Ga, and monazite La1‑xLnxPO4 with paramagnetic dopants Ln3+ (Ln = Nd, Gd, Dy, Ho, Er, Tm, Yb) and NMR nucleus 31P. For all homogeneously doped sample series the NMR visibility method could be applied and the NMR blind sphere radii were obtained, which lay typically in the Å to nm range. Additionally, the theoretical study of the NMR blind sphere radii was shown.
In section 3.3, the NMR visibility function was shown to be able to differentiate a heterogeneous doping scenario from a homogeneous one, as for heterogeneously doped samples a deviation from the visibility function was observed. The term “NMR homogeneity” was thus introduced for homogeneous samples tested by the NMR visibility method. Samples with higher NMR homogeneity were also shown to be positively correlated to better luminescence performance, including intensity and decay time.
Furthermore, in section 3.4, the NMR visibility function was extended to co-doped systems including La1‑x‑yGdxDyyPO4, La1‑x‑yNdxTmyPO4 and La1‑x‑yNdxHoyPO4. For La1‑x‑yNdxHoyPO4, a 3D NMR visibility map instead of the 2D NMR visibility curve was developed. As the radii of NMR blind spheres were in Å to nm range, the NMR homogeneity determined by the NMR visibility model was also on a similar length scale. Together with SEM-EDX mapping and SEM-CL techniques, co-doping homogeneity or heterogeneity can be systematically studied from Å to nm range.
Overall, the NMR visibility method has been shown to be useful both for theoretical NMR blind sphere studies and for applications in paramagnetic systems as long as NMR nuclei are present.