Surface and paramagnetic effects in pseudo-cubic NaYF4 nanoparticles by solid state 19F and 23Na NMR combining time and spectral resolution

NaYF₄ crystals and nanocrystals have attracted great attention in optical applications due to their exceptional ability to host rare-earth (RE) emitting ions in a beneficial lattice. In particular, they show good RE solubility since these ions replace the chemically similar Y ions. The α-NaYF₄ phase has a pseudo-cubic structure, i.e., locally, the cubic structure is distorted due to Y and Na distributed statistically in the face-centered cell. It leads to a quite significant heterogeneous broadening of ¹⁹F and ²³Na NMR spectra, reducing the structural information that can be obtained by high resolution solid state NMR. For instance, bulk sites and defects or surface sites are indistinguishable by magic angle spinning NMR. However, relaxation processes are mostly due to fluctuations of the dipolar coupling and/or quadrupolar coupling to the time-modulated electrical field gradient (EFG). Thus, for ²³Na NMR, relaxation processes are highly sensitive to local distortions. We demonstrate that in this case one can associate spectroscopy to relaxation measurements to resolve such structural regions. Another issue arises to characterize the RE distribution in the NaYF₄ lattice, which is a routine NMR task. Usually, the ¹⁹F and ²³Na NMR line broadening can be rationalized in terms of van Vleck’s second moment theory, i.e., (M₂)^1/2 ~ σ(ppm) ~ μ_eff(RE³⁺). A statistical distribution of RE in the lattice as a function of concentration leads to a linear line broadening behavior whereas saturation means RE clustering. But in such a case where there is a superposition of paramagnetic and heterogeneous broadening, this approach is obscured. We show that this investigation can be also done by associating spectroscopy with relaxation measurements. However, it is of ultimate importance to take into consideration the competition between dipolar, quadrupolar and Solomon’s relaxation mechanisms in order to weight the paramagnetic contribution. Our findings highlight the power of associating spectral and temporal resolution in NMR to resolve complex structural details in these important materials.