⁹³Nb solid state NMR of high surface area niobium oxides

The local environment of niobium in oxides reflects the perturbations in bond strength that affect the acidity of oxygen atoms in the structure. To understand the relationship between metal environment and properties, ⁹³Nb solid state NMR has been used to measure the electric field gradient and chemical shift anisotropy for layered niobates with either alkali cations or protons at the material surface. In order to determine these parameters, a variety of techniques have been applied to extract information for the multiple environments located in these oxides. Variable offset cumulative echo spectra were collected on static samples at multiple magnetic fields, 4.7 T, 9.4 T and 14.1 T. RAPT enhanced QPASS data were collected at 9.4 T to extract quadrupolar-coupling information without the influence of chemical shift anisotropy. Data from KCa₂Nb₃O₁₀ and an acid exchanged form were collected and two distinct quadrupolar environments were observed. Acid exchange altered the isotropic chemical shift but did not significantly affect the electric field gradient or the chemical shift anisotropy.