The structure and dynamics of inorganic systems: Solid-state NMR studies

Divalent metal nitrates of barium, calcium, lead, and strontium are isomorphous, crystallizing in the Pa3 cubic space group, and they form a continuous set of solid solutions. These cubic crystal systems have ionic sites surrounded by equivalent nearest neighbor ions. The positional coordinates of the cations usually have no degrees of freedom. With such properties, the nitrates are ideal models for calculating the electric field gradient in ionic crystal lattices, and comparing the calculations with results obtained from quadrupole coupling constants measured by nuclear magnetic resonance. The lattice is decomposed in a multipole expansion into point charges, dipole and quadrupole moments. In addition to the point charge, the most significant contributors to the field gradient are generally the induced dipoles, though in crystals with strong fixed dipoles such as water these may dominate. Fixed and induced quadrupoles contribute to a lesser extent.; Crystals grown from aqueous solutions containing mixtures of these nitrates incorporate both of the divalent ions into the lattice. Under the right conditions, all of the thirteen possible resonances, ranging from lead ions surrounded by twelve nearest neighbors, can be observed in a single spectrum. Quantitative analysis of NMR peaks by deconvolution and Monte Carlo simulation shows that solid solutions of PbxSr1-x(NO3) 2 and PbxBa1-x(NO3)2 tend to cluster slightly, to an extent of the magnitude expected from the van der Waals interaction energies within the crystal.; Since the lead atom is very sensitive to its local environment, a correlation between the chemical shift and structure was investigated using NMR powder patterns. An increase in mean Pb-O interatomic distance tends to move the isotropic chemical shifts to low frequency, whereas an increase in Pb-halogen interatomic distance shifts them to high frequency. In addition, complete information about the quadrupolar interactions and the chemical shieldings of the quadrupolar nuclei can be obtained from NMR powder patterns acquired under static and magic angle spinning.