Miniature electronic capacitors used in cell phones and other microwave communications devices require materials with very low dielectric loss and large, temperature independent dielectric constants. Major industrial effort in this area is devoted to developing new materials with optimized properties, that are also easy to manufacture economically. Perovskite ceramics are widely used for this purpose, since their properties can be "tailored" empirically by substitution at both A and B sites of the basic ABO3 crystal lattice. Traditional methods of materials characterization, including electron microscopy, X-ray diffraction, and dielectric relaxation measurements, are very useful in this effort. However, these techniques provide only indirect information about the microscopic origin of bulk properties. Most of the atomic nuclei in these materials are NMR-active, and modern solid state NMR techniques provide an exciting new source of information about local structure and dynamics.

Most nuclei in high performance dielectric ceramics have half-integer spin I greater than 1, with large quadrupole moments. This implies that very high magnetic fields are needed to reduce the second order quadrupole broadening enough to yield informative NMR spectra. This point is illustrated below in Figure 1, which shows ⁹³Nb MAS spectra of two representative materials obtained in our laboratory at 7.06T and 17.6T.

Figure 1

	7.05T	17.6T

The peak positions are determined by a combination of chemical shifts, that scale linearly with magnetic field, and isotropic second order quadrupole shifts, that scale inversely with magnetic field. This leads to significantly better resolution at 17.6T.The two materials differ only by the replacement of Ca²⁺ (ionic radius 0.99 Angstroms, electronegativity, 1.1) by Sr²⁺ (ionic radius 1.12 Angstroms, electronegativity, 0.95). In both materials, the B-site Nb5+ and Li+ cations ions are surrounded by six oxygen anions that form a distorted octahedron. The resulting electric field gradients at Nb⁵⁺ sites are a sensitive measure of the local Nb-O bond lengths and Nb-O-Nb bond angles. The multiplicity of peaks, resolved only at 17.6T, arise from NbO₆ subunits with different numbers of B-site Li⁺ and A-site Ca²⁺ or Sr²⁺ neighbors. We are in process of determining site-specific chemical shifts and quadrupole coupling constants for all the peaks, and interpreting their values with the aid of first principles quantum mechanical calculations on model structures.