43587-AC5
Atomic Structure of Pyrochlore Bismuth Zinc Niobate Thin Films
The unusual dielectric properties of bismuth-based pyrochlores, such as Bi1.5Zn1.0Nb1.5O7 (BZN), in particular their low dielectric loss, high dielectric constant and electric field tunability are believed to be closely related to the displacive and chemical disorder within the unit cell. Disorder in BZN is introduced by random substitution of Zn on both A- and B-sites and uncorrelated off-centering of the A and O' sites (O' denotes the seventh oxygen only bound to the A-site in the general pyrochlore formula A2B2O6O'). However, many aspects of the dielectric properties of cubic pyrochlores remain poorly understood. This report summarizes our research activities in this ACS PRF funded project to improve our understanding of the structure-property relationships of pyrochlores with unusual dielectric properties.
We performed atomic-scale structural analysis of BZN using chemically sensitive, atomic resolution scanning transmission electron microscopy (STEM) techniques, in particular high-angle annular dark-field imaging (HAADF). These studies yielded several surprising results that we still do not understand well. For example, along <110> HAADF images showed intensities in the B-site columns that were sometimes greater than those of the A-site columns containing Bi, contrary to what was expected given the strong atomic number sensitivity of these images. One possible explanation is that the off-centering of the A-site positions caused dechanneling or "cross-talk" of the electron probe and HAADF image simulations are currently underway. Bi-rich planar defects along (111) planes were seen in HAADF-STEM. These defects were not observed by conventional high-resolution transmission electron microscopy. Intensity analysis of images along <110> showed shifts of the lattice of 1/8[112] in the plane of the image. Furthermore, HAADF-STEM images of BZN films showed a lower symmetry than was expected from the average crystal structure. The lower symmetry of the images was due to a change in the intensity of every second {111} plane. Such a "superlattice" effect was not observed in an ideal pyrochlore, Y2Ti2O7. These results showed that care must be taking in relating the average atomic structure (as, for example, obtained in x-ray diffraction) to the dielectric properties of these materials.
The second approach taken in the project was to systematically vary off-centering and chemical disorder. Bi2Ti2O7 has the cubic pyrochlore structure with disordered displacements of both the Bi and O' sites,[1] making it a potential model material for bismuth pyrochlore dielectrics. However, in contrast to BZN, Bi2Ti2O7 is not a thermodynamically stable phase and decomposes into Bi2Ti4O11 and Bi4Ti3O12. It is therefore difficult to obtain in bulk form. We synthesized nearly phase-pure Bi2Ti2O7 thin films with the cubic pyrochlore structure by sputtering. Impurity phases, in particular Bi4Ti3O12, formed at high post-growth annealing temperatures. Electron diffraction patterns confirmed that films were predominantly cubic pyrochlore. The 244 reflections were present in diffraction patterns from individual grains along [011]. These reflections are forbidden in the ideal pyrochlore structure and have been explained with A-site displacive disorder. At 1 MHz, the dielectric constants were about 140 – 150 with negligible tunability and the dielectric loss was about 4x10-3. The dielectric loss increased with frequency. Several possible reasons may exist for the differences in the dielectric properties of BZN and Bi2Ti2O7. BZN shows a relatively wide distribution of random fields associated with structural and chemical disorder in its unit cell. The results showed that in addition to the structural disorder, chemical disorder (present in BZN but not in Bi2Ti2O7) is essential for achieving high room temperature dielectric constants and tunabilities in the bismuth based pyrochlores.
