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43587-AC5
Atomic Structure of Pyrochlore Bismuth Zinc Niobate Thin Films

Susanne Stemmer, University of California (Santa Barbara)

The unusual dielectric properties of bismuth-based pyrochlore thin films, such as Bi_1.5Zn_1.0Nb_1.5O₇ (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.  A possible approach to obtain such an understanding is to systematically vary structural features such as 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.  In the present project period we have attempted to synthesize phase-pure Bi₂Ti₂O₇ films by radio-frequency (rf) sputtering to conduct dielectric measurements that may give insights into the dielectric properties of Bi-based pyrochlore films.

Bi₂Ti₂O₇ thin films were grown by rf magnetron sputtering on bare and Pt-coated sapphire substrates at low substrate temperatures (~ 200 °C).  Post-deposition anneals were carried out at different temperatures to crystallize the films.  Nearly phase-pure Bi₂Ti₂O₇ thin films with the cubic pyrochlore structure were obtained at annealing temperatures up to 800 °C.  Impurity phases, in particular Bi₄Ti₃O₁₂, formed at higher temperatures.  Electron diffraction patterns confirmed that films were predominantly cubic pyrochlore, even at annealing temperatures as high as 900 °C.  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 a very small 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.  Simple models of hopping dipoles under the influence of uniformly distributed random fields predict that dielectric tunabilities increase for narrower random field distributions or in the absence of random fields.  Although random fields were likely different in Bi₂Ti₂O₇, because of the absence of chemical disorder and the somewhat larger A-site displacements, their relative magnitude and distribution are unknown.  Another possible explanation for the absence of tunability in Bi₂Ti₂O₇ is a dielectric relaxation, similar to BZN and other pyrochlores, but at temperatures above room temperature.  However, no increase in dielectric constant with temperature was observed for Bi₂Ti₂O₇, as would be expected if the dielectric relaxation temperature was approached from below.  It is possible that freezing of dipoles occurred at temperatures higher than the maximum temperature in the experiments.  Other explanations may exist for the absence of tunability in Bi₂Ti₂O₇, such as field-induced structural changes, and further studies are needed. 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 bismuth based pyrochlores.

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