Atomic-scale evidence for displacive disorder in bismuth zinc niobate pyrochlore.

Author(s) Jia, C.L.; Jin, L.; Chen, Y.H.; Urban, K.W.; Wang, H.
Journal Ultramicroscopy
Date Published 2018 Sep
Abstract

Pyrochlores characterized by the chemical formula ABO form an extended class of materials with interesting physical and chemical properties. The compound BiZnNbO is prototypical. Its excellent dielectric properties make it attractive, e.g. for capacitors, tunable microwave devices and electric-energy storage equipment. BiZnNbO shows an intriguing frequency-dispersive dielectric relaxation at 50 K ≤ T ≤ 250 K, which has been studied intensively but is still not fully understood. In this first study on a pyrochlore by atomic-resolution transmission electron microscopy we observe the Bi atoms on A sites since, due to their low nuclear charge, the contribution of Zn atoms to the contrast of these sites is negligible. We find in our [1¯00]and [112] oriented images that the position of the atomic intensity maxima do not coincide with the projected Wyckoff positions of the basic pyrochlore lattice. This supplies atomic-scale evidence for displacive disorder on split A-type sites. The Bi atoms are sessile, only occasionally we observe in time sequences of images jumps between individual split-site positions. The apertaining jump rate of the order of 0.1-1 Hz is by ten orders of magnitude lower than the values derived in the literature from Arrhenius plots of the low-temperature dielectric relaxation data. It is argued that these jumps are radiation induced. Therefore our observations are ruling out a contribution of Bi-atom jumps to low-temperature dielectric A sites-related relaxation. It is suggested that this relaxation is mediated by jumps of Zn atoms.

DOI 10.1016/j.ultramic.2018.05.009
ISSN 1879-2723
Citation Jia C-, Jin L, Chen Y-, Urban KW, Wang H. Atomic-scale evidence for displacive disorder in bismuth zinc niobate pyrochlore. Ultramicroscopy. 2018;192:57-68.

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