Atomic-Scale Study of Polar Order Across Temperatures in Ferroelectric Oxides

In ferroelectric oxides, the response of polar order to external field drives abundant ferroic properties, such as ferroelectricity, piezoelectricity and electrocaloric effect [1-3]. Temperature is one of the key factors that controls the evolution of polar order due to the competition between thermal fluctuation and polar interactions. Polar order generally emerges from disorder when temperature is reduced as thermal fluctuations subside. Visualization of the polar order across temperatures is compelling, not only for understanding temperature-dependent ferroic properties, but also for discovering emergent phenomena related to nano- or atomic-scale polar fluctuations [4-5].

Here, we directly visualize the evolution of polar order across temperature in two ferroelectric oxides using in situ scanning transmission electron microscopy from cryogenic to high temperatures. In the classical ferroelectric oxide BaTiO₃, we mapped the polar order across the high-temperature phase transition and observed a random but finite distribution of polar order above the Curie temperature [6], emphasizing the role of order-disorder transition in describing its ferroelectric-paraelectric transition. In contrast, we observed in the chemically doped system (BaTi_1−xZr_xO₃) that thermally driven fluctuations at high temperature gave way to a more ordered state and then, surprisingly, to a re-entrant disordered configuration at even lower temperature [7]. These atomic-scale visualizations highlight a rich landscape of polar order in ferroelectric oxides.

References:
1. K. J. Choi, et al., Science 306, 1005-1009 (2004)
2. F. Li, et al., Science 364, 364-368 (2019)
3. B. Nair, et al., Nature 575, 468-472 (2019)
4. H. Pan, et al., Science 374, 100-104 (2021)
5. M. Zhang, et al., Science 384 185-189 (2024)
6. Y. Zhang, et al., in preparation
7. Y. Zhang, et al., submitted