Q. Luo, Y. S. Lan, H. G. Zhang, P. Wang, C. F. Li(李川福), Z. B. Yan, and J.-M. Liu(刘俊明)

Significantly enhanced stability of hybrid improper ferroelectricity in Sr₃Zr₂O₇ by isovalent Ca substitution

Physical Review B 111, 174108 (2025)

The A₂A′B₂O₇-type two-layered Ruddlesden-Popper perovskite oxides as a novel class of ferroelectrics, i.e., hybrid improper ferroelectric (HIF) compounds, have recently been receiving increasing attention. However, the low stability of the ferroelectric state against elevated strain, along with the small spontaneous electric polarization (P), remains a major challenge for practical applications. Taking the HIF compound Sr₃Zr₂O₇ as an example, it was revealed in literature that the ferroelectric ground state will lose its stability by structural transition into nonpolar phases upon an epitaxial strain as small as ε ~ ±2%. The measured polarization in the polycrystalline state is only P ~ 0.30 µC/cm² and the predicted maximal P is ~ 6.75 µC/cm², which are insufficient for practical application. In this work, we address this issue numerically by employing first-principles calculations to explore the possibility to enhance the ferroelectric stability and polarization of Sr₃Zr₂O₇ via effective site-selective isovalent substitutions on the Sr site by a smaller Ca ion. Under a proper substitution scheme, the polar A2₁am ground state becomes further stabilized relative to the competing nonpolar Pnab phase, and the ground-state polarization can be doubled, while the estimated coercive field slightly increases. Simultaneously, the polar-to-nonpolar transition would not occur unless an unrealistically large strain is imposed. It is thus suggested that a site-selective isovalent substitution scheme would be a valuable way to improve the ferroelectric stability against mechanical fluctuations.

URL: https://doi.org/10.1103/PhysRevB.111.174108