In contrast to polar cation displacements driving oxides into noncentrosymmetric and ferroelectric states, inversion-preserving anion displacements, such as rotations or tilts of oxygen octahedra about cation coordination centers, are exceedingly common. More than one nonpolar rotational mode in layered perovskites can lift inversion symmetry and combine to induce an electric polarization through a hybrid improper ferroelectric (HIF) mechanism. This form of ferroelectricity expands the compositional palette to new ferroelectric oxides because its activity derives from geometric rather than electronic origins. Here, the new Ruddlesden-Popper HIF Sr 3 Zr 2 O 7 , which is the first ternary lead-free zirconate ferroelectric, is reported and room-temperature polarization switching is demonstrated. This compound undergoes a first-order ferroelectric-to-paraelectric transition, involving an unusual change in the "sense" of octahedral rotation while the octahedral tilt remains unchanged. Our experimental and first-principles study shows that the paraelectric polymorph competes with the polar phase and emerges from a trilinear coupling of rotation and tilt modes interacting with an antipolar mode. This form of hybrid improper "antiferroelectricity" is recently predicted theoretically but has remained undetected. This work establishes the importance of understanding anharmonic interactions among lattice degrees of freedom, which is important for the discovery of new ferroelectrics and likely to influence the design of next-generation thermoelectrics.

Hybrid improper ferroelectricity, which utilizes nonpolar but ubiquitous rotational/tilting distortions to create polarization, offers an attractive route to the discovery of new ferroelectric and multiferroic materials because its activity derives from geometric rather than electronic origins. Design approaches based on group theory and first principles can be utilized to explore the crystal symmetries of ferroelectric ground states, but in general do not make accurate predictions for some important parameters of ferroelectrics, such as Curie temperature (T C). Here, we establish a predictive and quantitative relationship between T C and the Goldschmidt tolerance factor, t, by employing n = 2 Ruddlesden-Popper (RP) A 3 B 2 O 7 as a prototypical example of hybrid improper ferroelectrics. The focus is placed on an RP system, (Sr 1−x Ca x) 3 Sn 2 O 7 (x = 0, 0.1, and 0.2), which allows for the investigation of the purely geometric (ionic-size) effect on ferroelectric transitions, due to the absence of the second-order Jahn-Teller active (d 0 and 6s 2) cations that often lead to ferroelectric distortions through electronic mechanisms. We observe a ferroelectric-to-paraelectric transition with T C = 410 K for Sr 3 Sn 2 O 7. We also find that the T C increases linearly up to 800 K with increasing the Ca 2+ content, i.e., with decreasing the value of t. Remarkably, this linear relationship is applicable to the suite of all known A 3 B 2 O 7 ferroelectrics, indicating that T C correlates with the simple crystal-chemistry descriptor, t, based on the ionic-size mismatch. This study provides a predictive guideline for estimating T C of a given material, which would complement the grouptheoretical and first-principles design approach. Additional ND and SXRD analyses, first-principles calculation results, and Mössbauer spectroscopy (PDF).