Thermally Controlled A-site Cation Ordering and Coupled Polarity in Double Perovskite NaLaZr2O6

A-site cation ordering in double perovskites is crucially important for their physical properties. In this study, polycrystalline samples of Zr-based double perovskite NaLaZr₂O₆ were synthesized via high-temperature solid-state reactions, and the influence of the heating temperature and cooling rate on their crystal structures was investigated using synchrotron X-ray diffractometry and optical second harmonic generation. The samples prepared at 1200 °C, followed by slow cooling to room temperature, crystallize in a polar P2₁am structure, exhibiting partial A-site cation ordering, with Na- and La-rich A-site layers alternately stacked along the c axis. Remarkably, this structure transforms to a previously reported nonpolar Pnam phase with a disordered A-site cation arrangement when the slow-cooled samples are reheated at 1300 °C or higher and then rapidly quenched to room temperature. Theoretical analyses reveal that in the P2₁am phase, the a^-a^-c⁺ octahedral rotations trigger antiparallel displacements of the A-site cations in the Na- and La-rich A-site layers to induce spontaneous polarization. This is in contrast to the case of the Pnam phase, in which the rotation-induced antiparallel displacements of the equivalent A-site cations are antipolar. This work represents a rare example of AA'B₂O₆ perovskites exhibiting layered A-site ordering and polarity and also demonstrates a novel mechanism for reversible thermal switching from polar to nonpolar states in perovskites.