Thermally Controlled -site Cation Ordering and Coupled Polarity in Double Perovskite NaLaZrO.

-site cation ordering in double perovskites is crucially important for their physical properties. In this study, polycrystalline samples of Zr-based double perovskite NaLaZrO 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 2 structure, exhibiting partial -site cation ordering, with Na- and La-rich -site layers alternately stacked along the axis. Remarkably, this structure transforms to a previously reported nonpolar phase with a disordered -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 2 phase, the octahedral rotations trigger antiparallel displacements of the -site cations in the Na- and La-rich -site layers to induce spontaneous polarization. This is in contrast to the case of the phase, in which the rotation-induced antiparallel displacements of the equivalent -site cations are antipolar. This work represents a rare example of 'O perovskites exhibiting layered -site ordering and polarity and also demonstrates a novel mechanism for reversible thermal switching from polar to nonpolar states in perovskites.