Hybrid Improper Ferroelectricity in Ho _2/3 Ta ₂ O ₆ Ceramics With <i>A</i> ‐site‐Deficient Double‐Perovskite Structures

ABSTRACT Although defects are inevitable in materials and generally degrade their performance, they can also serve as a pathway to realize compatible functionalities. In the present work, the dense single‐phase Ho 2/3 Ta 2 O 6 ceramics with A ‐site‐deficient double‐perovskite structures were prepared, and the room‐temperature ferroelectricity with a remanent polarization of approximately 0.3 µC/cm 2 and a coercive field of 470 kV/cm was observed. A first‐order improper ferroelectric phase transition was identified near the Curie temperature of 762 K, as confirmed by the temperature‐dependent dielectric response and DSC measurement. Moreover, the temperature‐dependent in situ XRD verifies that the paraelectric phase above the Curie temperature belongs to the P 4/ mmm space group. Based on group theory analysis, the polar Pb 2 1 m phase is established by the coupling of the in‐phase oxygen octahedral rotation and the out‐of‐phase oxygen octahedral tilt in the prototype P 4/ mmm phase, while this tetragonal phase is derived from the ideal simple perovskite through the A ‐site layered ordering of Ho 3+ cations and vacancies; thus, the hybrid improper ferroelectricity originates from the synergistic interaction between the A ‐site layered ordering and oxygen octahedral tilting. This work unambiguously demonstrates the hybrid improper ferroelectricity in Ho 2/3 Ta 2 O 6 ceramics with A ‐site layered ordered double‐perovskite structures and significantly expands the research scope of hybrid improper ferroelectricity into double‐perovskite systems.