Halogen Engineering Achieving Regulable Polar Axes and Large Spontaneous Polarization in Mixed‐Cation Metal Halide Hybrid Multiferroics

Multiaxial ferroelectrics with multiple equivalent polarization directions are free from the constraints of single‐crystal form and exhibit remarkable advantages in device applications. Halogen engineering enables precise modulation of the halogen composition in materials at the atomic scale, effectively constructing multiaxial ferroelectricity while significantly enhancing the spontaneous polarization. Here, based on mixed‐cation hybrid uniaxial ferroelectric ( S ‐3‐hydroxypyrrolidinium) 2 NH 4 BiBr 6 ( HDPIB ), we successfully synthesized a multiaxial ferroelectric ( S ‐3‐hydroxypyrrolidinium) 2 NH 4 BiCl 6 ( HDPIC ) via halogen substitution strategy on the inorganic framework. HDPIC exhibits a higher Curie temperature (388 K) than HDPIB (375 K). Meanwhile, HDPIC shows a drastically enhanced P s of 16.7 µC·cm −2 , which is nearly 12 times that of HDPIB and surpasses that of most mixed‐cation hybrid molecular ferroelectrics. Notably, the halogen substitution strategy effectively regulated the hydrogen bond network within the structure, inducing lattice symmetry breaking and successfully increasing the number of polar axes in HDPIC to 6. This design strategy differs from the conventional organic‐component substitution methods reported in previous studies. Our research establishes an effective route for designing multiaxial ferroelectrics and offers new insights into the exploration of high‐performance ferroelectric materials.