Near-Room-Temperature Magnetoelectric Coupling Engineered through Inversion-Breaking Tilts in a Bulk Perovskite Polytype.

Systematic strategies to design properties such as ferroelectricity or magnetoelectric coupling are well-established in simple perovskite materials, but they remain scarce in more complex framework structures. Using a hexagonal polytype of the ternary Manganite AMnO3 (A= Ba, Sr, Ca) as a model system, we introduce a symmetry-guided design principle in which an inversion-breaking rigid-unit mode (RUM) serves as a single structural instability generating both polar and ferromagnetic orders within a bulk antiferromagnetic material. Symmetry analysis and first-principles calculations reveal that cooperative tilts of the Mn2O9 bioctahedral dimers generate a spontaneous polarization, and in the antiferromagnetically ordered state, they also induce a ferromagnetic moment. High-resolution diffraction and magnetic susceptibility measurements show that the structural and magnetic orders persist as high as 450 and 280 K, respectively, highlighting the untapped potential of framework structures that deviate from simple perovskite motifs to be designed to host useful ferroic properties. Our approach establishes a transferable symmetry-based framework for engineering ferroelectric and magnetoelectric states across chemically diverse framework architectures.