Hard‐Sphere/Lone‐Pair Cation Ordering as a Designable Polar Order Parameter in Phosphate Open Frameworks

ABSTRACT

Polar materials, enabled by non‐centrosymmetric structures and macroscopic dipoles, underpin key functionalities (e.g., piezoelectricity, pyroelectricity, and second‐order nonlinear optics) and serve as model platforms for symmetry‐breaking and structure‐property coupling. Yet polar‐material discovery and design still rely heavily on a few classical structural prototypes, limiting predictable and controllable polarity across broader material families. Here we propose a channel‐site engineering strategy to induce polarity in P–O frameworks: cooperative pairing of non–off‐centering “hard‐sphere” filler cations with lone‐pair‐driven off‐centering cations promotes a long‐range zigzag ordering within one‐dimensional P–O channels as a symmetry‐breaking order parameter, converting an otherwise nonpolar framework into a polar structure. Guided by this concept, we synthesize and determine a new polar phosphate, NaPb _0.6 Ni ₃ (PO ₄ ) ₃ O (NPNP), crystallizing in polar Ima2 at room temperature with channels defined by the P–O framework and a Na–Pb channel chain. MEM electron‐density reconstruction directly resolves the Na–Pb zigzag order, clarifying how Na ⁺ site occupation and Pb ²⁺ lone‐pair off‐centering jointly stabilize the polar state. Spectroscopic/electrical measurements confirm thermal robustness and a persistent pyroelectric polar response. Mechanistically, the cation zigzag order triggers symmetry‐breaking, whereas the dominant dipole arises from cooperative distortions of the P–O channel framework, establishing symmetry‐engineering strategy for inducing polarity in open frameworks.