Cooperative Herringbone Hinging and Columnar Shifts in Perovskite‐Related Coordination Polymers

Casting a material’s structure into structural degrees of freedom (DOFs), elementary structural features that govern the material’s response to external stimuli, is a powerful concept for rationalizing material responsiveness. Elevating this concept into a materials design strategy requires both the discovery of new DOFs and a clear understanding of how their interplay generates emergent properties. In this work we expand the family of perovskite-related ( A 2+ )( B 2+ (C 2 N 3 ) 3 ) 2 coordination polymers by introducing phosphonium-based divalent A 2+ cations and we show how the cooperative interaction between DOFs leads to biaxial negative thermal expansion (NTE). The materials contain two key DOFs. One DOF originates from the geometric arrangement of the A 2+ cations within the 3D (( BX 3 ) 2 ) 2- ReO 3 -type network. The second DOF arises from translational columnar shifts. These two DOFs act cooperatively so that columnar shifts induce herringbone-type hinging of the A 2+ cations and, overall, causing biaxial NTE. The strength and character of this cooperativity are highly sensitive to subtle modifications of the A 2+ cation such as rigidification through the introduction of a double bond. The results provide a compelling blueprint for engineering material responsiveness through the deliberate design of cooperative structural DOFs.