Crossover between Tilt Families and Zero Area Thermal Expansion in Hybrid Prussian Blue Analogues

Materials in the family of Prussian blue analogues (C 3 H 5 N 2 ) 2 K[ M (CN) 6 ], where C 3 H 5 N 2 is the imidazolium ion and M =Fe, Co, undergo two phase transitions with temperature; at low temperatures the imidazolium cations have an ordered configuration ( C 2/ c ), while in the intermediate‐ and high‐temperature phases (both previously reported as R 3 ‾ m ) they are dynamically disordered. We show from high‐resolution powder neutron diffraction data that the high‐temperature phase has zero area thermal expansion in the ab ‐plane. Supported by Landau theory and single‐crystal X‐ray diffraction data, we re‐evaluate the space group symmetry of the intermediate‐temperature phase to R 3 ‾ . This reveals that the low‐to‐intermediate temperature transition is due to competition between two different tilt patterns of the [ M (CN) 6 ] 3− ions. Controlling the relative stabilities of these tilt patterns offers a potential means to tune the exploitable electric behaviour that arises from motion of the imidazolium guest.