Multiferroicity and thermal expansion of the layered metal–organic framework NH₄Cl₂Ni(HCOO)₂(NH₃)₂

We have investigated the magnetic, dielectric, pyroelectric, and thermal expansion properties of a layered perovskite metal–organic framework, NH₄Cl₂Ni(HCOO)₂(NH₃)₂. The material undergoes three phase transitions including a canted antiferromagnetic transition at ∼ 36 K, and two successive structural transitions around 100 K and 110 K, respectively. The temperature dependence of dielectric permittivity and pyroelectric current suggests that the structural transitions induce weak ferroelectricity along the c-axis and antiferroelectricity in the ab plane. A negative thermal expansion along the c-axis is observed between two structural phase transitions, which is ascribed to the abnormal shrinkage of interlayer hydrogen bonding length. Moreover, the ferroelectric/antiferroelectric phase transition temperature shifts towards a higher temperature under a magnetic field, suggesting certain magnetoelectric coupling in the paramagnetic phase. Our study suggests that the layered metal–organic frameworks provide a unique playground for exploring exotic physical properties such as multiferroicity and abnormal thermal expansion.