Multiferroicity and thermal expansion of the layered metal–organic framework [NH₄Cl]₂[Ni(HCOO)₂(NH₃)₂]

Abstract We have investigated the magnetic, dielectric, pyroelectric, and thermal expansion properties of a layered perovskite metal–organic framework, [NH 4 Cl] 2 [Ni(HCOO) 2 (NH 3 ) 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.