Pressure-Induced Magnetoelectric Effect in Both Magnetically Ordered and Paramagnetic States of the Metal–Organic Framework [NH₄][Co(HCOO)₃]

Metal–organic frameworks (MOFs) with perovskite-like structures exhibit rich physical phenomena due to their flexible architectures and multifunctional properties, such as magnetism, ferroelectricity, and magnetoelectric coupling. In this work, we investigate the pressure-tunable structural and physical properties of the MOF [NH4][Co(HCOO)3], which simultaneously exhibits spin-canted antiferromagnetism and ferroelectricity at low temperatures. High-pressure X-ray diffraction and Raman spectroscopy reveal two reversible structural phase transitions at 0.9–1.4 GPa and 4.3–6.7 GPa, with the first transition involving pronounced lattice distortion and compression of the CoO6 framework. Correspondingly, anomaly features are observed in both ac magnetic susceptibility and dielectric permittivity under pressures, indicating a strong coupling between structural and functional responses. Remarkably, pressure-induced magnetoelectric (ME) effects are detected in both the paramagnetic and magnetically ordered phases, attributed to modulation of the hydrogen bonding and Co–O–C–O–Co superexchange pathways. These findings demonstrate the feasibility of simultaneously tuning paramagnetic and spin-mediated ME effects through external pressure and provide new insights into the design of pressure-responsive multifunctional MOFs.