Strain Coupling and Dynamic Relaxation in a Molecular Perovskite-Like Multiferroic Metal-Organic Framework

Magnetic metal-organic frameworks (MOFs) with a perovskite structure AMX(3) are emerging single-phased multiferroics with different sources of magnetic and electric ordering. However, the atomic mechanism underlying the multiple ferroic coupling is convincingly clarified. In this work, large single crystals of [(CH3)(2)NH2] [Ni(HCOO)(3)] are synthesized and shown to exhibit a first-order ferroelectric phase transition at approximate to 178 K during heating and at approximate to 151 K during cooling, as confirmed by temperature-dependent differential scanning calorimetry, Raman scattering, and X-ray diffraction studies. Resonant ultrasound spectroscopy (RUS) is used to investigate the elastic and anelastic properties between 5 and 300 K. The RUS results show an abrupt disappearance of resonance peaks above the ferroelectric transition point of approximate to 178 K. This is probably due to the unfreezing of dimethylammonium cation motion which couples with local strain. Small changes in elastic properties associated with two known magnetic transition at approximate to 35 and approximate to 15 K, respectively, are indicative of weak magnetoelastic coupling. An apparent peak in acoustic loss accompanying the canted antiferromagnetic ordering (approximate to 35 K) and spin reorientation transition (approximate to 15 K) is attributed to dynamical magnetoelastic coupling on the RUS time scale of approximate to 10(-6) s. In comparison with the same MOF structures containing Mn2+ and Co2+, the smaller Ni2+ ions effectively generate an internal chemical pressure and induce a compressed ion force on the anion frameworks. This study opens up a new landscape to explore possibilities for ferroic-order coupling in molecular MOFs.