Investigation Of Elastic Properties And Multiferroic Properties Of Metal-organic Frameworks[?CH₃?₂NH₂][M?HCOO?₃]and[NH₄][M?HCOO?₃]

Multiferroic Metal-Organic Frameworks?MOFs?have broad application prospects in information storage,polymorphic memory components,magnetoelectric sensors,controllable devices and so on.The elastic properties indicate the bonding forces among atoms,and control the stability of the structure.It is of great theoretical and practical importance to investigate the elastic properties and multiferroic properties of MOFs.In this paper,multiferroic MOFs[?CH₃?₂NH₂][M?HCOO?₃]?M=Mn,Co,Ni,Mg?and[NH₄][M?HCOO?₃]?M=Mn,Co,Ni,Mg?were prepared and characterized.The elastic properties under low frequency,high stress and strain conditions from low temperature to room temperature were studied,and the anelastic properties and energy dissipation related to phase transitions were investigated.It is beneficial for the further understanding of the phase transition mechanisms and the developments and applications of multiferroic MOFs.MOFs,[?CH₃?₂NH₂][M?HCOO?₃]?M=Mn,Co,Ni,Mg?and[NH₄][M?HCOO?₃]?M=Mn,Co,Ni,Mg?,were prepared by hydrothermal method or diffusion method.At room temperature,X-Ray Diffraction?XRD?analysis showed that the structure of[?CH₃?₂NH₂][M?HCOO?₃]?M=Mn,Co,Ni,Mg?was perovskite-like with space group of rohombohedral R c,and the structure of[NH₄][M?HCOO?₃]?M=Mn,Co,Ni,Mg?was chiral with space group of hexagonal P6₃22.Infrared?IR?and Raman spectroscopy analysis indicated that the main vibrational modes of the functional groups in[?CH₃?₂NH₂][M?HCOO?₃]?M=Mn,Co,Ni,Mg?included the internal vibration of dimethylamine ions[?CH₃?₂NH₂]⁺and formate ions HCOO^-,and the lattice vibration of metal ions M²⁺.The main vibrational modes of the functional groups in[NH₄][M?HCOO?₃]?M=Mn,Co,Ni,Mg?included the internal vibration of amine ions NH₄⁺and formate ions HCOO^-,and the lattice vibration of metal ions M²⁺.The morphologies of the samples were studied by Scanning Electron Microscopy?SEM?and Transmission Electron Microscopy?TEM?.Determined by Differential Scanning Calorimetry?DSC?between 130 K and300 K,the phase transitions of[?CH₃?₂NH₂][M?HCOO?₃]?M=Mn,Co,Ni,Mg?occurred around 152-264 K?Mn:183-190 K,Co:152-161 K,Ni:172-180 K,Mg:261-264 K?.They were associated with the ferroelectric transition and the structural phase transition from rohombohedral R c to monoclinic Cc.Features of the first-order phase transitions were observed.They were mainly caused by the disorder-order transitions of the[?CH₃?₂NH₂]⁺ions.The phase transitions of[NH4][M?HCOO?3]?M=Mn,Co,Ni,Mg?occurred around 172 K-256 K?Mn:254-255 K,Co:172-200 K,Ni:180-199 K,Mg:254-256 K?,and they were associated with the ferroelectric transition and the structural phase transition from non-polar hexagonal P6₃22 to polar hexagonal P6₃,which were mainly caused by the disorder-order transitions of the NH₄⁺ions.Measurements of magnetic properties showed that the magnetic transition temperatures of[?CH₃?₂NH₂][M?HCOO?₃]?M=Mn,Co,Ni?were around 8.5-37.0 K?Mn:8.5 K,Co:15.0 K,Ni:37.0 K?,and the magnetic transition temperatures of[NH₄][M?HCOO?₃]?M=Mn,Co,Ni?were around 8.0-29.9 K?Mn:8.0 K,Co:10.0 K,Ni:29.9 K?.Fitting by Curie-weiss law indicated the anti-ferromagnetic ordering feature of the magnetic transitions.Dynamic Mechanical Analysis?DMA?was used to study the elastic properties at low frequencies?0.5-10 Hz?and under high stress and high strain conditions between130 K and 300 K,and the anelastic properties and energy loss associated with the ferroelectric transitions and the structural transitions.For[?CH₃?₂NH₂][M?HCOO?₃]?M=Mn,Co,Ni,Mg?,near the ferroelectric transition temperatures,the storage modulus E'showed the valley or kinks,and the loss modulus E''and the loss factor tan?showed peaks.The peak height decreased at higher frequency,and the peak temperature was roughly independent of the frequency,showing the features of the first-order phase transitions.The frequency dependence of the peak height followed the power law tan?=Afⁿ,with n value from-0.382 to-0.049.The obvious elastic anomalies and large energy loss near the ferroelectric transition temperature indicated strong coupling of ferroelasticity and ferroelectricity,and they were associated with the coupling of the local strain with the the freezing of the[?CH₃?₂NH₂]⁺ions and the mobility of the twin walls under applied stress.Near the ferroelectric transition temperature of[NH4][M?HCOO?3]?M=Mg?,the storage modulus showed the valley or kinks,and the loss modulus and the loss factor tan?showed broad peaks,with weak frequency dependences.