The Investigation On Large-sized Crystal Growth Of Metalorganic Frameworks M[（CH₃）₂NH₂]（COOH）₃（M=Mn,Mg）

Metal-organic frameworks（MOFs）,also known as coordination polymers,are metal ions and organic ligands linked by coordination bonds,forming a periodic network-like structure of the new functional materials.The magnetic properties of different transition metal ions and the advantages of flexible organic ligand framework make the magnetic MOFs become the hotspot of new ferroelectric materials.MOFs have a great variety of materials.At present,the research on MOFs is mainly focused on basic design and preparation,structural characterization and performance testing.However,few people have studied how to prepare high-quality large-size MOFs crystals.The close to powder small MOFs crystals can hardly be applied to photoelectric magnetic devices.Therefore,it is of great practical significance to successfully produce high quality and large size target MOFs single crystal.In this paper,the experimental scheme was improved.The temperature control of the MOFs prepared by the hydrothermal method and the crystallization method of the modified crystal size were studied.The Mn[（CH₃）₂NH₂]（COOH）₃and Mg[（CH₃）₂NH₂]（COOH）₃ MOFs with good morphology and high crystal quality were successfully designed and prepared.The samples were characterized by structure characterization,phase analysis and morphology analysis and the Mn[（CH₃）₂NH₂]（COOH）₃crystals were tested by magnetic analysis.The main contents and results are as follows:（1）MOFs material with chemical formula Mn[（CH₃）₂NH₂]（COOH）₃ was prepared by hydrothermal method,using magnetic Mn²⁺ as metal ion ligand and N,N-dimethylformamide aqueous solution as hot solvent.The crystal growth process was prolonged and the defect of crystal growth was reduced effectively.Mn DMF crystals with a size of up to 4×4×1 mm³ were prepared,which were flat hexahedrons.The XRD was used to characterize the phase composition,and the crystals grow layer by layer along the（012）direction.The infrared absorption spectrum（IR）further confirmed that the chemical formula.The SC-XRD results show that Mn DMF belong to trigonal system,space group is R3-CH,the lattice constant is a=8.3330（17）?,c= 22.947（11）?,γ=120°,V=1379.09（10）?,Z=6.Super depth 3D laser microscope shows surface morphology of the Mn DMF crystal intuitively.DSC was used to determine the first-order transformation temperature of the samples.MPMS test shows that the sample has a paramagnetic-antiferromagnetic phase transition at low temperature,so the sample is a new soft ferromagnetic material.（2）A new MOFs material with chemical formula Mg[（CH₃）₂NH₂]（COOH）₃ was designed and prepared by hydrothermal method,using Mg²⁺ as metal ion ligand and N,N-dimethylformamide aqueous solution as hot solvent.In the process of preparing heat saturated solution,the different temperature gradients of hydrothermal method was studied.The composition of the products was investigated at different temperature.The results of XRD showed that at 160 ℃,the product was the target product with the maximum size of 5×5×2 mm³.The XRD of the powder samples confirmed the phase composition.The XRD of the block samples showed that the crystals grow layer by layer along the（012）direction,which coincided with the three-dimensional surface topography of the samples.The IR showed that the organic functional groups in Mg DMF crystal were dimethylamine and formate,which further confirmed the chemical formula.The results of DSC showed that the thermal stability of Mg DMF was good at room temperature,and the first-order transformation temperature was determined.The relationship between solubility and temperature was determined by the temperature profile,and the product of the first decomposition phase of TGA was analyzed.At the same time,the effects of two different crystal growth methods on the crystal morphology were investigated.