Physical properties of metal-organic frameworks

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Metal-organic frameworks (MOFs) essentially consist of metal ions, which are two- or three-dimensionally connected via organic bridging molecules. By variation of the inorganic and organic units numerous combinations and hence a large variety of MOF materials with very different physical properties, can be synthesized. MOFs were investigated manifold due to their extreme large porosity in respect to gas storage, their photoluminescence, and their magnetic and electric properties such as ferroelectrics.The electric properties are closely linked to the elastic properties via the electro-mechanic coupling. However, only for single ferroelectric MOFs a part of their elastic properties was investigated so far. The aim of the project is to investigate the elastic properties of selected MOFs of metal formates and tartrates with the help of conventional ultrasound methods and in particular with the resonant ultrasound spectroscopy (RUS) technique as a function of temperature in order to grasp the main features of the elastic and thermoelastic behaviour of this group of MOFs for the first time. As the elastic properties reflect the three-dimensional bonding system of the crystal structure, ultrasound techniques like RUS show a highly sensitive probe for the detection of structural instabilities and phase transitions. For this purpose single crystals with several millimeters in size are required, which will be synthesized and grown in further experiments. Up to now most of MOFs which were known from literature were grown only in small crystals of a few millimeters and less.The results are supposed to give information on (1) to what extent ferroelectric MOFs show clear changes of the elastic behavior at the transition from the paraelectric to the ferroelectric phase, (2) whether magneto-elastic coupling, like it was found in [(CH3)2NH2][Co(HCOO)3], also exists in isotypic and structurally related MOFs, and (3) how this influences the elastic properties of multiferroic MOFs. Further, it will be determined whether some of the here to be examined ferroic MOFs show not only ferroelectric but also ferroelastic behaviour. Especially, the question is to be answered with respect to the group of tartrates, whether by substitution MOFs can be made, which exhibit similar ferroelectric and ferroelastic phase transitions to [(NH4)Li(C4H4O6)] H2O. Another aspect deals with the determination of the influence of the size of the organic bridging molecules and their hydrogen bonds on the elastic and thermoelastic behaviour of MOFs, and with the question, whether the anisotropy of the thermoelastic behaviour is influenced by the main directions/orientation of the hydrogen bonds.