Temperature And Pressure Induced Fe(?)Compound Spin Transition Phenomenon Research

Spin transition compound is a kind of bistable molecular magnetic materials.External factors such as temperature,pressure,light,magnetic field,can make its spin state change.At the same time,the magnetic and electrical properties,color,structure of compound will be changed.As a result,this kind of compound in storage,display,sensors and other devices has a broad application prospect.Temperature is a kind of tool for the initial study of spin transitions,but with the development of research,researchers have found that the pressure-induced spin transitions produce more interesting experimental phenomena with the help of high-pressure devices,but a reasonable and accurate explanation of these phenomena is a very big challenge.In this paper,the spin transition of 2D coordination polymer{Fe[bipy(ttr)₂]}_n induced by pressure and temperature and the effect of pressure on the temperature induced spin transition in a new 2D Hoffmann-type coordination polymer Fe(Fpz)₂[Pt(CN)₄]have been studied.Finally,the experimental phenomena are analyzed by means of theoretical model.The results of this paper are as follows:1.The spin transition of the coordination polymer{Fe[bipy(ttr)₂]}_n induced by temperature and pressure have been systematically studied by using the magnetic system SQUID and UV-Vis absorption spectra.The compound has been found to have unusual spin transition behavior.A complete and reversible spin transition with asymmetric hysteresis has been observed during the temperature induced spin transition under atmospheric pressure,which is transformed into a two-step spin transition at a pressure of 0.04GPa.There are an abrupt spin transition at high temperature and a gradual spin transition at low temperature.In addition,the transition temperature of the spin transition increases with increasing pressure,and the spin transition is stretched over a wide temperature range.At room temperature,the pressure-induced spin transition curve shows that the properties of polymer are reversible,but with an asymmetric hysteresis.These results indicate that the polymer is very sensitive to pressure and can be used in molecular pressure sensors.2.In the analysis of the two-step spin transition phenomenon induced by pressure and temperature in{Fe[bipy(ttr)₂]}_n,a theoretical model considering elastic interaction is used for the first time.The results of elastic energy and interaction parameters are almost identical.In the two-step spin transition induced by temperature,for each specific pressure,the parameters of the high temperature part are twice that of low temperature part,which explains abrupt transition of high temperature part and gradual transition of low temperature part.The parameters of the high temperature part increase with pressure increasing,while the parameters of the low temperature part are on the contrary.Maybe because the relaxation time under pressure decreases,and the contribution of the vibration component to the elastic energy and the interaction energy decreases.In addition,the method used to analyze the two-step spin transitions is of great significance to the study of the more complex multi-step spin transitions.3.The influence of pressure on the temperature induced spin transition of 2D Hoffmann-type coordination polymer Fe(Fpz)₂[Pt(CN)₄]has been studied by using the magnetic system SQUID.It is found that the transition temperature increases gradually with the increase of pressure,but the hysteresis width first decreases to almost zero and then increases again.The changes of all parameters related to spin transition process are studied by means of theoretical model.It is found that the ratio of the molecular interaction to the d level splitting energy reflects the hysteresis behavior.When pressure is 0.68GPa and 0.71GPa,the interaction between molecules hardly changes,while the intramolecular elastic energy increases,leading to the increase of splitting energy,so the hysteretic loop width decreases sharply.The hysteresis behavior may be due to the Fe(?)octahedral coordination along the z axis of local anisotropy caused by compression.The observed hysteresis width and transition temperature behavior can be explained by determining variations of all parameters related to the spin transition process and their composition effect.These results provide a new way to study the relationship between spin transition and temperature and pressure.