Study On The Pressure Response And Barocaloric Effect Of Several Typical Spin Crossover Compounds

As an important branch of molecular magnetic materials,spin crossover（SCO）compounds widely exist in coordination compounds with 3dⁿ（n=4-7）transition metals,which exhibit bistable behavior under external stimuli（temperature,light,pressure,etc.）.In the process of spin transition,SCO compounds are accompanied by rich physicochemical properties（such as thermal,color,electrical conductivity and magnetic properties）changes.Therefore,SCO compounds have great application potential in molecular switches,memory components,temperature/pressure sensors,barocaloric refrigeration and actuators,and have been widely concerned by researchers.However,the transition temperature(T_1/2)of most SCO compounds is far below room temperature,so its application has been limited.In order to accurately and controllably design compounds with temperature-induced spin transition（TIST）behavior near room temperature,a deeper understanding of the relationship between the behavior and structure of SCO compounds is necessary.Pressure,as another important thermal parameter besides temperature,can effectively regulate the intermolecular interaction and lattice stacking of compounds,which will provide a new angle for the study of spin transition behavior.In addition,the spin transition of SCO compounds is a first-order phase transition process,which is accompanied by huge entropy change and exhibits high-pressure sensitivity(dT_1/2/d>100 K GPa^-1).Therefore,SCO compounds are considered to be a class of excellent solid barocaloric refrigeration materials.However,the entropy change of most SCO compounds is less than 100 J kg^-1 K^-1,which is much lower than the entropy change associated with the phase transition of commercial refrigerants and plastic-crystalline materials,and theT_1/2 of most SCO compounds is much lower than or higher than room temperature,which is not conducive to practical application.Therefore,it is an important research direction to search for SCO compounds with large entropy change and high-pressure sensitivity near room temperature and explore their potential in the field of barocaloric refrigeration.In view of the above scientific problems,in this thesis,based on several typical two-dimensional Hoffmann-type SCO compounds,we explore the abnormal pressure response behavior of TIST and the effects of metal atom substitution and hydrostatic pressure on pressure-induced spin transition（PIST）.In addition,the barocaloric effect（BCE）of SCO compounds with large entropy change and SCO compounds with small hysteresis width(ΔT_1/2)near room temperature is also studied.This deepened the understanding of spin transition behavior and provided new insights for the design of SCO compounds to meet different application fields,and further enhanced the application potential of SCO compounds in the field of barocaloric refrigeration.The main research contents and conclusions of this thesis are as follows:1.Based on the self-made piston-cylinder press and diamond anvil press,the pressure response behavior of the TIST of compound[Fe（Fpz）₂M（CN）₄]（M=Pd,Ni;Fpz=F-pyrazine）and the influence of metal ion substitution on PIST were investigated by magnetic measurement system and high-pressure UV-Vis,Raman,IR and X-ray diffraction（XRD）spectra.It is also systematically discussed in combination with the elastic interaction model and the I-sing model.The experimental results show that TIST of[Fe（Fpz）₂Pd（CN）₄]（Fpz-Pd）shows anomalous pressure response behavior,T_1/2has nonlinear pressure dependence andΔT_1/2 decreases to saturation value with pressure.However,[Fe（Fpz）₂Ni（CN）₄]（Fpz-Ni）,which is always a high spin state（HS）at atmospheric pressure,undergoes incomplete TIST under pressure.High-pressure spectroscopy and structural studies show that both Fpz-Pd and Fpz-Ni are completely reversible at room temperature.The transition pressure(T_1/2)of Fpz-Ni is higher than that of Fpz-Pd,which is due to the stronger feedbackπbond between Pd and CN,resulting in more charge accumulation on the N atom.Therefore,Fpz-Pd has higher ligand field strength.Through the discussion of the elastic interaction model,it is found that the abnormal pressure response behavior of the TIST is the result of the synergistic effect of the enhanced interlayer interaction and the distortion of the octahedral coordination environment under pressure,which is also supported by the I-sing model.This study shows that interlayer interactions and octahedral coordination environments synergistically influence the behavior of SCO compounds at high pressure.2.In order to explore whether the significant enhancement of the interlayer interaction leads to the abnormal increase ofΔT_1/2 with pressure and whether the distortion of the coordination environment under critical pressure is a common behavior.The SCO compound[Fe（Isoq）₂M（CN）₄]（M=Pt,Pd;Isoq=Isoquinoline）,which has more interlayer interactions,was selected as the research object to study the pressure response of their TIST and PIST.It is found that the compounds[Fe（Isoq）₂Pt（CN）₄]（Isoq-Pt）and[Fe（Isoq）₂Pd（CN）₄]（Isoq-Pd）exhibit an abnormal increase ofΔ_1/2 at low-pressure range and a nonlinear pressure dependence ofT_1/2.This suggests that the interlayer interaction plays an important role in improving the system cooperativity and that octahedral distortion under a certain critical pressure may be a common phenomenon in two-dimensional Hoffmann-type SCO compounds.Due to the partial irreversible lattice distortion,ΔT_1/2 increases after pressure relief,suggesting that mechanical treatment（compression or grinding）may be an effective means to improve the system cooperativity.Under pressure,both compounds have completely reversible PIST,and Isoq-Pd has a higherT_1/2.Interestingly,Isoq-Pt and Isoq-Pd exhibit significantly different"rates"of spin transition with Isoq-Pt showing a faster spin transition behavior,because the difference in lattice mechanical properties between HS and the low spin state（LS）will affect the formation of the spin domain.In addition,since the non-hydrostatic pressure is not conducive to the formation of the spin domain,the spin transition"rate"of Isoq-Pt is significantly reduced in the pressure transfer medium with poor hydrostatic properties.This work has further deepened the understanding of the spin transition behavior and found for the first time that the lattice mechanical properties and non-hydrostatic pressure have a significant impact on the PIST of SCO compounds.3.High entropy change is the basis for BCE materials to show excellent cooling performance.A SCO compound{Fe（pz）₂（BH3CN）₂}（pz=pyrazine）with low molar mass and a statically disordered pyrazine ligand was selected to investigate the effect of reorientation behavior on the phase transition entropy of SCO compounds and the BCE of{Fe（pz）₂（BH₃CN）₂}.It was found that the FWHM of the stretching vibration mode of pyrazine gradually increased with the transition from LS to HS,indicating that the frequency of reorientation movement of pyrazine ligands increased,and a third entropy change sources-reorientation entropy change was introduced into SCO compounds besides the electronic and vibrational entropy change,but this part of entropy change accounted for a small proportion in the total entropy change.By the quasi-direct method,it is found that the entropy associated with the spin transition at{Fe（pz）₂（BH₃CN）₂}becomes～202 J kg^-1 k^-1 and exhibits a high-pressure sensitivity(d_1/2/d≈188 K GPa^-1).Due to the hysteresis behavior,the reversible isothermal entropy change and adiabatic temperature change of{Fe（pz）₂（BH3CN）₂}are only 103J kg^-1 k^-1 and～0 K at 1 kbar.However,at sufficiently high driving pressures,the reversible isothermal entropy change will exceed 200 J kg^-1 k^-1,and the reversible adiabatic temperature change can reach～47 K,which is much higher than all SCO compounds reported in BCE studies,and comparable to plastic-crystalline materials and two-dimensional perovskite barocaloric refrigeration materials.This work shows that SCO compounds with small molar mass may exhibit excellent BCE,and it is found that the introduction of reorientation entropy change has no obvious influence on BCE.4.Materials with reversible BCE at low driving pressure near room temperature have greater application potential.A SCO compound[Fe L（NCS）₂]（L=N¹,N³-bis（（1-propyl-1H-1,2,3-triazol-4-yl）methylene）-2,2-dimethylpropane-1,3-diamine）with a small hysteresis width near the freezing point was selected as the research object.The reversibility of its PIST was first investigated by high-pressure UV-Vis,Raman,IR and XRD,and then the BCE of[Fe L（NCS）₂]and its reversibility were investigated by high-pressure calorimetry.The results of various high-pressure experiments show that[Fe L（NCS）₂]can complete the spin transition process at room temperature under the pressure of 3 kbar,and the compound can completely recover to HS after pressure relief.Through quasi-direct discussion,it is found that[Fe L（NCS）₂]exhibits a reversible isothermal entropy change of～114 J kg^-1 k^-1 and a reversible adiabatic temperature change of～16 K under a driving pressure of 1 kbar.More importantly,[Fe L（NCS）₂]showed a high reversible cooling efficiency CPR=2.09,higher than most solid barocaloric refrigeration materials,due to the reversible adiabatic temperature change increased by the change in the type of spin transition from"single step"to"multi-step".This work further proves that SCO compounds are a class of high-efficiency barocaloric refrigerants suitable for low driving pressure.