Invention Of A High Pressure Magnetic Measurement Device And Research Of Temperature And Pressure Induced Spin Transition

Spin-crossover complexes are magnetic materials with electronically bistable properties.By controlling external conditions such as temperature,pressure,light radiation and magnetic field,the 3d orbital electrons of the metal ion at the centre of the complex can be jumped,resulting in a transition between high and low spin states,a phenomenon known as spin transition（spin shift）or spin-crossover.The most extensively studied is the Fe（II）-based spin-crossover complex,in which the 3d electron configuration of Fe（II）changes from t_2g⁴e_g²（HS）to t_2g⁶e_g⁰（LS）during the spin transition process causing a strong change in the metal-ligand covalent bond（～0.2（?）shortening,～10%）,resulting in a change in crystal structure and volume,which in turn leads to changes in the crystal symmetry,optical,electrical,magnetic and mechanical properties of the compound.The physical and chemical properties of the compounds,such as optical,electrical,magnetic and mechanical properties,are significantly altered.The multifunctional properties of spin-crossover complexes are a positive guide for the design of thermal/pressure sensitive sensors,brakes and data memories at the molecular and even atomic level.Temperature was the first means of regulating spin transitions and many more have been developed to date,such as the application of pressure,light,electric and magnetic fields and the introduction of guest molecules.The development of pressure-controlled spin transitions has been relatively slow due to the specialisation of high-pressure generating devices（often designed in-house）and the limitations of means of characterisation at high pressures.In this thesis,we have designed the first high-pressure device in China capable of generating pressures up to 5 GPa and suitable for MPMS,which can be used to study the pressure-temperature synergistic regulation of the spin transition process,and have also investigated the temperature and pressure induced spin transition of liquid crystal Fe（II）complexes with the general formula[Fe（C_n-tren H）]（Cl O₄）₂,and Theoretical simulations and analysis of the experimental results were carried out based on the elastic interaction model.The main results obtained in this thesis are as follows:1.A new diamond-to-top anvil piston-cylinder pressure device for MPMS has been designed with reference to the working principle of diamond-to-top anvil devices.The pressure device is made of a non-magnetic Ni Cr Al alloy and has a quasi-cylindrical shape with a height of 59 mm and a diameter of 8.5 mm,which is characterised by its compact size,low magnetic background signal,high working pressure and pressure stability.A diamond with an anvil diameter of 600μm,a copper sheet with a thickness of approximately 300μm as a gasket,potassium bromide as a pressure transfer medium and ruby as a pressure calibrator were used to measure the pressure in the pressure chamber and the device was found to achieve a working pressure of approximately 5 GPa.The magnetic moment of the nickel metal placed in the pressure device was measured at room temperature under atmospheric pressure and compared to the standard magnetic moment of nickel measured under the same conditions.Based on the actual test data the background signal of the pressure device can be deducted from the sample signal to improve the accuracy of the test results.2.High-pressure variable-temperature magnetization measurements were performed on the samples[Fe（C_n-tren H）]（Cl O₄）₂（n=3,8,16,abbreviated as 5C3,5C8,5C16）using MPMS,and the results showed that the spin transition patterns of all three samples remained hysteresis-free with pressure,indicating that the temperature-induced spin transitions of the samples were reversible at different pressures.The three samples remain highly sensitive to pressure,mainly because the transition temperature TC increases rapidly with increasing pressure:sample 5C3 is particularly sensitive to pressure,and a wide temperature range magnetisation measurement at 0.205 GPa shows that 5C3 is no longer able to complete the complete spin transition;samples 5C8 and 5C16 at 0.356 GPa and 0.384 GPa respectively The transition process remained relatively intact for samples 5C8 and 5C16 at 0.356 GPa and 0.384 GPa respectively.Theγ_HS-P curves of 5C3 were plotted according to the change in intensity of the characteristic peaks of the two spectra,and the curves showed a hysteretic transition mode due to the use of a potassium bromide solid with relatively poor hydrostatic pressure as the pressure transfer medium in the IR spectra,while the UV-Vis absorption spectra using silicone oil as the pressure transfer medium showed a hysteretic transition mode.The UV-Vis absorption spectra curve using silicone oil as the pressure transfer medium exhibits a hysteresis-free mode,which agrees well with the magnetic measurements.3.The phenomenon of temperature-induced spin transition at different pressures of sample 5C3 was analysed using an elastic interaction model.The theoreticalγ_HS-T curves were fitted to the experimentally obtainedγ_HS-T curves to compare the values of the non-monotonically varying elastic interaction energyΔ_elasticand the intermolecular interaction energyΓ.Calculation of the difference between the two revealed that the reason for the increasing transition temperature of 5C3 under pressure was that the difference between the two parameters,Δ_elastic-Γ,was greater than 0 and remained increasing,and the trend of theΔ_elastic-Γ.The trend of the difference with pressure is generally consistent with the trend of the change in transition temperatureΔT_1/2.It can therefore be concluded that the non-linear change inΔ_elastic-Γunder pressure leads to a non-linear change in the 5C3 transition temperature.