The Preparations Of Stannate Pyrochlore Oxides A₂Sn₂O₇ And Their High Pressure Physical Properties Research

Pyrochlore is a potential candidate for permanently disposing of high-level radioactive waste in safe geologic repositories due to its chemical stability,low swelling,and excellent radiation tolerance.Due to its special structural and flexibility elements choosing,the material can present many exotic phenomena and new properties,which cause a lot of attention.Pressure is an effective way to turn the property of materials.Combining with other measurement methods,the in-situ measurement under high pressure makes it possible to explore the properties of these materials.In this thesis,combined with the high pressure test technology,the structural evolution and electronic property of A₂Sn₂O₇ under high pressure were systematically studied by angle-dispersive X-ray diffraction?ADXRD?,Raman,fluorescence and alternate current?AC?impedance spectra.Firstly,we have successfully synthesized stannate A₂Sn₂O₇?A=La?Eu?Nd?Sm?Ho?by using the solid state method and high temperature and high pressure?HTHP?method.The results of in-situ high pressure measurements of pyrochlore A₂Sn₂O₇ are listed as follow:1)We conducted a systematic investigation of the high-pressure crystal and electronic structure using in situ ADXRD,Raman and fluorescence spectrum,impedance,and DFT calculations.The strong room temperature near infare photoluminescent?PL?of La₂Sn₂O₇ was recorded at ambient pressure.With increasing pressure,the NIR luminescence signal shown a blue shift withpressure and quenched above 7.3 GPa.The pressure-induced phase decomposition of small amounts metallic Sn was observed above 2 GPa,accompanied with the emergence of visible multi-band PL.The variation of the visible PL spectrum intensity with pressure plateaued between 6.6 GPa and 16.6 GPa,which illustrates a well-defined pressure window that yielded flat maximum PL.Pressure exceeded 23.7 GPa,the visible PL quenched.This unexpected appearance of visible PL in La₂Sn₂O₇ was originated from the Sn2+ 5s5 p to 5s2 photon transitions,while the Sn O6 distortion and Sn-O1-Sn bond angle bent are crucial to pressure-induced multi-band PL emergence and quenching.2)In-situ high pressure ADXRD,Raman spectroscopy,photoluminescence and alternating current?AC?impedance spectroscopy studies are performed in Eu₂Sn₂O₇ to investigate the responses of structural evolution,optical emission and electrical transport properties.The abrupt change of the oxygen atomic position parameter x without breaking the crystal symmetry is accompanied with disappearing of Raman mode and around 67% increase of the bulk modulus.It indicates a pressure-induced second-order iso-structural transition,which suppresses the trigonal distortion in the Sn O6 octahedron but enhances the local symmetry distortion of Eu O8 hexahedron.Anomalous luminescence 5D0?7F1/5D0?7F2 intensity ratio of Eu3+ 4f-4f transition confirms the frustrated compete of Eu O8 and Sn O6 distortion.A rapid increase in gain resistance of four orders of magnitude by applied pressure is observed in AC impedance till 16.6 GPa,and implying pressure enhanced insulator state.All these observations indicate a pressure-induced lattice distortion,accompanied with the anomalous luminescence of Eu3+ 4f-4f transition and four orders of magnitude increase in gain resistance.Thus it will enlarge an opening gap at Fermi energy and is followed by releasing lattice distortion at higher pressures.3)The structural stabilities of Nd₂Sn₂O₇,Sm₂Sn₂O₇ and Ho₂Sn₂O₇ under high pressure have been investigated by ADXRD,Raman spectroscopy andphotoluminescence.A pressure-induced phase transition from cubic?Fd-3m?structure to orthorhombic?Pnma?structure at 26.5 GPa was observed in Nd₂Sn₂O₇ and the phase transition was mainly caused by the pressure-induced rearrangement of Sn O6 and Eu O8 polyhedron,which lead to the disorder of anions and cations,thus phase transition accured.Almost all of the Raman peaks disappeared above 34.5 GPa in Sm₂Sn₂O₇,implying pressure induced amorphization.We see the disorder to order transition in Ho₂Sn₂O₇ by high pressure Raman measurements,which accompanied with the abnormal increase of unit cell volume and the disappear of abnormal Raman peaks.Above 26.8 GPa,pressure-induced photoluminescence and amorphization were observed.