Preparations And High Pressure Physical Properties Of Pyrochlore-type Pb₂Ta（Nb）₂O₇

As complex functional material,pyrochlore oxides have the characteristics of different A and B ions,so that they can have many excellent properties.In recent years,a great deal of research has been done on this type of material,including magnetic properties,ionic conductivity,photocatalytic properties and solidification of nuclear radioactive wastes,etc.For example,the discovery of ferroelectricity in Cd₂Nb₂0₇ had aroused great interest and further investigations.Pressure can easily change the crystal structure of a substance.Therefore,it can be used in combination with other analytic methods to explore the structure and property changes of materials.In this thesis,Pb₂Ta（Nb）₂O₇ were prepared and characterized by combinations of X-ray diffraction,Raman spectrum and scanning electron microscope（SEM）.And the structure behavior of the samples under high pressure was studied by synchrotron radiation X-ray diffraction technique and Raman spectroscopy technique.The specific research results are as follows:1)Two different structures of Pb₂Ta₂O₇ and a type of Pb₂Nb₂O₇ were successfully obtained by solid state method and high pressure and high emperature（HPHT）method.Through the Rietveld refinement of X-ray iffraction data and the identification of Raman vibration modes,the results howed that the samples prepared by solid state method are both in hombohedral structure（R-3）.And the sample prepared by HPHT had cubic tructure（Fd-3m）.SEM showed that the crystallinity and compactness of the amples by HPHT were better and more conducive to crystal growth.2)High pressure Raman spectroscopy and high pressure synchrotron radiation -ray diffraction experiments were carried out on rhombohedral Pb₂Ta₂O₇. he results of high pressure Raman spectroscopy showed that the Raman odes disappeared at 251 cm^-1,355 cm^-1（10.8 GPa）and 587 cm^-1,765 cm^-1, 44 cm^-1（15.2 GPa）in the pressure range of 10.8¹8.3 GPa.And the aman modes merged at 321 cm^-1 and 375 cm^-1（18.3 GPa）to produce a ew Raman peaks at 336 cm^-1,and the new Raman modes appeared at 524cm^-1,693 cm^-1 and 901 cm^-1（10.8 GPa）.It indicated that structural phase ransition might be taking place in the rhombohedral structure Pb₂Ta₂O₇ in his pressure range.The high pressure XRD results showed that the sample tarted to change from rhombohedral structure（R-3）to cubic structure Fd-3m)above 13.6 GPa,and this phase transition was related to the nisotropy of compressibility in the lattice.The change of the bulk modulus ndicated that the high-pressure phase of this material was more difficult to ompress and this structure was more stable.3)High pressure Raman spectroscopy experiments were carried out on the ubic structure Pb₂Ta₂O₇.The results showed that the Raman modes isappeared at 143 cm^-1,367 cm^-1 and 777 cm^-1 above 16.7 GPa and the new aman modes appeared at 103 cm^-1 and 718 cm^-1.A pressure-induced phase ransition might be taking place.