The Preparations Of A₂B₂O₇-Type Complex Oxides And Their Physical Properties Under High Pressure

In recent years, A2B2O7-type complex oxides have played an important role in not only basic research but also a variety of practical applications, duo to their unique electrical and magnetic properties. High pressure can help people find and discover more new material and exotic properties. Combined with other measurement methods, the in-situ measurement under high pressure becomes an efficient approach to explore the properties of these materials. In this thesis, combined with the high pressure test technology, the structural characteristics and physical properties of A2B2O7-type complex oxides under high pressure were systematically studied by Raman, angle-dispersive X-ray diffraction (ADXRD) and alternate current (AC) impedance spectra.Firstly, by using the solid state method, sol-gel method and high temperature and high pressure (HTHP) method, we have successfully synthesized germinate A2Ge2O7 (A=In, Sc, Ho), zirconate A2Zr2O7 (A=Y, Eu), pyrochlore In2Mn2O7 and thortveitite Mn2V2O7. The results of in-situ high pressure measurements of A2B2O7-type complex oxides are listed as follow:1) We have successfully synthesized monoclinic In2Ge2O7, Sc2Ge2O7 and tetragonal by using solid state method and cubic In2Ge2O7, Sc2Ge2O7 and 7 by using HTHP method. The structural stabilities of germinate (A=In, Sc, Ho) under high pressure have been investigated by ADXRD. For monoclinic In2Ge2O7 and Sc2Ge2O7, an irreversible structural phase transition from monoclinic (C2/m) to another monoclinic(P21/c) phase has been found at 6.6 GPa and 4.7 GPa, respectively. This phase transition is caused by the anisotropic compression in this structure. For Ho2Ge2O7 with the tetragonal structure, pressure-induced amorphization has been found, which is suggested to be associated with the breaking-up of infinite chains of edge-shared polyhedron group Ho4O20.On the contrary, for the cubic In2Ge2O7, Sc2Ge2O7 and there was no sign of a structural phase transition, indicating the cubic Sc2Ge2O7 and Ho2Ge2O7 at high pressures are very stable. Furthermore, the pressure dependent electrical resistance of the monoclinic In2Ge2O7 shows dramatic change at 6.4 GPa, where it can be attributed to the observed pressure-induced structural phase transition.2) We have successfully synthesized the defect fluorite structure Y2Zr2O7 and the pyrochlore Eu2Zr2O7 using standard solid state reaction method. The structural stabilities of Y2Zr2O7 and Eu2Zr2O7 under high pressure have been investigated by ADXRD and Raman spectroscopy. For Y2Zr2O7, a pressure-induced phase transition from cubic (Fm-3m) structure to orthorhombic(Pnma) structure at 27.3 GPa was observed and the phase transition was irreversible. The high-pressure orthorhombic (Pnma) phase has longer average cation-anion bonding distance than cubic (Fm-3m) structure which was proved by the increase of coordination number. For Eu2Zr2O7, High-pressure Raman studies have shown that two new peaks appear around 21.2 GPa, located at 266å'Œ 541 cm-1, which exhibited clear evidence for a change of structural symmetry. The transition process was irreversible. The results of Rietveld refinements from in situ ADXRD data indicate that the ordered pyrochlore (Fd-3m) transformed to a defect-cotunnite structure(Pnma) at 26.5 GPa. The phase transition was irreversible and the transformation process was mainly caused by the accumulation of antisite defects of the cation sublattice and Frenkel defects on the anion sublattice by characterizing the degree of disorder of anions and cations. Besides, the<Zr-O> bonds should play a more important role in this phase transformation than the <Eu-O> bond.3) We have successfully synthesized the pyrochlore In2Mn2O7 using HTHP combined sol-gel method. The structural stabilities of In2Mn2O7 under high pressure have been investigated by ADXRD. There was no sign of a structural phase transition, indicating the pyrochlore In2Mn2O7 at high pressures is much stable.4) We have successfully synthesized the thortveitite Mn2V2O7 using standard solid state reaction method method. The structural stabilities of Mn2V2O7 under high pressure have been investigated by ADXRD. Two pressure-induced phase transitions were observed at 5.7 GPa and 13.3 GPa, respectively.