The Preparations Of New Type Double Perovskite Oxides And Their Physical Properties Under High Pressure

ABO₃ perovskite oxides have played an important role in the field of physics, chemistry and material science etc., because of the appearances of the superconductivity, magnetoresistance and other exotic physical properties in these compounds. In addition, the studies of double perovskite oxides（general formula A2B′B″O₆） have also attracted many attentions, due to their similar structural characteristics.In this thesis, by using the high temperature and high pressure（HTHP） method and solid-state reaction method, we have successfully synthesized La₂BTiO₆（B = Mn, Fe） and Ba₂SmMO₆（M = Sb, Bi） double perovskite oxides. Their crystal structures and crystallinity have been investigated by ambient pressure XRD, TEM and Raman spectra. The high pressure structural stabilities of Ba₂SmMO₆（M = Sb, Bi） have been examined by using angle-dispersive synchrotron X-ray powder diffraction and Raman spectra measurements at room temperature. The main results of the thesis are shown as following:1) By using the HTHP method, we have successfully synthesized ideal cubic La₂BTiO₆（B = Mn, Fe） for the first time.2) Ba₂SmMO₆（M = Sb, Bi） with cubic structure have been synthesized by solid-state reaction method and their Raman spectra have been investigated. Raman modes were assigned using the method of factor group analysis. For Ba₂ SmSbO₆, the peaks of 763.0, 569.6 and 375.0 cm-1 were assigned to the symmetric oxygen stretch vibration of the TeO₆ octahedra as A1 g mode, an oxygen asymmetric stretching vibration of the TeO₆ octahedra as Eg mode and an oxygen bending motion in the TeO₆ octahedra as F2g（1） mode, respectively. The peak of 101.7 cm-1 was considered to be the crystal lattice translation mode F2g（2）, which is related with A ion.3) For Ba₂SmSbO₆ and Ba₂ SmBiO₆, high-pressure Raman studies have shown that new peaks appear around 15.0 GPa and 4.85 GPa, respectively. The results were clear evidences for a possible change of structural symmetry. And the transition process was reversible.4) The in-situ high-pressure behaviors of double perovskite Ba₂SmSbO₆ had been investigated using angle-dispersive synchrotron X-ray powder diffraction in a diamond anvil cell（DAC） at room temperature. The results showed that Ba₂SmSbO₆ underwent a phase transition at 14.9 GPa from cubic（Fm-3m） structure to monoclinic（P21/n） structure, which also confirmed the observed high-pressure Raman results. This phase transition is caused by the SmO₆ and SbO₆ octahedral rearrangements.