High-Pressure And High-Temperature Flux Synthesis And Characterization Of Lanthanide And Transition Metal Oxides

Pressure is, like temperature, a basic thermodynamic variable which can be used to transform matter from one state to another. The application of high pressure results in a densification of solids, often accompanied by dramatic changes in physical properties, such as electrical conductivity, optical absorption, magnetism, and resistance to shear or compressive stress. Solids synthesized under high pressure conditions tend to have increased first- or second-nearest neighbor coordination numbers, and often unusual valence states for the constituent atoms. The same solids decompressed to ambient conditions also can exhibit unusual and interesting material properties, due to their metastable'stretched'state, relative to their equilibrium volume within the high pressure stability field. The pressure variable in synthesis also provides a pathway to unusual metastable compounds, not normally observed in room pressure syntheses, derived from highly energetic precursor materials, by careful design of the pressure–temperature treatment conditions used.Melt salts were used as flux under high temperature and high pressure, which are similar to water under hydrothermal condition. Their solubility of metals and metal oxides is favorable for the growth of crystals. In this thesis, NaCl or KOH was adopted, and the effects of pressure to the growth of crystal are discussed.The rod-like and sheet-like single crystals of La(OH)3 were obtained under high pressure(0.2GPa) and ambient pressure, respectively, with the same concentration of KOH and temperature. PBC theory revealed that shape of crystals is related to the binding energy of the bonds in their structure. In this thesis, we found that the sheet-like shape of the crystals grown under ambient pressure is due to the differrence between the equatorial and the apical La-O bonds in its structure,which is significantly suppressed under high pressure. Therefore, the rod-like crystals were obtained under high pressure.Since their unusual insulating and magnetic properties were first recognized in the 1930s, the first-row transition metal monoxides have provided considerable challenges to commonly accepted theories of electronic structure and bonding. Manganese oxides were most important ones which have special 3d orbital electron and various valence states. Single crystals of Mn2O3, Mn3O4, MnO were grown under ambient pressure, 50MPa and 200MPa, respectively, with the same raw material MnO2. The crystals of Mn2O3 was grown from MnO2 with KOH as flux in vacuum at the temperature of 450℃and ambient pressure. When the pressure of 50MPa was applied to this reaction, the crystals of Mn3O4 was obtained instead. It is because the deoxidization of MnO2 was accelerated by the pressure and the manganese oxide with the short bond distance was obtained. The temperature dependence magnetic susceptibility of this compound showed that it is ferromagnetic with the low spin state. John-Teller effect may influence the magnetic coupling while distorting the structure in this manganese oxide.When the pressure of 200MPa was applied, Mn3O4 was deoxidized and the crystals of MnO were obtained. The classical antiferromagnet MnO has been widely studied as a representative of the family of first-row transition-metal monoxides for more than seventy years. At temperature above its ordering (Neel) temperature TN of 118K, the MnO lattice has the cubic NaCl-type structure; the transitions from this paramagnetic phase to an antiferromagnetic phase at TN is accompanied by a cubic-to-rhombohedral lattice distortion and provide benchmarks with which to test and develop our understanding of magnetic phenomena at atomic scale. Therefore, understading the structure of MnO crystals is important. To date, all the structural parameters of MnO were based on powder neutron or X-ray diffraction and no single crystal diffraction data were obtained before. In this thesis, the crystals of MnO with the normal structure and the distorted structure were obtained with different starting materials.When Manganese oxide and Lanthanum oxide reacted together under high temperature and high pressure with NaCl flux, Single crystals of LaMnO3 were obtained. The single crystal X-ray diffractions showed that the La-O bonds in this structure are almost not change and the Mn-O bonds are shorter than in the structure of sample which was obtained under ambient pressure. The temperature dependence magnetic susceptibility behaved that it is not antiferromagnet as normal but ferrimagnet. Same to the other manganese oxides, the octahedron of Mn(III)O6 in this structure was also distorted. It must be the causation of the special magnet. So many special characters of RE manganites with perovskite structure were owing to the distortion of MnO6 octahedron by the adulteration or the different synthesize conditions. We obtained single crystal of RE manganites with special structure under high pressure. A revolution must be broken by more novel structures which were obtained under high pressure and a completed high pressure phase diagram of RE manganites. They just are our aim in the future works.