High Pressure-induced Structural Phase Transitions In Typical Metal Oxides Containing Oxygen Octahedra

Due to the unique and excellent chemical and physical properties and extensivepotential applications in the fields of optical, electronic device and multi-functionalmaterials, metal oxides, such as rare earth sesquioxide, perovskite oxides and spineloxides have been the subject of intense studies. The studies on the structural phasetransition and physical properties of materials by means of high pressure experimentaltechnology have been an important research subject in high pressure physical science,and it has important scientific significance and potential application value forobtaining new structures, synthesizing new materials, discovering new phenomenaand revealing new rules. However, there still exists some urgent problems to solve inthe pressure-induced phase transition studies of the above mentioned three typicalmetal oxides containing oxygen octahedral and oxygen, including the new type ofpressure-induced phase transitions, novel high pressure structures of metal oxidescontaining oxygen octahedra and the influence of nano effects on the phase transitionbehavior. Therefore, we carried out the related researches about the above mentionedproblems.In this thesis, we performed high pressure experimental researches on severaltypical metal oxide materials, such as rare earth sesquioxide, perovskite oxides andspinel oxides by using diamond anvil cell (DAC) to make detailed studies on theirstructural phase transitions under pressure. In addition, we study the high pressurebehavior of oxygen confined in the one-dimensional nano channels of AFI by in situhigh pressure experiments.1) The influence of nano effects on the phase transitions of one-dimensionalrare-earth sesquioxides nanomaterials and spinel oxides nanomaterials, and discoveries of new type of pressure-induced transition and abnormal compressionbehavior.Taking the rare-earth sesquioxides Gd2O3/Er3+nanorods and spinel oxides Co3O4as typical examples, we have performed high pressure studies by using in situsynchrotron X-ray diffraction and Raman spectroscopy. Compared with the bulkmaterials and nanoparticles, we found that Gd2O3/Er3+nanorods undergo a differentphase transition process. The experimental results show that it does not transformform cubic phase to hexagonal phase under high pressure, but transforms into anamorphous phase. The TEM observation of the quenched samples clearly shows thatthe samples almost maintain their pristine nanorod shape. This is the first time that wediscovered the pressure-induced amorphous structure in the Gd2O3/Er3+nanorods. Theexperimental results provide a further understanding of the influence of nano effectsand special morphology on the phase transition behavior of rare-earth sesquioxides.For spinel oxides Co3O4, we found the complete phase transition process. Importantly,it is found the abnormal compression behavior of the discontinuous volume change inCo3O4nanomaterials. It is suggested that the unique cation distribution and thenano-size effect play vital roles in the high pressure behaviors of Co3O4. These resultsenrich the understanding of high pressure behavior in nano spinels and have animportant significance of the phase transition mechanism and preparation of newfunctional materials.2) Discoveries of a new type of reconstructive phase transition and new highpressure phases in perovskites.Taking CaZrO3and BaZrO3perovskites as examples, we have performed thehigh pressure studies by using in situ synchrotron X-ray diffraction. In the study of thehigh pressure behavior of CaZrO3up to50.1GPa, we found that the orthorhombicphase CaZrO3can be stable up to30GPa, then it transforms into a new high pressuremonoclinic structure. This new high pressure phase of CaZrO3is found for the firsttime, and it is different from the high pressure structures that were reported for theother perovskite oxides in previous studies. It is a new type of reconstructive phase transition. In addition, we found that a phase transition from cubic phase (Pm3m) totetragonal phase (I4/mcm) takes place at a pressure of17.2GPa in BaZrO3. This phasetransition was found for the first time. It is proposed that the distorted and rotation ortilting of octahedra play an important role in the high pressure behavior of perovskites.These results provide a new understanding of phase transition behaviors of perovskiteoxides and greatly enrich the content of phase transition of perovskite oxides.3）Discovery of the new structure change of oxygen confined in the channels ofAlPO4-5under high pressure.The high pressure behaviors of oxygen confined inside the AFI channels havebeen studies by in situ X-ray diffraction and Raman spectroscopy for the first time. Itis found that the liquid oxygen solidifies to β phase in bulk oxygen at about6.0GPa.Compared with bulk oxygen, the Raman mode of confined oxygen has an obviouschange at6.0GPa. The result indicates that the confined oxygen undergoes a phasetransition from liquid oxygen to γ phase at6.0GPa. We obtained the low temperatureγ-O2through the confinment effect for the first time. In addition, the XRD experimentresults show that the transition pressure of crystalline-to-amorphous phase transitionfor AFI obviously increased from8.5GPa to17.5GPa, indicating that oxygenmolecules can be inserted into the channels of AFI single crystals. The experimentalresults show a great significance for designing the one-dimensional nanometerfunctional materials and an important application prospect for the capture and storageof small molecules.