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Crystal chemistry, polymorphisms, and phase transformations of lithium-containing metal phosphates
- Alternative Title
- Sung-Chul Kim
- Author(s)
- 김성철
- Alternative Author(s)
- Sung-Chul Kim
- Advisor
- 김승주
- Department
- 일반대학원 에너지시스템학과
- Publisher
- The Graduate School, Ajou University
- Publication Year
- 2017-02
- Language
- eng
- Alternative Abstract
- This thesis focuses on the crystal structures and phase transformations of lithium metal phosphates LiMPO4 (M = Ca, Sr, Ba). It shows that a correlation exists between the nonlinear optical property and the crystal structure. The synthesis of a new complex metal phosphate was also proposed based on the lattice design concept. Each compound was synthesized by solid state reaction or the molten salt method. X-ray diffraction (XRD), thermophysical analysis, field emission scanning electron microscopy (FE-SEM), Raman spectroscopy, measurement of optical second harmonic generation, and impedance analysis were employed for structural analysis and characterization. Chapter 1 describes the crystal structure and the physical properties of inorganic phosphates, gives an introduction to phase transformation, describes the procedure for crystal structure determination using powder diffraction, and states the purpose of this paper. Spinodal decomposition, which is one of the unique phase transformation phenomena, is described along with the classification according to the phase transition mechanism. ABPO4 (A, B = alkali, alkaline earth metal)-type inorganic phosphates have different coordination bonds between metal ions and PO4 units depending on the sizes of the A and B ions. The structures are closely related to tridymite, olivine, and glaserite minerals. ABPO4 compounds have attracted much attention as phosphors, optical materials, and electrode materials for secondary batteries due to their structural stability. However, the detailed structures remain unclear. The crystal structure, phase transformation and physicochemical properties of lithium metal phosphates, LiMPO4 (M = Ca, Sr, Ba), are discussed. The synthesis, crystal structure, and electrochemical properties of a new lithium metal phosphate Li2Sr2Al(PO4)3, with an intergrowth-type 2-D layered structure, are also discussed. In Chapter 2, the crystal structure of LiSrPO4 and the phase transformation with temperature are analyzed. LiSrPO4 exists in two polymorphs: a monoclinic phase at room temperature, and a hexagonal phase at high temperatures. In Particular, spinodal decomposition and reentrant phase formation, which are rare in ceramics, have been reported for the first time. Phase decomposition into Sr3(PO4)2 and Li3PO4 occurred at about 650 °C; the separated phases disappeared and returned to the hexagonal LiSrPO4 phase at 830 °C. The phase transformations were influenced by the morphology of the sample. The bulk sample did not show phase decomposition on cooling because it allows larger strain than the powder sample, which is an energy barrier that prevents phase decomposition. Chapter 3 shows the structural change of LiBaPO4 with temperature. Two kinds of polymorphs were confirmed, which were different from the crystal structures previously assumed to be orthorhombic or hexagonal. LiBaPO4 exhibited a monoclinic to hexagonal phase transition above 600 °C. In Chapter 4, a comparative study on the crystal structures and the phase trans-formation behavior of LiMPO4 (M = Ca, Sr, Ba) was undertaken. The correlations between the crystal structure and the second harmonic generation (SHG) and Raman scattering were investigated. The phase transformations in LiMPO4 were related to the lattice energy of each phase and can simply be compared by the lattice volumes. Phase separation occurs when the lattice volume difference between M3(PO4)2 and LiMPO4 is sufficiently large. The trend of SHG observed in LiMPO4 was consistent with the structural distortion of the polyhedron calculated using the bond-valence method. In Chapter 5, the synthesis, crystal structure, and ionic conductivity of the new lithium metal phosphate, Li2Sr2Al(PO4)3, with a layered structure are discussed. Li2Sr2Al(PO4)3 is a new intergrowth-type compound in which two different phosphate units, LiO4 and PO4 groups are alternately stacked. The ionic conductivity of this compound was measured using an impedance analyzer.
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- Graduate School of Ajou University > Department of Energy Systems > 4. Theses(Ph.D)
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