The Structure Study Of Nanocrystals In Rare-earth Doped Oxyfluoride Glass Ceramics

Rare-earth (RE) doped oxyfluoride glass ceramics (GCs) have been shown to be a novel and promising luminescent material with a perspective of application in devices such as amplifiers, solid-state lasers, fiber lasers, three-dimensional displays, photon sources, telecommunications, and so on. The structure of the nanocrystals in GCs influences the optical properties of RE ions such as the luminescent efficiency intensively. The main purpose of this thesis are to obtain and to describe the crystal field environment of the RE ions and the thermodynamic mechanism related to the structure, which has a guiding significance to the development of this material.RE doped oxyfluoride glasses and nanocrystalline GC have been prepared and studied by Energy Dispersive X-ray Spectroscopy (EDS) and X-ray Diffraction (XRD) aiming at investigating the structure and the symmetry of the nanocrystal as well as the site of the RE ion. To soive the problem encountered by previous researchers due to glass host interference, we use the "Thermal Induction and Corrosion Treatment" to etched off glass matrix and released the fluoride nanocrystal, which is more convenient for EDS measurement.Mainly based on the EDS and XRD analyses, a tetragonal phase model with the chemical formula as PbREFs has been proposed in this thesis for the first time. The RE-doping mechanism is considered to be RE3+ substitution for Pb2+ as well as interstitial F" charge compensation. This model has successfully described and interpreted the two specific crystalline phases with the same space group observed at 460℃-500℃and 520℃-560℃, respectively. Furthermore, the validity of the model has been proved by the more accurate Rietveld refinement analyses.Moreover, the lattice parameters have been carefully analyzed and calculated according to XRD and High Resolution Transmission Electron Microscopy (HRTEM). A super "pseudo-cubic" cell based on our tetragonal model may give a good explanation to the probable previous cubic-symmetry-misunderstanding by researchers. Eu3+ ion as a structural probe instead of Er3+/Yb3+ has been used to characterize and measure the local symmetry of the nanocrystal. The emission spectra of Eu3+ doped samples further support the model proposed from the perspective of symmetry. Additionally, the thermodynamic mechanism of phase transition and the thermal stability related to the structure of nanocrystals in GC have been studied and supported by first principle calculations based on Density Functional Theory (DFT) and experimental methods.The structure and thermal stability of the nanocrystal and clear crystalline field environment of the rare-earth ion reported here have far-reaching significance to the optical investigations and further applications of rare-earth doped oxyfluoride glass ceramics.