Studies Of Electrical, And Luminescence Characteristics And Microstructure Of Eu³⁺-doped Ca₉Znli(PO₄)₇

As one of the most important activator ions, Eu³⁺ is widely applied in red luminescent materials. The luminescence of Eu³⁺ being the well-known probe ion is highly affected by the surrounding environments in a lattice. The luminescence from 5D0â†'7F0 transitions of Eu³⁺ ions can present different emission spectra and decay profiles when their crystallographic surroundings have even small changes. The surrounding environment of Eu³⁺ ions doped in a host can be elucidated by applying the site-selective excitation and emission spectra technique. And this also can provide the symmetry of different luminescence centers in the matrix and then give details of the host structure.In this work, the Eu³⁺-doped Ca₉ZnLi(PO₄)₇ was prepared by high temperature solid-state reaction and their crystal structures and morphologies were investigated. The photoluminescence excitation and emission spectra, the luminescence decay curves, the thermal stability of luminescence and their potential applications as a white LED phosphor were studied. The crystallographic sites of Eu³⁺ doped in Ca₉ZnLi(PO₄)₇ were investigated by the laser site-selective excitation and emission spectroscopy together with the luminescence decay curves. The well-crystallized Ca₉ZnLi(PO₄)₇ ceramics were prepared by the reactive pressureless sintering at atmospheric pressure. The Ca₉ZnLi(PO₄)₇ ceramics were investigated by dielectric and complex impedance spectroscopy.In the chapter three, the red-emitting phosphors were synthesized by conventional solid state method. Eu³⁺-doped Ca₉ZnLi(PO₄)₇? were investigated by X-ray diffraction (XRD), Differential Thermal Analysis (DTA) and Fourier Transform Infrared spectroscopy (FT-IR). The results indicate that the Eu³⁺-doped Ca₉ZnLi(PO₄)₇ has theβ-Ca₃(PO₄)₂ whitlockite-like structure with the space group of R3c and Z=6. Eu³⁺ ions replace Ca₂+ ions in the Ca₉ZnLi(PO₄)₇ lattices. The excitation and emission spectra, decay curves and the dependence of luminescence on temperature were studied. The excitation spectra indicate that this phosphor can be effectively excited by near ultraviolet light, which well matches the emission wavelength of near-UV LED chips. The phosphor shows intense red emission attributed to 5D0â†'7F2 electric dipole transitions. The luminescence has high quenching temperature and can keep stable color purity (x=0.65, y=0.34) with increasing temperature. Because of its good excitation profile and stable luminescence properties at high temperature, Ca₉ZnLi(PO₄)₇:Eu³⁺ could be a potential red emitting phosphor for the near UV chip-based white light-emitting diodes.In the chapter four, the local structure and luminescent properties of Eu³⁺ ions doped in Ca₉ZnLi(PO₄)₇ were discussed on the base of the spectroscopic probe of Eu³⁺ by the site-selective excitation and emission spectroscopy. The temperature-dependent emission spectra and luminescence decay curevs of Eu³⁺ were investigated. The result presents four crystallographic sites are occupied by Eu³⁺ ions. The site selective excitation and emission spectra for Eu³⁺ sites present a statistical and random occupation with similar crystallographic environment in the lattices. Under the excitation into 5D0, the Eu³⁺ shows unusual fluorescence decay properties, i.e., the lifetime becomes longer with the increasing of temperature from 10 to 300 K.In the chapter five, thr well-crystallized Ca₉ZnLi(PO₄)₇ ceramics were prepared by reactive pressureless sintering at atmospheric pressure. The dielectric and electrical properties were investigated over a wide frequency range by complex impedance spectroscopy at different temperatures. A dielectric anomaly was observed at 440℃, which might be related to the phase transition. The impedance Cole–Cole plot revealed that the electrical properties depend strongly on frequency and temperature. Two relaxation dispersions of the electrical parameters were found and analyzed in terms of bulk and grain-boundary ionic transfer processes. These results suggest that the conduction process is of the mixed type.In the chapter six, Nd³⁺ doped NaBi(WO₄)₂ single crystals were successfully grown by modified-Bridgman method. The crystallographic Nd³⁺ doped NBW were investigated by? optical absorption spectrum, luminescence and decay curves. The up-conversion luminescence has several blue emission bands, Ssome violet bands. In order to investigate up-conversion mechanisms, the decay behavior and the dependence of the up-conversion luminescence on laser powers were investigated. The temporal decay behavior of the up-conversion luminescence indicates that the two-photon excited state absorption of the Nd³⁺ ions NaBi(WO₄)₂ crystal was the dominant mechanism in this crystal.The novelties of this desertation are the following: the light-emitting characteristics and luminescence decays of the Eu³⁺-doped Ca₉ZnLi(PO₄)₇ were studied; The microstructure of the Eu³⁺-doped Ca₉ZnLi(PO₄)₇, the temperature-dependent crystal sites and luminescence properties were investigated by site-selective excitation and emission spectroscopy of Eu³⁺. It provided a useful reference for the further development and application of rare earth doped Ca₉ZnLi(PO₄)₇. A dielectric anomaly of the Ca₉ZnLi(PO₄)₇ ceramics suggested that the conduction process was of the mixed type.