Studies Of Luminescence Characteristics And Microstructure Of Eu^2+,3+-doped ABPO₄（A=Li, Na, K, B=Mg, Ca, Sr, Ba）

Rare-earth （RE） doped materials have been widely applied in display, fluorescentlamp, WLED and many other fields. The phosphates with ABPO₄formula （A and B aremono-and divalent cations, respectively） are a large family of mono-phosphates withthe different structure types. These compounds have been considered to be efficientluminescent hosts due to its excellent physical, chemical and thermal stability and haverepresented excellent application value in PDP, FED and LED areas.The absorption and emission spectra of Eu²⁺usually comprise broad bands due totransitions between the4f7ground state and the crystal field components of the4f⁶5d¹excited state configuration. The wavelengths of excitation and emission bands stronglydepend on the host crystal. So the choice of the hosts greatly affects the opticalproperties of Eu²⁺ions.As one of the most important activator ions, Eu³⁺is widely applied in redluminescent materials. The luminescence of Eu³⁺being the well-known probe ion ishighly affected by the surrounding environments in a lattice. The luminescence from⁵D₀�'⁷F₀transitions of Eu³⁺ions can present different emission spectra and decayprofiles when their crystallographic surroundings have even small changes. Thesurrounding environment of Eu³⁺ions doped in a host can be elucidated by applying thesite-selective excitation and emission spectra technique. And this also can provide thesymmetry of different luminescence centers in the matrix and then give relation betweenthe microscopic structure and luminescence properties.In this work, the phosphates with ABPO₄（A=Li, Na, K, B=Mg, Ca, Sr, Ba） with various structure types were selected to be the host material and can promote rare earthions reduction and stability in host lattice due to their PO₄tetrahedra rigid networkstructure. Eu²⁺,3+-doped ABPO₄（A=Li, Na, K, B=Mg, Ca, Sr, Ba） were prepared byhigh temperature solid-state reaction. Their crystal structures, luminescence properties,thermal stability and microscopic crystal structure were investigated. Their potentialapplications as a new phosphor were discussed in display, fluorescent lamp, in particular,in WLED field.In the chapter two, the formation of LiMgPO₄: Eu²⁺was confirmed by X-raypowder diffraction measurement to be single LiMgPO₄phase. It belongs to the orderedolivine-type structure with the space group Pnma. The photoluminescence excitationand emission spectra and decay curves were measured. At low temperature it is foundthat the emission line of the4f7（6P7/2）�'4f⁷(⁸S_7/2) transition is overlapped with thezero-phonon line at nearly the same position of360nm. The influences of temperatureon the luminescence spectra and decay times were investigated. The site-selectiveexcitation and emission spectra have been investigated in the⁵D₀�'⁷F₀region by using apulsed, tunable and narrowband dye laser. The excitation spectra consist of the strongestline at579.0nm （M） and a weak broad peak at579.9nm (M’. The results clearlyindicate that the Eu³⁺ions occupy one intrinsic crystallographic site Eu³⁺（M） and otherdisturbed sites Eu³⁺（M’） in LiMgPO₄. The charge compensation mechanisms of Eu³⁺doping in LiMgPO₄were discussed. This is helpful for the luminescence investigationof RE ions doped in this kind of phosphate host.In the chapter three, Eu²⁺-doped KMgPO₄was prepared by high-temperaturesolid-state reaction. Photoluminescence excitation spectrum measurements show thatthe phosphor can be efficiently excited by near UV light from230to400nm andpresents a dominant luminescence band centered at470nm due to the4f⁶5d¹�'4f7transition of Eu²⁺ions at room temperature. The influence of temperature on theluminescence and decay times were investigated. There are two distinct Eu²⁺sites inKMgPO₄lattices, which present different luminescence and lifetimes. The assignmentsof the Eu²⁺sites were discussed on the base of the luminescence properties and the structure of KMgPO₄crystal. The unusual increase of the decay time of the4f65demission in KMgPO₄from10K to room temperature was reported.In the chapter four, the luminescence of NaMgPO₄:Eu²⁺was given an insightinvestigation. The phosphor can be excited by UV-visible light from220to430nm torealize emission from4f⁶5d¹�'4f⁷(⁸S_7/2) transition in the blue range. The4f7（6P7/2）�'4f⁷(⁸S_7/2) transition in the4f7electronic configuration of Eu²⁺at360nm was observedat low temperature. The luminescence absolute quantum efficiency, the crystallographicsites and the microstructure of Eu²⁺ions doped in NaMgPO₄lattices were reported. Themultiple sites structure of Eu²⁺ions in NaMgPO₄lattices was discussed. Theluminescence quenching temperatures and the thermal activation energy forNaMgPO₄:Eu²⁺were obtained from the temperature dependent （18–300K）luminescence decay curves. Eu²⁺ions have the “disordered environment” in NaMgPO₄lattices. The relation between the luminescence thermal stabilities and the crystalstructures were discussed.In the chapter five, Eu²⁺-activated LiBaPO₄phosphor was synthesized byconventional solid-state reaction. The photoluminescence excitation and emissionspectra, the temperature dependent luminescence intensities （12-450K） and decaycurves of the phosphor were investigated. With the increasing of temperatures, theemission bands of LiBaPO₄:Eu²⁺show the abnormal blue-shift and the decreasing ofemission intensity. The natures of the Eu²⁺emission in LiBaPO₄, e.g., the luminescencequenching temperature, and the activation energy for thermal quenching （ΔE）, werereported. The afterglow fluorescence was detected in LiBaPO₄:Eu²⁺phosphor. Thesite-selective excitation in the⁵D₀�'⁷F₀region for Eu³⁺ions, emission spectra and decaycurves have been investigated using a pulsed, tunable and narrowband dye laser todetect the micro-structure and crystallographic surrounding of Eu³⁺,2+at Ba2+sites inLiBaPO₄. The multiple sites structure of Eu²⁺and Eu³⁺ions in LiBaPO₄lattices wassuggested. The lower quenching temperature, afterglow and luminescence mechanismwere discussed. The Photoluminescence quantum efficiencies of LiBaPO₄:Eu²⁺weremeasured and compared with the reported phosphors. Different from the published data on LiBaPO₄:Eu²⁺, this investigation indicate that LiBaPO₄:Eu²⁺is not a good phosphorcandidate applied in white light emitting diode.In the chapter six, the phase formation of NaBaPO₄:Eu²⁺was confirmed by X-raypowder diffraction measurements. No impurity lines were observed and the structurewith the space group P3m1is isotypic with that of glaserite. The photoluminescenceexcitation and emission spectra, and the luminescence quantum efficiency of Eu²⁺ionswere investigated. The dependence of luminescence intensities on temperatures, and thetemperature-dependent decay times of Eu²⁺doped NaBaPO₄were measured anddiscussed. The natures of the Eu²⁺emission in NaBaPO₄, e.g., the Stokes shifts, theluminescence quenching temperature （T0.5）, the activation energy for thermal quenching（ΔE） were reported. The phosphor shows an excellent thermal stability on temperaturequenching effects. With the increasing of temperature, the emission bands show theabnormal blue-shift.The laser site-selective excitation and emission spectra have been investigated inthe⁵D₀�'⁷F₀region by using a pulsed, tunable and narrowband dye laser. The excitationspectra corresponding to the⁷F₀�'⁵D₀transition consist of two transitions at579.6nmEu（I） and578.9nm Eu（II）, indicating the Eu³⁺ions occupy two crystallographic sites ofBa2+ions. The decay lifetimes of the two Eu³⁺sites were measured. Twocrystallographic sites for Eu³⁺ions doped in NaBaPO₄lattice were assigned from theluminescence characteristic and structure features. Meanwhile, the charge compensationmechanism of Eu³⁺doping in NaBaPO₄was discussedIn the chapter seven, two Eu^2+,3+-doped mono-phosphate hosts NaSrPO₄andKBaPO₄with β-K2SO₄structure have been selected to investigate the luminescenceproperties and thermal stabilities. KBaPO₄:Eu²⁺shows one emission band peaking at420nm, however, the emission spectra of NaSrPO₄:Eu²⁺have an asymmetric spectrum.The phase formation, the decay curves and the luminescence absolute quantumefficiencies were measured. The luminescence quenching temperatures and the thermalactivation energy for NaSrPO₄:Eu²⁺and KBaPO₄:Eu²⁺were obtained from thetemperature dependent （10-435K） luminescence intensities and decay curves. KBaPO₄:Eu²⁺presents only one emission center, however, Eu²⁺ions have the“disordered environment” in NaSrPO₄lattices. This was analogically analyzed by thesite-selective emission spectra and the excitation spectra of the⁷F₀�'⁵D₀transitions ofEu³⁺ions in the hosts using a pulsed, tunable and narrowband dye laser. In KBaPO₄, theEu³⁺ions could be distributed in the host with a high “ordered state” in only one site inthe lattices. However, the multiple sites structure of Eu³⁺ions with highly disordereddistributions in NaSrPO₄lattices was suggested. The relation between the luminescencethermal stabilities and the crystal structures were discussed.In the chapter eight, the photoluminescence excitation and emission spectra wereinvestigated. The results showed that LiCaPO₄:Eu²⁺can be efficiently excited by theincident lights of280-420nm, which well match with the emissions of near-UV LEDs.The phosphor showed bright blue luminescence. The temperature and concentrationdependence of luminescence intensities were investigated. The phosphor has anexcellent thermal stability on temperature quenching effects. The luminescence decayand the color coordinates were discussed in order to further investigate its potentialapplications for white light-emitting diode phosphors pumped by near-UV chip.In the chapter nine, a green-emitting phosphor, Eu²⁺-activated KCaPO₄, wassynthesized by conventional solid-state reaction. The phase formation was confirmed byX-ray powder diffraction measurements. The photoluminescence excitation andemission spectra, the temperature dependent luminescence intensities （293-438K） anddecay curves of the phosphor were measured. With the increasing of temperature, theemission bands show the abnormal blue-shift with broadening bandwidth. The naturesof the Eu²⁺emission in KCaPO₄, e.g., the chromaticity coordinates, the luminescencequenching temperature, activation energy for thermal quenching （ΔE）, were reported.The excitation spectra have been investigated in the⁵D₀�'⁷F₀region for Eu³⁺ions inKCaPO₄by using a pulsed, tunable and narrowband dye laser to detect the cation site ofCa2+in KCaPO₄. The multiple sites structure of Eu²⁺ions in KMgPO₄lattices weresuggested and discussed.The novelties of this dissertation are as follows: the structure characteristics, photoluminescence properties and decay curves of Eu^2+,3+-doped ABPO₄（A=Li, Na, K,B=Mg, Ca, Sr, Ba） were systematically studied. The features about microstructure ofABPO₄and crystallographic site-occupations of Eu ions were firstly investigated by thesite-selective excitation and emission spectra. The luminescence performances ofdifferent crystallographic sites have been investigated. The relation between themicroscopic structure and luminescence properties of Eu ions was insight discussed intheory to obtain the influence of the structure characteristic on luminescence features. Ina word, this provided a helpful reference for further development and application of rareearth doped ABPO₄phosphor.