Studies On Structural And Luminescence Properties Of Rare Earth Ions Doped Solid Solution Materials

Rare earth ions not only have abundant energy levels but also have three kinds of electronic transitions including f-f transition,f-d transition and charge transfer,providing excitation and emission spectra covering various wavelength bands from infrared to visible to ultraviolet for the rare earth ions activated luminescence materials.Accordingly,they have been extensively used in the fields of solid state lasers,scintillators,white LEDs and biomarkers and so on.Since their luminescence properties strongly denpends on the structure and composition of the host crystals as well as varies widely with the type of rare earth ions,searching a suitable crystal as the host followed by optimizing the composition is the most common strategy for designing and improving the luminescence properties of materials,thereby obtaining anticipated luminescence materials.Along the way,the structure-property relations can be established which will in turn provide theoretical guidance for developing novel materials.This dissertation first selects two kinds of inorganic crystals with high physical and chemical stabilities,i.e.K₂Al₂B₂O₇?KAB?and Ca₃Sc₂Si₃O₁₂?CSS?,as the host crystals for Eu²⁺ and Yb³⁺/Tb³⁺,respectively.And then solid solutions are formed through isovalent or aliovalent substitution with high doping concentrations to regulating the luminescence properties.Besides,the effects of the crystal structureand composition on the luminescence properties and the detailed energy transfer mechanism are also investigated thoroughly,thus constructing reasonable structure-properties relations.The main research contents and the obtained results are as follows:Firstly,a new blue-emitting phosphor,KAB:Eu²⁺ was synthesized by solid state reaction and the photoluminescence properties were studied in detail.The obtained KAB:Eu²⁺phosphor shows bright blue emission centered at 450 nm under the excitation of light ranging from 230 to 420 nm which matches well with the emission of n-UV LED chips.Two different emissions peaking at 447 and 470 nm were observed for KAB:Eu²⁺,originating from the presence of two different Potassium sites in the KAB host,which was confirmed by their different lifetimes.The optimal concentration was determined to be 2 mol% and the critical distance was calculated by using two methods proposed by Blasse and Dexter,respectively,which verified that the energy transfer between Eu²⁺ ions occurs through the electric dipole-dipole interactions.Temperature dependence of the photoluminescence demonstrates that the PL intensity of KAB:0.04Eu²⁺ retains 82% when the temperature is raised up to 150 ?.The PL intensity of the as-synthesized KAB:Eu²⁺sample achieves ?58% that of the commercial blue BAM:Eu²⁺phosphor under theexcitation of 325 nm.As discussed,the developed KAB:Eu²⁺ phosphor is a potential blue-emitting component for n-UV excited w-LEDs.Secondly,through cation substitution we developed color tunable solid solution phosphors K2-2xNa2xAl2B2O7:Eu²⁺,of which the emission wavelength can be tuned from 450 to 487 nm with increasing Na doping concentration.Based on the analysis of the changing trends of the excitation and emission spectra,we found that the obvious redshift of emission mainly originates from the increased Stokes shift of the emission,while the effect of the changes of the crystal splitting and the centroid shift on the lowest 5d level offset by each other,leading to the lowest 5d level changing a little.Furthermore,we introduced the inductive effect of neighboring cation to explain these changes of Stokes shift as well as the centroid shift,which wasconfirmed by the observed “abnormal” redshift of the excitation and emission spectra in Ga³⁺ replacing Al³⁺.Besides,by analyizing the changes in the relative position of the lowest 5d level in host forbidden band and Stokes shift,the degradation of thermal stability with Na+ doping is ascribed to the increased Stokes shift,for which the energy of 5d electron may be easily lost in the form of heat via the crossing point in configurational coordinate;however,the enhanced quenching by Ga³⁺doping is due to the much narrower bandgap,leading to obvious thermal ionization of 5d electron.Thirdly,we have obtained RE³⁺ heavily doped single-phase silicate garnet CSS via the sol-gel method followed by high temperature sintering.the results of Rietveld structural refinement indicate that multiple chemical unit substitutions of [REO8],[AlO₆]and [AlO4] respectively for [Ca O8],[ScO₆] and [SiO4] polyhedra can act as charge compensator for each other to promote the doping level of RE³⁺ up to 20mol%,which is high enough for most of the RE³⁺-doped luminescence materials.And the obtained single-phase CSS powder provides more possibilities for fabricating translucent or even transparent ceramic CSSAt last,in Yb³⁺ and Tb³⁺ heavily codoped CSS,intense cooperative UCL was observed and confirmed by the pump power dependence of the UCL intensity and the fluorescence decay curves.Structurally,antisite defects Yb³⁺ for Sc³⁺ and aliovalent substitution of Yb³⁺for Ca²⁺in CSS are both in favor of clustering into closely spaced Yb³⁺–Yb³⁺pair,markedly enhancing the efficiency of cooperative UC process.Compared with the garnets YAG,YGG,and GGG,CSS has the highest UCL intensity when doping with 10 mol% Yb³⁺ and 8 mol% Tb³⁺ and its intensity is37 times higher than that of YAG,making Yb³⁺and Tb³⁺codoped CSS a potential candidate for optical applications like tunable UC laser.