A series of iso-structural La5(Si2+xB1–x)(O13–xNx):Ce3+ phosphors with apatite structure have been prepared. A combination of powder X-ray diffraction and neutron scattering technique was employed to explore the crystal structural evolution and the rigid nature from oxy- to oxynitride-based apatites, and some local structures were also characterized by HRTEM and 29Si NMR data, respectively. The new La5(Si2+xB1–x)(O13–xNx):Ce3+ solid solution phosphors gave continuously controlled emission from 421 nm [La5Si2BO13:Ce3+, end-member (x = 0)] to 463 nm (La5Si3O12N:Ce3+, end-member (x = 1)). Substitution of B3+ and O2– by Si4+ and N3– in La5(Si2+xB1–x)(O13–xNx):Ce3+ phosphors produced more covalency into the crystal field environment around the Ce3+ ions inducing the red-shifted emission, further improving the thermal stability of the oxynitride-based apatite phosphors. The proposed approach from oxy- to oxynitride based iso-structural phases could significantly contribute to future research in designing complex solid solution phosphors.
Bond valence method illustrates the relation between valence and length of a particular bond type. This theory has been used to predict structure information, but the effect is very limited. In this paper, two indexes, i.e., global instability index (GII) and bond strain index (BSI), are adopted as a judgment of a search-match program for prediction. The results show that with GII and BSI combined as judgment, the predicted atom positions are very close to real ones. The mechanism and validity of this searching program are also discussed. The GII & BSI distribution contour map reveals that the predicted function is a reflection of exponential feature of bond valence formula. This combined searching method may be integrated with other structure-determination method, and may be helpful in refining and testifying light atom positions.
The union of structural and spectroscopic modeling can accelerate the discovery and improvement of phosphor materials if guided by an appropriate principle. Herein, we describe the concept of “chemical unit cosubstitution” as one such potential design scheme. We corroborate this strategy experimentally and computationally by applying it to the Ca2(Al1–xMgx)(Al1–xSi1+x)O7:Eu2+ solid solution phosphor. The cosubstitution is shown to be restricted to tetrahedral sites, which enables the tuning of luminescent properties. The emission peaks shift from 513 to 538 nm with a decreasing Stokes shift, which has been simulated by a crystal-field model. The correlation between the 5d crystal-field splitting of Eu2+ ions and the local geometry structure of the substituted sites is also revealed. Moreover, an energy decrease of the electron–phonon coupling effect is explained on the basis of the configurational coordinate model.
A series of phosphors, Eu-doped Ca(Al/Si)2N2(N1−xOx), derivatives of CaAlSiN3, were synthesized by alloy-nitridation method. We demonstrated that their emission peaks can be tuned from 650 nm to 610 nm by oxygen preferential substitutions of nitrogen located at one of two crystallographic sites. Two luminescent centers corresponding to two types of Eu2+-coordination modes, i.e. EuNI2NII3 and EuNI2NII2O, were identified and accounted for the emission band structures, emission band shifts with oxygen/nitrogen substitutions, and the dependence of peak position and integrated emission intensity on temperature. As a typical example, Ca(Al/Si)2N2(N0.80O0.20):0.02Eu showed intense orange–red emission peaking at 622 nm and kept the feature of excellent chemical stabilities, which would have potential applications in fabricating the white light-emitting diode. The excellent luminescent properties of these materials, such as wavelength-tunable red emission and excellent chemical stabilities, make them practical for use in typical LED package.
Oxonitridosilicate phosphors with compositions of (Y1−xCex)2Si3O3N4 (x=0−0.2) have been synthesized by solid state reaction method. The structures and photoluminescence properties have been investigated. Ce3+ ions have substituted for Y3+ ions in the lattice. The emission and excitation spectra of these phosphors show the characteristic photoluminescence spectra of Ce3+ ions. Based on the analyses of the diffuse reflection spectra and the PL spectra, a systematic energy diagram of Ce3+ ion in the forbidden band of sample with x=0.02 is given. The best doping Ce content in these phosphors is ∼2 mol%. The quenching temperature is ∼405 K for the 2 mol% Ce content sample. The luminescence decay properties were investigated. The primary studies indicate that these phosphors are potential candidates for application in three-phosphor-converted white LEDs.
Oxonitridosilicate phosphors with compositions of (La1−xCex)5Si3O12N (x = 0–0.1) have been synthesized. The XRD analyses show that all the compounds are single polycrystalline La5Si3O12N phase. La atoms occupy two crystallographic sites in the structure. Two groups of photoluminescence spectra have been observed and can be ascribed to the excitation and emission of the two types of Ce3+ photoluminescence centers (Ce(1)3+ and Ce(2)3+) in the crystallographic sites of La(1) and La(2). The energy transfer between the two types of photoluminescence centers has been discussed. Schematic energy levels of Ce3+ ions at the two crystallographic sites are given. Luminescence concentration quenching occurs when Ce content is more than 3 mol%. The quenching temperature is evaluated to be about 406 K for the 3 mol% Ce content sample. This study shows these phosphors potential candidates for application in three-phosphor-converted white LEDs.
The common understanding of the negative relationship between bond lengths and crystal-field splitting (CFS) is renewed by Ce3+ doped garnets in this work. We represent the contradictory relationship between structure data and spectroscopic crystal-field splitting in detail. A satisfactory explanation is given by expressing crystal-field splitting in terms of crystal-field parameters, on the basis of structural data. The results show that not only the bond length, but also the geometrical configuration have influence on the magnitude of crystal-field splitting. Also it is found that the ligand oxygen behaves differently with regard to multiple site substitution in garnet structure.
Copyright 2021.北京科技大学 光功能材料与器件实验室. All Rights Reserved.