Abstract:The atomistic structure, energetics, and electronic structure of single substitutional Ce and La defects and double substitutional Ce-La defects in Ce,La-codoped yttrium aluminum garnet (YAG) Y 3 Al 5 O 12 have been studied by means of first-principles periodic boundary conditions density-functional theory calculations. Single substitution of Y by Ce or by La produces atomistic expansions around the impurities which are significantly smaller than the ionic radii mismatches and the overall lattice distortions a… Show more
“…Since Ce 3+ has in 8-fold coordination larger ionic radius compared to Y 3+ [28], in Ce:YAG crystal a significant expansion of the distorted cubic local structure (limited to the nearest neighbors coordination O 2À sphere) takes place. The estimated two Ce 3+ -O 2À distances, 2.38 Å and 2.52 Å [27], are much larger than those of Y 3+ -O 2À 2.31 Å and 2.44 Å in pure YAG, and the expansion is anisotropic; these experimental results are in agreement with the theoretical estimation of structure relaxation near Ce 3+ in YAG [29]. The symmetry of Ce 3þ Y center can be considered as D 2d , the parent group of D 2 , as the pressure data show [30].…”
“…Since Ce 3+ has in 8-fold coordination larger ionic radius compared to Y 3+ [28], in Ce:YAG crystal a significant expansion of the distorted cubic local structure (limited to the nearest neighbors coordination O 2À sphere) takes place. The estimated two Ce 3+ -O 2À distances, 2.38 Å and 2.52 Å [27], are much larger than those of Y 3+ -O 2À 2.31 Å and 2.44 Å in pure YAG, and the expansion is anisotropic; these experimental results are in agreement with the theoretical estimation of structure relaxation near Ce 3+ in YAG [29]. The symmetry of Ce 3þ Y center can be considered as D 2d , the parent group of D 2 , as the pressure data show [30].…”
“…Despite the fact that this phosphor material was proposed almost 45 years ago [1], and that it has been very thoroughly investigated from the point of view of optical spectroscopy [2][3][4][5][6] and electronic structure [6,7], to the best of our knowledge an experimental investigation of the local structure of the impurity in the garnet structure has never been reported so far. For this reason, we have found it interesting to undertake a detailed study of the location of the Ce3+ ions in YAG using Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy, with the aim of gaining more insight on the spectroscopic behaviour of this valuable material, and to compare the experimental results with the theoretical ones [7,8]. We are also confident that the present structural results will allow a solid foundation for future improved electronic structure calculation on YAG:Ce.…”
“…Especially, white light can be generated through a simple combination of blue-emitting devices and yellow emitting phosphor YAG:Ce 3+ , i.e., phosphor-converted LEDs (PC-LEDs), which have been successfully commercialized [9]. Therefore, adjusting luminescence properties of YAG:Ce 3+ attract much attention via codoping with another lanthanide ions (La 3+ , Pr 3+ , Tb 3+ , Sm 3+ codoping) [10,11,12,13,14] or modification of the host-lattice by substitution (Lu/Y, Ga/Al, Si/Al, N/O, Si-N/Al-O) [15,16,17,18,19,20]. …”
To modify the luminescence properties of Ce3+-doped Y3Al5O12 (YAG) phosphors, they have been coated with a carbon layer by chemical vapor deposition and subsequently heat-treated at high temperature under N2 atmosphere. Luminescence of the carbon coated YAG:Ce3+ phosphors has been investigated as a function of heat-treatment at 1500 and 1650 °C. The 540 nm emission intensity of C@YAG:Ce3+ is the highest when heated at 1650 °C, while a blue emission at 400–420 nm is observed when heated at 1500 °C but not at 1650 °C. It is verified by X-ray diffraction (XRD) that the intriguing luminescence changes are induced by the formation of new phases in C@YAG:Ce3+-1500 °C, which disappear in C@YAG:Ce3+-1650 °C. In order to understand the mechanisms responsible for the enhancement of YAG:Ce3+ emission and the presence of the blue emission observed for C@YAG:Ce3+-1500 °C, the samples have been investigated by a combination of several electron microscopy techniques, such as HRTEM, SEM-CL, and SEM-EDS. This local and cross-sectional analysis clearly reveals a gradual transformation of phase and morphology in heated C@YAG:Ce3+ phosphors, which is related to a reaction between C and YAG:Ce3+ in N2 atmosphere. Through reaction between the carbon layer and YAG host materials, the emission colour of the phosphors can be modified from yellow, white, and then back to yellow under UV excitation as a function of heat-treatment in N2 atmosphere.
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