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Title: Ce–O Covalence in Silicate Oxyapatites and Its Influence on Luminescence Dynamics

Abstract

Cerium substituting gadolinium in Ca2Gd8(SiO4)6O2 occupies two intrinsic sites of distinct coordination. The coexistence of an ionic bonding at a 4F site and an ionic–covalent mixed bonding at a 6H site in the same crystalline compound provides an ideal system for comparative studies of ion–ligand interactions. Experimentally, the spectroscopic properties and photoluminescence dynamics of this white-phosphor are investigated. An anomalous thermal quenching of the photoluminescence of Ce3+ at the 6H site is analyzed. Theoretically, ab initio calculations are conducted to reveal the distinctive properties of the Ce–O coordination at the two Ce3+ sites. The calculated eigenstates of Ce3+ at the 6H site suggest a weak Ce–O covalent bond formed between Ce3+ and one of the coordinated oxygen ions not bonded with Si4+. The electronic energy levels and frequencies of local vibrational modes are correlated with specific Ce–O pairs to provide a comparative understanding of the site-resolved experimental results. On the basis of the calculated results, we propose a model of charge transfer and vibronic coupling for interpretation of the anomalous thermal quenching of the Ce3+ luminescence. The combination of experimental and theoretical studies in the present work provides a comprehensive understanding of the spectroscopy and luminescence dynamics of Ce3+ inmore » crystals of ionic–covalent coordination.« less

Authors:
; ; ;
Publication Date:
Research Org.:
Argonne National Lab. (ANL), Argonne, IL (United States)
Sponsoring Org.:
USDOE Office of Science (SC), Basic Energy Sciences (BES) (SC-22)
OSTI Identifier:
1396039
DOE Contract Number:
AC02-06CH11357
Resource Type:
Journal Article
Resource Relation:
Journal Name: Journal of Physical Chemistry. C; Journal Volume: 118; Journal Issue: 29
Country of Publication:
United States
Language:
English
Subject:
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY; ion-ligand interaction; luminescence quenching; phosphors; vibronic coupling

Citation Formats

Zhai, Dengyun, Ning, Lixin, Huang, Yucheng, and Liu, Guokui. Ce–O Covalence in Silicate Oxyapatites and Its Influence on Luminescence Dynamics. United States: N. p., 2014. Web. doi:10.1021/jp5049293.
Zhai, Dengyun, Ning, Lixin, Huang, Yucheng, & Liu, Guokui. Ce–O Covalence in Silicate Oxyapatites and Its Influence on Luminescence Dynamics. United States. doi:10.1021/jp5049293.
Zhai, Dengyun, Ning, Lixin, Huang, Yucheng, and Liu, Guokui. Thu . "Ce–O Covalence in Silicate Oxyapatites and Its Influence on Luminescence Dynamics". United States. doi:10.1021/jp5049293.
@article{osti_1396039,
title = {Ce–O Covalence in Silicate Oxyapatites and Its Influence on Luminescence Dynamics},
author = {Zhai, Dengyun and Ning, Lixin and Huang, Yucheng and Liu, Guokui},
abstractNote = {Cerium substituting gadolinium in Ca2Gd8(SiO4)6O2 occupies two intrinsic sites of distinct coordination. The coexistence of an ionic bonding at a 4F site and an ionic–covalent mixed bonding at a 6H site in the same crystalline compound provides an ideal system for comparative studies of ion–ligand interactions. Experimentally, the spectroscopic properties and photoluminescence dynamics of this white-phosphor are investigated. An anomalous thermal quenching of the photoluminescence of Ce3+ at the 6H site is analyzed. Theoretically, ab initio calculations are conducted to reveal the distinctive properties of the Ce–O coordination at the two Ce3+ sites. The calculated eigenstates of Ce3+ at the 6H site suggest a weak Ce–O covalent bond formed between Ce3+ and one of the coordinated oxygen ions not bonded with Si4+. The electronic energy levels and frequencies of local vibrational modes are correlated with specific Ce–O pairs to provide a comparative understanding of the site-resolved experimental results. On the basis of the calculated results, we propose a model of charge transfer and vibronic coupling for interpretation of the anomalous thermal quenching of the Ce3+ luminescence. The combination of experimental and theoretical studies in the present work provides a comprehensive understanding of the spectroscopy and luminescence dynamics of Ce3+ in crystals of ionic–covalent coordination.},
doi = {10.1021/jp5049293},
journal = {Journal of Physical Chemistry. C},
number = 29,
volume = 118,
place = {United States},
year = {Thu Jul 24 00:00:00 EDT 2014},
month = {Thu Jul 24 00:00:00 EDT 2014}
}
  • Energy transfer phenomena in rare-earth metaphosphate compounds have been investigated. For monoclinic GdP/sub 3/O/sub 9/-Ce,Tb (1%) at 300 K, it is shown that energy transfer does not take place from Ce/sup 3 +/ to Gd/sup 3 +/, but from Gd/sup 3 +/ to Ce/sup 3 +/. Moreover, no energy migration over the Gd/sup 3 +/ sublattice was observed. In the case of orthorhombic La/sub 1-x/Gd/sub x/P/sub 3/O/sub 9/-Ce,Tb (1%) (x less than or equal to 0.68) on the other hand, the excitation energy migrates from the Ce/sup 3 +/ ion to the Tb/sup 3 +/ traps via the Gd/sup 3more » +/ sublattice. The energy migration over the Gd/sup 3 +/ ions is probably one-dimensional. In CeP/sub 3/O/sub 9/ rapid energy migration occurs over the Ce/sup 3 +/ ions.« less
  • Highlights: • A novel yellow-emitting alkaline and alkaline-earth silicate Na{sub 2}Ca{sub 3}Si{sub 6}O{sub 16}:Eu{sup 2+} was first developed. • Under excitation with UV or near UV light the silicate presents broad emission band centered at 580 nm. - Abstract: Yellow-emitting phosphors of Na{sub 2}Ca{sub 3}Si{sub 6}O{sub 16}:Eu{sup 2+} was prepared by wet chemistry sol–gel method. X-ray powder diffraction and SEM measurements were applied to characterize the structure and morphology, respectively. The luminescence properties were investigated by the photoluminescence excitation and emission spectra, decay curve (lifetimes), CIE coordinates and the internal quantum efficiencies. The excitation spectra can match well with themore » emission light of near UV-LED chips (360–400 nm). Na{sub 2}Ca{sub 3}Si{sub 6}O{sub 16}:Eu{sup 2+} presents a symmetric emission band from 4f{sup 6}5d{sup 1} ⟶ 4f{sup 7}({sup 8}S{sub 7/2}) transitions of Eu{sup 2+} ions on doping below 3.0 mol%. On increasing Eu-doping levels, the sample contains two kinds of emission centers, i.e., Eu{sup 2+} and Eu{sup 3+} ions, which present the characteristic broad band (5d ⟶ 4f) and narrower (4f ⟶ 4f) luminescence lines, respectively. The energy transfer, the luminescence thermal stability (activation energy ΔE for thermal quenching) and luminescence mechanism of Na{sub 2}Ca{sub 3}Si{sub 6}O{sub 16}:Eu{sup 2+} phosphors were discussed by analyzing the relationship between the luminescence characteristics and the crystal structure.« less
  • A puzzle is pointed out concerning the low-temperature behavlor of La{sub 2}CuO{sub 4} and YBa{sub 2}Cu{sub 3}O{sub 6,} the insulating parents'' of the high-{ital T}{sub {ital c}} superconductors, and indicate a possible solution. The measured ordered magnetic moment is about 0.64 {mu}{sub {ital B}} per Cu, in agreement with the value calculated on the basis of the Heisenberg Hamiltonian. But this calculation is within a nearly purely ionic or localized picture of the Cu {ital d} hole, the reduction from 1.1 {mu}{sub {ital B}} ({ital g}{approx}2.2) being entirely due to spin fluctuations. Thus the almost perfect agreement leaves no roommore » for the moment reduction due to covalence or 3{ital d}-2{ital p} hybridization, expected to be large in these oxides. A possible explanation might lie in the fact that an ostensibly symmetry-based argument relating the calculated moment (in the symmetry-unbroken state) to that measured is not valid, as has recently been shown.« less
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