Xiao-Jun Kuang

Promising Oxonitridosilicate Phosphor Host Sr3Si2O4N2: Synthesis, Structure, and Luminescence Properties Activated by Eu2+ and Ce3+/Li+ for pc-LEDs

A novel oxonitridosilicate phosphor host Sr(3)Si(2)O(4)N(2) was synthesized in N(2)/H(2) (6%) atmosphere by solid state reaction at high temperature using SrCO(3), SiO(2), and Si(3)N(4) as starting materials. The crystal structure was determined by a Rietveld analysis on powder X-ray and neutron diffraction data. Sr(3)Si(2)O(4)N(2) crystallizes in cubic symmetry with space group Pa ̅3, Z = 24, and cell parameter a = 15.6593(1) Å. The structure of Sr(3)Si(2)O(4)N(2) is constructed by isolated and highly corrugated 12 rings which are composed of 12 vertex-sharing [SiO(2)N(2)] tetrahedra with bridging N and terminal O to form three-dimensional tunnels to accommodate the Sr(2+) ions. The calculated band structure shows that Sr(3)Si(2)O(4)N(2) is an indirect semiconductor with a band gap ≈ 2.84 eV, which is close to the experimental value ≈ 2.71 eV from linear extrapolation of the diffuse reflection spectrum. Sr(3-x)Si(2)O(4)N(2):xEu(2+) shows a typical emission band peaking at ~600 nm under 460 nm excitation, which perfectly matches the emission of blue InGaN light-emitting diodes. For Ce(3+)/Li(+)-codoped Sr(3)Si(2)O(4)N(2), one excitation band is in the UV range (280-350 nm) and the other in the UV blue range (380-420 nm), which matches emission of near-UV light-emitting diodes. Emission of Sr(3-2x)Si(2)O(4)N(2):xCe(3+),xLi(+) shows a asymmetric broad band peaking at ~520 nm. The long-wavelength excitation and emission of Eu(2+) and Ce(3+)/Li(+)-doped Sr(3)Si(2)O(4)N(2) make them attractive for applications in phosphor-converted white light-emitting diodes.

Solid-State (29)Si NMR and neutron-diffraction studies of Sr(0.7)K(0.3)SiO(2.85) oxide ion conductors.

K/Na-doped SrSiO3-based oxide ion conductors were recently reported as promising candidates for low-temperature solid-oxide fuel cells. Sr0.7K0.3SiO2.85, close to the solid-solution limit of Sr1-xKxSiO3-0.5x, was characterized by solid-state (29)Si NMR spectroscopy and neutron powder diffraction (NPD). Differing with the average structure containing the vacancies stabilized within the isolated Si3O9 tetrahedral rings derived from the NPD study, the (29)Si NMR data provides new insight into the local defect structure in Sr0.7K0.3SiO2.85. The Q(1)-linked tetrahedral Si signal in the (29)Si NMR data suggests that the Si3O9 tetrahedral rings in the K-doped SrSiO3 materials were broken, forming Si3O8 chains. The Si3O8 chains can be stabilized by either bonding with the oxygen atoms of the absorbed lattice water molecules, leading to the Q(1)-linked tetrahedral Si, or sharing oxygen atoms with neighboring Si3O9 units, which is consistent with the Q(3)-linked tetrahedral Si signal detected in the (29)Si NMR spectra.

Far infrared reflection spectrum and IR-active modes of MgTiO3

A MgTiO3 ceramic sample was synthesized by solid-state reactions and its far infrared (IR) reflection spectrum recorded. The IR-active vibrational modes of MgTiO3 were calculated using first-principle calculations and the modes were represented by linear combinations of the symmetry coordinates. Using the eigenfrequencies obtained by the density functional theory calculations as initial values, we fitted the infrared reflection spectrum using the four-parameter semiquantum (FPSQ) model and the Lorentz three-parameter classical model. The parameters (ΩjTO, ΩjLO, γjTO, γjLO in FPSQ fitting and Ωj, Sj, γj in the three-parameter model fitting) of seven of the eight modes predicted by group theory analysis were obtained. The transverse optical modes (in FPSQ) are assigned as 279cm−1 (Au), 383cm−1 (Au), 472cm−1 (Au), 673cm−1 (Au) and ∼262cm−1 (Eu), 317cm−1 (Eu), 416cm−1 (Eu), 531cm−1 (Eu). The modes Au (279cm−1), Eu (317cm−1), Au (472cm−1), and Eu (531cm−1), which contain larger components of cation vibrations,...

Double substitution induced tunable luminescent properties of Ca3−xYxSc2−xMgxSi3O12:Ce3+ phosphors for white LEDs

Y3+–Mg2+ pairs were incorporated into the Ca3Sc2Si3O12 for Ca2+–Sc3+ sites to investigate their structural and optical properties systematically, their potential applications in white light-emitting diodes, as well as the coexistent shrinkage effects of the first sphere and the second sphere based on the Rietveld refinement analysis of X-ray diffraction data. Due to the substitution of Y3+–Mg2+, an obvious red shift of the emission occurs. CIE chromaticity coordinates, quantum efficiency as well as thermal stability suggest that this double substitution is an efficient way to tune the luminescence properties of phosphors. The white LED devices were also successfully fabricated which demonstrates that such a kind of double substitution will be a promising way to broaden the applications of phosphors in advanced lighting in the future.

First-principle calculation and assignment for vibrational spectra of Ba(Mg1/3Nb2/3)O3 microwave dielectric ceramic

1:2 B-site cation ordered Ba(Mg1/3Nb2/3)O3 ceramic was synthesized using conventional solid-state reaction at 1600 °C for 12 h. The structure parameters were obtained through Rietveld refinement of X-ray diffraction data. The Raman peak frequencies were obtained by Lorenz fitting on Raman spectrum. Four-parameter semiquantum model was used to fit the infrared (IR) reflectivity spectrum, and the fitted parameters were used to calculate the dielectric permittivity e and dielectric loss tanδ. A total of 9 active Raman and 16 active IR modes were obtained using first-principle calculations based on density functional theory with local density approximation. All of the vibrational modes were assigned and represented by linear combinations of the symmetry coordinates deduced using group theory analysis. The Raman mode with the highest frequency A1g(4) (789 cm−1) can be described as the breathing vibration of NbO6. The IR modes Eu(1) (149 cm−1) and A2u(2) (212 cm−1), which can be described as the twisting vibrat...

The structure, anion order, and Ce3+ luminescence of Y4Al2O9–Y4Si2O7N2 solid solutions

Oxonitridoalumosilicates Y4Al2−2xSi2xO9−2xN2x (0.00 ≤ x ≤ 1.00) were prepared and the substitution effect of [Al3+O2−] pairs by [Si4+N3−] pairs in Y4Al2O9–Y4Si2O7N2 solid solutions on the structure and its corresponding influence on the Ce3+ activated luminescence properties are investigated. Y4Al2−2xSi2xO9−2xN2x (0.00 ≤ x ≤ 1.00) shows two solid solution regions, x = 0.00–0.40 for solid solution I, x = 0.50–1.00 for solid solution II from X-ray and neutron diffraction analysis. In the solid solution I, the Ce3+ emission is mainly in the UV range with the maximum wavelength at 350 nm, and the intensity decreases rapidly with the increase of the N content. In the solid solution II, the Ce3+ doped samples show blue-green luminescence with the maximum wavelength at ∼450 nm and the wide excitation bands extend from UV to 450 nm. The phosphors in the solid solution II are promising candidates for white-LED applications.

Unraveling the correlation between oxide-ion motion and upconversion luminescence in β-La2Mo2O9:Yb3+,Er3+ derivatives

An optical approach is an alternative method to give insight into the oxide-ion motion in oxide-ion conductors. Herein, we illustrate the correlation between upconversion (UC) luminescence in Yb3+–Er3+ and oxide-ion motion in a β-La2(Mo,W)2O9 series. The break points at ∼150 °C in the logarithmic UC emission intensity ratio of I525/I660 or I660/I550 (the three emission peaks of Er3+) versus temperature imply oxide-ion jumps, whereas the slopes of these plots above 150 °C suggest the capacity of oxide-ion motion. Specifically, the larger the absolute slope values, the higher the oxide-ion conduction capacity. Due to the pinning feature of the Fermi level contributed by the Mo–O bonds in β-La2Mo2O9 both with and without anion-Frenkel defects or W dopants, as revealed by density functional theory calculations, β-La2Mo2O9 remains electronically insulating. Thus, the increase in activation energy and decline in conductivity with an increase in W contents at low temperature (<400 °C) are likely attributed to the higher barrier for the formation of new anion-Frenkel defects. This research gives another perspective on oxide-ion conductors via an optical probe.

Transport and Magnetic Properties of MgFeVO4

A new spinel compound, MgFeVO4, was prepared by solid-state reactions. On the basis of the Mossbauer spectrum, it can be deduced that both Fe and V in MgFeVO4 are trivalent. Structure refinements based on X-ray and neutron diffraction data indicated that V3+ ions are likely to occupy the octahedral site, whereas Fe3+ and Mg2+ ions take both octahedral and tetrahedral sites. The formula of the compound can be represented as (Mg1-xFex)[MgxFe1-xV]O4 (x=0.638). The transport measurements indicate that MgFeVO4 is an n-type semiconductor with the hopping mechanism below 170 K and thermally activated mechanism at high temperatures. The DC and AC magnetic data show the antiferromagnetic interactions and spin glass behavior in MgFeVO4. The time-dependent magnetic relaxation and the exchange bias effect related to the spin glass phase are also analyzed. The curve fittings give long flipping times and large n values, indicating that strongly interacting clusters rather than individual spins are the predominant spin glass features.