Chun-Hai Wang

Photoluminescence and Raman Spectra of Double-Perovskite Sr2Ca ( Mo / W ) O6 with A- and B-Site Substitutions of Eu3 +

3+ have been synthesized using solid-state reactions and characterized by X-ray diffraction, Raman spectroscopy, and photoluminescence measurement. Raman spectra are used to identify the Aand B-site substitutions, because specific Raman peaks corresponding to different ions motion are sensitive to each situation. Raman data reveal that both the A- and B-site substituted solid solutions are formed. The photoluminescence intensity of the B-site substituted Sr2CaMoO6 is evidently higher than that of the A-site substituted phosphor. The WO6 group introduced into Sr2CaMoO6:Eu 3+ ,Li + acts as an energy obstacle to prevent energy transfer among MoO6 groups, leading to more energy being trapped by Eu 3+ , and a higher photoluminescence intensity is obtained. The phosphor with optimized composition Sr2Ca0.80Li0.10Eu0.10Mo0.10W0.90O6 shows a better luminescence intensity than the commercial phosphor Y2O2S:Eu under 395 nm excitations.

First-principles calculations of mechanical and electronic properties of silicene under strain

We perform first-principles calculations of mechanical and electronic properties of silicene under strains. The in-plane stiffness of silicene is much smaller than that of graphene. The yielding strain of silicene under uniform expansion in the ideal conditions is about 20%. The homogeneous strain can introduce a semimetal-metal transition. The semimetal state of silicene, in which the Dirac cone locates at the Fermi level, can only persist up to tensile strain of 7% with nearly invariant Fermi velocity. For larger strains, silicene changes into a conventional metal. The work function is found to change significantly under biaxial strain. Our calculations show that strain tuning is important for applications of silicene in nanoelectronics.

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.

Assignment of Raman-active vibrational modes of MgTiO3

MgTiO3 ceramic sample was synthesized and its Raman spectra were recorded. The Raman-active vibrational modes of MgTiO3 were calculated using first-principle calculations (density functional theory). Based on experimental data and calculation results, the Raman peaks were assigned as 225 cm−1 (Ag), 306 cm−1 (Ag), 398 cm−1 (Ag), 500 cm−1 (Ag), 715 cm−1 (Ag) and 281 cm−1 (Eg), 328 cm−1 (Eg), 353 cm−1 (Eg), 486 cm−1 (Eg), 641 cm−1 (Eg). The assignment was supported by the polarized Raman spectrum. Meanwhile, the symmetry coordinates of MgTiO3 primitive cell were analyzed and employed to expand the Raman-active modes.

Long Wavelength Extension of the Excitation Band of LiEuMo2O8 Phosphor with Bi3 + Doping

Bi-doped red phosphor series LiEu 1-x Mo 2 O 8 :xBi were prepared by solid-state reactions which present bright red luminescence (615 nm) dominated by a 5 D 0 → 7 F 2 transition of Eu 3+ . With Bi 3+ doping, the excitation bands of the phosphors extend to longer wavelength (from 350 to 380 nm), and the excitation efficiency for optimized Bi 3+ content is dramatically increased, which favors the phosphors for the white-light ultraviolet light emitting diode applications. The luminescent model for the phosphors studied in this work is proposed. Because the 6p level of Bi 3+ is immerged in the conduction band (CB), the excitation 6s → 6p of Bi 3+ is overlapped by the excitation 6s → CB, which results in extension of the excitation band. The absorbed energy by the excitation 6s → CB can be effectively transferred to Eu 3+ , but the 6p → 6s emission of Bi 3+ disappears.

O/N ordering in the structure of Ca3Si2O4N2 and the luminescence properties of the Ce3+ doped material

A white powder of oxonitridosilicate Ca3Si2O4N2 was synthesized at 1450 °C from CaCO3, Si3N4 and SiO2. The crystal structure was refined using X-ray and neutron powder diffraction data with subsequent Rietveld refinements (Rwp = 7.35 for XRD and Rwp = 5.53 for NPD). The phase has a cubic unit cell with space group Pa (no. 205), cell parameters a = 15.0739(2) A and Z = 24. In contrast with most layered oxonitridosilicates, such as MSi2O2N2 (M = Ca, Sr, Ba), Ba3Si6O9N4 and Ce4Si4O5N6, Ca3Si2O4N2 is constructed of highly corrugated 12-membered rings of [Si12O24N12], which is composed of [SiO2N2] tetrahedra. N atoms fully occupy the bridging sites and O atoms fully occupy the terminal sites. This structure is also supported by the 29Si NMR measurement. The photoluminescence of Ce3+ doped Ca3Si2O4N2 shows that the phosphor has a typical shouldered emission band peaking at 470 nm and the phosphor can be efficiently excited in the range 300–400 nm, which makes it an attractive candidate phosphor for the application in phosphor-converted light-emitting diodes (pc-LEDs).

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,...

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...

Synthesis, Structure, and Characterization of the Series BaBi1−xTaxO3 (0 ≤ x ≤ 0.5)

The series BaBi(1-x)Ta(x)O(3) (0 <or= x <or= 0.5) has been synthesized by traditional solid-state reactions. Their structures are analyzed by the combinational use of X-ray, neutron, and converged-beam electron diffractions. They all crystallize in P1. FTIR and Raman data confirm that an inversion center is absent in the crystal structure of the series BaBi(1-x)Ta(x)O(3) (0 <or= x <or= 0.5). No obvious P-E (polarization-electrical field) loops are observed for them.

Synthesis, structural characterization, and physical properties of the new transition metal oxyselenide Ce2O2ZnSe2.

The quaternary transition metal oxyselenide Ce2O2ZnSe2 has been shown to adopt a ZrCuSiAs-related structure with Zn(2+) cations in a new ordered arrangement within [ZnSe2](2-) layers. The color of the compound changes as a function of cell volume, which can vary by ∼0.4% under different synthetic conditions. At the highest, intermediate, and lowest cell volumes, the color is yellow-ochre, brown, and black, respectively. The decreased volume is attributed to oxidation of Ce from 3+ to 4+, the extent of which can be controlled by synthetic conditions. Ce2O2ZnSe2 is a semiconductor at all cell volumes with experimental optical band gaps of 2.2, 1.4, and 1.3 eV for high, intermediate, and low cell volume samples, respectively. SQUID measurements show Ce2O2ZnSe2 to be paramagnetic from 2 to 300 K with a negative Weiss temperature of θ = -10 K, suggesting weak antiferromagnetic interactions.