Investigation of multicomponent chemical short-range order in NiZr2

The local atomic configuration of multicomponent chemical short-range order (MCSRO) in NiZr2 has been investigated by means of molecular dynamics simulation (MD) in a wide temperature range. The potential functions for the system based on the embedded atom method are constructed and the parameters are obtained by fitting the structure and properties of NiZr2 crystal. The static structures such as pair distribution functions and the distribution of coordination number have been calculated. The local atomic configurations of the MCSI~Os in the melt were demonstrated as distorted coordination polyhedron of the compound structure and/or the structure similar to cubooctahedron analogues. It is indicated by the results of MD simulation that above the melting point the atomic packing of long-range order disappears, but the chemical interaction of coordinated atoms still exists, which leads to the formation of various MCSROs with atomic configurations similar to the stable or metastable unit cell of NiZr2 compound. When the system is just melted, many icosahedral polyhedron configurations appear, which decrease as the over-heating temperature increases.     

First-principles study on with nitrogen and anatase TiO2 codoped praseodymium

<正>The crystal structures,electronic structures and optical properties of nitrogen or/and praseodymium doped anatase TiO₂ were calculated by first principles with the plane-wave ultrasoft pseudopotential method based on density functional theory.Highly efficient visible-light-induced nitrogen or/and praseodymium doped anatase TiO₂ nanocrystal photocatalyst were synthesized by a microwave chemical method.The calculated results show that the photocatalytic activity of TiO₂ can be enhanced by N doping or Pr doping,and can be further enhanced by N+Pr codoping.The band gap change of the codoping TiO₂ is more obvious than that of the single ion doping,which results in the red shift of the optical absorption edges.The results are of great significance for the understanding and further development of visible-light response high activity modified TiO₂ photocatalyst.The photocatalytic activity of the samples for methyl blue degradation was investigated under the irradiation of fluorescent light.The experimental results show that the codoping TiO₂ photocatalytic activity is obviously higher than that of the single ion doping.The experimental results accord with the calculated results.     

Zero-field splitting parameters and local structures for tetragonal Cr2＋ centers in Cr2＋-doped ZnSe semiconductors

The inter-relation between zero-field splitting （ZFS） parameters and local lattice structures of the （CrSe4）6 clusters in ZnSe semiconductors has been established by using the complete diagonalization （of the energy matrix） method. On the basis of this, the local lattice distortions, the ZFS parameters D, a, F and the optical spectrum for Cr2＋ ions doped into ZnSe are theoretically investigated, and the contributions of the spin singlets have been taken into account. The calculated ZFS parameters are in good agreement with the experimental values. From our calculations, the tetragonal distortion parameters AR = 0.091A and Aθ = 4.28° of Cr2＋ in ZnSe are acquired, and the results suggest that there exists a tetragonal expansion distortion for the local lattice structure of （CrSe4）6- clusters in ZnSe crystals. The influence of the spin singlets on ZFS parameters is also discussed, indicating that the contributions to ZFS parameters a and F cannot be ignored.     

Structural analysis and crystal-field calculations of Nd^3＋ in Gdx Lu1-x TaO4（x = 0.85） polycrytalline

The crystal structural parameters of Nd 3+-doped rare earth orthotantalate Gd x Lu 1 x TaO 4(x = 0.85) are determined by applying the Rietveld refinement to its X-ray diffraction,and its emission and excitation spectra at 77 K are analysed.The relativistic model of ab initio self-consistent DV-Xα method,which is applied to the cluster NdO 8 in Gd x Lu 1 x TaO 4,and the effective Hamiltonian model are used to investigate its spin-orbit and crystal-field parameters.The free-ions and crystal-field parameters are fitted to the experimental energy levels at 77 K with a root-mean-square deviation of 14.92 cm 1.According to the crystal-field calculations,96 levels of Nd 3+ are assigned.Finally,the fitting results of free-ions and crystal-field parameters are compared with those already reported for Nd 3+:YAlO 3.The results indicate that the free-ion parameters are similar to those of the Nd3+ in Gdx Lu1-x TaO4 and YAlO 3 hosts,and the crystal-field interaction of Nd3+ in Gdx Lu1-x TaO4 is stronger than that in YAlO 3.     

Electronic structures and thermoelectric properties of solid solutions CuGa_(1-x) In_xTe_2 : A first-principles study

The electronic structures of solid solutions CuGal_xlnxTe2 are systematically investigated using the full-potential all-electron linearized augmented plane wave method. The calculated lattice parameters almost linearly increase with the increase of the In composition, which are in good agreement with the available experimental results. The calculated band structures with the modified Becke-Johnson potential show that all solid solutions are direct gap conductors. The band gap decreases linearly with In composition increasing. Based on the electronic structure calculated, we investigate the thermoelectric properties by the semi-classical Boltzmann transport theory. The results suggest that when Ga is replaced by In, the bipolar effect of Seebeck coefficient S becomes very obvious. The Seebeck coefficient even changes its sign from positive to negative for p-type doping at low carrier concentrations. The optimal p-type doping concentrations have been estimated based on the predicted maximum values of the power factor divided by the scattering time.     

Potential energy curves and spectroscopic properties of X2∑ and A2П states of 13C14N

The potential energy curves(PECs) of X2+Σand A2Π states of the CN molecule have been calculated with the multireference configuration interaction method and the aug-cc-pwCV5Z basis set. Based on the PECs, all of the vibrational and rotational levels of the13C14N molecule are obtained by solving the Schro¨dinger equation of the molecular nuclear motion.The spectroscopic parameters are determined by fitting the Dunham coefficients with the levels. Both the levels and the spectroscopic parameters are in good qualitative agreement with the experimental data available. The analytical potential energy functions are also deduced from the calculated PECs. The present results can provide a helpful reference for future spectroscopy experiments or dynamical calculations of the molecule.     

Optical spectra and local structure of Eu3+ ions doped in Nb2O5-La2O3-B2O3-BaO glasses

The xNb2O5-(15-x)La2O3-40B2O3-45BaO (x = 5, 7.5, 12.5 mol%) glasses doped with Eu3+ ions in 1mol% are fabricated by the melting method. The Fourier transform infrared (FTIR) spectra, phonon sideband spectra, emission and excitation spectra of the glasses are measured. The crystal field parameter and coordination number of Eu3+ ions in the glasses are obtained according to the splitting of their 5D0 - 7F1levels. The intensity parameters Ω2 and Ω4 of Eu3+ ions for optical transition are calculated from their emission spectra in terms of reduced matrix U(t) (λ= 2,4,6) character for optical transitions. The results indicate that the intensity parameters Ω2 and Ω4 increase with the increase of Nb2O5 content, suggesting that the symmetry becomes lower, the band of Eu and O atoms becomes stronger and the covalence increases with the increase of Nb2O5 content.     

Empowering perovskite PbTiO3 nanoparticles with enhanced up-conversion luminescence and thermal sensitivity by introducing Er3+ dopant

Up-convertion(UC)perovskite Er³⁺-doped Pb TiO3(Er-PTO)nanoparticles with green and red emissions were synthesized via the hydrothermal method.The UC properties were manipulated by adjusting the concentration of Er³⁺ ions dopant.The green emission intensity was decreased as the doping concentration increased from 1% to 4%(mole fraction),whereas the red emission intensity was increased.The influences of Er³⁺ ions on the temperature-sensing performance were further investigated.The results demonstrated that Er-PTO nanoparticles with doping 1% Er³⁺ ions possessed a sensitivity of3.1×10^-3 K^-1 at 475 K,presenting a high potential in optical heating devices.     

Study on energy up-conversion in Yb, Ho:YAG crystal

Optic parameters, such as the probabilities of radiative and non-radiative transition and the cross-relaxation probability between Yb3+ and Ho3+ ions in Yb,Ho:YAG crystal, are calculated on the basis of Judd-Ofelt and Dexter theories. The energy up-conversion process is analyzed by solving the transition rate-equations. The results show that (1) the intensity of the green fluorescence relates to the square of the concentration of the active ions; (2) the intensity increases with the concentration of sensitive ions as well, but the increasing rate goes rather too slow; (3) the efficiency of the energy up-conversion relates with the speed of the energy up-conversion and the quantum efficiency of the transiting from upper level to lower level.     

Structures of surface—and bulk—dispersion phases of NiO／Al2O3

The structural features of surface-disperison and bulk-dispersion states of NiO on/in γ-Al2O3 have been studied by x-ray absorption fine structure (XAFS) in combination with x-ray diffraction(XRD) and XPS.The results obtained from XRD and XAFS show that surface and buld dispersions of Ni^2 ions need different conditions and have different diffraction features and different corrdination structures.In both the surace-dispersion phase and the solid solution,Ni^2 ions seem to prefer to keep the coordination mode in NiO,which is quite different from that in the stoichioetric spinel.The Ni/Ai intensity ratios of XPS at different etching depths of samples have a maximum and the distribution of Ni^2 ions in the solid solution is not homogeneous.Both it and the coordination features provide evidence of the transition process of dispersing from surface to bulk.Copyright 2001 John Wiley and Sons,Ltd.     

Spectral investigation of Sm3＋/yb3＋co-doped sodium tellurite glass

YAG-Ce,Nd,and Yb phosphors with a triple-doped system are prepared by conventional solid-state reaction method.The fluorescence emission and excitation spectra are measured and analyzed.The influences of Yb3+ doping concentration on the emission of Yb3+ and Nd3+ in YAG-Ce,Nd,and Yb are studied.The fluorescence decay spectra,lifetime,and energy transfer efficiency of Ce3+ in different host materials of YAG-Ce and Yb,and YAG-Ce,Nd,and Yb are also compared.Furthermore,the trends of fluorescence decay spectra and the lifetimes of Nd3+ and Yb 3+ in YAG-Ce,Nd,and Yb with the increase of Yb3+ concentration are discussed.Results indicate that YAG-Ce,Nd,and Yb are good candidates for downconverting phosphor,with energy transfer efficiency reaching as high as 82.8%.     

Calculating Hamiltonian parameters for Yb^3＋ in a low-symmetry lattice site, and fitting the structure and levels of Yb^3＋ ：RETaO4 （RE = Gd, Y, and Sc）

An iterative method is used to find the values of the Hamiltonian parameters for Yb^3＋ in a given low-symmetry crys- talline site. Samples of Yb^3＋ ：RETaO4 （RE = Gd, Y, and Sc） were prepared and their structures were determined. Based on the obtained structural data, their orbital-spin parameters and crystal field parameters were fitted by the superposition model （SM）. Using the crystal field parameters obtained by the SM fitting as the initial parameters, the Hamiltonian parameters were fitted iteratively. The calculated and experimental energy levels for Yb^3＋：RETaO4 are consistent, and the maximal mean-root-square deviation is only 2.84 cm^- 1, indicating that the method is effective to determine the Hamiltonian parameters of Yb^3＋ in low-symmetry crystalline sites.     

Local Lattice Distortion Around VPb^2- in PbWO4 and the Origin of the 420nm Luminescence Band

The lattice structures around one lead vacancy VPb^2- in PbWO4 are studied within the framework of the fullrelativistic density functional theory. Using the conjugate gradient method, we optimize the geometry of the ions around VPb^2- by moving the ions within the sub-cell until specified tolerance is satisfied. We obtain the geometric positions of ions around VPb^2- after optimization. The calculated results indicate that there would be local lattice distortion caused by the existence of VPb^2-. The electronic structures of both the optimized and pre-optimized PWO containing VPb^2- have been calculated. The density of states of the distorted PWO indicates that the energy gap between the 2p state of O^2- and the 5d state of W^6 is 3.90eV, which shows that the distorted W-O tetrahedron may be the 42Onm luminescence centre.     

Structural analysis and crystal-field calculations of Nd³⁺ in Gd_x Lu_1-x TaO₄（x = 0.85） polycrytalline

The crystal structural parameters of Nd 3+-doped rare earth orthotantalate Gd x Lu 1 x TaO 4(x = 0.85) are determined by applying the Rietveld refinement to its X-ray diffraction,and its emission and excitation spectra at 77 K are analysed.The relativistic model of ab initio self-consistent DV-Xα method,which is applied to the cluster NdO 8 in Gd x Lu 1 x TaO 4,and the effective Hamiltonian model are used to investigate its spin-orbit and crystal-field parameters.The free-ions and crystal-field parameters are fitted to the experimental energy levels at 77 K with a root-mean-square deviation of 14.92 cm 1.According to the crystal-field calculations,96 levels of Nd 3+ are assigned.Finally,the fitting results of free-ions and crystal-field parameters are compared with those already reported for Nd 3+:YAlO 3.The results indicate that the free-ion parameters are similar to those of the Nd3+ in Gdx Lu1-x TaO4 and YAlO 3 hosts,and the crystal-field interaction of Nd3+ in Gdx Lu1-x TaO4 is stronger than that in YAlO 3.     

Optical spectra and local structure of Eu~(3+) ions doped in Nb_2O_5-La_2O_3-B_2O_3-BaO glasses

The xNb2O5-(15-x)La2O3-40B2O3-45BaO (x = 5, 7,5, 12.5 mol%) glasses doped with Eu3 mol% are fabricated by the melting method. The Fourier transform infrared (FTIR) spectra, phonon sideband spectra, emission and excitation spectra of the glasses are measured. The crystal field parameter and coordination number of Eu3 ions in the glasses are obtained according to the splitting of their 5D0 - 7F1 levels. The intensity parametersΩ2 andΩ4 of Eu3 ions for optical transition are calculated from their emission spectra in terms of reduced matrix U(t) (λ= 2, 4, 6) character for optical transitions. The results indicate that the intensity parametersΩ2 andΩ4 increase with the increase of Nb2O5 content, suggesting that the symmetry becomes lower, the band of Eu and O atoms becomes stronger and the covalence increases with the increase of Nb2O5 content.     

Effect of Hf~(4+) doping on structure and enhancement of upconversion luminescence in Yb:Tm:LiNbO_3 crystals

A series of Yb:Tm:LiNbO_3 crystals doped with x mol% Hf~(4+)ions(x = 2, 4, and 6) were grown by the Czochralski method. The dopant occupancy and defect structure of Hf:Yb:Tm:LiNbO_3 crystals were investigated by x-ray diffraction and infrared transmission spectra. The influence of Hf~(4+)ions concentration on UV–VIS–NIR absorption spectra of Hf:Yb:Tm:LiNbO_3 crystals was discussed. The upconversion luminescence of Hf:Yb:Tm:LiNbO_3 crystals was obtained under 980 nm excitation. Strong emissions were observed at 475 nm in the blue wavelength range and 651 nm in the red wavelength range. Remarkably, enhancement of the red and blue upconversion luminescence was achieved by tridoping Hf~(4+)ions.     

Shape controllable synthesis and enhanced upconversion photoluminescence of β-NaGdF_4:Yb~(3+), Er~(3+) nanocrystals by introducing Mg~(2+)

Different concentrations of Mg~(2+) -doped hexagonal phase NaGdF_4:Yb~(3+), Er~(3+)nanocrystals(NCs) were synthesized by a modified solvothermal method. Successful codoping of Mg~(2+)ions in upconversion nanoparticles(UCNPs) was supported by XRD, SEM, EDS, and PL analyses. The effects of Mg~(2+)doping on the morphology and the intensity of the upconversion(UC) emission were discussed in detail. It turned out that with the concentration of Mg~(2+)increasing, the morphology of the nanoparticles turn to change gradually and the UC emission was increasing gradually as well. Notably the UC fluorescence intensities of Er~(3+)were gradually improved owing to the codoped Mg~(2+)and then achieved a maximum level as the concentration of Mg~(2+)ions was 60 mol% from the amendment of the crystal structure of β-NaGdF_4:Yb~(3+),Er~(3+)nanoparticles. Moreover, the UC luminescence properties of the rare-earth(Yb3+, Er~(3+)) ions codoped NaGdF_4 nanocrystals were investigated in detail under 980-nm excitation.     

2.0-μm emission and energy transfer of Ho~(3+)/Yb~(3+) co-doped LiYF_4 single crystal excited by 980 nm

Ho3+/Yb3+co-doped Li YF4 single crystals with various Yb3+concentrations and ～ 0.98 mol% Ho3+concentration are grown by the Bridgman method under the conditions of taking Li F and YF3 as raw materials and a temperature gradient(40°C/cm–50°C/cm) for the solid–liquid interface.The luminescent performances of the crystals are investigated through emission spectra,infrared transmittance spectrum,emission cross section,and decay curves under excitation by 980 nm.Compared with the Ho3+single-doped Li YF4 crystal,the Ho3+/Yb3+co-doped Li YF4 single crystal has an obviously enhanced emission band from 1850 nm to 2150 nm observed when excited by a 980-nm diode laser.The energy transfer from Yb3+to Ho3+and the optimum fluorescence emission around 2.0 μm of Ho3+ions are investigated.The maximum emission cross section of the above sample at 2.0 μm is calculated to be 1.08×10-20cm2 for the Li YF4 single crystal of1-mol% Ho3+and 6-mol% Yb3+according to the measured absorption spectrum.The high energy transfer efficiency of88.9% from Yb3+to Ho3+ion in the sample co-doped by Ho3+(1 mol%) and Yb3+(8 mol%) demonstrates that the Yb3+ions can efficiently sensitize the Ho3+ions.     

Characterization of Undoped and Cu-Doped ZnO Thin Films Deposited on Glass Substrates by Spray Pyrolysis

Undoped and copper doped zinc oxide （ZnO） thin films have been prepared on glass substrates by spray pyrolysis technique. The films were doped with copper using the direct method by addition of a copper salt （CuCl2） in the spray solution of ZnO. Variation of structural, electrical, optical and thermoluminescence （TL） properties with doping concentrations is investigated in detail.     

Preparation and optical spectroscopy of phosphate glasses containing divalent europium ions

P2O5.BaO.Na2O.K2O glasses doped with various content of Eu2O3 were prepared using high temperature melting method, and the Eu2+ ions in the phosphate glasses were obtained with the aid of the reductive action of silicon powder. The fabricating conditions, fluorescence, excitation spectra of the glasses were then studied. The glasses containing europium show a broad emission band at 450 nm and sharp bands from 580 to 650 nm, and the co-existence of Eu2+ and Eu3+ is identified. Also, a good glass with a dominant proportion and large quantity of Eu2+ ions can be obtained by the reductive action of silicon powder and proper processing.