Jun Wen

Spectroscopic Distinctions between Two Types of Ce3+ Ions in X2-Y2SiO5: A Theoretical Investigation

The Ce(3+) ions occupying the two crystallographically distinct Y(3+) sites both with C1 point group symmetry in the X2-Y2SiO5 (X2-YSO) crystal are discriminated by their spectroscopic properties calculated with ab initio approaches and phenomenological model analyses. Density functional theory (DFT) calculations with the supercell approach are performed to obtain the local structures of Ce(3+), based on which the wave function-based embedded cluster calculations at the CASSCF/CASPT2 level are carried out to derive the 4f → 5d transition energies. From the ab initio calculated energy levels and wave functions, the crystal-field parameters (CFPs) and the anisotropic g-factor tensors of Ce(3+) are extracted. The theoretical results agree well with available experimental data. The structural and spectroscopic properties for the two types of Ce(3+) ions in X2-YSO are thus distinguished in terms of the calculated local atomic structures, 4f → 5d transition energies, and spectral parameters.

Red-shift of vanadate band-gap by cation substitution for application in phosphor-converted white light-emitting diodes

Eu3+ doped YVO4 is excellent traditional red phosphor, but shifting its efficient excitation band to a longer wavelength is necessary for application in white light-emitting diodes, and is a big challenge that calls for good solution. Following the observation that cation substitution usually is an effective way to manipulate the band-gap of phosphors, we carried out ab initio calculation for the band-gap of pure and Bi3+ doped LnVO4 (Ln = Y, Lu, and Sc). The results show that the band-gap decreases with the cationic radius from Y3+ to Sc3+, and the doping of Bi3+ further decreases their band-gaps. The experimental results on the Eu3+ doped and Eu3+, Bi3+ co-doped LnVO4 (Ln = Y, Lu, and Sc) phosphors confirm this. In particular, the excitation of Eu3+ and Bi3+ co-doped ScVO4 is shifted to approach 400 nm, which fulfills the excitation requirement of near-ultraviolet-based white light-emitting diodes, and implies that Eu3+ and Bi3+ co-doped ScVO4 should be a promising candidate as red phosphor in white lig...

Crystal-field analyses for trivalent lanthanide ions in LiYF4

Abstract Based on the completely parametric crystal-field model, the energy level parameters, including free-ion parameters and crystal-field parameters, obtained by fitting the experimental energy level data sets of Ln 3+ in LiYF 4 were systematically analyzed. The results revealed that the regular variation trends of the major parameters at relatively low site symmetry still existed. The g factors of ground states were calculated using the parameters obtained from least-squares fitting. The results for Ce 3+ , Nd 3+ , Sm 3+ , Dy 3+ and Yb 3+ were in good agreement with experiment, while those of Er 3+ deviated from experiment dramatically. Further study showed that the g factors depended strongly on B 6 4, and a slightly different B 6 4 value of −580 cm −1 led to g factors agreeing well with the experimental values.

Crystal field interactions between Ce3+ ion and fluoride ligands: a theoretical investigation

Superposition model (SM) analyses combined with ab initio calculations are presented to investigate crystal field interactions between Ce3+ ions and their fluoride ligands. On the basis of energies and wave functions derived from complete active space self-consistent field (CASSCF) embedded cluster calculations, 4f and 5d crystal field parameters (CFPs) for Ce3+ in three typical fluoride compounds (Cs2NaYF6, CaF2 and KMgF3) are extracted, showing a good agreement with available experimental data. Moreover, the SM intrinsic parameters, which are usually used to depict physical interactions between the dopant and its ligands, are determined from the ab initio calculated CFPs. The aim of this work is to obtain the quantitative dependence of intrinsic parameters on geometric structures around Ce3+ in fluoride compounds. Thus, the spectral parameters and further the spectroscopic properties of Ce-doped fluoride compounds would be conveniently predicted according to the calculations and analyses in the present work.

Thermodynamic Stabilities, Electronic Properties, and Optical Transitions of Intrinsic Defects and Lanthanide Ions (Ce3+, Eu2+, and Eu3+) in Li2SrSiO4

Geometric structures, electronic properties, thermodynamic stabilities, and optical transitions of intrinsic defects (vacancies and antisite defects) and lanthanide ions (Ce3+, Eu2+, and Eu3+) in Li2SrSiO4 (LSSO) host are studied by theoretical calculations combined with hybrid density functional theory, the multireference configuration interaction method, and empirical models. Calculations on the defect formation energies and the ab initio simulations of 4f → 5d electronic transitions for Ce3+ ions determine the most possible charge-compensation mechanism and accurately identify excitation bands in experimental spectra for LSSO:Ce3+ phosphors. On the basis of previously reported experimental spectra of Ce3+- and Eu3+-doped LSSO phosphors as well as a series of empirical models developed by Dorenbos, the locations of the 4f ground states and the lowest 5d excited states of Ln3+ and Ln2+ ions in the host (illustrated by the host-referred binding energy scheme, i.e., the HRBE scheme) are obtained, which is useful for the investigation of the electron-transfer and spectroscopic properties in lanthanide-doped LSSO. Moreover, thermodynamic and optical transition energy levels related to intrinsic defects and lanthanide ions (with various charge states) are derived from total energy calculations. The mechanisms of thermoluminescence (TL) and long-lasting luminescence (LLL) in LSSO:Eu2+,Dy3+ phosphors and especially the contributions of oxygen vacancies ( VO) and Dy3+ dopants are then interpreted. The aim of this study is thus to deeply understand the mechanisms of charge compensation, TL, and LLL in lanthanide-doped phosphors from theoretical calculations and analyses.