Lixin Ning

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.

Electronic properties and 4f → 5d transitions in Ce-doped Lu2SiO5: a theoretical investigation

The electronic properties and the 4f → 5d transitions of the dopant Ce3+ ions located at the two crystallographic lutetium sites of Lu2SiO5 (LSO) are investigated using the hybrid density functional theory (DFT) with the HSE06 functional and the wavefunction-based embedded cluster methods, respectively. The HSE06 calculations give a band gap of 6.35 eV for LSO, which agrees well with the reported experimental values between 6.4 and 6.8 eV. It is found that Ce3+ prefers strongly to occupy the seven-coordinated (Lu1) site over the six-coordinated (Lu2) one. The energy gaps between the occupied Ce3+ 4f band and the valence band maximum of the host are predicted to be 2.81 and 3.07 eV for CeLu1 and CeLu2 substitutions, respectively, which are close to the experimental data of 2.6–2.9 eV. Based on the wavefunction-based CASSCF/CASPT2 embedded cluster calculations for the energies of the Ce3+ 4f1 and 5d1 levels, the experimentally observed 4f → 5d transition bands are identified in association with the two substitutions. The predicted transition energy and intensity patterns for CeLu1 substitution are in fairly good agreement with those of the experimental absorption spectrum. The variations of the two lowest 4f → 5d transition energies with the substitutions are finally discussed in terms of the changes of the centroid-energy difference and the crystal-field splitting with the local coordination geometries.

Computational design of inorganic nonlinear optical crystals based on a genetic algorithm

Based on the results of density functional theory calculations, a theoretical method to design inorganic nonlinear optical (NLO) crystals for second harmonic generation (SHG) is presented. In this method, a specialized genetic algorithm (GA) is developed to search the stable structures of the inorganic crystal with known compositions and study the noncentrosymmetric stable structures and the second-order nonlinear optical properties by calculating the corresponding SHG coefficients. Unlike normal GA techniques, the main feature of the present method is that the coordination fashions of the building units are introduced to construct the structures of individuals during the GA procedure, which can obviously improve the efficiency and success rate of obtaining the stable structure of the inorganic crystals. As typical examples, two ternary compounds, AgGaS2 and LiAsSe2 crystals are considered, and besides the structures observed experimentally, the geometries and optical performance of other metastable (or more stable) phases have been explored. Our results clearly demonstrate that the present method can provide a feasible way to design and optimize new inorganic NLO crystals.

Structural properties and 4f → 5d absorptions in Ce-doped LuAlO3: a first-principles study

The structural properties and the 4f → 5d absorptions of Ce-doped LuAlO(3) have been studied using the density functional theory-based generalized gradient approximation PBE + U (PBE: Perdew, Burke and Ernzerhof) and wavefunction-based embedded cluster calculations, respectively. The PBE or PBE + U calculations reveal that the substitution of Ce for Lu induces a strongly anisotropic distortion of the local atomic structure around the dopant site, which is largely insensitive to the value of U for the Ce 4f states. The calculated electronic structures depend explicitly on the value of U, and a value of U ≈ 6 eV is determined by comparison with experimental x-ray photoelectron data for CeAlO(3). On the basis of the PBE-optimized structure, CASSCF/CASPT2 (complete-active-space self-consistent-field/second-order perturbation theory) embedded cluster calculations for the Ce(3+) 4f → 5d transitions yield energy and intensity patterns in fairly good agreement with those estimated from the experimental absorption spectrum. A Mulliken spin population analysis for the 5d(1) states shows that the origins of the states are significantly different from being caused by the cubic crystal field, confirming an earlier conclusion as regards the origin of the 5d(1) states based on semiempirical molecular orbital calculations. The importance of spin-orbit effects on the energies and wavefunctions of the Ce 5d(1) states is highlighted.

DFT calculations of crystal-field parameters for the lanthanide ions in the LaCl3 crystal.

We present herein a calculation of crystal-field (CF) parameters for the lanthanide ions in the LaCl(3) crystal, using the density functional theory-based orbital-free embedding formalism in conjunction with the effective Hamiltonian method. The calculated values for the second- and fourth-rank CF parameters agree fairly well with the experimental data. It is found that the effect of ligand polarization plays a crucial role in determining the sign and magnitude of the second-rank CF parameters, while the effect of Pauli repulsion between 4f electrons and the ligands is important for the fourth-rank CF parameters. The usefulness of the present approach is illustrated by a study of the relative preference of Eu(3 + ) occupation on the two distinct Pb(2 + ) sites in the KPb(2)Cl(5) crystal. The computational approach and results discussed in this work are useful for a better understanding and simulation of the CF effect in the lanthanide-containing systems.

Deposition of (WO3)3 nanoclusters on the MgO(001) surface: A possible way to identify the charge states of the defect centers

Periodic density functional theory calculations have been performed to study the most stable structure of the (WO(3))(3) nanocluster deposited on the MgO(001) surface with three kinds of F(S) centers (F(S)(0), F(S)(+), and F(S)(2+)). Our results indicate that the configuration of (WO(3))(3) cluster, including the cyclic conformation and the heights of three W atoms, and the oxidation states are sensitive to the charge state of the F(S) center. It is interesting that the electron-riched F(S) (0) vacancy on the MgO(001) surface can act as a promoting site to enhance the W-W interaction and the W(3)O(3) cyclic conformation is maintained, while the skeleton of cluster becomes flexible when (WO(3))(3) is adsorbed on the electron-deficient vacancy (F(S)(+) and F(S)(2+)). Accordingly, three F(S)-centers exhibit different arrangements of X-ray photoelectron spectra, the scanning tunneling microscopy images, and the vibrational spectra after depositing (WO(3))(3) cluster. Present results reveal that the (WO(3))(3) cluster may be used as a probe to identify the different F(S) centers on the MgO(001) surface.

Density Functional Study on Structures and Relative Stability of Gd(H2O)n3+ (n = 8,9)

Density functional theory calculations were performed to study the structures and relative stability of the gadolinium complexes, Gd(H2O)n3+ (n = 8,9), in vacuo and in aqueous solution. The polarizable continuum model with various radii for the solute cavity was used to study the relative stability in aqueous solution. The calculated molecular geometries for n = 8 and 9 obtained in vacuo are consistent with those observed in experiments. It was found that while the nona-aqua complex is favored in the gas phase, in aqueous solution the octa-aqua conformation is preferred. This result, independent of the types of cavities employed, is in agreement with the experimental observation. The reliability of the present calculation was also addressed by comparing the calculated and experimental free energy of hydration, which revealed that the UA0, UAHF, and UAKS cavities are most appropriate when only the first solvation shell is treated explicitly.

First Principles Study on the 4f-5d Transition of Ce3+ in LuAlO3

We have investigated properties of the compound LuAlO3:Ce3+ associated with the Ce3+ 4f-5d transition by using the periodic density functional theory. A hybrid functional has been used for the 4f states and a constrained approach has been employed for the excited 5d state. It is found that the average distance between Ce3+ and the eight nearest-neighbor O atoms decrease by 0.05 A on going from 4f to 5d state. The calculated Stokes shift is in good agreement with experiment. Based on the optimized structure around Ce3+, the energy level scheme of the 5d states has been evaluated using the angular overlap model, in reasonable agreement with experiment.

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.

Origin of the green persistent luminescence of Eu-doped SrAl2O4 from a multiconfigurational ab initio study of 4f7 → 4f65d1 transitions

The green persistent luminescence of Eu2+ in SrAl2O4 has been widely applied in self-sustained night vision due to its long duration and ability to be excited by sunlight. However, the question of which substitutional site in the lattice is responsible for the green emission remains unresolved. Herein, we perform multiconfigurational ab initio calculations at the CASSCF/CASPT2 level with the spin–orbit effect on 4f7 → 4f65d1 (4f → 5d) transitions of Eu2+ located at the Sr1 and Sr2 sites of SrAl2O4. By comparison of the calculated transition energies with data from experimental low-temperature excitation spectra, the green luminescence is unambiguously assigned to Eu2+ at the Sr2 site. In addition, by using the present results as a calibration, the accuracies of the literature-reported DFT-based approaches to the Eu2+ 4f → 5d transition energies are discussed. This investigation is expected to motivate more exploration of multiconfigurational ab initio methodology to elucidate luminescence properties of Eu2+-activated complex phosphors.