A. Yu. Kuznetsov

Electronic structure of lithium tetraborate Li2B4O7 crystals. Cluster calculations and x-ray photoelectron spectroscopy

The results of an investigation of the electronic structure of lithium tetraborate crystals using experimental (x-ray photoelectron spectroscopy) and theoretical (quantum-chemical modeling) methods are reported. The experimental spectrum of the valence-band states of the crystal lies 2–15 eV below the Fermi level and is due primarily to boron-oxygen groups (B4O9). The quantum-chemical calculations were performed self-consistently, using the standard variant of the scattered-wave method in the model of a cluster embedded in a lattice of point charges. The data obtained on the partial contribution of the model densities to the one-electron spectrum of the [B4O9]6 cluster make it possible to interpret the fine structure of the experimental spectrum of the valence-band states.

First-principles calculations of the electronic structure and plastic properties of CsCl, CsBr, and CsI crystals

The electronic structure and plastic properties are investigated for a number of alkali halide crystals (CsCl, CsBr, CsI). First-principles calculations are carried out within the Hartree-Fock and density-functional theory approximations using several variants of the exchange-correlation functional, including the hybrid exchange technique. The results obtained with the use of five methods are compared with the available experimental data. The tendencies revealed in the variations in the band parameters and plastic properties of the crystals under investigation are analyzed.

Embedded Cluster Calculations of the Electron Structure of the Ce3+ Impurity in Lu2SiO5 Crystals with Allowance for Crystal Lattice Relaxation and Polarization

We present results of an investigation of the electron structure of the Ce 3+ impurity in the Lu 2 SiO 5 crystal with allowance for lattice polarization and relaxation. The calculations were performed by invoking an embedded cluster technique based on a synthesis of the scattered wave method and the molecular statics method. A discussion is provided of the effect that the amount of defect-produced lattice distortion has on calculation results.

First-principles calculations of the electronic and spatial structures of the Ba1−xLaxF2+x system within the supercell model

The electronic and spatial structures of the La impurity in BaF2 are investigated using the ab initio method of linear combinations of atomic orbitals based on the Hartree-Fock approximation in the supercell model. Calculations are performed using the CRYSTAL-98 software package. Possible models of defects are discussed. The MOLSTAT computer code is employed to estimate the energies of formation of defects and their parameters. The influence of defects on the BaF2 band structure is analyzed.

Molecular-statics simulation of the cerium impurity in LSO crystals

A model is proposed for the Lu2SiO5 crystal with cerium impurity, and the defect formation energy, ion relaxation energy, and the defect-induced changes of the Madelung potentials are calculated. The calculations show substitution of the cerium ion for the lutetium ion in the Lu1 position to be energetically preferred.

Embedded cluster calculation of the Ce3+ impurity in Lu2SiO5 crystals including crystal lattice relaxation and polarization

Results of electronic structure studies of the Ce3+ impurity in Lu2SiO5 crystals allowing for lattice polarization and relaxation are given. The calculations were performed by the embedded cluster method combining the scattered wave and molecular static methods. The effect of lattice deformation induced by the defect on the results of calculations is discussed.

Theoretical Investigation of the Electronic and Spatial Structures of the La Impurity in a BaF2 Crystal

The results of theoretical investigations of the electronic structure of BaF2 and LaF3 crystals and BaF2–LaF3 complexes without and with La impurity are presented. Changes in the structure of the state density, electronic spectrum, and effective charges caused by defects are analyzed. It is demonstrated that the NNN position of the [La3+Fi-] defect in BaF2 is more stable than the NN position.

Application of the Modified Exchange Correlation Potential to the Scattered Wave Cluster Method: Some Problems

Previously, J. B. Krieger, Y. Li, and G. J. Iafrate suggested a new form of exchange correlation potential for electronic structure calculations (KLI form). This paper deals with some problems of adaptation of the KLI formalism to the scattered wave method in the cluster model. The basic approximations of the model (frozen core, muffin tin, and the form of the exchange correlation functional) are examined from the viewpoint of their effect on the results of calculations, considering the given form of the exchange correlation potential. The results of test calculations are given for isolated H, He, Mg, and Xe atoms and H2O, CH4, and CO molecules.

Criteria of parameter selection for quantum chemical simulation of the electronic structure of rare-earth ions by the scattered wave cluster method

Electronic structure calculations of the Ce3+ ion in terms of a molecular cluster model in a quasirelativistic approximation are reported. The influence of the self-interaction correction and variations of muffintin parameters on the calculation results is discussed. A method is proposed for estimating the 4f→5d transition energy in the cerium ion using the SCF-SW formalism.

Kinetics of nonequilibrium processes excited in broad-band dielectrics by synchrotron radiation

Luminescence spectroscopy with subnanosecond time resolution is used to study features of nonequilibrium processes excited in several broad-zone dielectrics (mainly inorganic scintillators) by pulses of synchrotron radiation (SR). When excitation density exceeds a certain level, which is different for each material, there is an abrupt change in the kinetics of relaxation of the nonequilibrium states. This change is accompanied by nonuniform broadening or shortwave shifting of the luminescence spectrum and a drop in quantum light yield. The decay time for natural luminescence decreases by 1–3 orders, to nanoseconds, and is independent of temperature within the range 80–450 K. The build-up stage disappears in the kinetics of luminescence of Ce3+-centers and decay time is reduced by a factor of 2–4. Density effects are found to be independent of the conditions under which the material is exposed to SR. A model is proposed in which density effects are related to nonradiative energy transfer from the upper excited states of the luminescence centers to external quenching centers. The contribution of the space charge induced by SR is also examined.