0000000000315983
AUTHOR
N. E. Christensen
showing 7 related works from this author
First-principles and semiempirical calculations forFcenters inKNbO3
1997
The linear muffin-tin-orbital method combined with density functional theory (local approximation) and the semiempirical method of the intermediate neglect of the differential overlap (INDO) based on the Hartree-Fock formalism are used for the study of the $F$ centers (O vacancy with two electrons) in cubic and orthorhombic ferroelectric KNbO$_3$ crystals. Calculations for 39-atom supercells show that the two electrons are considerably delocalized even in the ground state of the defect. Their wave functions extend over the two Nb atoms closest to the O vacancy and over other nearby atoms. Thus, the $F$ center in KNbO$_3$ resembles electron defects in the partially-covalent SiO$_2$ crystal (…
First-principles and semi-empirical calculations for bound hole polarons in KNbO3
1999
The ab initio linear muffin-tin-orbital (LMTO) formalism and the semi-empirical method of the Intermediate Neglect of the Differential Overlap (INDO) based on the Hartree-Fock formalism are combined for the study of the hole polarons (a hole trapped nearby the cation vacancy) in a cubic phase of KNbO3 perovskite crystals. The 40-atom and 320-atom supercells were used, respectively. We predict existence of both, one-site and two-site (molecular) polarons with close optical absorption energies (0.9 eV and 0.95 eV). The relevant experimental data are discussed.
Quantum Mechanical Modelling of Pure and Defective KNbO3 Perovskites
2000
Ab initio electronic structure calculations using the density-functional theory (DFT) are performed for KNbO3 with and without defects. Ferroelectric distortive transitions involve very small changes in energies and are therefore sensitive to DFT-approximations. This is discussed by comparing results obtained with the local density approximation (LDA) to those where generalized gradient approximations (GGA) are used. The results of ab initio calculations for F-type centers and bound hole polarons are compared to those obtained by a semiempirical method of the Intermediate Neglect of the Differential Overlap (INDO), based on the HartreeFock formalism. Supercells with 40 and 320 atoms were us…
A comparative study of the atomic and electronic structure of F centers in ferroelectric KNbO3: Ab initio and semi-empirical calculations
1998
Abstract The linear muffin-tin-orbital method combined with density functional theory (in a local density approximation) and the semi-empirical method of the intermediate neglect of the differential overlap (INDO) based on the Hartree-Fock formalism are used for the supercell study of the F centers (O vacancy with two electrons) in cubic and orthorhombic ferroelectric KNbO3 crystals. The two electrons are found to be considerably delocalized even in the ground state of the defect. Their wave functions extend over the two Nb atoms closest to the O vacancy and over other nearby atoms. Thus, the F center in KNbO3 resembles much more electron defects in the partly covalent SiO2 crystal (the so-…
First-Principles Simulations of Interstitial Atoms in Ionic Solids
1995
The atomic and electronic structure of the radiation-induced interstitial atoms in MgO and KCl crystals representing two broad classes of ionic solids are calculated and compared. The first-principles full potential LMTO method is applied to a 16-atom supercell. For both crystals the energetically most favourable configuration is a dumbbell centered at a regular anion site. Its (110) and (111) orientations are very close in energy which permits the dumbbell to rotate easily on a lattice site. The mechanism and the relevant activation energy for thermally activated diffusion hops from the dumbbell equilibrium position to the cube face and cube center are discussed in the light of the availab…
Interstitial-oxygen-atom diffusion in MgO.
1996
configuration is the ~111! dumbbell centered at a regular oxygen site, whereas face-centered and cube-centered configurations are higher in energy by 1.45 eV and 3.57 eV, respectively. The~111! configuration isclose in energy to the ~110! configuration, which allows the dumbbell to rotate easily on a lattice site. In allthese four cases the interstitial oxygen atom attracts considerable additional electron density from its nearestregular O
First-principles and semiempirical Hartree-Fock calculations for F centers in KNbO3 and Li impurities in KTaO3
1998
The LMTO method based on the density-functional theory and the semi-empirical INDO method based on the Hartree--Fock formalism are used for the supercell study of the F centers in cubic and orthorhombic ferroelectric KNbO3 crystals. Two electrons are found to be considerably delocalized even in the ground state of the defect. The absorption energies were calculated by means of the INDO method using the Delta-SCF scheme after a relaxation of atoms surrounding the F center. As an example of another type of point defect in perovskite, an isolated Li impurity in KTaO3 as well as interacting Li pairs are considered in the supercell approach, using the supercells of up to 270 atoms. The off-cente…