Modelling of defects and surfaces in perovskite ferroelectrics

Abstract

The results of electronic structure calculations for different terminations of SrTiO 3 (100) and (110) perovskite thin films are discussed. These calculations are based on the ab initio Hartree-Fock (HF) method and Density Functional Theory (DFT). Results are compared with previous ab initio plane-wave LDA and classical Shell Model (SM) calculations. Calculated considerable increase of the Ti -O chemical bond covalency nearby the surface is confirmed by experimental data. Our quantum chemical calculations performed by means of the intermediate neglect of differential overlap (INDO) method confirm the existence of self-trapped electrons in KNbO 3 , KTaO 3 and BaTiO 3 crystals. The relevant lattice relaxation energies are 0.21 eV, 0.27 eV and 0.24 eV, and the optical absorption energies 0.78 eV, 0.75 eV and 0.69 eV, respectively. We suggest a theoretical interpretation of the so-called green luminescence (2.2 -2.3 eV) in ABO 3 perovskite crystals as a result of the recombination of electrons and holes forming the ChargeTransfer-Vibronic-Exciton (CTVE). The calculated luminescence energies for SrTiO 3 , BaTiO 3 , KNbO 3 and KTaO 3 perovskite crystals are in a good agreement with the experimentally observed energies.1. Introduction Thin films of ABO 3 perovskite ferroelectrics (e.g., SrTiO 3 ) are important for many technological applications, where the surface structure is important [1-4]. The SrTiO 3 (100) surface relaxation has been experimentally studied by means of several techniques, e.g. low energy electron diffraction (LEED), reflection high-energy electron diffraction (RHEED), medium energy ion scattering (MEIS) and surface X-ray diffraction (SXRD) measurements [5][6][7][8][9]. Several ab initio [10][11][12][13][14][15][16][17] and shell model (SM) [18,19] theoretical studies were dedicated to the (100) surface of BaTiO 3 and SrTiO 3 crystals. In order to study the dependence of the surface relaxation properties on exchange-correlation functionals and localized/plane wave basis sets used in most of calculations, recently [20] we performed a detailed comparative study of SrTiO 3 surface relaxation based on a number of different techniques. In this paper we analyze the relevant electronic structure of the (001) surface with emphasis on the electronic charge redistribution and gap modification, and present preliminary results for the atomic structure of the (110) surface.The properties of ABO 3 ferroelectric perovskites are influenced by point defects, primarily vacancies. For instance, laser second harmonic generation in KNbO 3 is reduced by the blue-light-induced absorption (BLIIRA) [21]. Despite the fact that such light-induced absorption is well known for many perovskites, the effect was studied only in a few papers. Relatively little is known about intrinsic point defects in ABO 3 . A broad absorption band around 2.7 eV has been observed in electron-irradiated

Hybrid DFT calculations of the atomic and electronic structure for ABO3 perovskite (001) surfaces

“…[26]. In this paper, we used this new BS which differs from our previous calculations [27,[29][30][31] by inclusion of polarizable d-orbitals on O ions. We demonstrated that this leads to better agreement of calculated optical gaps with experimental data.…”

“…In this paper, we continue our recent theoretical studies of the surfaces of perovskite materials. Our studies were carried out using both semiempirical SM [28] and ab initio HF and DFT methods [27,[29][30][31] and were dedicated mostly to the SrTiO 3 (0 0 1) surfaces. We studied the effects of different type of Hamiltonians (varied from DFT-LDA to HF with a posteriori corrections) on surface properties and atomic structure.…”

Stabilization Mechanisms of LaFeO3 (010) Surfaces Determined with First Principles Calculations

“…Although their dimensions are slightly different due to the octahedral tilting, the and (101) surfaces can be expected to be similar. The corresponding surfaces in various other perovskites, including BaTiO 3 , SrTiO 3 , and LaAlO 3 , have been extensively examined experimentally and theoretically 20–26 …”

“…The corresponding surfaces in various other perovskites, including BaTiO 3 , SrTiO 3 , and LaAlO 3 , have been extensively examined experimentally and theoretically. [20][21][22][23][24][25][26] The surface slab models are built with a (1 Â 1) surface unit cell within the plane of the surface. To determine the energy dependence on surface unit cell size, we compare the total energies of the LaO surface termination with (1 Â 1) and (2 Â 2) surface unit cells, where each slab has a thickness of 11 atomic layers.…”

Photo-Activated Low Temperature, Micro Fuel Cell Power Source

“…17,18,19,20 Our group has extensive experience with SrTiO 3 as a photoelectrode, 21 with its high temperature electrical properties, its defect structure and transport characteristics and its optical properties. 8,10,13 We have successfully grown high quality thin films of both BaTiO 3 and SrTiO 3 by pulsed laser deposition (PLD) and we can, by control of growth conditions, obtain textured films with desired crystallographic orientations and grain size.…”