The physics of dynamical atomic charges: the case of ABO3 compounds

Based on recent first-principles computations in perovskite compounds, especially BaTiO3, we examine the significance of the Born effective charge concept and contrast it with other atomic charge definitions, either static (Mulliken, Bader...) or dynamical (Callen, Szigeti...). It is shown that static and dynamical charges are not driven by the same underlying parameters. A unified treatment of dynamical charges in periodic solids and large clusters is proposed. The origin of the difference between static and dynamical charges is discussed in terms of local polarizability and delocalized transfers of charge: local models succeed in reproducing anomalous effective charges thanks to large atomic polarizabilities but, in ABO3 compounds, ab initio calculations favor the physical picture based upon transfer of charges. Various results concerning barium and strontium titanates are presented. The origin of anomalous Born effective charges is discussed thanks to a band-by-band decomposition which allows to identify the displacement of the Wannier center of separated bands induced by an atomic displacement. The sensitivity of the Born effective charges to microscopic and macroscopic strains is examined. Finally, we estimate the spontaneous polarization in the four phases of barium titanate.Comment: 25 pages, 6 Figures, 10 Tables, LaTe

Phonons and related properties of extended systems from density-functional perturbation theory

This article reviews the current status of lattice-dynamical calculations in crystals, using density-functional perturbation theory, with emphasis on the plane-wave pseudo-potential method. Several specialized topics are treated, including the implementation for metals, the calculation of the response to macroscopic electric fields and their relevance to long wave-length vibrations in polar materials, the response to strain deformations, and higher-order responses. The success of this methodology is demonstrated with a number of applications existing in the literature.Comment: 52 pages, 14 figures, submitted to Review of Modern Physic

Pressure dependence of static and dynamic ionicity of SiC polytypes

The ionicity of SiC polytypes and its pressure dependence is investigated by employing ab initio calculations using density-functional theory within the local-density approximation. Two approaches are used to quantify ionicity: a static one introduced by Garcia and Cohen involving a decomposition of the valence-charge density into symmetric and antisymmetric contributions with respect to the bond center and a dynamic one connected with Born effective charges. Both approaches yield increasing values of the ionicity which is in accordance with existing experimental data for 3C and 6H SiC for low hydrostatic pressure ( $\lt$= 0.2 Mbar). A decrease of ionicity at ultrahigh pressures (up to 1.2 Mbar) as quoted for 6H SiC cannot be confirmed by our calculations. We discuss possible explanations of such a discrepancy

SiC-Elektronik. Teilvorhaben: Berechnung der Energiebandstruktur von SiC-Polytypen Schlussbericht

Detailed knowledge of the electronic structure is prerequisite for the design of electronic devices. For SiC polytypes this knowledge was lacking, in particular for near-band edge states. Our project aimed to filling this gap and to provide by means of ab-initio band calculations detailed information required for the design of devices. These informations include effective masses, density of states and carrier concentrations in conduction and valence bands of SiC polytypes. (orig.)SIGLEAvailable from TIB Hannover: DtF QN1(57,51) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekBundesministerium fuer Bildung, Wissenschaft, Forschung und Technologie, Bonn (Germany)DEGerman