Born Effective Charges of Barium Titanate: band by band decomposition and sensitivity to structural features

The Born effective charge tensors of Barium Titanate have been calculated for each of its 4 phases. Large effective charges of Ti and O, also predicted by shell model calculations and made plausible by a simplified model, reflect the partial covalent character of the chemical bond. A band by band decomposition confirms that orbital hybridization is not restricted to Ti and O atoms but also involves Ba which appears more covalent than generally assumed. Our calculations reveal a strong dependence of the effective charges on the atomic positions contrasting with a relative insensitivity on isotropic volume changes.Comment: 13 page

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

Cr3+ CENTERS IN THE LaMgAl11O19 LASER HOST : THEORETICAL STUDY THROUGH A MOLECULAR ORBITAL MODEL

No abstract availabl

Calcul des multiplets de spin de l'ion Cr

La luminescence de Cr3+ a été étudiée de manière extensive et la dépendance du spectre d’absorption avec la distance Cr-0 a été mise en évidence1,2’3. Dans le présent travail on a calculé les intégrales électrostatiques (a, b, c, d, e, f, g, h, i, j)7, nécessaires à la détermination des multiplets de spin. Mais du fait de la délocalisation de l’orbitale eg dans la région de l’outersphère, nous nous sommes ramenés au modèle proposé par Tanabe et Sugano1. L’influence de la distance Cr-0 (0.19 à 0.22 nm) sur la structure électronique a été étudiée, et à partir des diagrammes de termes, on a discuté les résultats expérimentaux relatifs aux grenats dopés au chrome