A microscopic study of barium titanate

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Abstract
Using Variational Density Functional Perturbation Theory, the dielectric and effective charge tensors of cubic Barium Titanate have been calculated as second derivatives of the total energy with respect to electric field and phonon perturbations. Born effective charges of Ti and O‖ are surprisingly large. Anomalous contributions reflect how orbital hybridizations are dynamically affected by atomic displacements. This has been examined in the frame of a band by band decomposition. In this description Ba appears more covalent than generally assumed. The LDA greatly overestimates the dielectric constant. This feature can be corrected with a semi-empirical “scissors” correction.