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DF: Dielektrische Festkörper

DF 2: Elektrische und optische Eigenschaften II

DF 2.7: Vortrag

Montag, 24. März 2003, 16:50–17:10, HSZ/403

Lattice dynamics in LiNbO₃ and LiTaO₃ — •Andrei Postnikov¹, Vasile Caciuc^1,2, Patrice Bourson³, and Marc Fontana³ — ¹Universität Osnabrück – Fachbereich Physik, 49069 Osnabrück — ²Institut für Festkörperforschung – Forschungszentrum Jülich, 52425 Jülich — ³MOPS-CLOES, Supélec, 2 rue E.Belin, F-57070 Metz Cedex, France

Zone-center phonons in ferroelectric lithium niobate are discussed in comparison with lithium tantalate. Despite the similarities in crystalline and electronic structure of these materials, the shifts in the positions of their Raman lines cannot be trivially related to differences in Nb/Ta masses. Instead, they can be traced back to different compositions of (some) eigenvectors, estimated from first-principles total-energy calculations. For stoichiometric substances, frozen-phonon calculations done with the full-potential linearized augmented plane wave method (WIEN97) help to obtain the eigenvectors of the transversal optical E-modes in LiTaO₃, to be compared with earlier obtained results on LiNbO₃. For LiNbO₃, the use of numerical orbital tight-binding method (SIESTA) allows to try enlarged unit cells and address the issues of phonon dispersion, intrinsic defects (almost unavoidable in this material), and anharmonicity.