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DF: Fachverband Dielektrische Festkörper

DF 3: Nonlinear dielectrics, phase transitions, relaxors

DF 3.2: Vortrag

Montag, 26. März 2012, 15:20–15:40, EB 107

Comparison of hydrostatic and chemical pressure in lead-free Na_0.5Bi_0.5TiO₃-based materials studied by first-principles methods — •Melanie Gröting¹, Silke Hayn¹, Igor Kornev², Brahim Dkhil², and Karsten Albe¹ — ¹Materialwissenschaft, TU Darmstadt, Darmstadt, Germany — ²Laboratoire SPMS, Ecole Centrale Paris, Chatenay-Malabry, France

Na_0.5Bi_0.5TiO₃-based materials show extraordinarily high strains and are thus promising environmentally friendly alternatives to the toxic lead-containing standard piezoelectric materials. Na_0.5Bi_0.5TiO₃ is considered as a model relaxor ferroelectric with a complex perovskite structure having two different cations (Na⁺ and Bi³⁺) on the A-site.

In this contribution, the influence of hydrostatic pressure and chemical substitution on ferroelectric instabilities and octahedral tilting in Na_0.5Bi_0.5TiO₃-based solid solutions is studied by first-principles methods. In pure Na_0.5Bi_0.5TiO₃ under positive pressure an orthorhombic Pbnm phase is stabilized above few GPa in good agreement with experimental data. While the polarization is "killed" leaving only tilts under positive pressure, for negative pressure the polarization survives whereas the tilts are suppressed leading to a tetragonal P4mm phase. We find that solid solutions of Na_0.5Bi_0.5TiO₃ with CaTiO₃ or BaTiO₃ show the same sequence of phase transitions as pure Na_0.5Bi_0.5TiO₃ under hydrostatic pressure and identify the origin of chemical pressure. Chemical substitution leads to a combination of both misfit and modulus effects due to differences in ionic sizes and chemical bonding properties of the different A-cations, respectively.