Date of Graduation

8-2012

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Physics (PhD)

Degree Level

Graduate

Department

Physics

Advisor/Mentor

Laurent Bellaiche

Committee Member

Huaxiang Fu

Second Committee Member

William F. Oliver III

Third Committee Member

Gregory J. Salamo

Fourth Committee Member

Douglas E. Spearot

Keywords

Pure sciences, Applied sciences, Barium titanate, Dielectric response, Ferroelectrics, Molecular dynamics, Perovskites, Strontium titanate

Abstract

A first-principles-based effective Hamiltonian scheme which incorporates coupling between ferroelectric (FE) and antiferrodistortive (AFD) motions is applied to Pb(Zr,Ti)O3 alloys. It validates the existence of two modes of E symmetry (rather than the single E(1TO) soft mode) in the 50-75 cm-1 range for temperatures smaller than 200 K and for compositions falling within the Rhombohedral R3c phase. Coupling between long-range-ordered FE and AFD motions is shown to be the cause of the additional mode and more insight into its nature is provided. This scheme is further used to reveal a field-induced anticrossing involving FE and AFD degrees of freedom for Ti composition of 45%.

Molecular dynamics (MD) simulations, across the morphotropic phase boundary (MPB) of disordered Pb(Zr,Ti)O3 solid solutions at 10 K, confirms the existence of similar additional modes in the the monoclinic Cc and tetragonal I4cm phases. Lifting of degeneracy of E modes in the Cc phase each giving A' + A'' modes is seen in accordance with group theory predictions. In particular, a compositional-induced anticrossing occurring within the bridging Cc state is revealed, and the difference in frequency between A' and A'' modes in the Cc state is linked to a quantity introduced here and termed the monoclinic depth. Analytical models are further developed to reproduce and better understand characteristics of these modes across the MPB.

Furthermore, a Fermi resonance (FR) emerging from the nonlinear coupling between ferroelectric motions and tiltings of oxygen octahedra is exposed. This FR manifests itself as the doubling of a nominally single FE mode in a purely FE phase, when the resonant frequency of the FE mode is close to the first overtone of the tiltings. It is shown, through the use of an analytical model (that captures the essence of the effect), that the FR is the result of a nonlinear coupling that is proportional to the spontaneous polarization of the material.

MD simulations incorporating a first-principles-based effective Hamiltonian scheme consisting of FE and strain degrees of freedom, are conducted on (Ba,Sr)TiO3 (BST) bulks and epitaxially strained SrTiO3 (STO) thick films at finite temperature. The appearance of a central mode (CM) is confirmed and splitting of soft mode (SM) into out-of-plane and in-plane modes is predicted for strained STO films and two CM's are predicted for Ba0.5Sr0.5TiO3 in FE phase. Symmetries of modes in FE phases originating from the soft-mode are discussed. Electrostriction energy is shown to be governing the Curie temperature Tc and determine the type of FE phase transition each system undergoes. The comparatively large electrostriction

energy in BST systems is also pointed to be behind the emergence of the CM in PE phase of them.

Moreover, MD simulation are performed of BST bulks and epitaxially strained STO thick films to obtain dielectric tunability &tau(E), as a function of electric field applied along the polarization. Landau-Devoshire theroy based fittings are shown to inaccurately describe &tau(E) in the low-field regime and the presence of strong CM in this regime is claimed to be the cause of this discrepancy in these systems.

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