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Comment: 6 pages, MRS Spring 2002 proceedings
The ground-state ferroelectric distortion of PbTiO3 thin films is studied using a first-principles effective Hamiltonian to describe the structural energetics at a microscopic level. Under short-circuit electrical and zero-stress mechanical boundary conditions, (0 0 1) films as thin as one unit cell are found to support a stable perpendicular polarization. Size effects in the layer-by-layer ferroelectric distortion are discussed. The continuum limit of the first-principles effective Hamiltonian has the same form as the widely-used Landau-Ginzburg expression for the free energy near T-c, so that the present results can be directly related to those of previous phenomenological studies. In particular, a microscopic interpretation of the extrapolation length delta is proposed.
The c(2$\times$2) reconstruction of (001) PbTiO$_3$ surfaces is studied by means of first principles calculations for paraelectric (non-polar) and ferroelectric ([001] polarized) films. Analysis of the atomic displacements in the near-surface region shows how the surface modifies the antiferrodistortive (AFD) instability and its interaction with ferroelectric (FE) distortions. The effect of the surface is found to be termination dependent. The AFD instability is suppressed at the TiO$_2$ termination while it is strongly enhanced, relative to the bulk, at the PbO termination resulting in a c(2x2) surface reconstruction which is in excellent agreement with experiments. We find that, in contrast to bulk PbTiO$_3$, in-plane ferroelectricity at the PbO termination does not suppress the AFD instability. The AFD and the in-plane FE distorti...
Comment: submitted to PRB, completely revised version after referee report
Comment: 27 pages (Sections IVB, IVE rewritten; PRB April 15 II 1995 issue)
Comment: 11 pages, in RevTex, 3 uuencoded figures
The application of first-principles methods to the study of ferroelectric oxides is reviewed. While the main focus is on the perovskites, particularly the most-studied compounds BaTiO3, PbTiO3, and SrTiO3, other oxide families, including LiNbO3, layered perovskites, nitrites and nitrates, and electronic and magnetic ferroelectrics, are included. Results are presented for crystal structure, polarization and dielectric and piezoelectric coefficients. The identification of lattice instabilities through the computation of phonon dispersion relations for a high-symmetry reference phase is presented. Results at nonzero temperature, obtained through effective Hamiltonian and interatomic potential approaches, are given. Calculations for solid solutions, defects, thin films, superlattices and nanostructures are described. Challenges and prospec...
Comment: 12 pages, 2 figures, 7 tables, submitted to PRB
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