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Using a first-principles approach based on density-functional theory, we find that a large tetragonal strain can be induced in PbTiO3 by application of a negative hydrostatic pressure. The structural parameters and the dielectric and dynamical properties are found to change abruptly near a crossover pressure, displaying a "kinky" behavior suggestive of proximity to a phase transition. Analogous calculations for BaTiO3 show that the same effect is also present there, but at much higher negative pressure. We investigate this unexpected behavior of PbTiO3 and discuss an interpretation involving a phenomenological description in terms of a reduced set of relevant degrees of freedom.

Using a combination of first-principles and effective-Hamiltonian approaches, we map out the structure of BaTiO3 under epitaxial constraints applicable to growth on perovskite substrates. We obtain a phase diagram in temperature and misfit strain that is qualitatively different from that reported by Pertsev et al. [Phys. Rev. Lett. 80, 1988], who based their results on an empirical thermodynamic potential with parameters fitted at temperatures in the vicinity of the bulk phase transitions. In particular, we find a region of 'r phase' at low temperature where Pertsev et al. have reported an 'ac phase'. We expect our results to be relevant to thin epitaxial films of BaTiO3 at low temperatures and experimentally-achievable strains. PACS numbers: 77.55.+f, 77.80.Bh, 77.84.Dy, 81.05.Zx The perovskite oxide barium titanate (BaTiO 3 ) is a prototypical ferroelectric, an insulating solid whose macroscopic polarization can be reoriented by the application of an electric field [1]. In the perovskite ferroelectrics, it is well known both experimentally and theoretically that the polarization is also strongly coupled to strain [2], and thus that properties such as the ferroelectric transition temperature and polarization magnitude are quite sensitive to external stress.Experimentally, the properties of ferroelectrics in thin film form generally differ significantly from those in the bulk [3]. While many factors are expected to contribute to these differences, it has been shown that the properties of perovskite thin films are strongly influenced by the magnitude of the epitaxial strain resulting from latticematching the film to the substrate. For example, Yoneda et al.[4] used molecular-beam epitaxy (MBE) to grow BaTiO 3 (lattice constant of 4.00Å) on (001)-oriented SrTiO 3 (lattice constant of 3.91Å); they found that the ferroelectric transition temperature exceeds 600 • C, to be compared to the bulk Curie temperature of T C = 130 • C. Other studies have shown that the amount of strain in BaTiO 3 /SrTiO 3 superlattices on SrTiO 3 substrates strongly influences properties including the observed polarization, phase transition temperature, and dielectric constant [5,6,7,8].In a seminal paper, Pertsev, Zembilgotov and Tagantsev [9] introduced the concept of mapping the equilibrium structure of a ferroelectric perovskite material versus temperature and misfit strain, thus producing a "Pertsev phase diagram" (or Pertsev diagram) of the observable epitaxial phases. The effect of epitaxial strain is isolated from other aspects of thin-film geometry by computing the structure of the bulk material with homogeneous strain tensor constrained to match a given substrate with square surface symmetry [10]. In addition, short-circuit electrical boundary conditions are imposed, equivalent to ideal electrodes above and beneath the film [9]. Given the recognized importance of strain in determining the properties of thin-film ferroelectrics, Pertsev diagrams have proven to be of enormous interest to experimentalists seeking to interpret the ...

Interatomic potentials are determined in the framework of a shell model used to simulate the structural instabilities, dynamical properties, and phase transition sequence of BaTiO 3 . The model is developed from first-principles calculations by mapping the potential energy surface for various ferroelectric distortions. The parameters are obtained by performing a fit of interatomic potentials to this energy surface. Several zero-temperature properties of BaTiO 3 , which are of central importance, are correctly simulated in the framework of our model. The phase diagram as a function of temperature is obtained through constant-pressure molecular dynamics simulations, showing that the non-trivial phase transition sequence of BaTiO 3 is correctly reproduced. The lattice parameters and expansion coefficients for the different phases are in good agreement with experimental data, while the theoretically determined transition temperatures tend to be too small.

Molecular dynamics simulations of first-principles-based effective Hamiltonians for Pb(Sc{1/2}Nb{1/2})O(3) under hydrostatic pressure and for Pb(Mg{1/3}Nb{2/3})O(3) at ambient pressure show clear evidence of a relaxor state in both systems. The Burns temperature is identified as the temperature below which dynamic nanoscale polar clusters form, pinned to regions of quenched chemical short-range order. The effect of pressure in Pb(Sc{1/2}Nb{1/2})O(3) demonstrates that the stability of the relaxor state depends on a delicate balance between the energetics that stabilize normal ferroelectricity and the average strength of random local fields which promote the relaxor state.