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In this project, a computational investigation utilizing density functional theory methods is carried out to elucidate the differences in stereochemical lone-pair activity of Pb2+ and Sn2+ A-site ions in epitaxial polar ATiO3 perovskites. The contrasting tendencies for the lead- and tin-based compounds to form different phases--Amm2 for the former versus Cm for the latter-under biaxial tension are connected to the amount of charge concentrated within the lone pair lobes. Specifically, phases are energetically more preferable when as much charge as possible is dissipated out of the lobe, thus lowering the cost of Coulomb repulsions between the lone pair and the surrounding oxygen cage. Although a strong band-gap tuning was predicted in (fictitious) SnTiO3 during the tensile P4mm → Cm phase transformation [see W. D. Parker, J. M. Rondinelli, and S. M. Nakhmanson, Phys. Rev. B 84,245126 (2011)], we find the same effect to be considerably weaker in PbTiO3. The insights gained about the electronic-level underpinnings of transitional behavior in such lone-pair-active epitaxial ferroelectrics may be used in the design of a new generation of more efficient electromechanical and electrooptical devices. [ABSTRACT FROM AUTHOR]

Motivated by the reported high dielectric response of monoclinic β-Ba2TiO4 as well as its affinity for absorbing small molecules, we investigate its structural, electronic, and vibrational properties with density functional theory (DFT). DFT-based structural optimization obtains lattice parameters and bond lengths within a few percent of experimentally observed values, with specific details depending on the choice of exchange-correlation functional. Although, for both the local density approximation (LDA) and generalized gradient approximation (GGA) functionals employed, the DFT calculations produce a wide-band-gap insulating state for β-Ba2TiO4 (with an indirect gap of 4.1 eV or greater), they do not agree on the energetic ordering of this phase with respect to its perovskite-like Ruddlesden-Popper (RP) polymorph. Simulations that utilize LDA place the β phase 0.30 eV higher, while those using the Perdew-Burke-Ernzerhof GGA functional place it 0.22 eV lower than the RP one, leaving the question of the degree of perovskite-like phase metastability under epitaxial growth conditions unresolved. Comparison of the formula unit volumes of the A2TiO4 and ATiO3 polymorphs (A = Sr, Ba) reveals that both Ba2TiCO4 structures possess much more open geometries-more so for the β than for the RP phase-than their isostoichiometric Sr-based counterparts and all of the examined ATiO3 compounds, in line with the demonstrated propensity of the Ba-based 2-1-4 oxides to capture molecules like CO2 and H20. However, an analysis of vibrations and their contributions to the static dielectric permittivity tensor of β-Ba2TiO4 indicates that, unlike the perovskite RP phase, this structure does not exhibit strong polar lattice distortions, which results in a rather low value for its average static dielectric constant. [ABSTRACT FROM AUTHOR]

With the help of first-principles-based computational techniques, we evaluate the structural, vibrational, and polar properties of a fictitious Ruddlesden-Popper-type Ba2TiO4 compound that is subjected to a range of epitaxial strains. We demonstrate that, depending on the value of the applied strain, this compound exhibits different types of incommensurate distortions, which does not happen in the similar, but better cation-size-balanced, Ca- and Sr-based structures. Furthermore, at moderate tensile strains this material develops spontaneous polarization (0.17 C/m² at 1.5% strain) that is directed along the perovskite planes. By virtue of having the same symmetry, planar incommensurate and polar distortions can interact with each other, which results in a variety of complicated ground-state structural arrangements whose properties could be fine tuned by epitaxial strain engineering. [ABSTRACT FROM AUTHOR]

Toxicity of lead and bismuth has motivated an active search for isovalent ferroelectric oxides free of these elements. Using first-principles density-functional calculations, we survey Sn(II) titanates with SnTiO3 stoichiometry to evaluate the phase stability of polar and nonpolar polymorphs: we predict a tetragonal perovskite P4mm phase with a large axial ratio (c/a = 1.134) and ferroelectric polarization (1.28 C/m²) to be the ground-state equilibrium structure. We also show that heteroepitaxial thin films of perovskite SnTiO3 promote the stereochemical lone-pair activity and simultaneously enable control over the direction of the net electric polarization and magnitude of the electronic band gap. Finally, we examine the consequence of antisite defects on the polar cation displacements by studying the substitution of Sn on Ti sites. We demonstrate that local metallic screening resulting from site substitution diminishes the magnitude of the polar distortions but does not completely quench it. Based on these calculations, we suggest that polar perovskite SnTiO3 ferroelectrics are viable thin-film alternatives to Pb- and Bi-containing oxides. [ABSTRACT FROM AUTHOR]

We evaluated metatarsal lengths from roentgenograms in a group with metatarsal stress fractures and compared them with those of a control group having no foot symptoms to determine if a short first metatarsal could be implicated as a cause for metatarsal stress fractures. By statistical analysis we showed that the lengths of the first and second metatarsals in the group with stress fractures did not differ from those in a randomly selected control group. In addition, we determined that to be "short," the first metatarsal must be less than 73% of the length of the second metatarsal, and to be "long," the first metatarsal must be more than 94% of the length of the second metatarsal. [ABSTRACT FROM PUBLISHER]