Piezoelectric materials, which convert mechanical to electrical energy and vice versa, are typically characterized by the intimate coexistence of two phases across a morphotropic phase boundary. Electrically switching one to the other yields large electromechanical coupling coefficients. Driven by global environmental concerns, there is currently a strong push to discover practical lead-free piezoelectrics for device engineering. Using a combination of epitaxial growth techniques in conjunction with theoretical approaches, we show the formation of a morphotropic phase boundary through epitaxial constraint in lead-free piezoelectric bismuth ferrite (BiFeO3) films. Electric field-dependent studies show that a tetragonal-like phase can be reversibly converted into a rhombohedral-like phase, accompanied by measurable displacements of the surface, making this new lead-free system of interest for probe-based data storage and actuator applications.

We calculate the effect of epitaxial strain on the structure and properties of multiferroic bismuth ferrite, BiFeO 3 , using first-principles density functional theory. We investigate epitaxial strain corresponding to an (001)-oriented substrate and find that, while small strain causes only quantitative changes in behavior from the bulk material, compressive strains of greater than 4% induce an isosymmetric phase transition accompanied by a dramatic change in structure. In striking contrast to the bulk rhombohedral perovskite, the highly strained structure has a c/a ratio of ∼1.3 and five-coordinated Fe atoms. We predict a rotation of polarization from [111] (bulk) to nearly [001], accompanied by an increase in magnitude of ∼50%, and a suppression of the magnetic ordering temperature. Our calculations indicate critical strain values at which the two phases might be expected to coexist and shed light on recent experimental observation of a morphotropic phase boundary in strained BiFeO 3 .

Recent reports on epitaxial BiFeO 3 films show that the crystal structure changes from nearly rhombohedral ("R-like") to nearly tetragonal ("T-like") at strains exceeding ≈-4.5%, with the "Tlike" structure being characterized by a highly-enhanced c/a ratio. While both the "R-like" and the "T-like" phases are monoclinic, our detailed x-ray diffraction results reveal a symmetry change from M A and M C type, respectively, at this "R-like"-to-"T-like" transition. Therefore, the ferroelectric polarization is confined to different (pseudocubic) planes in the two phases. By applying additional strain or by modifying the unit cell volume of the film by substituting Ba for Bi, the monoclinic distortion in the "T-like" M C phase is reduced, i.e. the system approaches a true tetragonal symmetry. Therefore, in going from bulk to highly-strained films, a phase sequence of rhombohedral(R)-to-monoclinic("R-like" M A )-to-monoclinic("T-like" M C )-totetragonal(T) is observed. This sequence is otherwise seen only near morphotropic phase boundaries in lead-based solid-solution perovskites (i.e. near a compositionally induced phase instability), where it can be controlled by electric field, temperature, or composition. Our results now show that this evolution can occur in a lead-free, stoichiometric material and can be induced by stress alone.3

Heterointerface stabilization of a distinct nonpolar BiFeO3 phase occurs simultaneously with changes in octahedral tilts. The resulting phase arises via suppression of polarization by a structural order parameter and can thus be identified as anti-ferroelectric (Fe displacements - bottom panel). The phase is metastable and can be switched into a polar ferroelectric state (top panel) under an applied electric bias.

We investigate the effect of epitaxial strain on [001]-oriented LaAlO3 using first-principles density functional calculations. We find a series of structural phase transitions between states characterized by different patterns of tilting of the AlO6 octahedra. By tuning the biaxial strain from compressive to tensile, we induce an evolution in the crystal structure in which the tilt axis changes from outof-plane to in-plane, corresponding to space groups I4/mcm and Imma. We also study the effect of uniaxial relaxation of the usual biaxial epitaxial constraint and explore this as a mechanism for selectively stabilizing different patterns of octahedral tilts.

Orientational anisotropies are calculated from molecular dynamics simulations of bulk water and the Na(+) and H(+) forms of hydrated Nafion and then compared with corresponding experimental values. The extended jump model of Laage and Hynes is applied to water reorientations for each system, and the anisotropies are explored as a product of hydrogen bond restricted "wobble-in-a-cone" reorientations and that due to the discrete jumps of hydrogen bond reorganization. Additionally, the timescales of hydrogen bond switching and proton transport are presented for bulk water and the H(+) form of hydrated Nafion. The short time scale of proton hopping is found to be independent of Nafion water loading, suggesting the short time dynamics of proton hopping are relatively insensitive to the level of hydration. Furthermore, the long time decay for the forward rate of hydrogen bond switching is shown to be identical to the long time decay in the forward rate of proton hopping, for bulk water and all water loadings of Nafion investigated, suggesting a unified process.

In recent years, there has been an increased interest in octahedral rotations in perovskite materials, particularly on their response to strain in epitaxial thin films. The current theoretical framework assumes that rotations are affected primarily through the change in inplane lattice parameters imposed by coherent heteroepitaxy on a substrate of different lattice constant. This model, which permits prediction of the thin-film rotational pattern using firstprinciples density functional theory, has not been tested quantitatively over a range of strain states. To assess the validity of this picture, coherent LaAlO 3 thin films were grown on SrTiO 3 , NdGaO 3 , LaSrAlO 4 , NdAlO 3 , and YAlO 3 substrates to achieve strain states ranging from +3.03% to-2.35%. The out-of-plane and in-plane octahedral rotation angles were extracted from the intensity of superlattice reflections measured using synchrotron x-ray diffraction. Density functional calculations show that no measurable change in intrinsic defect concentration should occur throughout the range of accessible strain states. Thus, the measured rotation angles were compared with those calculated previously for defect-free films. [Hatt and Spaldin, Phys. Rev. B 82, 195402 (2010)]. Good agreement between theory and experiment was found, suggesting that the current framework correctly captures the appropriate physics in LaAlO 3 .

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