No abstract

AFe 1/2 B 1/2 O3(A-Ba, Sr, Ca; B-Nb, Ta, Sb) ceramics were synthesized and temperature dependencies of the dielectric permittivity were measured at different frequencies. The experimental data obtained show very high values of the dielectric permittivity in a wide temperature interval that is inherent to so-called high-k materials. The analyses of these data establish a Maxwell-Wagner mechanism as a main source for the phenomenon observed.

Perovskite oxides form a fascinating class of materials because they possess many active degrees of freedom that result in a large variety of physical effects. One important structural parameter controlling the behavior of perovskites is the tilting of the oxygen octahedral. Among other properties, this tilting is coupled with the electric and magnetic orders, which leads to novel and potentially useful phenomena; recent examples include new mechanisms for improper and triggered ferroelectricity, rich phase diagrams, and novel chiral phases, counter‐intuitive behaviors of ferroelectric and multiferroic films, and weak ferromagnetism in otherwise antiferromagnetic materials. Interestingly, most perovskites present the same tilted structures, which are few in number and fairly simple. In contrast, here we use different theoretical methods to show that a complete new family of stable phases, all displaying complex and nano‐twinned tilting patterns (as well as other anomalous properties), exists in multiferroic BiFeO3 and related compounds.

Lattice dynamics for five ordered PbMg 1/3 Nb 2/3 O3 supercells were calculated from first principles by the frozen phonon method. Maximal symmetries of all supercells are reduced by structural instabilities. Lattice modes corresponding to these instabilities, equilibrium ionic positions, and infrared reflectivity spectra were computed for all supercells. Results are compared with our experimental data for a chemically disordered PMN single crystal.states (GS) with symmetries that are lower than those dictated by chemical ordering]. ( 2) to compute the lattice dynamics for the same set of ordered PMN supercells, and to compare the results with experimental data, e.g. by comparing simulated IR reflectivity spectra with the experimental one. The goal is to find which ordered supercell most closely approximates the experimental case of local 1:1 order.

Two tilted triclinic phases were found via synchrotron X-ray diffractions in the mixedphase regions of highly strained BiFeO 3 films. First-principles calculations suggest that these two triclinic phases originate from a phase separation of a single monoclinic state accompanied by elastic matching between the two phase-separated states and further reveal that the ease of phase transition between these two energetically close phases is responsible for the large piezoelectric responses observed in Zhang et al., Nat Nano 6, 98 (2011).3

Epitaxial strain has recently emerged as a powerful means to engineer the properties of ferroelectric thin films, for instance to enhance the ferroelectric Curie temperature (T(C)) in BaTiO(3). However, in multiferroic BiFeO(3) thin films an unanticipated strain-driven decrease of T(C) was reported and ascribed to the peculiar competition between polar and antiferrodistortive instabilities. Here, we report a systematic characterization of the room-temperature ferroelectric and piezoelectric properties for strain levels ranging between -2.5% and +1%. We find that polarization and the piezoelectric coefficient increase by about 20% and 250%, respectively, in this strain range. These trends are well reproduced by first-principles-based techniques.

A first-principles-based approach is used to show (i) that stress-free ferroelectric nanodots under open-circuit-like electrical boundary conditions maintain a vortex structure for their local dipoles when subject to a transverse inhomogeneous static electric field, and, more importantly, (ii) that such a field leads to the solution of a fundamental and technological challenge: namely, the efficient control of the direction of the macroscopic toroidal moment. The effects responsible for such striking features are revealed and discussed.

A first-principles-based effective Hamiltonian is used to investigate low-temperature properties of Ba(Zr,Ti)O(3) relaxor ferroelectrics under an increasing dc electric field. This system progressively develops an electric polarization that is highly nonlinear with the dc field. This development leads to a maximum of the static dielectric response at a critical field, E(th), and involves four different field regimes. Each of these regimes is associated with its own behavior of polar nanoregions, such as shrinking, flipping, and elongation of dipoles or change in morphology. The clusters propagating inside the whole sample, with dipoles being parallel to the field direction, begin to form at precisely the E(th) critical field. Such a result, and further analysis we perform, therefore, reveal that field-induced percolation of polar nanoregions is the driving mechanism for the transition from the relaxor to ferroelectric state.