Polar Instabilities Research Articles

Effect of pressure on the band gap and the local FeO6environment in BiFeO3

BiFeO${}_{3}$ exhibits a complex phase-transition sequence under pressure associated with changes in octahedron tilts and displacements of Bi${}^{3+}$ and Fe${}^{3+}$ cations. Here, we investigate the local structure of Fe${}^{3+}$ as a function of pressure through absorption crystal-field spectroscopy in the 0--18 GPa range. We focus on the influence of phase transitions on the Fe${}^{3+}$ off-center displacement through the energy ($E$) and oscillator strength (${f}_{d\ensuremath{-}d}$) of the ${}^{4}$T${}_{1}$ and ${}^{4}$T${}_{2}$ Fe${}^{3+}$ (3${d}^{5}$) bands observed below the band gap (${E}_{\mathrm{gap}}$ $=$ 2.49 eV) at 1.39 and 1.92 eV, respectively, at ambient conditions. Pressure induces linear redshift of both ${}^{4}$T${}_{1}$ and ${}^{4}$T${}_{2}$ bands, consistent with the compression of the FeO${}_{6}$ octahedron under pressure. On the other hand, the transition oscillator strength (${f}_{d\ensuremath{-}d}=3\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}5}$), enabled by both the exchange mechanism and the off-center Fe${}^{3+}$ distortion, slightly increases with pressure. The absence of notable anomalies in the variation of $E$($P$) and ${f}_{d\ensuremath{-}d}$($P$) through the phase sequence from the ferroelectric rhombohedral $R$3$c$ phase to the nonpolar orthorhombic Pnma phase suggests a persisting off-center position of the Fe${}^{3+}$. While this local polarity is correlated and expected in the ferroelectric $R$3$c$ phase, its presence in the high-pressure nonpolar Pnma phase indicates the presence of local polar instabilities.

Strain dependence of polarization and piezoelectric response in epitaxial BiFeO3 thin films

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 (TC) in BaTiO3. However, in multiferroic BiFeO3 thin films an unanticipated strain-driven decrease of TC 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.

Theory of prospective perovskite ferroelectrics with double rocksalt order

Using first-principles methods, we predict the energy landscape and ferroelectric states of double perovskites of the form AA$'$BB$'$O$_6$ in which the atoms on both the A and B sites are arranged in rock-salt order. While we are not aware of compounds that occur naturally in this structure, we argue that they might be realizable by directed synthesis. The high-symmetry structure formed by this arrangement belongs to the tetrahedral $F\bar{4}3m$ space group. If a ferroelectric instability occurs, the energy landscape will tend to have minima with the polarization along tetrahedral directions, leading to a rhombohedral phase, or along Cartesian directions, leading to an orthorhombic phase. We find that the latter scenario applies to CaBaTiZrO$_6$ and KCaZrNbO$_6$, which are weakly ferroelectric, and the former one applies to PbSnTiZrO$_6$, which is strongly ferroelectric. The results are modeled with a fourth- or fifth-order Landau-Devonshire expansion, providing good agreement with the first-principles calculations. Computations of zone-center soft modes are also carried out in order to characterize the polar and octahedral-rotation instabilities in more detail. Prospects for synthesis of ferroelectric materials belonging to this class are discussed.

Phase transitions and piezoelectric properties of SrBi2Ta2O9 by molecular dynamics simulations

The phase transition sequence of SrBi2Ta2O9 (SBT) and the local microscopic dynamics near the ferroelectric transition are investigated using a shell model with parameters fitted to first-principle calculations. We show that the complex interplay between polar and nonpolar instabilities leads to the presence of two phase transitions. In this way the existence of an intermediate orthorhombic paraelectric phase, characterized by the rotation of the TaO6 octahedra, is demonstrated without using any explicit experimental data as input. The local polarization dynamics does not provide any indication of a relaxation process near the ferroelectric transition. Finally, dielectric and piezoelectric coefficients along crystallographic directions are investigated.

Evidence for Room-Temperature Multiferroicity in a Compound with a Giant Axial Ratio

In the search for multiferroic materials magnetic compounds with a strongly elongated unit-cell (large axial ratio c/a) have been scrutinized intensely. However, none was hitherto proven to have a switchable polarization, an essential feature of ferroelectrics. Here, we provide evidence for the epitaxial stabilization of a monoclinic phase of BiFeO3 with a giant axial ratio (c/a=1.23) that is both ferroelectric and magnetic at room temperature. Surprisingly, and in contrast with previous theoretical predictions, the polarization does not increase dramatically with c/a. We discuss our results in terms of the competition between polar and antiferrodistortive instabilities and give perspectives for engineering multiferroic phases.

Temperature-driven phase transitions in SrBi2Ta2O9 from first-principles calculations

The phase transition sequence of SrBi2Ta2O9 is investigated using a shell model with parameters fitted to first-principles calculations. We show that the complex interplay between polar and nonpolar instabilities leads to the presence of two phase transitions, corroborating the existence of an intermediate orthorhombic paraelectric phase. This phase is characterized by the rotation of the TaO6 octahedra around the a axis. We show that this phase can also be detected from the dielectric response of the material. The present approach constitutes a powerful tool for a theoretical prediction of intermediate phases, not yet observed experimentally, in other Aurivillius compounds.

Ground state of Ca-doped strontium titanate: Ferroelectricity versus polar nanoregions

The structural properties of Ca-doped $\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_{3}$ (SCT) have been investigated by x-ray and neutron diffractions and first-principles calculations. The results of diffraction experiments on ${\mathrm{Sr}}_{0.96}{\mathrm{Ca}}_{0.04}\mathrm{Ti}{\mathrm{O}}_{3}$ show a nonferroelectric $I4∕mcm$ phase at low temperature. Density-functional calculations on the ${\mathrm{Sr}}_{1\ensuremath{-}x}{\mathrm{Ca}}_{x}\mathrm{Ti}{\mathrm{O}}_{3}$ system, which were performed in supercells with $x=0.125$ Ca concentration, show that zone center polar soft modes and antiphase antiferrodistortive modes [which correspond to the acoustic zone-boundary modes of pure strontium titanate (STO)] are strongly coupled and confirm that polar instabilities do exist in SCT in the directions perpendicular to the $c$ tetragonal axis and not along. Moreover, the energetic stabilization induced in the tilted phase by such distortions, as well as the associated polarization, is of the same order of magnitude as that in pure tilted tetragonal STO (if the ${E}_{u}$ ferroelectric mode would freeze). These results suggest that polar instabilities originating from the (weak) off-center displacements of ${\mathrm{Ca}}^{2+}$ ions $(0.08\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}})$ are not likely to directly polarize the host matrix by an electrostatic mechanism. Instead, we suggest the possible role of random fields in inducing the presence of disordered polar clusters, which is similar to polar nanoregions in relaxor materials.

Soft mode induced structural instabilities

Abstract Structural instabilities and especially those which are related to polar instabilities and ferroelectric phase transitions are investigated within a modified φ4-model. Instead of describing these types of phase transitions within a rigid ion model with anharmonic ion-ion interactions the electronic degrees of freedom are seriously taken into account and shown to renormalize the temperature dependence of the soft mode frequency. Using a self-consistent phonon approximation quantitative agreement of theoretical calculations with experimental data is achieved for the temperature dependences of soft modes and related quantities.