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Register a new userCooperative Couplings between Octahedral Rotations and Ferroelectricity in Perovskites
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The structure of ABO3 perovskites is dominated by two types of unstable modes, namely, the oxygen octahedral rotation (AFD) and ferroelectric (FE) mode. It is generally believed that such AFD and FE modes tend to compete and suppress each other. Here we use first-principles methods to show that a dual nature of the AFD-FE coupling, which turns from competitive to cooperative as the AFD mode strengthens, occurs in numerous perovskite oxides. We provide a unified model of such a dual interaction by introducing novel high-order coupling terms, and explain the atomistic origin of the resulting new form of ferroelectricity in terms of universal steric mechanisms. We also predict that such a novel form of ferroelectricity leads to atypical behaviors, such as an enhancement of the electric polarization under hydrostatic pressure.
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We model short-period superlattices of WO$_3$ and ReO$_3$ with first-principles calculations. In fully-relaxed superlattices, we observe that octahedral tilts about an axis in the planes of the superlattices do not propagate from one material, despite the presence of the corner-shared oxygen atoms. However, we find that octahedral rotation is enhanced within WO$_3$ layers in cases in which strain couples with native antiferroelectric displacements of tungsten within their octahedral cages. Resulting structures remain antiferroelectric with low net global polarization. Thermodynamic analysis reveals that superlattices with sufficiently thick ReO$_3$ layers, the absolute number being three or more layers and the Re fraction $geq 50%$, tend to be more stable than the separated material phases and also show enhanced octahedral rotations in the WO$_3$ layers.
We analyze the hypothetical link between octahedral straightening and increased conductivity inside the domain walls of BiFeO3. Our calculations for 109 degree walls predict a lattice parameter expansion of c.a. 1 percent in the direction perpendicular to the wall, and an associated straightening of the octahedral rotation angle of 4 degrees, which is comparable to that observed in the high temperature metallic phase of BiFeO3. On the other hand, in the closely related family of rare-earth orthoferrites, straighter octahedra do not correlate with increased bandgap, which suggests that the correlation between octahedral straightening and bandgap reduction in BiFeO3 is perhaps fortuitous and not necessarily the cause of increased conductivity at the walls.
Epitaxial strain is a proven route to enhancing the properties of complex oxides, however, the details of how the atomic structure accommodates strain are poorly understood due to the difficulty of measuring the oxygen positions in thin films. We present a general methodology for determining the atomic structure of strained oxide films via x-ray diffraction, which we demonstrate using LaNiO3 films. The oxygen octahedral rotations and distortions have been quantified by comparing the intensities of half-order Bragg peaks, arising from the two unit cell periodicity of the octahedral rotations, with the calculated structure factor. Combining ab initio density functional calculations with these experimental results, we determine systematically how strain modifies the atomic structure of this functional oxide.
The rotation of octahedra (octahedral tilting) is common in ABO3 perovskites and relevant to many physical phenomena, ranging from electronic and magnetic properties, metal-insulator transitions to improper ferroelectricity. Hydrostatic pressure is an efficient way to tune and control octahedral tiltings. However, the pressure behavior of such tiltings can dramatically differ from one material to another, with the origins of such differences remaining controversial. In this work, we discover several new mechanisms and formulate a set of simple rules that allow to understand how pressure affects oxygen octahedral tiltings, via the use and analysis of first-principles results for a variety of compounds. Besides the known A-O interactions, we reveal that the interactions between specific B-ions and oxygen ions contribute to the tilting instability. We explain the previously reported trend that the derivative of the oxygen octahedral tilting with respect to pressure (dR/dP) usually decreases with both the tolerance factor and the ionization state of the A-ion, by illustrating the key role of A-O interactions and their change under pressure. Furthermore, three new mechanisms/rules are discovered. We further predict that the polarization associated with the so-called hybrid improper ferroelectricity could be manipulated by hydrostatic pressure, by indirectly controlling the amplitude of octahedral rotations.
The effects of octahedral tilting of RbANb2O7 (A = Bi, Nd) compounds was studied using density-functional theory. In this compound, the structural phase transition was correlated with two octahedral tilting modes (a-a-c0 tilting and a0a0c+ tilting), and magnitude of the octahedral tilting mode was analyzed in the optimized structure. The theoretical results correlated well with the recent experimental results on the ferroelectricity of RbBiNb2O7. The hybrid improper ferroelectricity resulting from the coupling of two octahedral tilting modes and off center displacement mode was analyzed by group theory and symmetry mode analysis. The detailed relationship of the tilting modes to the structural phase transition and the detailed physical properties of ferroelectricity are also presented.
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