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Register a new userOctahedral tilting and ferroelectricity in RbANb2O7 (A = Bi, Nd) from first principles
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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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The effect of the octahedral tilting of ASnO3 (A = Ca, Sr, Ba) parent compound and bi-color ASnO3/BSnO3 superlattice (A, B = Ca, Sr, Ba) was predicted from density-functional theory. In the ASnO3 parent compound, the structural phase transition as a function of the A-site cation size was correlated with the magnitude of the two octahedral tilting modes (a-a-c0 tilting and a0a0c+ tilting). The magnitude of the octahedral tilting modes in the superlattices was analyzed quantitatively and found to be associated with that of the constituent parent materials. The ASnO3/BSnO3 superlattices showed hybrid improper ferroelectricity resulting from the coupling of two octahedral tilting modes (a-a-c0 tilting and a0a0c+ tilting), which are also responsible for the structural phase transition from a tetragonal to orthorhombic phase. The ferroelectricity due to A-site mirror symmetry breaking is a secondary order parameter for an orthorhombic phase transition in the bi-color superlattice and is related to the {Gamma}5- symmetry mode. The coupling between the tilting modes and ferroelectric mode in the bi-color superlattice of ASnO3/BSnO3 was analyzed by group theory and symmetry mode analysis.
The octahedral tilting and ferroelectric-like structural transition of LiOsO3 metallic perovskite [Nature Materials 12, 1024 (2013)] was examined using first-principles density-functional theory. In LiOsO3, a-a-a- octahedral titling mode is responsible for the cubic to rhombohedral structural transition, which is stable phase at room temperature. At low temperatures, a non-centrosymmetric transition to a rhombohedra phase was realized due to zone center phonon softening. The phase transition behavior of LiOsO3 can be explained fully by density functional calculations and phonon calculations. The electronic structure and Fermi surface changes due to the electron lattice coupling effect are also presented. The carrier density of state across the phase transition is associated with the resistivity, heat capacity, and susceptibility.
We report the relationship between epitaxial strain and the crystallographic orientation of the in-phase rotation axis and A-site displacements in Pbnm-type perovskite films. Synchrotron diffraction measurements of EuFeO3 films under strain states ranging from 2% compressive to 0.9% tensile on cubic or rhombohedral substrates exhibit a combination of a-a+c- and a+a-c- rotational patterns. We compare the EuFeO3 behavior with previously reported experimental and theoretical work on strained Pbnm-type films on non-orthorhombic substrates, as well as additional measurements from LaGaO3, LaFeO3, and Eu0.7Sr0.3MnO3 films on SrTiO3. Compiling the results from various material systems reveals a general strain dependence in which compressive strain strongly favors a-a+c- and a+a-c- rotation patterns and tensile strain weakly favors a-a-c+ structures. In contrast, EuFeO3 films grown on Pbnm-type GdScO3 under 2.3% tensile strain take on a uniform a-a+c- rotation pattern imprinted from the substrate, despite strain energy considerations that favor the a-a-c+ pattern. These results point to the use of substrate imprinting as a more robust route than strain for tuning the crystallographic orientations of the octahedral rotations and A-site displacements needed to realize rotation-induced hybrid improper ferroelectricity in oxide heterostructures.
Titanates with the perovskite structure, including ferroelectrics (e.g., BaTiO$_3$) and ferromagnetic ones (e.g., YTiO$_3$), are important functional materials. Recent theoretical studies predicted multiferroic states in strained EuTiO$_3$ and titanate superlattices, the former of which has already been experimental confirmed. Here, a first-principles calculation is performed to investigate the structural, magnetic, and electronic properties of Y half-substituted LaTiO3. Our results reveal that the magnetism of Y$_{0.5}$La$_{0.5}$TiO$_3$ sensitively depends on its structural details because of the inherent phase competition. The lowest energy state is the ferromagnetic state, resulting in 0.25 $mu_{rm B}$/Ti. Furthermore, some configurations of Y$_{0.5}$La$_{0.5}$TiO$_3$ exhibit hybrid improper polarizations, which can be significantly affected by magnetism, resulting in the multiferroic properties. Because of the quenching disorder of substitution, the real Y$_{0.5}$La$_{0.5}$TiO3 material with random A-site ions may exhibit interesting relaxor behaviors.
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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