Rotation Of Oxygen Octahedra Research Articles

Doping-driven structural distortion in the bilayer iridate (Sr1−xLax)3Ir2O7

Neutron single crystal diffraction and rotational anisotropy optical second harmonic generation data are presented resolving the nature of the structural distortion realized in electron-doped (Sr$_{1-x}$La$_x$)$_3$Ir$_2$O$_7$ with $x=0.035$ and $x=0.071$. Once electrons are introduced into the bilayer spin-orbit assisted Mott insulator Sr$_3$Ir$_2$O$_7$, previous studies have identified the appearance of a low temperature structural distortion and have suggested the presence of a competing electronic instability in the phase diagram of this material. Our measurements resolve a lowering of the structural symmetry from monoclinic $C2/c$ to monoclinic $P2_1/c$ and the creation of two unique Ir sites within the chemical unit cell as the lattice distorts below a critical temperature $T_S$. Details regarding the modifications to oxygen octahedral rotations and tilting through the transition are discussed as well as the evolution of the low temperature distorted lattice as a function of carrier substitution.

Learning from data to design functional materials without inversion symmetry

Accelerating the search for functional materials is a challenging problem. Here we develop an informatics-guided ab initio approach to accelerate the design and discovery of noncentrosymmetric materials. The workflow integrates group theory, informatics and density-functional theory to uncover design guidelines for predicting noncentrosymmetric compounds, which we apply to layered Ruddlesden-Popper oxides. Group theory identifies how configurations of oxygen octahedral rotation patterns, ordered cation arrangements and their interplay break inversion symmetry, while informatics tools learn from available data to select candidate compositions that fulfil the group-theoretical postulates. Our key outcome is the identification of 242 compositions after screening ∼3,200 that show potential for noncentrosymmetric structures, a 25-fold increase in the projected number of known noncentrosymmetric Ruddlesden-Popper oxides. We validate our predictions for 19 compounds using phonon calculations, among which 17 have noncentrosymmetric ground states including two potential multiferroics. Our approach enables rational design of materials with targeted crystal symmetries and functionalities.

Locked octahedral tilting in orthorhombic perovskites: At the boundary of the general rule predicting phase transitions

Mainly ruled by oxygen octahedral rotations, perovskite oxides can exhibit zone boundary transitions (ZBTs) either with $d{T}_{c}/dP>\phantom{\rule{0.16em}{0ex}}0$ or $d{T}_{c}/dP<\phantom{\rule{0.16em}{0ex}}0$. Synchrotron structural investigations at high pressure conditions place $\mathrm{YA}{\mathrm{l}}_{0.25}\mathrm{C}{\mathrm{r}}_{0.75}{\mathrm{O}}_{3}$ orthorhombic perovskite at the boundary of ZBTs. The absence of changes in the octahedral tilting and a volume reduction with pressure exclusively controlled by an isotropic polyhedral compression set $\mathrm{YA}{\mathrm{l}}_{0.25}\mathrm{C}{\mathrm{r}}_{0.75}{\mathrm{O}}_{3}$ as the first finding of a possible asymptote at the Clapeyron relation for predicting ZBTs in perovskites. Furthermore, the discovery of a ``locked-tilt perovskite'' can pave the way to a new class of functional materials.

Impact of interfacial coupling of oxygen octahedra on ferromagnetic order in La0.7Sr0.3MnO3/SrTiO3 heterostructures

La0.7Sr0.3MnO3, a half-metallic ferromagnet with full spin polarization, is generally used as a standard spin injector in heterostructures. However, the magnetism of La0.7Sr0.3MnO3 is strongly modified near interfaces, which was addressed as “dead-layer” phenomenon whose origin is still controversial. Here, both magnetic and structural properties of La0.7Sr0.3MnO3/SrTiO3 heterostructures were investigated, with emphasis on the quantitative analysis of oxygen octahedral rotation (OOR) across interfaces using annular-bright-field imaging. OOR was found to be significantly altered near interface for both La0.7Sr0.3MnO3 and SrTiO3, as linked to the magnetism deterioration. Especially in La0.7Sr0.3MnO3/SrTiO3 superlattices, the almost complete suppression of OOR in 4 unit-cell-thick La0.7Sr0.3MnO3 results in a canted ferromagnetism. Detailed comparisons between strain and OOR relaxation and especially the observation of an unexpected La0.7Sr0.3MnO3 lattice c expansion near interfaces, prove the relevance of OOR for the magnetic properties. These results indicate the capability of tuning the magnetism by engineering OOR at the atomic scale.

Competing Structural Instabilities in the Ruddlesden–Popper Derivatives HRTiO4 (R = Rare Earths): Oxygen Octahedral Rotations Inducing Noncentrosymmetricity and Layer Sliding Retaining Centrosymmetricity

We report the observation of noncentrosymmetricity in the family of HRTiO4 (R = Eu, Gd, Dy) layered oxides possessing a Ruddlesden–Popper derivative structure, by second harmonic generation and synchrotron X-ray diffraction with the support of density functional theory calculations. These oxides were previously thought to possess inversion symmetry. Here, inversion symmetry is lifted by rotations of the oxygen-coordinated octahedra, a mechanism that is not active in simple perovskites. We observe a competition between rotations of the oxygen octahedra and sliding of a combined unit of perovskite–rocksalt–perovskite blocks at the proton layers. For the smaller rare earth ions, R = Eu, Gd, and Dy, which favor the octahedral rotations, noncentrosymmetricity is present but the sliding is absent. For the larger rare earth ions, R = Nd and Sm, the octahedral rotations are absent, but the sliding at the proton layers is present to optimize the length and direction of hydrogen bonding in the crystal structure. Th...

Concurrent magnetic and structural reconstructions at the interface of (111)-oriented La0.7Sr0.3MnO3/LaFeO3

We observe an induced switchable magnetic moment of $1.6\ifmmode\pm\else\textpm\fi{}0.40\phantom{\rule{0.16em}{0ex}}{\ensuremath{\mu}}_{\mathrm{B}}/\mathrm{Fe}$ for the nominally antiferromagnetic $\mathrm{LaFe}{\mathrm{O}}_{3}$ extending two to four interface layers into the non--charge transfer system $\mathrm{L}{\mathrm{a}}_{0.7}\mathrm{S}{\mathrm{r}}_{0.3}\mathrm{Mn}{\mathrm{O}}_{3}\text{/}\mathrm{LaFe}{\mathrm{O}}_{3}\text{/}\mathrm{SrTi}{\mathrm{O}}_{3}(111)$. Simultaneously a mismatch of oxygen octahedra rotations at the interface implies an atomic reconstruction of reduced symmetry at the interface, reaching two to five layers into $\mathrm{LaFe}{\mathrm{O}}_{3}$. Density functional theory of a structure with atomic reconstruction and different correlation strength shows a ferrimagnetic state with a net Fe moment at the interface. Together these results suggest that engineered oxygen octahedra rotations, affecting the local symmetry, affect electron correlations and can be used to promote magnetic properties.

Polarization twist in perovskite ferrielectrics.

Because the functions of polar materials are governed primarily by their polarization response to external stimuli, the majority of studies have focused on controlling polar lattice distortions. In some perovskite oxides, polar distortions coexist with nonpolar tilts and rotations of oxygen octahedra. The interplay between nonpolar and polar instabilities appears to play a crucial role, raising the question of how to design materials by exploiting their coupling. Here, we introduce the concept of ‘polarization twist’, which offers enhanced control over piezoelectric responses in polar materials. Our experimental and theoretical studies provide direct evidence that a ferrielectric perovskite exhibits a large piezoelectric response because of extended polar distortion, accompanied by nonpolar octahedral rotations, as if twisted polarization relaxes under electric fields. The concept underlying the polarization twist opens new possibilities for developing alternative materials in bulk and thin-film forms.

Structural and magnetic field effects on spin fluctuations inSr3Ru2O7

We investigate the evolution of magnetic excitations in Sr$_3$Ru$_2$O$_7$ using a three band tight binding model that takes into account the influence of Mn and Ti dopant ions. The effect of dopant ions on the Sr$_3$Ru$_2$O$_7$ band structure has been included by taking into account the dopant induced suppression of the oxygen octahedral rotation in the tight binding band structure and changes in electron occupation. We find that the low energy spin fluctuations are dominated by three wave vectors around Q=$((0,0),(\pi/2,\pi/2))$, and $(\pi,0)$ which compete with each other. As the octahedral rotation is suppressed with increasing doping, the three wave vectors evolve differently. In particular, the undoped compound has dominant wavevectors at Q=$((0,0),(\pi/2,\pi/2))$, but doping Sr$_3$Ru$_2$O$_7$ leads to a significant enhancement in the spin susceptibility at the Q=$(\pi,0)$ wavevector bringing the system closer to a magnetic instability. All the features calculated from our model are in agreement with neutron scattering experiments. We have also studied the effect of a c-axis Zeeman field on the low energy spin fluctuations. We find that an increasing magnetic field suppresses the AFM fluctuations and leads to stronger competition between the AFM and FM spin fluctuations. The magnetic field dependence observed in our calculations therefore supports the scenario that the observed nematic phase in the metamagnetic region in Sr$_3$Ru$_2$O$_7$ is intimately related to the presence of a competing ferromagnetic instability.

Interfacially engineered oxygen octahedral rotations and their impact on strain relief in coherently grown SrRuO3 films

We report synchrotron x-ray diffraction investigations of interfacially engineered oxygen octahedral rotations and their impact on strain relief in perovskite $\mathrm{SrRu}{\mathrm{O}}_{3}$ films. We show that octahedral rotations with distinct patterns and magnitudes can be accommodated into coherently grown films. The $\mathrm{SrRu}{\mathrm{O}}_{3}$ film grown directly on the $\mathrm{GdSc}{\mathrm{O}}_{3}$ substrate has the $\mathrm{Ru}{\mathrm{O}}_{6}$ octahedral rotation with the ${a}^{\ensuremath{-}}{b}^{+}{c}^{\ensuremath{-}}$ pattern in the Glazer notation and the rotation angles of ${\ensuremath{\alpha}}_{\mathrm{rot}}=6.6\ifmmode\pm\else\textpm\fi{}0.{2}^{\ensuremath{\circ}}, {\ensuremath{\beta}}_{\mathrm{rot}}=5.5\ifmmode\pm\else\textpm\fi{}0.{2}^{\ensuremath{\circ}}$, and ${\ensuremath{\gamma}}_{\mathrm{rot}}=3.6\ifmmode\pm\else\textpm\fi{}0.{2}^{\ensuremath{\circ}}$. On the other hand, when a 1-nm-thick $\mathrm{BaTi}{\mathrm{O}}_{3}$ layer without $\mathrm{Ti}{\mathrm{O}}_{6}$ rotations is inserted between the $\mathrm{SrRu}{\mathrm{O}}_{3}$ and $\mathrm{GdSc}{\mathrm{O}}_{3}$, the $\mathrm{SrRu}{\mathrm{O}}_{3}$ has the $\mathrm{Ru}{\mathrm{O}}_{6}$ rotation with ${a}^{\ensuremath{-}}{b}^{0}{c}^{+}$, and ${\ensuremath{\alpha}}_{\mathrm{rot}}=5.6\ifmmode\pm\else\textpm\fi{}0.{8}^{\ensuremath{\circ}}$ and ${\ensuremath{\gamma}}_{\mathrm{rot}}=3.6\ifmmode\pm\else\textpm\fi{}0.{8}^{\ensuremath{\circ}}$. These results indicate that there are some degrees of freedom in the octahedral rotations accommodated in $\mathrm{SrRu}{\mathrm{O}}_{3}$ depending on the interface structure and that the ${\ensuremath{\gamma}}_{\mathrm{rot}}$ rotations play the important roles in the film's structural properties when the rotation about the ${[010]}_{\mathrm{pc}}$ axis is blocked. We also found that the strain relief in the film is influenced by the interfacially engineered octahedral rotations. The interfacial $\mathrm{BaTi}{\mathrm{O}}_{3}$ layer results in the in-plane periodic lattice modulation in the t-SRO film, allowing for the anisotropic relief of the substrate-induced strain. The results highlight the importance of the interface structure as a factor, determining not only octahedral rotations in coherently grown SRO films but also the strain reliefs in them.

Controlling Octahedral Rotations in a Perovskite via Strain Doping.

The perovskite unit cell is the fundamental building block of many functional materials. The manipulation of this crystal structure is known to be of central importance to controlling many technologically promising phenomena related to superconductivity, multiferroicity, mangetoresistivity, and photovoltaics. The broad range of properties that this structure can exhibit is in part due to the centrally coordinated octahedra bond flexibility, which allows for a multitude of distortions from the ideal highly symmetric structure. However, continuous and fine manipulation of these distortions has never been possible. Here, we show that controlled insertion of He atoms into an epitaxial perovskite film can be used to finely tune the lattice symmetry by modifying the local distortions, i.e., octahedral bonding angle and length. Orthorhombic SrRuO3 films coherently grown on SrTiO3 substrates are used as a model system. Implanted He atoms are confirmed to induce out-of-plane strain, which provides the ability to controllably shift the bulk-like orthorhombically distorted phase to a tetragonal structure by shifting the oxygen octahedra rotation pattern. These results demonstrate that He implantation offers an entirely new pathway to strain engineering of perovskite-based complex oxide thin films, useful for creating new functionalities or properties in perovskite materials.

Controlled lateral anisotropy in correlated manganite heterostructures by interface-engineered oxygen octahedral coupling.

Controlled in-plane rotation of the magnetic easy axis in manganite heterostructures by tailoring the interface oxygen network could allow the development of correlated oxide-based magnetic tunnelling junctions with non-collinear magnetization, with possible practical applications as miniaturized high-switching-speed magnetic random access memory (MRAM) devices. Here, we demonstrate how to manipulate magnetic and electronic anisotropic properties in manganite heterostructures by engineering the oxygen network on the unit-cell level. The strong oxygen octahedral coupling is found to transfer the octahedral rotation, present in the NdGaO3 (NGO) substrate, to the La2/3Sr1/3MnO3 (LSMO) film in the interface region. This causes an unexpected realignment of the magnetic easy axis along the short axis of the LSMO unit cell as well as the presence of a giant anisotropic transport in these ultrathin LSMO films. As a result we possess control of the lateral magnetic and electronic anisotropies by atomic-scale design of the oxygen octahedral rotation.

Emergent Noncentrosymmetry and Piezoelectricity Driven by Oxygen Octahedral Rotations in n = 2 Dion–Jacobson Phase Layer Perovskites

The loss of centrosymmetry via oxygen octahedral rotations is demonstrated in the n = 2 Dion–Jacobson family of layered oxide perovskites, A′LaB2O7 (A′ = Rb, Cs; B = Nb, Ta). Ab initio density functional theory calculations predict that all four materials should adopt polar space groups, in contrast to the results of previous experimental studies that have assigned these materials to the centrosymmetric P4/mmm space group. Optical second harmonic generation experiments confirm the presence of a noncentrosymmetric phase at ambient temperature. Piezoresponse force microscopy experiments also show that this phase is piezoelectric. To elucidate the symmetry‐breaking and assign the appropriate space groups, the crystal structure of CsLaNb2O7 is refined as a function of temperature from synchrotron X‐ray diffraction data. Above 550 K, CsLaNb2O7 adopts the previously determined centrosymmetric P4/mmm space group. Between 550 and 350 K, the symmetry is lowered to the noncentrosymmetric space group Amm2. Below 350 K, additional symmetry lowering is observed as peak splitting, but the space group cannot be unambiguously identified.

Origin of the magnetization and compensation temperature in rare-earth orthoferrites and orthochromates

We consider orthoferrite and orthochromite perovskites with the formula $\mathit{RM}{\mathrm{O}}_{3}$, where $R$ is a lanthanide and $M$ is Fe or Cr. We identify an atomistic interaction that couples the magnetic moments of the $R$ and $M$ cations with the oxygen octahedral rotations characterizing these crystals. We show that this interaction results in an effective magnetic field acting on the $R$ atoms, which in turn explains several intriguing features of these compounds, such as the existence (or absence) of a net magnetization associated with the $R$ sublattice and its crystallographic direction. Further, considered together with the couplings described by L. Bellaiche et al. [J. Phys.: Condens. Matter 24, 312201 (2012)], this interaction explains why some $\mathit{RM}{\mathrm{O}}_{3}$ compounds display a compensation temperature at which the magnetization cancels and changes sign, while others do not.

Ferroelectric control of metal–insulator transition

We propose a method of controlling the metal-insulator transition of one perovskite material at its interface with a another ferroelectric material based on first principle calculations. The operating principle is that the rotation of oxygen octahedra tuned by the ferroelectric polarization can modulate the superexchange interaction in this perovskite. We designed a tri-color superlattice of (BiFeO$_3$)$_N$/LaNiO$_3$/LaTiO$_3$, in which the BiFeO$_3$ layers are ferroelectric, the LaNiO$_3$ layer is the layer of which the electronic structure is to be tuned, and LaTiO$_3$ layer is inserted to enhance the inversion asymmetry. By reversing the ferroelectric polarization in this structure, there is a metal-insulator transition of the LaNiO$_3$ layer because of the changes of crystal field splitting of the Ni $e_g$ orbitals and the bandwidth of the Ni in-plane $e_g$ orbital. It is highly expected that a metal-transition can be realized by designing the structures at the interfaces for more materials.

A glimpse at topological structures in multiferroic materials

A glimpse at topological structures in multiferroic materials

Electrostatic tuning of oxygen octahedral rotations in ferroelectric perovskites

Using first-principles methods, we investigate the joint effects of epitaxial strain and electrostatic coupling on the appearance of oxygen octahedral rotations, O6, in ferroelectric perovskites. We find that by varying the relative content of ferroelectric and paraelectric layers in epitaxially strained superlattices the size of O6 can be efficiently tuned.

Interplay of coupling between strain and rotation in ferroelectric SrZrO3/SrTiO3 superlattices

The combination of oxygen octahedral rotation and epitaxial strain provides a unique opportunity to tune the ferroelectric properties of perovskite superlattices. Here, through first-principles calculations, we demonstrate that the oxygen octahedral rotation predominates the ground state and ferroelectric properties of SrZrO3/SrTiO3 superlattices. The predicted ground state combines the ferroelectric distortion and antiferrodistortive modes simultaneously. The structure–strain phase diagrams of the superlattices are calculated with and without octahedral rotations, which elucidate the interplay of coupling between epitaxial strain and octahedral rotation. It is found that the presence of octahedral rotation not only lowers the energy but also changes the sequence of phase transition from c–r–aa to c–r, in which the coupling of rotation and strain induces an out-of-plane polarization that transforms aa-phase into r-phase.

A Crystal-Chemical Framework for Relaxor versus Normal Ferroelectric Behavior in Tetragonal Tungsten Bronzes

Tetragonal tungsten bronzes (TTBs), an important class of oxides known to exhibit ferroelectricity, undergo complex distortions, including rotations of oxygen octahedra, which give rise to either incommensurately or commensurately modulated superstructures. Many TTBs display broad, frequency-dependent relaxor dielectric behavior rather than sharper frequency-independent normal ferroelectric anomalies, but the exact reasons that favor a particular type of dielectric response for a given composition remain unclear. In this contribution the influence of incommensurate/commensurate displacive modulations on the onset of relaxor/ferroelectric behavior in TTBs is assessed in the context of basic crystal-chemical factors, such as positional disorder, ionic radii and polarizabilities, and point defects. We present a predictive crystal-chemical model that rationalizes composition–structure–properties relations for a broad range of TTB systems.

Residual tensile stress in robust insulating rhombohedral Bi1−xLaxFe1−yTiyO3 multiferroic ceramics and its ability to pin ferroelectric polarization switching

Robust insulating rhombohedral Bi1−xLaxFe1−yTiyO3 multiferroic ceramics with 0.02 ≤ x ≤ 0.12 and 0.01 ≤ y ≤ 0.08 are prepared by a refined solid-state reaction electroceramic processing. Residual internal tensile stresses existed in the ceramics according to unit cell volume enlargement observed by X-ray diffraction and frequency redshifts of Raman modes related to Bi motion and oxygen octahedral rotation detected by Raman scattering measurements. Residual internal tensile stresses in the ceramics are believed to originate from structural phase transitions through an intermediate paraelectric rhombohedral phase with a negative thermal expansion coefficient in the transformation from paraelectric cubic to ferroelectric rhombohedral phases. All of the rhombohedral Bi1−xLaxFe1−yTiyO3 ceramics exhibited a pinched polarization versus electric field hysteresis loop indicative of ferroelectric subswitching. We argue that the residual internal tensile stresses are responsible for such ferroelectric polarization subswitching behavior in the Bi1−xLaxFe1−yTiyO3 ceramics.

Characterization of tetragonal phases of SrRuO3 under epitaxial strain by density functional theory

Using density functional theory calculations we study the effect of strain on the magnetic and structural properties of tetragonal SrRuO3. All four different oxygen octahedra rotation patterns that are in agreement with tetragonal symmetry are considered and compared with the bulk-like orthorhombic structure. We find that among the tetragonal structures the phase with I4 /mcm symmetry is energetically most favorable. This structure is also lower in energy than the orthorhombic phase for compressive and tensile strain larger than −2.1% and +3.8%, respectively. The stability of the magnetic moment is found to be greatly dependent on the octahedra rotation pattern. The magnetic moment of the zero or one-tilt systems is quenched under tensile strain, while ferromagnetic order is preserved in the two or three-tilt systems.