Deformation Of Octahedra Research Articles

La2SrSc2O7: A-Site Cation Disorder Induces Ferroelectricity in Ruddlesden-Popper Layered Perovskite Oxide.

Rational design of ferroelectrics in layered perovskites, like n = 2 Ruddlesden-Popper (RP) phase A3B2O7, has been achieved by the hybrid-improper ferroelectric (HIF) mechanism, in which an electric polarization is induced via a trilinear coupling to nonpolar BO6 octahedral rotation and tilt distortions around crystallographic axes. In the present work, hybrid improper ferroelectricity in n = 2 RP-type La2SrSc2O7 induced by the disordering of Sr2+/La3+ cations on the A-sites in rocksalt ([Sr/La]Rs = 25/75) and perovskite ([Sr/La]Pv = 50/50) layers is demonstrated through experimental and theoretical investigations. The ferroelectric A21am structure (a-a-c+ in Glazer notation) at room temperature and the second-order phase transition to paraelectric Amam structure (a-a-c0) at TC ∼ 600 K are determined by a combination of X-ray and neutron diffraction and optical second harmonic generation. The ferroelectric hysteresis loop measurements prove the switchable electric polarization indicative of ferroelectricity. These results represent an unprecedented example of ferroelectricity in the n = 2 RP family of Ln2AB2O7 with inequivalent Ln3+ and A2+ cations. Combining the abovementioned experimental results with the first-principles calculations, we verify the role of Sr/La distributions in regulating the interlayer rumpling, which, in addition to the structural tolerance factor, is key to controlling the structural distortions of RP phases. The stabilization of the ferroelectric, a-a-c+ distorted structure is a consequence of the disordered Sr/La distribution on the A-sites, which suppresses the rumpling-induced octahedral deformations in competition with the octahedral rotations and thus enables the concurrence of a0a0c+ rotations and a-a-c0 tilts required for the HIF mechanism. This work demonstrates the possibility of altering the crystal symmetry of RP phases through the A-site cation disorder and provides a complementary approach to the rational design of new HIF materials.

Insights into the Impact of Fe-Doped CsPbCl3 Perovskites on Stability and Luminescence: A First-Principles Investigation

The transition metal-doped CsPbX3 family is regarded as promising materials to be applied in optoelectronic devices. However, the understanding of the underlying luminescence mechanism is still limited. In our study, the mechanism of dual emission for Fe-doped CsPbCl3 is determined from the first-principles methods. Fe doping can cause lattice compression and octahedral deformation and importantly improve the stability of CsPbCl3 well. The spin-included band structure calculation shows that the Fe concentration mainly modulates the energy level position of eg↓ (Fe 3d spin-down state). The eg↓ location in the band gap and near the conduction band minimum can induce efficient energy transfer from the conduction band minimum to the eg↓ state, which leads to the electronic transition eg↓ → t2g↓ of the Fe ion further. The experimental dual emission is confirmed by the calculated band gaps between band edges and between eg↓ and t2g↓ states (Fe 3d spin-down state). According to the analysis of density of states, the eg↓ and t2g↓ states are broad in the larger octahedral deformation, which suggests the experimental broadband emission of the Fe ion. The optical absorption of CsPbCl3 is improved and blue-shifted upon Fe doping, also in agreement with experiments. Our findings provide the understanding for designing high-performance doped perovskite optoelectronic devices.

The evolution of structure and magnetic properties of GdCr1-Fe O3 (0 ≤ x ≤ 0.5) polycrystalline samples

In this paper, the evolution of crystal structures and magnetic properties of GdCr 1- x Fe x O 3 (0 ≤ x ≤ 0.5) samples with different doping levels was systematically investigated by powder X-ray diffraction, Fourier transform infrared spectra, positron annihilation spectra, and temperature-dependent and field-dependent magnetization measurements. The lattice constants and Cr(Fe)O 6 octahedral deformation increase with Fe doping showing an anomaly at x = 0.4 which is further confirmed by positron annihilation and Fourier transform spectra. Fe doping in GdCrO 3 can suppress the magnetization reversal in the parent phase as well as its antiferromagnetic ordering temperature T N at low doping levels but enhance T N at high doping levels. A possible spin flip or flop transition under field-cooled-cooling process occurs for the samples of x = 0.3 and 0.4 at low temperature. The coercive fields of weak ferromagnetic state with different doping levels at 10 and 100 K also exhibit a local maximum at around x = 0.4 which is accordingly correlated to the concentrations of the vacancy defects in the sample. • The information of cationic vacancy defects is analyzed by positron annihilation technique. • The correlation between lattice parameters and vacancy defects is unveiled. • Defects in GdCr 1- x Fe x O 3 samples have a great impact on their magnetic properties. • Chemical doping and defect engineering are effective tools to manipulate R CrO 3 materials.

Incommensurate Modulation and Competing Ferroelectric/Antiferroelectric Modes in Tetragonal Tungsten Bronzes

The nanoscale structure of Sr0.61Ba0.39Nb2O6, a classic uniaxial relaxor ferroelectric crystallizing with the tetragonal tungsten bronze (TTB) structure and exhibiting an incommensurate modulation, has been determined by atomistic refinements using combined data from variable-temperature neutron total scattering, extended X-ray absorption fine structure, and three-dimensional single-crystal diffuse scattering. We found the modulation to arise from the intergrowth of structural slabs featuring distinct types of octahedral rotations directed to minimize octahedral deformations. This modulation involves displacements of the A cations (Sr and Ba) but exerts no significant effect on the polar displacements of Nb. Our results demonstrated the coexistence of competing polar Γ3– and antipolar Γ2– modes for off-centering of Nb above the nominal ferroelectric transition temperature of 350 K. The nanoscale-correlated antipolar distortions, which exhibit their largest amplitudes at 425 K, are suppressed below the transition. We identified coupling between the polar off-centering of Nb on its two symmetrically distinct sites as a fundamental characteristic controlling the ferroelectric-to-relaxor crossover in TTBs. We then used it to propose a simple microscopic interpretation for the previously established empirical trend that links crystal–chemical parameters in these systems to their polar response.

Interplay between Oxygen Octahedral Rotation and Deformation in the Acentric ARTiO4 Series toward Negative Thermal Expansion

Interplay between Oxygen Octahedral Rotation and Deformation in the Acentric <i>AR</i>TiO₄ Series toward Negative Thermal Expansion

John-Teller Distortion in Octahedral hexaaquacopper (II) Transition for S=1/2, d9 Metal Complex

Jahn–Teller effect (JTE) describe how specific electrons distributions induce geometric deformation in molecules or ions. The distortion of Jahn–Teller (JT) takes place because of the uneven occupation of orbitals with the identical energies is disapproving. To prevent such inappropriate electronic arrangements, molecules distort (lose regularity), causing these orbitals to become non-degenerate. This distortion is most frequently seen in octahedral compounds, at which two axial bonds could be squeezed (compressed) or extend (elongated), leading to bond lengths that the different from equator bonds. The degeneracy of electronic states really isn’t accurately measured, but it would be measured precisely by the disparity in a concentration of electron among metal and the ligands upon on axil direction. The density functional theory has been used to calculate bond length in both cases geometry for , octahedral metal-complexes, the majority ordinary mechanism of distortion is elongation (z-out). In perfectly octahedral complexes deformation, the orbitals energy is changed more than the orbitals energy, causing elongation z-out distortion. The d-levels energy would be splitting in a ligand field throughout transition metal centers can frequently result in degenerate electron configurations that are subject to Jahn–Teller effects. In six-coordinate complexes of copper II ions. It is also significant in fewer regular oxidation levels of other transition ions with that count.

Crystal structure and octahedral deformation of orthorhombic perovskite ABO3: Case study of SrRuO3

Using density functional theory with local spin density approximation and taking SrRuO3 as an example, the crystal structure of ABO3 perovskite oxide and the rectangular deformation of the BO6 octahedron were studied. It was found that the degree of octahedral rectangular deformation of orthorhombic SrRuO3 is much larger than Jahn–Teller distortion, therefore, cannot be ignored. The lattice constant a is greater than b in SrRuO3 (as well as LaCrO3, LaGaO3, and PbRuO3) because the side length of rectangle along x direction, aoct, is larger than that along y direction, boct; and the tilting angle of the octahedron, φT, is smaller than arccos (boct/aoct). We found that in ABO3 oxide, BO6 octahedral rectangular deformation has a universality and, consequently, aoct ≠ boct. We discussed in detail the changes of lattice constants caused by rectangular deformation of the octahedron and divided orthorhombic ABO3 oxides into the following three categories: aoct ​> ​boct with a >b; aoct ​> ​boct with a <b; and aoct ​≤ ​boct with a <b. The relationship between the rectangular deformation and the rotation (tilting) angle or tolerance factor is analyzed. As the symmetry and lattice constants of SrRuO3 are rather inconsistent, we have analyzed and clarified them. The influence of rectangular deformation of the octahedron on the electronic structure and magnetic properties of SrRuO3 was also calculated using first principles.

Fluorescence properties of &amp;lt;italic&amp;gt;A&amp;lt;/italic&amp;gt;&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt;InCl&amp;lt;sub&amp;gt;5&amp;lt;/sub&amp;gt;&amp;middot;H&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt;O ∶Sb&amp;lt;sup&amp;gt;3+&amp;lt;/sup&amp;gt;(&amp;lt;italic&amp;gt;A&amp;lt;/italic&amp;gt;=Cs, Rb, K)

The photoluminescence of self-trapped excitons is a common luminescence behavior of metal halide perovskite fluorescent materials. The ability of Lattice deformation has a significant impact on its self-trapped exciton luminescence. The 0-dimensional inorganic perovskite <italic>A</italic>₂InCl₅·H₂O (<italic>A</italic>=Cs, Rb, K) doped with Sb³⁺ is characterized by X-ray diffraction, fluorescence spectroscopy and Raman spectroscopy. The effect of octahedral deformation of [InCl₅H₂O]^2-/[SbCl₅H₂O]^2- due to cation radius change on the fluorescence properties is investigated. The decrease of the cation radius causes the decrease of the interplanar spacing of material, the increase of band gapand the blue shift of the excitation spectrum. At the same time, the electro-acoustic coupling effect of material is enhanced, then the non-radiative recombination process is enhanced, the Stokes shift increases, the spectral line broadens, the spectrum red shift happens, and the photoluminescence quantum yield decreases. This paper provides an effective way to regulate the self-trapped excitons luminescence of perovskites.

Role of octahedral deformation in the broad-band emission in Mn-doped lead halide perovskite: First-principles investigation for the case of CsPbX3 (X = Cl, Br, I)

Doping manganese ions (Mn2+) in lead halide perovskites (LHPs) has been proven to be an efficient strategy to achieve excellent light-emitting performance. A self-trapped exciton on a MnX6 (X = Cl, Br, I) octahedron and associated energy transfer play critical roles in the broad-band emission of Mn-doped LHPs. In this work, we performed first-principles study on the effect of octahedral MnX6 deformation on the light emission in Mn-doped LHPs. It is revealed that the broad-band luminescence peak is associated with Mn 3d orbitals. Our results revealed that octahedral MnX6 deformation, which enhances the dispersion of the associated bands, plays an important role in the broad-band emission in Mn-doped LHPs. Furthermore, octahedral MnX6 deformation shows dependence on the halogen element and structural phase of LHPs. Our results proved that tuning octahedral MnX6 deformation is an efficient way to achieve the regulation of the luminescence performance of Mn-doped LHPs. Our predictive results may inspire further studies on the applications of doped Mn perovskites.

Pressure-induced octahedral tilting distortion and structural phase transition in columbite structured NiNb2O6

High-pressure behavior of the technologically important compound NiNb2O6 adopting a columbite-type orthorhombic structure at ambient pressure and temperature conditions was investigated using synchrotron x-ray powder diffraction, Raman spectroscopic measurements, and first-principles calculations. The x-ray diffraction data indicate the occurrence of irreversible pressure-induced structural phase transition in the studied compound beyond 9 GPa. The high-pressure phase is found to be monoclinic with space group P2/m. The large volume collapse (∼4.4%) at the transition indicates the nature of the transition to be of the first order. There is a change in oxygen anion coordination number around Nb from 6 to 8; however, the coordination number around Ni remains 6. The experimental pressure–volume data when fitted to the Birch−Murnaghan equation of states yield the value of ambient pressure bulk modulus (B0) as 178.7 (17) GPa for the orthorhombic phase and 244 (6) for the high-pressure monoclinic phase. The changes in Raman spectra indicate the distortion of NbO6 octahedra resulting in structural phase transitions. The logarithmic variation of unit cell volume (V) with optical lattice mode frequency (ν) helped us to calculate their respective Grüneisen parameters (γ) and it also supports the instability of the orthorhombic columbite structure of NiNb2O6 beyond 9 GPa, originated due to strong octahedral deformation of NbO6 octahedra. The variation of structural, electronic, and optical properties with pressure has also been discussed through first-principles calculations based on density functional theory using the revised Perdew–Burke–Ernzerh of generalized gradient approximation. The theoretical bandgap collapses and the distortion of NbO6 octahedra through the O chains with pressure are emphasized from the density of states data.

Orientation-dependent low cycle fatigue performance of nickel-base single crystal superalloy at intermediate temperature range

Slip deformation is the predominant fatigue damage form of nickel-based single crystals at intermediate temperature range (near 760℃). Under different crystal orientations, the dominant slip system may be an octahedral or cubic slip system. This paper explores the dominant slip systems under different crystal orientations through crystal plastic finite element analysis, and determines the fatigue damage parameter that can characterize both the octahedral deformation and the cubic deformation. The results show that within a certain orientation range close to the <111> orientation, the dominant slip system of the nickel-based single crystal is the cubic slip system. For an orientation far deviating from the <111> orientation, the dominant slip system is the octahedral slip system. Through finite element calculation, this paper gives the distribution diagram of the dominant slip system in the standard pole figure. The fatigue data of nickel-based single crystals with different orientations in the existing literature are used for verification, and the results show that the dominant slip system can be effectively determined by using the distribution diagram, and the proposed fatigue damage parameter can effectively characterize the fatigue damage of different slip systems. Compared with the condition where the distribution of the dominant slip system is not considered, the method proposed in this paper has effectively improved the fatigue life prediction ability.

Magnetic properties of [formula omitted][formula omitted][formula omitted] epitaxial thin films grown on [formula omitted](0 0 1)

Epitaxial La0.67Sr0.33MnO3 thin films play important roles in several applications due to their half-metallic conductivity and high spin polarization. Since these properties are very sensitive to minor structural changes arising from slightly different growing conditions it is important to understand their effect on the electronic and magnetic properties of the resulting samples. Here, we study two sets of La0.67Sr0.33MnO3 thin films, thicker (90 nm) and thinner (7 nm) ones, focusing on the influence of laser fluence and post-annealing oxygen partial pressure on their electronic structure and magnetism. Regardless of the preparation conditions, the thicker films present a tensile strain that leads to MnO6 octahedral deformation as detected by Raman active modes. The thinner films exhibit sizable in-plane magnetic anisotropy which depends on the sample preparation that leads to distinct surface morphology and electronic properties (Mn4+/Mn3+ ratio).

First-principles calculations of oxygen octahedral distortions in LaAlO3/SrTiO3(001) superlattices.

The size, shape and connectivity of oxide octahedra are essential for understanding and controlling the emergent functional properties of ABO3 perovskites. Using first-principles calculations, we systematically studied the oxygen octahedral rotation and deformation in LaAlO3/SrTiO3(001) superlattices. Superlattices with electron- or hole-doped interfaces, or both, are compared. The results showed that there are at least three different types of oxygen octahedral distortions in these superlattices, which is more than what had previously been reported in the literature. We demonstrate that interfacial oxygen octahedral coupling and hole-doping, in addition to epitaxial strain, are the key factors underlying the formation of multiple types of oxygen octahedral rotations in these systems. We confirm that oxygen octahedral rotations and deformations play an essential role in insulator-metal transitions. Furthermore, octahedral distortion leads to ferroelectricity like dipole formation with the polarization vector always pointing to the positively charged interfaces.

Group-Theoretical Classification of Aristotypes of Cation and Anion Orders in Perovskites

A group-theoretical classification of cation and anion orders of crystals with a perovskite structure is presented. Aristotypes are idealized structures (phases), from which the low-symmetry ordered modifications of perovskites are formed as a result of rotations and deformation of octahedra, as well as the cooperative Jahn–Teller effect and other physical phenomena. Seven aristotype classes have been distinguished, which are characterized by the atomic ordering in each of Wyckoff positions and simultaneous atomic ordering in several positions. The critical and secondary (improper) order parameters, describing the formation of ordered structures, are determined. The classification proposed makes it possible to systematize a variety of known phases, establish genetic structural links between low-symmetry ordered structures, and design new materials.

Exploring new approaches towards the formability of mixed-ion perovskites by DFT and machine learning.

Recent years have witnessed a growing effort in engineering and tuning the properties of hybrid halide perovskites as light absorbers. These have led to the successful enhancement of their stability, a feature that is often counterbalanced by a reduction of their power-conversion efficiency. In order to provide a systematic analysis of the structure-property relationships of this class of compounds we have performed density functional theory calculations exploring fully inorganic ABC3 chalcogenide (I-V-VI3), halide (I-II-VII3) and hybrid perovskites. Special attention has been given to structures featuring three-dimensional BC6 octahedral networks because of their efficient carrier transport properties. In particular we have carefully analyzed the role of BC6 octahedral deformations, rotations and tilts in the thermodynamic stability and optical properties of the compounds. By using machine learning algorithms we have estimated the relations between the octahedral deformation and the bandgap, and established a similarity map among all the calculated compounds.

Spectroscopic criteria for identification of nuclear tetrahedral and octahedral symmetries: Illustration on a rare earth nucleus

We formulate criteria for identification of the nuclear tetrahedral and octahedral symmetries and illustrate for the first time their possible realization in a Rare Earth nucleus 152Sm. We use realistic nuclear mean-field theory calculations with the phenomenological macroscopic-microscopic method, the Gogny-Hartree-Fock-Bogoliubov approach and the general point-group theory considerations to guide the experimental identification method as illustrated on published experimental data. Following group-theory the examined symmetries imply the existence of exotic rotational bands on whose properties the spectroscopic identification criteria are based. These bands may contain simultaneously states of even and odd spins, of both parities and parity doublets at well defined spins. In the exact-symmetry limit those bands involve no E2-transitions. We show that coexistence of tetrahedral and octahedral deformations is essential when calculating the corresponding energy minima and surrounding barriers and that it has a characteristic impact on the rotational bands. The symmetries in question imply the existence of long-lived shape-isomers and, possibly, new waiting point nuclei -- impacting the nucleosynthesis processes in astrophysics -- and an existence of 16-fold degenerate particle-hole excitations. Specifically designed experiments which aim at strengthening the identification arguments are briefly discussed.

Tuning Perpendicular Magnetic Anisotropy by Oxygen Octahedral Rotations in (La_{1-x}Sr_{x}MnO_{3})/(SrIrO_{3}) Superlattices.

Perpendicular magnetic anisotropy (PMA) plays a critical role in the development of spintronics, thereby demanding new strategies to control PMA. Here we demonstrate a conceptually new type of interface induced PMA that is controlled by oxygen octahedral rotation. In superlattices comprised of La_{1-x}Sr_{x}MnO_{3} and SrIrO_{3}, we find that all superlattices (0≤x≤1) exhibit ferromagnetism despite the fact that La_{1-x}Sr_{x}MnO_{3} is antiferromagnetic for x>0.5. PMA as high as 4×10^{6} erg/cm^{3} is observed by increasing x and attributed to a decrease of oxygen octahedral rotation at interfaces. We also demonstrate that oxygen octahedral deformation cannot explain the trend in PMA. These results reveal a new degree of freedom to control PMA, enabling discovery of emergent magnetic textures and topological phenomena.

Perovskite solid solutions La0.75Bi0.25Fe1−xCrxO3: Preparation, structural, and magnetic properties

Solid solutions of La0.75Bi0.25Fe1−xCrxO3 (x = 0.1, 0.25, 0.5, and 0.75) prepared by conventional solid state reaction have been studied by means of X-ray powder diffraction (XRPD), neutron powder diffraction (NPD) and magnetic measurements. The NPD and XRPD patterns indicate orthorhombic structure (space group Pnma) for all compositions in the whole temperature range investigated (4–900K). The lattice parameters of La0.75Bi0.25Fe1−xCrxO3 were found to decrease with the Cr content. It was established that the Fe3+ and Cr3+ cations are randomly positioned at the B-site of the perovskite structure.All samples order antiferromagnetically below transition temperatures that decrease with increasing Cr content, from around 700K for x = 0.1 to about 300K for x = 0.75. The antiferromagnetic arrangement of the Fe3+/Cr3+ magnetic moments in the B-site is of G-type along the x-axis (Gx mode) with propagation vector k = (0,0,0) for all concentrations of Cr. Effects of the composition on several structural distortion parameters were investigated and an anomalous variation of the octahedral deformation with Cr content was found. Whilst the overall octahedral deformation varies irregularly with increasing Cr content, the octahedral tilting was found to decrease monotoneously.

Formation and distortion of iodidoantimonates(III): the first isolated [SbI6]3- octahedron.

The ability to intentionally construct, through different types of interactions, inorganic-organic hybrid materials with desired properties is the main goal of inorganic crystal engineering. The primary deformation, related to intrinsic interactions within inorganic substructure, and the secondary deformation, mainly caused by the hydrogen bond interactions, are both responsible for polyhedral distortions of halogenidoantimonates(III) with organic cations. The evolution of structural parameters, in particular the Sb-I secondary- and O/N/C-H...I hydrogen bonds, as a function of temperature assists in understanding the contribution of those two distortion factors to the irregularity of [SbI6]3- polyhedra. In tris(piperazine-1,4-diium) bis[hexaiodidoantimonate(III)] pentahydrate, (C4H12N2)3[SbI6]2·5H2O (TPBHP), where the isolated [SbI6]3- units were found, distortion is governed only by O/N/C-H...I hydrogen bonds, whereas in piperazine-1,4-diium bis[tetraiodidoantimonate(III)] tetrahydrate, (C4H12N2)[SbI4]2·4H2O (PBTT), both primary and O-H...I secondary factors cause the deformation of one-dimensional [{SbI4}n]n- chains. The larger in spatial dimensions piperazine-1,4-diium cations, in contrast to the smaller water of crystallization molecules, do not significantly contribute to the octahedral distortion, especially in PBTT. The formation of isolated [SbI6]3- ions in TPBHP is the result of specific second coordination sphere hydrogen bond interactions that stabilize the hybrid structure and simultaneously effectively separate and prevent [SbI6]3- units from mutual interactions. The temperature-induced changes, further supported by the analysis of data retrieved from the Cambridge Structural Database, illustrate the significance of both primary and secondary distortion factors on the deformation of octahedra. Also, a comparison of packing features in the studied hybrids with those in the non-metal containing piperazine-1,4-diium diiodide diiodine (C4H12N2)I2·I2 (PDD) confirms the importance and hierarchy of different types of interactions.

A brief review on perovskite multiferroics

ABSTRACTThe paper first introduces the basic physics behind perovskite multiferroics. Then the recent progress on simple, complex perovskites, and manganites are briefly reviewed. The review discloses two important directions on achievement multiferrocity in perovskite: pay with strain engineering and adjust octahedral deformation. The authors also suggest that specific issues like leakage current and characterization platforms are also very important for the study of multiferroics.