Oxygen Octahedron Research Articles

Structural evolution and its effect on multiferroic properties in magnetoelectric 0.67Sm0.12Bi0.88FeO3–0.33BaTiO3 ceramics by tuning the cooling rate

The effects of cooling rate on structure and multiferroic properties were investigated in the lead-free 0.67Sm0.12Bi0.88FeO3–0.33BaTiO3 ceramics. Cooling in different medium induces the variations of phase fractions, c/a ratio and the distortion degree of oxygen octahedron, which can effectively regulate the multiferroic properties at room temperature. The high-temperature P4mm phase transforms to the Pm3m phase with the decrement of the cooling rate. The magnetoelectric coupling effect is nonlinearly related to the cooling rate, which is induced by the corresponding distortion in FeO6 octahedron. The ceramic cooled in water shows the largest remnant polarization of ~ 20.9 μC/cm2, while the ceramic cooled in furnace shows large remnant magnetization and magnetoelectric coupling coefficient of ~ 0.50 emu/g and 7.3 mV/(cm Oe), respectively.

Sintering behavior, microwave dielectric properties of Ca0.66Ti0.66Nd0.34Al0.34O3 ceramics revealed by microstructure and Raman scattering

Ca0.66Ti0.66Nd0.34Al0.34O3(CTNA) ceramics were prepared by solid-state reaction method and their sintering behavior, microwave dielectric properties, and microstructure were investigated. CTNA ceramics were examined by Raman scattering to reveal the correlation of perovskite structure with the microwave properties. It found that the area of peak A1g(O) stretch mode reflects the quality factor Q × f value in the perovskite materials. This concludes that the oxygen octahedron plays an important role of the material's microwave performance. Sintering temperature affected the unit cell volume and the lattice parameters of the CTNA ceramics, the grain size distribution, the Gibbs free energy, the thermal expansion coefficient (CTE), and the dielectric properties (εr, Q×f, τf). The samples sintered at 1500 °C displayed the best dielectric constant and Q×f, as 44.94 and 41922.22 GHz, respectively.

Emergence of Topological Hall Effect in a SrRuO3 Single Layer.

Topological Hall effect (THE), appearing as bumps and/or dips in the Hall resistance curves, is considered as a hallmark of the skyrmion spin texture originated from the inversion symmetry breaking and spin-orbit interaction. Recently, Néel-type skyrmion is proposed based on the observed THE in 5d transition metal oxides heterostructures such as SrRuO3 /SrIrO3 bilayers, where the interfacial Dzyaloshinskii-Moriya interaction (DMI), due to the strong spin-orbit coupling (SOC) in SrIrO3 and the broken inversion symmetry at the interface, is believed to play a significant role. Here the emergence of THE in SrRuO3 single layers with thickness ranging from 3 to 6 nm is experimentally demonstrated. It is found that the oxygen octahedron rotation in SrRuO3 also has a significant effect on the observed THE. Furthermore, the THE may be continuously tuned by an applied electrical field. It is proposed that the large SOC of Ru ions together with the broken inversion symmetry, mainly from the interface, produce the DMI that is responsible for the observed THE. The emergence of the gate-tunable DMI in SrRuO3 single layer may stimulate further investigations of new spin-orbit physics in strong SOC oxides.

Intrinsic structural distortion and magnetic interaction in LuxSm1-xCrO3 compounds

Effects of the substitution of nonmagnetic metal Lu with different content on the magnetic property of LuxSm1-xCrO3 (0 < x < 1) are reported here for the sake of investigating the relationship between structural distortion and the magnetism of these materials. The structure of the synthesized samples was characterized by x-ray diffraction with Rietveld refinements and Raman spectroscopy at ambient conditions. Analysis of the recorded magnetization data reveals the Neel temperature changes with a reduction of the average radius of the A-site ion, and the tilting angle of the oxygen octahedron. Molecular field theory was used to calculate the strengths of the exchange interactions (J). It was found that the strength of the interaction J (reflected in the changes in the Neel temperatures) decreases with the proportion of replacement of Sm3+ by Lu3+, ascribing to the changes of the oxygen octahedron tilt angle. Meanwhile, coupling effects between Cr3+ and Sm3+ ions are examined with enhanced susceptibilities below TN due to the polarization of Sm3+ moments.

A New Phase Boundary of Oxygen Octahedral Tilt-Untilt Transition in Bi &lt;sub&gt;0.5&lt;/sub&gt;Na &lt;sub&gt;0.5&lt;/sub&gt;TiO &lt;sub&gt;3&lt;/sub&gt;-PbTiO &lt;sub&gt;3&lt;/sub&gt; Piezoelectric Ceramics

The investigation on the fine structure evolutions around morphotropic phase boundary (MPB) is important to understand the origin of high piezoelectricity in ferroelectric ceramics. Here, we discovered a new boundary associated with oxygen octahedron tilt-untilt phase transition by exploring the average and local structure evolutions in the vicinity of the MPB in (Bi0.5Na0.5)TiO3-PbTiO3 (BNT-PT) ceramics. It is found that the local structure transition due to oxygen octahedron rotation is decoupled with the phase transition from rhombohedral to tetragonal phase at MPB in the BNT-PT. Such a local tilt-untilt transition is responsible to the large high-field-induced electrostrain at MPB of the BNT-PT. The crucial difference in MPB between the BNT-PT and the classical piezoelectric Pb(Zr1-xTix)O3 is discussed by linking the structure evolutions to the performance at the MPBs. This work helps us to figure out the origin of the unique large high-field-induced electrostrain in BNT-based ceramics, and may be helpful to the design of high-performance piezoelectric ceramics in the future.

Atomic-Environment-Dependent Thickness of Ferroelastic Domain Walls

Domain walls are of increasing interest in ferroelectrics because of their unique properties and potential applications in future nanoelectronics. However, the thickness of ferroelastic domain walls remains elusive due to the challenges in experimental characterization. Here, we determine the atomic structure of ferroelastic domain walls and precisely measure the polarization and domain wall thickness at picometer scale using annular bright field imaging in an aberration-corrected scanning transmission electron microscope. We find that the domain wall thickness in PbZr0.2Ti0.8O3 and PbTiO3 thin films is typically about one-unit cell, across which the oxygen octahedron distortion behavior is in excellent agreement with first principles calculations. Remarkably, wider domain walls about two-unit cells in thickness are also observed for those domains walls are coupled with dislocations and underwent a compressive strain. These results suggest that the thickness of ferroelastic domain walls highly depends on their atomic environments. This study can help us to understand the past debatable experimental results and provide further insights into control of domain walls via strain engineering for their possible applications in nanoelectronics.

Correlation of crystalline and magnetic structures of barium ferrites with dual ferroic properties

Abstract The spontaneous polarization has been found out in solid solutions of barium ferrites partially substituted with In, Al, Ga and Sc ions that is an evidence of their ferroelectric properties. For explanation reasons of appearance ferroelectric properties in these materials have been investigated features of crystal and magnetic structures with using X-ray and neutron time-of-flight diffraction methods in a wide temperature range. The crystal structure of barium ferrites has been refined in the frameworks of centrosymmetric P 6 3 / mmc (No. 194) and non- centrosymmetric P 6 3 mc (No. 186) space groups. According to analysis of neutron patterns, the distortion of oxygen octahedrons (tetrahedrons and trigonal-bipyramids) has been observed in unit cells both space groups. It was shown that a key role in appearance of spontaneous polarization can be associated with the absence of inversion center in unit cell.

Features of crystalline and magnetic structure of barium ferromolybdate in a wide temperature range

Abstract The evolution of crystalline and magnetic structures of Ba 2 FeMoO 6 in a wide temperature range (20–473 K) has been investigated by time-of-flight neutron diffraction method. Analysis of the bond lengths showed that the phase transition of Ba 2 FeMoO 6 compound from cubic (Fm-3 m) to tetragonal (I4/mmm) structure is accompanied by distortions (∼1%) of oxygen octahedrons BO 6 . The magnetoresistance of Ba 2 FeMoO 6 has been measured in a wide range of temperatures and magnetic fields. The field dependence of magnetoresistance is demonstrative for systems with weighty contribution of the intergrain conduction.

Microstructure and ferroelectric properties of (Ca1−xSrx)3(Ti1−yMny)2O7 ceramics

(Ca1−xSrx)3(Ti1−yMny)2O7 (CSTMO) ceramics with hybrid improper ferroelectricity were prepared by conventional solid-state reaction method. Effect of Sr2+ and Mn4+ contents on microstructure, dielectric and ferroelectric properties have been systematically investigated. The XRD results show that CSTMO ceramics is single phase with orthorhombic structure. The lattice constants and average grain size increase with the increasing of Sr2+ content and decrease with the increasing of Mn4+ content. The introduction of Mn4+ deteriorates the densification of CSTMO ceramics to some extent, and the substitution of Sr2+ with higher content can improve the densification by promoting a rapid interdiffusion movement via grain-boundary. (Ca0.8Sr0.2)3(Ti0.9Mn0.1)2O7 ceramics present the highest dielectric constant and dielectric loss, while the dielectric loss of (Ca0.8Sr0.2)3(Ti0.95Mn0.05)2O7 ceramics has a significant reduction. The leakage current of CSTMO ceramics decreases with the increasing of Sr2+ and Mn4+ content. The leakage behavior suggests that the Ohmic conduction and Schottky emission dominate the leakage current of CSTMO ceramics. Moreover, the remnant polarization decreases with the increasing of Sr2+ because of its suppression for the rotating and tilting angle (a−a−c+) of oxygen octahedron. And the electrical polarization decreases with the increasing of Mn4+ content due to the instability of MnO6 rotation.

Structural Features, Physicochemical, and Optical Characteristics of Lithium Niobate Crystals Grown from Boron-Doped Melts

Chemical interactions in the Li2O−B2O3−Nb2O5 system, as well as certain features of crystallization of LiNbO3 crystals growing from melts containing nonmetal impurities, are considered. It is shown that boron changes the structure of the melt and has a significant impact on the structure and physical characteristics of LiNbO3 : В crystals, practically not entering the lithium niobate structure. Notable changes and certain features were found in the Raman spectra of grown LiNbO3 : В crystals, which indicates changing the sequence of the main cations and vacancies along a crystal polar axis and distortion of the oxygen octahedrons. Meanwhile, the distortion of the oxygen octahedrons is anisotropic. LiNbO3 : В crystals have a higher structural homogeneity than congruent crystals, and are close in the number of NbLi defects to a crystal with a stoichiometric composition, differing from it by a substantially less photorefraction effect.

Epitaxially strained SnTiO3 at finite temperatures**Project supported by the National Natural Science Foundation of China (Grant Nos. 11574246, 51390472, U1537210, and 11564010), the National Basic Research Program of China (Grant No. 2015CB654903), the Natural Science Foundation of Guangxi Zhuang Autonomous Region (Grant Nos. GA139008 and AA138162), and the “111” Project of China (Grant No. B14040).

By combining the effective Hamiltonian approach and direct ab initio computation, we obtain the phase diagram of SnTiO3 with respect to epitaxial strain and temperature. This demonstrates the complex features of the phase diagram and provides an insight into this system, which is a presumably simple perovskite. Two triple points, as shown in the phase diagram, may be exploited to achieve high-performance piezoelectric effects. Despite the inclusion of the degree of freedom related to oxygen octahedron tilting, the ferroelectric displacements dominate the structural phases over the whole misfit strain range. Finally, we show that SnTiO3 can change from hard to soft ferroelectrics with the epitaxial strain.

Magnetic Anisotropy Controlled by Distinct Interfacial Lattice Distortions at the La1- xSr xCoO3/La2/3Sr1/3MnO3 Interfaces.

Interface engineering is an important approach leading to multifunctional artificial materials. Although most of the previous works focused on the effects of the rotation/tilting of interfacial oxygen octahedron on perovskite multilayers, here, we report on a new kind of lattice distortion characterized by an off-center shift of the Mn ions within the MnO6 oxygen octahedra at the interfaces of La1- xSr xCoO3/La2/3Sr1/3MnO3/La1- xSr xCoO3/LaAlO3 trilayers ( x = 0-1/3), which drives the initially perpendicularly aligned magnetic axis of the La2/3Sr1/3MnO3 (LSMO) film toward the in-plane direction, though the film is in a strongly compressive state. It is further found that the magnetic anisotropy considerably depends on the content of Sr in La1- xSr xCoO3, enhancing as x decreases. The maximal anisotropy constant at 10 K is +2.5 × 106 erg/cm3 for the trilayers with x = 0, whereas it is -1.5 × 105 erg/cm3 for a bare LSMO film on LaAlO3. On the basis of the analysis of X-ray absorption spectroscopy and the results of density functional theory calculations, we found that the off-center displacement of the Mn ions has caused a strong orbital reconstruction at interfaces, resulting in the anomalous spin orientation against magnetoelastic coupling.

Oxygen vacancy micrometric enrichment in high-performance lead-free piezoelectric ceramic (Bi0.5Na0.5)0.94Ba0.06TiO3: A synchrotron radiation study

A multiscale description of (Bi0.5Na0.5)0.94Ba0.06TiO3 (BNBT6) ceramic is presented. High piezoelectric and electromechanical parameters (d33 = 167 pC/N, kt = 48.6%) were experimentally determined. The ceramic’s crystal structure, after poling, was determined from synchrotron X-ray diffraction. The Rietveld analysis reveals a highly textured BNBT6 perovskite with R3c symmetry. μ-X ray fluorescence on both unpoled and poled ceramics provided evidence of micrometric inhomogeneities in the elemental fluorescence intensities. μ-X ray absorption near edge spectroscopy (μ-XANES) at Ti K-edge spectra present anomalous absorption intensities in some energy intervals. First-principles modeling and linear combination analysis of μ-XANES spectra suggest local variations in the structure of the oxygen octahedron around titanium. The integrated analysis of experiments points toward the existence of micrometric clusters with an enrichment of Bi-O vacancies, distributed randomly at distances of tens of microns. Mentioned microstructural characteristics are consistent with the experimentally determined high piezoelectric and electromechanical parameters.

Coupling reactions and collapsing model in the roasting process of recycling metals from LiCoO2 batteries

The wider application of lithium ion batteries brings more production of spent ones to be recovered. There is a pyrometallurgical treatment by getting negative and positive materials burning together. The paper provides theoretical analysis and support for the feasibility of the treatment. Positive material graphite's burning and pyrolysis of negative material lithium cobalt oxide can make up as a coupling reaction, decreasing the decomposition temperature of lithium cobalt oxide from 1426 K to 1173 K. From the point view of crystal structure, the stronger attraction of graphite to oxygen than lithium and cobalt make oxygen octahedrons insides lithium cobalt oxide unstable to break down. We definite the phenomenon as the oxygen octahedrons collapsing model. Coupling reaction theory and crystal structure analysis indicate that graphite does promote the pyrolysis of lithium cobalt oxide. The study can guide industrial application to recover lithium ion batteries whose structure is similar to oxygen octahedrons at a comparatively low temperature.

Different Additives Doped Ca-Nd-Ti Microwave Dielectric Ceramics with Distorted Oxygen Octahedrons and High Q × f Value.

We investigated the structure and antireduction of Ti4+ in orthorhombic perovskite Ca0.61Nd0.26TiO3 (CNT) ceramic by doping with three different additives (Cr2O3, MnO2, and SnO2). The X-ray diffraction patterns showed that the main phase of orthorhombic perovskite was formed in all the samples. In addition, the substitution of M (M = Cr3+, Mn4+, Sn4+) for Ti4+ resulted in the oxygen octahedral distortion and changes of order degree of B-site, which was confirmed by Raman spectra. The εr and τf values were concerned with the average ionicity of B–O bond fi(B-ave) and the linear expansion coefficient α, respectively. The X-ray photoelectron spectra indicated that three different additives could restrain the reduction of Ti4+, which was beneficial to the improvement of the Q × f value for CNT ceramic. High Q × f value of 16 123 GHz was obtained in the CNT + 1 mol % Cr2O3 ceramic compared with Q × f value of 11 207 GHz in pure CNT ceramic.

Flux mechanism of compound flux on ash and slag of coal with high ash melting temperature

Abstract The melting temperature of Z coal ash was reduced by adding calcium–magnesium compound flux (WCaO/WMgO = 1). In the process of simulated coal gasification, the coal ash and slag were prepared. The transformation of minerals in coal ash and slag upon the change of temperature was studied by using X-ray diffraction (XRD). With the increase of temperatures, forsterite in the ash disappears, while the diffraction peak strength of magnesium spinel increases, and the content of the calcium feldspar increases, then the content of the amorphous phase in the ash increases obviously. The species and evolution process of oxygen, silicon, aluminum, calcium, magnesium at different temperatures were analyzed by X-ray photoelectron spectroscopy (XPS). The decrease of the ash melting point mainly affects the structural changes of silicon, aluminum and oxygen. The coordination of aluminum and oxygen in the aluminum element structure, e.g., tetracoordinated aluminum oxide, was changed. Tetrahedral [AlO4] and hexacoordinated aluminoxy octahedral [AlO6] change with the temperature changing. The addition of Ca2+ and Mg2+ destroys silica chain, making bridge oxide silicon change into non-bridge oxysilicon; and bridge oxygen bond was broken and non-bridge oxygen bond was produced in the oxygen element structure. The addition of calcium and magnesium compound flux reacts with aluminum oxide tetrahedron, aluminum oxide octahedron and silicon tetrahedron to promote the breakage of the bridge oxygen bond. Ca2+ and Mg2+ are easily combined with silicon oxide and aluminum oxide tetrahedron and aluminum. Oxygen octahedrons combine with non-oxygen bonds to generate low-melting temperature feldspars and magnesite minerals, thereby reducing the coal ash melting temperatures. The structure of kaolinite and mullite was simulated by quantum chemistry calculation, and kaolinite molecule has a stable structure.

Study of materials structure physics of isomorphic LiNbO3 and LiTaO3 ferroelectrics by synchrotron radiation X-ray diffraction

Electron charge density studies of stoichiometric LiNbO3 and LiTaO3 ferroelectrics have been carried out by analyzing the synchrotron radiation X-ray powder diffraction data using a combination of the Rietveld and maximum entropy methods. The clear relationships between the Nb(Ta)–O bond length, the electron charge density on the Nb(Ta)–O bond, and the phase transition temperature TC are revealed for isomorphic structures. Nb(Ta)–O bonding plays an important role in the elevation of TC. The TC in LiNbO3 being higher than that in LiTaO3 is attributed to the larger lattice distortion of the Nb–O oxygen octahedron in LiNbO3. The validity of estimating TC for LiNbO3 family crystals from the degree of lattice distortion in the ferroelectric structure is discussed.

Optical spectroscopy of Rh3+ ions in the lanthanum-aluminum oxide systems

A series of the Rh-containing samples supported on pure γ-Al2O3, La-doped γ-Al2O3, LaAl11O18 and LaAlO3 was prepared and characterized. Diffusion of Rh3+ ions from the surface of the support into its bulk causing the rapid deactivation of the catalyst in a model reaction of CO oxidation under the prompt thermal aging conditions was shown to be accelerated by a presence of lanthanum. The amount of rhodium disappeared from the surface during the high temperature treatment was estimated by a testing reaction of ethane hydrogenolysis and an electron paramagnetic resonance spectroscopy with spin probe. In order to characterize rhodium ions diffused into the bulk of the support at different calcination temperatures, the optical spectroscopic methods (diffuse reflectance UV–vis and photoluminescence spectroscopies) were applied. Rh3+ ions were shown to be applicable as a luminescence probe to follow the process of their bulk diffusion and localization within the matrix of the support, as well as for the structure identification of the La-containing sites responsible for the alumina thermal stabilization. The obtained data on luminescence of Rh3+ ions indicate that already at 800 °C the oxygen octahedrons corresponding to LaAl11O18 structure are being formed in the local coordination surroundings of isolated La ion in the 4%La2O3-Al2O3 support. It is important to notice that the structures of such kind can be formed in La-doped aluminas only. There are no octahedron positions with sufficient values of the crystal field for the formation of luminescence centers of Rh3+ ions in pure γ-Al2O3.

Atomic structure of manganese-doped yttrium orthoaluminate

Abstract Using hybrid exchange-correlation functional within density functional theory we have performed first-principle total energy calculations of Mn-doped yttrium orthoaluminate (YAlO3). Its equilibrium atomic structure has been predicted through optimization of coordinates of all atoms using a supercell approach. In our research both Mn3+ and Mn2+ ions have been substituted for the host alumina atom at orthorhombic Pbnm unit cell of YAlO3. F-center has been implemented as charge-compensating defect in case, when Mn2+ dopant is under study. In this study we thoroughly analyze the atomic displacements in seven nearest to Mn ion coordination spheres. Insertion of isoelectronic substitutional point defect Mn3+ leads to modest expansion of distorted oxygen octahedra, while the cage of Y atoms closest to the dopant remains mostly unchanged. Insertion of compensating F-center in the 1st oxygen coordination sphere breaks the symmetry of YAlO3 and heavily distorts the surrounding oxygen octahedron. That is why the Mn2+-F-center bonds is contracted about 32% with respect to reference Mn-O bond of Mn3+-doped orthorhombic YAlO3. Distortion is also well pronounced at Y coordination spheres of Mn2+/YAlO3. The influence of atomic relaxation near the Mn-dopants to the electronic structure of YAlO3 is also discussed.

Ultrafast Relaxation Dynamics of the Antiferrodistortive Phase in Ca Doped SrTiO_{3}.

The ultrafast dynamics of the octahedral rotation in Ca:SrTiO_{3} is studied by time-resolved x-ray diffraction after photoexcitation over the band gap. By monitoring the diffraction intensity of a superlattice reflection that is directly related to the structural order parameter of the soft-mode driven antiferrodistortive phase in Ca:SrTiO_{3}, we observe an ultrafast relaxation on a 0.2ps timescale of the rotation of the oxygen octahedron, which is found to be independent of the initial temperature despite large changes in the corresponding soft-mode frequency. A further, much smaller reduction on a slower picosecond timescale is attributed to thermal effects. Time-dependent density-functional-theory calculations show that the fast response can be ascribed to an ultrafast displacive modification of the soft-mode potential towards the normal state induced by holes created in the oxygen 2p states.