Distortion Of Oxygen Octahedra Research Articles

Enhanced Pseudocapacitance in Multicomponent Transition-Metal Oxides by Local Distortion of Oxygen Octahedra.

Anomalously high pseudocapacitance of a metal oxide was observed when Ni, Co, and Mn were mixed in a solid solution. Analysis by X-ray absorption near-edge spectroscopy (XANES) identified a wider redox swing of Ni as the origin of the enlarged pseudocapacitance. Ab initio DFT calculations revealed that aliovalent species resulting from the copresence of multiple transition metals can generate permanent local distortions of [NiO6] octahedra. As this type of distortion breaks the degenerate eg level of Ni(2+), the Jahn-Teller lattice instability necessary for the Ni(2+/3+) redox flip can be effectively diminished during charge-discharge, thus resulting in the significantly increased capacitance. Our findings highlight the importance of understanding structure-property correlation related to local structural distortions in improving the performance of pseudocapacitors.

Enhanced Pseudocapacitance in Multicomponent Transition‐Metal Oxides by Local Distortion of Oxygen Octahedra

AbstractAnomalously high pseudocapacitance of a metal oxide was observed when Ni, Co, and Mn were mixed in a solid solution. Analysis by X‐ray absorption near‐edge spectroscopy (XANES) identified a wider redox swing of Ni as the origin of the enlarged pseudocapacitance. Ab initio DFT calculations revealed that aliovalent species resulting from the copresence of multiple transition metals can generate permanent local distortions of [NiO6] octahedra. As this type of distortion breaks the degenerate eg level of Ni2+, the Jahn–Teller lattice instability necessary for the Ni2+/3+ redox flip can be effectively diminished during charge–discharge, thus resulting in the significantly increased capacitance. Our findings highlight the importance of understanding structure–property correlation related to local structural distortions in improving the performance of pseudocapacitors.

Local atomic arrangement and scintillation properties of Eu- and Ce-doped NaYP2O7

Direct determination of rare earth location and local environment in NaYP2O7 are presented. Undoped and Ln-doped NaYP2O7 (Ln=Eu, Ce) were produced via PVA-assisted sol–gel method. Lattice parameters were determined from Rietveld refinement, showing monoclinic structure. XAS results suggested Eu3+ and Ce3+ are incorporated into NaYP2O7 host in substitution to Y3+ site, with first coordination shell formed by six oxygen ions. Measurements at Eu edge showed a single peak in R space for Eu–O distribution. In this case, uniform interatomic distances implied to absence of significant disorder. Analysis at Ce edge presented different behavior, with Ce–O distribution characterized by a split peak in R space. Nearest neighborhood was found to be distributed with Ce occupying an off-center position in Y site. Under X-ray excitation, 5D0→7FJ emission lines of Eu3+ were identified for NaYP2O7:Eu. NaYP2O7:Ce presented a broad emission formed by 5d→2FJ transitions of Ce3+, with the superposition attributed to the effect of distorted oxygen octahedra around the dopant ions.

Oxygen octahedra distortion induced structural and magnetic phase transitions in Bi1−xCaxFe1−xMnxO3 ceramics

The co-doping of Ca and Mn in respective Bi and Fe-sites of BiFeO3 lattice leads to structural transition from rhombohedral (R3c space group) to orthorhombic (Pbnm space group) crystal symmetry. The tilt angle for anti-phase rotation of the oxygen octahedra of BiFeO3 at room temperature is observed to be ∼13.8°. It decreases with the increase in the co-doping percentage which suggests the composition-driven structural phase transition. The remnant magnetization for sample with 15% of co-doping becomes about 16 times that of BiFeO3. It may be attributed to the suppression of cycloid spin structure and uncompensated spins at the surface of nanocrystallites. Further increase in co-doping percentage results in the sharp reduction of remnant magnetization due to the dominant contribution from the collinear antiferromagnetic ordering in the Pbnm space group. The Arrott plot analysis clearly indicates the composition-driven crossover from the antiferromagnetic to weak ferromagnetic ordering and vice versa. Electron spin resonance results provide the evidence for the composition-driven phase transitions from an incommensurate spin cycloidal modulated state to one with nearly homogeneous spin order. The band gap (2.17 eV) of BiFeO3 measured using UV-Vis spectra was supported by the resonance Raman spectra.

Research Update: Interface-engineered oxygen octahedral tilts in perovskite oxide heterostructures

Interface engineering of structural distortions is a key for exploring the functional properties of oxide heterostructures and superlattices. In this paper, we report on our comprehensive investigations of oxygen octahedral distortions at the heterointerface between perovskite oxides SrRuO3 and BaTiO3 on GdScO3 substrates and of the influences of the interfacially engineered distortions on the magneto-transport properties of the SrRuO3 layer. Our state-of-the-art annular bright-field imaging in aberration-corrected scanning transmission electron microscopy revealed that the RuO6 octahedral distortions in the SrRuO3 layer have strong dependence on the stacking order of the SrRuO3 and BaTiO3 layers on the substrate. This can be attributed to the difference in the interfacial octahedral connections. We also found that the stacking order of the oxide layers has a strong impact on the magneto-transport properties, allowing for control of the magnetic anisotropy of the SrRuO3 layer through interface engineering. Our results demonstrate the significance of the interface engineering of the octahedral distortions on the structural and physical properties of perovskite oxides.

The multiferroic properties of BiFe0.5Mn0.5O3 and BiFeO3/BiMnO3 superlattice films

BiFe0.5Mn0.5O3 (BFMO) and BiFeO3/BiMnO3 superlattice (BFO/BMO) films were epitaxially grown on (111) SrTiO3 substrates by pulsed laser deposition. The ferroelectricity of both BFMO and BFO/BMO has been confirmed by piezoresponse force microscopy. Weak ferromagnetism was observed for both samples. The magnetization of BFO/BMO is smaller than that of BFMO, confirming the antiferromagnetic Fe3+-O2−-Mn3+ interaction. Two anomalies at low temperature, T1 = 235 K and T2 = 42 K for BFMO and T1 = 198 K and T2 = 77 K for BFO/BMO, were observed in temperature dependent magnetization (M-T) curves. The peaks at T1 in zero field cooled (ZFC) M-T curves for both samples can be understood by the onset of Fe3+-O2−-Mn3+ interaction. The peak at T2 for BFMO can be understood by the blocking of ferromagnetic Mn-rich clusters due to the inhomogeneous distribution of Mn. The peak at T2 for BFO/BMO observed in both field cooled and ZFC M-T curves has been ascribed to the enhanced single-ion magnetocrystalline anisotropy due to the highly distorted oxygen octahedra in the ordered structure of Fe3+ and Mn3+ ions along (111) orientation.

Oxygen-octahedral distortion and electronic correlation induced semiconductor gaps in ferrimagnetic double perovskite Ca2MReO6 (M = Cr, Fe)

Motivated by experimental nonmetallic features and high magnetic Curie temperatures in double perovskite Ca2CrReO6 and Ca2FeReO6, we systematically investigate the structural, electronic, and magnetic properties of Ca2MReO6 (M = Cr, Fe).

External pressure and composition effects on the atomic and electronic structure of SnWO4

The atomic and electronic structure of tin tungstates, α-SnWO4, α-Sn1.03W0.99O4 and β-SnWO4, was studied by the W L3-edge X-ray absorption spectroscopy and first-principles linear combination of atomic orbital (LCAO) calculations based on the hybrid exchange-correlation density functional (DFT)/Hartree–Fock (HF) scheme. It was found that the crystal structure of both α-phases is built up of strongly distorted WO6 octahedra, whereas that of β-SnWO4 is composed of nearly regular WO4 tetrahedra. In addition, there are distorted SnO6 octahedra in both α- and β-phases. The metal–oxygen octahedra distortion is explained by the second-order Jahn–Teller effect. The influence of pressure on the structure of α-SnWO4 and β-SnWO4 was studied in detail based on the calculated equations of state. The compressibility of β-SnWO4 was found to be larger than that of α-SnWO4. The existence of the insulator-to-metal transition was theoretically predicted in α-SnWO4 at about 16GPa and was explained by a symmetrization of metal–oxygen octahedra leading to a strong interaction of Sn 5s, W 5d and O 2p states and closing of band gap.

Strong Dependence of Oxygen Octahedral Distortions in SrRuO3 Films on Types of Substrate-Induced Epitaxial Strain

This work investigates the effects of types of epitaxial strain on the structural and magneto-transport properties of SrRuO3 (SRO) thin films grown on (110)ortho NdGaO3 (NGO) and (110)ortho SmScO3 (SSO) substrates that result in a −1.66% compressive strain and a +1.63% tensile strain, respectively. Although the epitaxial strains induced by the NGO and SSO substrates are almost equal in magnitude, the film properties were found to be strongly dependent on the type of strain. High-resolution scanning transmission electron microscopy revealed that the compressively strained SRO films possess a tetragonal structure with no octahedral tilts, while the tensilely strained SRO films undergo a thickness-dependent transition from a monoclinic structure with octahedral tilts to a tetragonal structure with small tilts. These findings indicate that octahedral tilt propagation from the substrate into the film is preferentially induced under tensile rather than compressive strain. We further found that the magneto-transport properties of SRO films exhibit a significant dependence on the type of the epitaxial strain, demonstrating the close correlation between strain-induced octahedral distortions and magnetic anisotropy. These results highlight the important role of the type of the epitaxial strain on the structural and physical properties of epitaxial thin films.

Microwave dielectric properties of 0.2SrTiO3-0.8Ca0.61Nd0.26Ti1−xAl4x/3O3 ceramics

Abstract The phase structure, microstructure and the dielectric properties of 0.2SrTiO3-0.8Ca0.61Nd0.26Ti1−xAl4x/3O3 (0.2 ≤ x ≤ 0.5) ceramics sintered at 1420–1580 °C with conventional solid-state route had been investigated. It was found that the orthorhombic perovskite phase with a small amount of unknown second phase was obtained in the compositional range of 0.2 ≤ x ≤ 0.5. Partial substitution of Ca2+ and Nd3+ by Sr2+ on the A site resulted in a slight increase in the dielectric constant (ɛr) for Ca0.61Nd0.26Ti1−xAl4x/3O3 ceramics. With an increasing of x, the interchange of Ti4+ and Al3+ on the B site caused the increase in quality factor (Q·f). Moreover the temperature coefficient of the resonant frequency (τf) gradually reduced to close to zero with a rise of x from 0.2 to 0.5. And the variation of the τf values with the change of tolerance factor (t) and oxygen octahedral distortion (Δ) was discussed as well. An optimized microwave dielectric properties with ɛr ∼ 48.3, Q·f ∼ 40,700 GHz (at 4.98 GHz) and τf ∼ 3.6 ppm/°C could be obtained as x = 0.5 after being sintered at 1520 °C for 4 h.

Microstructures and microwave dielectric properties of Li2ZnTi3O8 ceramics prepared by reaction-sintering process

The Li2ZnTi3O8 ceramic was prepared by reaction-sintering process and its microwave dielectric properties were investigated. With increasing sintering temperature or dwell time, the relative densities, er values and Q × f values of Li2ZnTi3O8 ceramic increased initially and then decreased. The smaller τf value of Li2ZnTi3O8 ceramic sintered at 975 °C was due to the low degree of oxygen octahedral distortion. When the sintering temperature exceeded 975 °C, the τf values of Li2ZnTi3O8 ceramics were independent of sintering temperature. The tetragonal-shaped grains tend to appear in Li2ZnTi3O8 ceramics sintered at higher temperature and longer dwell time, which was dominated by phase boundary reaction mechanism. Typically, The Li2ZnTi3O8 ceramic sintered at 1,025 °C for 6 h had a maximum relative density of 97.4 % and good microwave dielectric properties of er = 25.8, Q × f = 77,100 GHz, τf = −12.4 ppm/°C. The results show that the reaction-sintering process is a simple and effective method to produce dense Li2ZnTi3O8 ceramics with excellent dielectric properties.

The oxygen octahedral distortion induced magnetic enhancement in multiferroic Bi1−xYbxFe0.95Co0.05O3 powders

The Bi1−xYbxFe0.95Co0.05O3 (x range from 0 to 0.25) powders have been prepared by the sol–gel method. A phase transition from rhombohedral R3c to orthorhombic Pbnm via a biphasic region has been observed. The Rietveld refinement and Raman scattering analyses suggest an increase of the rotation of the oxygen octahedra ω for x⩽0.05. The remanent magnetization of Bi1−xYbxFe0.95Co0.05O3 is enhanced substantially at the R3c phase region. The enhancement of the magnetization is attributed to the augment of the Dzyaloshinsky-Moriya interaction and suppression of the spin cycloid, which is determined by ω.

X-ray powder diffraction refinement of PbTi(1−x)FexO(3−δ) solid solution series

A series of iron-doped lead titanate PbTi(1−x)FexO(3−δ) samples in the x = 0–1 composition range was prepared using sol–gel synthesis at a calcination temperature of 700 °C. The room temperature CuKα powder diffraction data collected from x = 0, 0.005, 0.01, 0.025, 0.05, 0.075, 0.1, 0.125, 0.15, 0.175, 0.2, and 0.3 samples were analyzed using the Rietveld method. Magnetoplumbite (PbFe12O19) secondary phase formation was observed at compositions x = 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, and 1. Fe-composition dependent decrease of tetragonal distortion has been observed. According to Vegard's law, the B-site iron solubility limit in the lead titanate host was found to be approximately 10 mol% (x = 0.1). Homogeneous distribution of dopants below the solubility limit was evidenced by the linear behavior of unit-cell parameters. The absence of a cubic phase and the persistence of distorted oxygen octahedra indicated the existence of ferroelectric properties even at the x = 0.3 composition. Beyond x = 0.3, the P4mm tetragonal model was determined to be invalid possibly because of oxygen defect driven structural changes, mainly tilting Fe-polyhedra.

Structural characterization of amorphous YCrO3 from first principles

We perform a theoretical prediction of the structure of amorphous YCrO3. We obtained equivalent amorphous structures by means of two independent first principles density functional theory based methods: molecular dynamics and stochastic quenching. In our structural analysis we include radial and angle distribution functions as well as calculations of bond lengths and average coordination numbers. We find Cr+3 atoms situated in slightly distorted oxygen octahedra throughout the amorphous structures and that the distribution of these octahedra is disordered. The presence of the same Cr+3 local environments that give rise to ferroelectricity in the orthorhombic perovskite structure suggests that the amorphous phase of YCrO3 may also exhibit ferroelectric properties.

Electrical properties of [Li 0.04(K 0.5Na 0.5) 0.96− xAg x](Nb 1− ySb y)O 3 ceramics

Dependence of electrical properties on the structural characteristics of Li 0.04(K 0.5Na 0.5) 0.96(Nb 1− y Sb y )O 3 (LKNNS ( x = 0, 0.00 ≤ y ≤ 0.10)) and [Li 0.04(K 0.5Na 0.5) 0.96− x Ag x ](Nb 0.925Sb 0.075)O 3 (LKNANS (0.01 ≤ x ≤ 0.05, y = 0.075)) were investigated. The oxygen octahedral distortion was dependent on Ag + and/or Sb 5+ content which affected to the phase transition temperature of LKNNS and LKNANS ceramics. The orthorhombic–tetragonal and tetragonal–cubic phase transition temperatures ( T O–T, T C ) of the specimens were decreased with increasing of average octahedral distortion. With increasing of Sb 5+ content, the electromechanical coupling factor ( k p), piezoelectric constant ( d 33) and dielectric constant ( ɛ r) of the sintered specimens were increased up to y = 0.075, and then decreased. These results could be attributed to the shift of T O–T to near room temperature for Li 0.04(K 0.5Na 0.5) 0.96(Nb 0.0925Sb 0.075)O 3.

Relationships between crystal structure and electrical properties of Li 0.055[Ag x(K 0.5Na 0.5) 1− x] 0.945(Nb 1− yTa y)O 3 ceramics

Electrical properties of perovskite Li 0.055[Ag x (K 0.5Na 0.5) 1− x ] 0.945(Nb 1− y Ta y )O 3 (0.00 ≤ x ≤ 0.04, 0.01 ≤ y ≤ 0.09) ceramics were investigated based on the structural characteristics. A morphotropic phase boundary (MPB) between orthorhombic and tetragonal phase was detected through the entire range of compositions. With increasing of Ta 5+ content, the dielectric constant ( ɛ r ), piezoelectric coefficient ( d 33) and electromechanical coupling factor ( k p ) of Li 0.055(K 0.5Na 0.5) 0.945(Nb 1− y Ta y )O 3 ceramics were increased up to y = 0.07 and then decreased, while mechanical quality factor ( Q m) was increased. However, the ɛ r , d 33, k p and Q m of Li 0.055[Ag x (K 0.5Na 0.5) 1− x ] 0.945(Nb 0.07Ta 0.03)O 3 ceramics were not changed remarkably with Ag + content. The dependence of temperature coefficient of k p ( TCk p ) on the oxygen octahedral distortion was also discussed by Raman-active vibrations modes.

Microwave dielectric properties of (Zn 1/3B 2/3) 0.5(Ti 0.8M 0.2) 0.5O 2 (B = Nb 5+, Ta 5+, M = Sn 4+, Ge 4+) ceramics

Dielectric properties of (Zn 1/3Nb (2− x)/3 Ta x/3 ) 0.5(Ti 0.8Sn 0.1Ge 0.1) 0.5O 2 ( x = 0, 1, 2) and/or (Zn 1/3Nb 1/3Ta l/3) 0.5(Ti 0.8Sn 0.2(l− y) Ge 0.2 y ) 0.5O 2 ( y = 0, 0.5, 1) were investigated at the microwave frequencies. For the compositions with single phase of rutile structure, the dielectric constant ( K) of specimens was not only dependent on the dielectric polarizabilities, but also on the bond length ratio of apical bond ( d apical ) to equatorial bond ( d equatorial ) of oxygen octahedron in the unit cell. Temperature coefficients of the resonant frequencies ( TCF) of the specimens with B = Nb 5+ and/or M = Sn 4+ was larger than those with B = Ta 5+ and/or M = Ge 4+. These results could be attributed to the changes of the degree of oxygen octahedral distortion. Quality factors ( Qf) of the specimens with B = Ta 5+ and/or M = Sn 4+ were larger than those with B = Nb 5+ and/or M = Ge 4+.

Dielectric properties of rutile structure (Zn1/3B2/3)x(TiySn1ߝy)1ߝxO2 (B=Nb5+,Ta5+) at microwave frequencies

The microwave dielectric properties of rutile (Zn₁/₃ B₂/₃)x(TiySn(1-y))(1-x)O₂ (B=Nb⁵+,Ta⁵+, 0.05 ≤ x ≤ 0.35, y = 0.2, 0.8) were investigated with structural characteristics. A single phase with tetragonal rutile (P4₂/mnm, no. 136, Z = 2) structure was obtained through the entire range of compositions. With an increase of (Zn₁/₃B₂/₃)O₂ content, the dielectric constant (K) of sintered specimens with (Ti₀.₂Sn₀.₈)O₂ increased, whereas that of the specimens with (Ti₀.₂Sn₀.₈)O₂ decreased. The temperature coefficient of resonant frequency (TCF) of sintered specimens was dependent upon oxygen octahedral distortion and/or cations bond valence of the rutile structure. The quality factor (Qf) was dependent upon (Zn₁/₃B₂/₃)O₂ content, and the specimens with (Ti₀.₂Sn₀.₈)O₂ showed larger Qf values than those with (Ti₀.₂Sn₀.₈)O₂ for the same content of (Zn₁/₃B₂/₃)O₂.

Quantifying octahedral rotations in strained perovskite oxide films

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.

A rapid one-step mechanosynthesis and characterization of nanocrystalline CaFe 2O 4 with orthorhombic structure

CaFe 2O 4 nanopowders were prepared via single-step mechanochemical processing of two various mixtures of precursors: simple oxides (CaO and α-Fe 2O 3) and elemental metal and oxide powders (Ca and α-Fe 2O 3). The mechanically induced evolution of the CaO/α-Fe 2O 3 and Ca/α-Fe 2O 3 mixtures was followed by 57Fe Mössbauer spectroscopy, XRD, and HR-TEM. The mechanosynthesis of CaFe 2O 4 proceeds very rapidly if starting from the metal/oxide system; after only 1 h of the mechanochemical treatment performed at room temperature, the synthesis of the complex oxide is almost completed. This is in a strong contrast to the conventional solid-state synthesis of CaFe 2O 4, which requires prolonged exposure (∼20 h) at considerably elevated temperatures (∼1400 K). It was revealed that mechanosynthesised CaFe 2O 4 nanoparticles with an average size of about 15 nm possess the core–shell structure consisting of an ordered inner core surrounded by a disordered surface shell with the thickness of about 1.9 nm. The main structural features of the surface shell of nanoparticles are distorted oxygen octahedra.