Effects Of Structural Distortion Research Articles

Boosting Photocatalytic Nitrogen Fixation via Nanoarchitectonics Using Oxygen Vacancy Regulation in W-Doped Bi2MoO6 Nanosheets.

Ammonia and nitrates are key raw materials for various chemical and pharmaceutical industries. The conventional methods like Haber-Bosch and Ostwald methods used in the synthesis of ammonia and nitrates, respectively, result in harmful emission of gases. In recent years, the photocatalytic fixation of N2 into NH3 and nitrates has become a hot topic since it is a green and cost-effective approach. However, the simultaneous production of ammonia and nitrates has not been studied much. In this regard, we have synthesized W-doped Bi2MoO6 nanosheets in various molar ratios and demonstrated their potential as efficient photocatalysts for the simultaneous production of NH3 and NO3- ions under visible light irradiation. It was found that one of the catalysts (BMWO0.4) having an optimal molar ratio of doped tungsten showed the best photocatalytic NH3 production (56 μmol h-1) without using any sacrificial agents along with the simultaneous production of NO3- ions at a rate of 7 μmol h-1. The enhanced photocatalytic activity of the synthesized photocatalysts could be ascribed to oxygen vacancy defects caused by Mo substitution by a more electronegative W atom. Furthermore, density functional theory calculations verified the alteration in the band gap after doping of W atoms and also showed a strong chemisorption of N2 over the photocatalyst surface leading to its activation and thereby enhancing the photocatalytic activity. Thus, the present work provides insights into the effect of structural distortions on tailoring the efficiency of materials used in photocatalytic N2 fixation.

First-principles investigation of the Metal-insulator transitions in Ti-substituted tetragonal rutile VO2

The effect of Ti-substitution on the structural, electronic and magnetic properties of VO2 in its high-temperature rutile phase is investigated. The crystal symmetry reduces with Ti concentration in V1-xTixO2 (x = 0.25, 0.5, 0.75) due to the breaking of mirror symmetries in the VO6 and TiO6 octahedra. The compounds V0.75Ti0.25O2 and V0.5Ti0.5O2 are found to be ferrimagnetic metals. These two compounds encounter metal-to-half-metal transition for the application of Coulomb repulsion of U = 1 eV and 0.5 eV respectively, exhibiting ferromagnetism. On the other hand, VO2 encounters metal-to-half-metal transition for x = 0.75 in V1-xTixO2 (the corresponding compound is V0.25Ti0.75O2), showing ferromagnetism. More interestingly, all three substituted compounds encounter metal-insulator transition (MIT) for U = 4 eV, 2.5 eV and 2 eV respectively, preserving ferromagnetism. In the absence of U, orbital fluctuation is observed among the three V-t2g states (V-dx2-y2, dyz, dxz). The sharing of the available single V-3d electron by the three V-t2g states results in the metallicity of V0.75Ti0.25O2 and V0.5Ti0.5O2, and half-metallicity of all three compounds. In the insulating phases, electron correlation is triggered by the application of U which diminishes the orbital fluctuation among the three V-t2g states. The single V-3d electron occupies the V-dx2-y2 state, leading to V4+-V4+ and Ti4+-Ti4+ charge ordering. Therefore, the simultaneous effect of structural distortion, U and V4+-V4+, Ti4+-Ti4+ charge ordering is responsible for the MIT in V0.75Ti0.25O2, V0.5Ti0.5O2 and V0.25Ti0.75O2. The cooperative effect of p-d hybridization and V–O antiferromagnetic coupling results in ferromagnetism in the insulating phases of all three compounds.

The effect of Cr-substitution on the structural, electronic and magnetic properties of rutile VO2

The effect of Cr-substitution on the structural, electronic and magnetic properties of rutile VO2 is investigated in this study. Pure VO2 is found to be a nonmagnetic metal, whereas all three Cr- substituted compounds [V1-xCrxO2 (VCO) with x = 0.25, 0.5, 0.75] are found to exhibit ferrimagnetic metallic behaviour. However, pure and all VCO compounds encounter metal to half-metal transition for the application of U = 2 eV, showing ferromagnetism. VO2, the VCO compounds V0.75Cr0.25O2 and V0.5Cr0.5O2 undergo metal–insulator transition (MIT) for U = 5 eV, 5.5 eV and 3.5 eV respectively, preserving ferromagnetism. Besides, V0.25Cr0.75O2 preserves its ferromagnetic half-metallic character even application of U as high as 6 eV. The breaking of mirror symmetries in the VO6/CrO6 octahedra induces structural distortions in all three VCO compounds. The MIT in pure VO2 is caused by the transfer of electrons from the V-dyz and dxz states to the dx2- y2 states. The simultaneous effect of structural distortion, V4+- V4+ and Cr3+-V5+ charge ordering and U is responsible for the MIT in V0.75Cr0.25O2. On the other hand, the simulteneous effect of structural distortion, transfer of electrons from V-t2g to Cr-t2g states and U is responsible for the MIT in V0.5Cr0.5O2. Although electron transfer is commenced from V-t2g to Cr-t2g states in V0.25Cr0.75O2, the latter cannot be completely occupied by the transferred electrons, resulting in the non-opening of a band gap near the Fermi level. The p-d hybridization between O-2p and V-3d/Cr-3d states, and V/Cr-O antiferromagnetic coupling facilitates ferromagnetism in insulating VO2, V0.75Cr0.25O2, V0.5Cr0.5O2 and half-metallic V0.25Cr0.75O2.

Engineering Specific Mo–O Bond Stretching to Activate Lattice Oxygen in V-Doped Bi2MoO6 for Enhanced Oxidative Dehydrogenation of 1-Butene

Structural distortion has been demonstrated to regulate lattice oxygen activity, but its precise regulation has always been a thorny problem, especially for the oxidative dehydrogenation of hydrocarbons. Herein, by synthesizing a series of V-doped Bi2MoO6 catalysts, mechanistic insight into the effect of structural distortion on the catalytic activity of oxidative dehydrogenation of 1-butene was provided. Comprehensive characterizations and kinetics tests revealed that the V-doped Bi2MoO6-induced active lattice oxygen initiates 1-butene activation by abstracting the C–H bond, which is a rate-determining step. Combined with density functional theory results, an overwhelming effect of the bond length between Mo and apical oxygen atom in the MoO6 octahedron on lattice oxygen activation and butadiene formation pathway was proposed. The stretching Mo–Oapical bond elicited by V doping enhances the abstraction of hydrogen and accelerates the redox cycle. The sample with a 5% V content possesses the maximum Mo–Oapical bond length, enabling superior catalytic performance at an 87.6% 1,3-butadiene yield.

The tuning of temperature stability in ultralow loss (Ba/Sr) zirconate microwave dielectric

The ceramic composition of Ba1- x Sr x (Zr0.96Ga0.02Nb0.02)O3, where x = 0, 0.1, 0.2, 0.4, 0.5, 0.7, 0.9 and 1, was prepared and its crystal structure, microstructure, Raman and dielectric properties are analyzed. As revealed by powder x-ray diffraction analysis, the crystal structure transforms from cubic (Pm-3m) at 0 ≤ x ≤ 0.2 via tetragonal (Cmcm) at 0.2 < x ≤ 0.5 to orthorhombic (Pbnm) at x > 0.5. This is in agreement with the structural transformations previously reported for the Ba1- x Sr x ZrO3 and Ba1- x Ca x ZrO3 solid solution systems. Raman spectroscopy consistently supports these findings. The effects of structural distortion, grain size and density of samples on the microwave dielectric properties are discussed. With increasing Sr content from x = 0 to 0.2, the Q-factor of Ba1- x Sr x (Zr0.96Ga0.02Nb0.02)O3 ceramics drastically decreases from 18,000 to 3,000 at 10 GHz. The ceramics with a temperature coefficient of −1.4 ppm/K, dielectric constant of 37 and Q-factor of 2,900 at the resonant frequency f = 10 GHz was obtained at x = 0.5.

A study of the crystal structure of a Tb-Dy-Fe alloy during annealing via rietveld analysis

The crystal structure and magnetic structure of Tb-Dy-Fe compounds before and after annealing heat treatment were researched, in order to further study the influence factors on the magnetic performance besides the orientation and phases. The Rietveld method was used to refine the structural data and obtain small changes in the structure. The results confirm the existence of tetragonal and rhombohedral phases at room temperature, which is consistent with previous findings. Both samples had tetragonal structures and rhombohedral distortion structures at room temperature. The lattice constants of the rhombohedral structure were a = 5.18160 Å and c = 12.72178 Å, while the volume diagonal of the tetragonal cell was 12.69513 Å, indicating the amount of change between the two structures. The volume proportion of the hexagonal phase increased from 25% to 51% after heat treatment. The refinements made on the basis of the neutron patterns indicated that the refined Tb/Dy and Fe moments are antiparallel, while the average atomic magnetic moment of Tb/Dy increased in the annealed state compared with that in the as-cast state. By combining all the analytical results, we discussed the effect of structural distortion on the magnetostriction of Tb-Dy-Fe.

Cation-size mismatch as a predictive descriptor for structural distortion, configurational disorder, and valence-band splitting in II-IV-N2 semiconductors

The II-IV-N2 class of heterovalent ternary nitrides has gained significant interest as alternatives to the III-nitrides for electronic and optoelectronic applications. In this study, we apply first-principles calculations based on density functional theory to systematically investigate the effects of structural distortions due to cation size mismatch on the configurational disorder of the cation sublattice and the valence band structure in this class of materials. We find that larger size mismatch between the group-II and the group-IV cations results in stronger lattice distortions from the ideal hexagonal ratio, which in turn inhibits the propensity of these materials toward octet-rule violating cation disorder. We also demonstrate that the formation energy of a single cation antisite pair, which is fast and simple to calculate, is a strong indicator of a material's propensity toward disorder. Furthermore, the breaking of in-plane symmetry leads to a splitting of the top three valence bands at Γ, which is also directly related to the magnitude of structural distortions. Our work demonstrates that the structural and functional properties of the II-IV-N2 materials can be finely tuned through controllable structural distortions that stem from the choice of cations.

Coupled spin–orbital fluctuations in a three orbital model for 4d and 5d oxides with electron fillings n = 3, 4, 5—application to NaOsO3, Ca2RuO4 and Sr2IrO4

A unified approach is presented for investigating coupled spin–orbital fluctuations within a realistic three-orbital model for strongly spin–orbit coupled systems with electron fillings n = 3, 4, 5 in the t 2g sector of d yz , d xz , d xy orbitals. A generalized fluctuation propagator is constructed which is consistent with the generalized self-consistent Hartree–Fock approximation where all Coulomb interaction contributions involving orbital diagonal and off-diagonal spin and charge condensates are included. Besides the low-energy magnon, intermediate-energy orbiton and spin–orbiton, and high-energy spin–orbit exciton modes, the generalized spectral function also shows other high-energy excitations such as the Hund’s coupling induced gapped magnon modes. We relate the characteristic features of the coupled spin–orbital excitations to the complex magnetic behavior resulting from the interplay between electronic bands, spin–orbit coupling, Coulomb interactions, and structural distortion effects, as realized in the compounds NaOsO3, Ca2RuO4, and Sr2IrO4.

Effects of structural distortion on the optoelectronic properties and reactivity of fullerenes: a DFT study

Effects of structural distortion on the optoelectronic properties and reactivity of fullerenes: a DFT study

Anisotropic ferroelectric distortion effects on the RKKY interaction in topological crystalline insulators

Manipulation of electronic and magnetic properties of topological materials is a topic of much interest in spintronic and valleytronic applications. Perturbation tuning of multiple Dirac cones on the (001) surface of topological crystalline insulators (TCIs) is also a related topic of growing interest. Here we show the numerical evidence for the ferroelectric structural distortion effects on the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction between two magnetic impurity moments on the SnTe (001) and related alloys. The mirror symmetry breaking between Dirac cones induced by the ferroelectric distortion could be divided into various possible configurations including the isotropically gapped, coexistence of gapless and gapped, and anisotropically gapped phases. Based on the retarded perturbed Green’s functions of the generalized gapped Dirac model, we numerically find the RKKY response for each phase. The distortion-induced symmetry breaking constitutes complex and interesting magnetic responses between magnetic moments compared to the pristine TCIs. In the specific case of coexisted gapless and gapped phases, a nontrivial behavior of the RKKY interaction is observed, which has not been seen in other Dirac materials up until now. For two impurities resided on the same sublattices, depending on the distortion strength, magnetic orders above of a critical impurity separation exhibit irregular ferromagnetic ⇔ antiferromagnetic phase transitions. However, independent of the impurity separation and distortion strength, no phase transition emerges for two impurities resided on different sublattices. This essential study sheds light on magnetic properties of Dirac materials with anisotropic mass terms and also makes TCIs applications relatively easy to understand.

Effect of structural distortions on articular cartilage permeability under large deformations.

The permeability of articular cartilage has a key role in load support and lubrication in diarthrodial joints. The microstructural rearrangement and consequent alteration in permeability caused by the large deformations undergone by cartilage have been previously modelled with a multi-scale approach. At the microscopic scale, the tissue is regarded as a homogeneous fluid-filled proteoglycan matrix reinforced by collagen fibres. A material point is described by a representative element of volume (REV), comprising a collagen fibre surrounded by a jacket of fluid-saturated proteoglycan matrix. At the macroscopic scale, the statistical orientation of the fibres is accounted for via averaging of the REV over all possible directions. The previous models accounted for volumetric deformation and fibre reorientation, but did not consider the cross-sectional distortion of the REV, which changes the widths of the fluid channels in different directions. We account for REV cross-sectional distortion and demonstrate its effects by simulating confined compression tests for the superficial, middle and deep zones of articular cartilage. The proposed model captures published experimental results that were not reproduced correctly by the previous models, and shows that each factor (volumetric deformation, fibre reorientation, REV cross-sectional distortion) can be dominant, depending on fibre orientation and amount of compression, implying that all three factors should be accounted for when modelling cartilage permeability.

Confluence of structural distortion and A -site composition in the band gaps of perovskite niobate and tantalate photocatalysts

The effects of structural distortion and $A$-site composition on the band gaps and band-edge orbitals of perovskite niobate and tantalate photocatalysts are explored using density-functional theory and crystal-field theory. When filled $d$ orbitals are introduced to the $A$ site by replacing alkali metal cations with silver, the crystal-field splitting energies of the $A$-site $d$ orbitals play a dominant role in determining the effect of silver in narrowing the band gap. It is demonstrated that an intuitive, geometry-based metric can usually predict within 0.1 eV the extent to which the presence of silver narrows the band gap. The ability to make such predictions is both valuable and potentially generalizable toward the design of photocatalysts.

Structural distortions of fullerene C60n (n = 0 to −6) by first principle density functional theory

The anions of C60 fullerene have geometrically multiple distortions. These geometric changes of C60 were directly related to the orbital changes called structural distortion effects. In this study, neutral C60 fullerene and its −1 to −6 anions were optimized by using DFT method. The geometric changes and the electronic properties were also investigated. Unrestricted calculations were applied to investigate the energy gap changes and density of states (DOS) spectrums as well. The electron localization function (ELF) along with the natural bond orbital (NBO) calculations were presented the considerable images from the changes of electron properties of C60 fullerene. The results show that in −5 and −6 anions lost π bonds and also can see creation of non-bonding lone pair in −6 anion. The calculations on the vibrational levels, structural changes such as changes in diameters and dipole moments of C60n (n = 0 to −6) were investigated as well. The most stable spin forms of the optimized C60 structures were chosen to observe structural distortions by using the DFT computational method. For some of the anions the wide changes of forms, energies of the valance orbitals and the energy states were investigated for the low and high spins. Also, the spin channel phenomenon was studied by using the results of DFT-B3LYP/6-31G∗ calculations. The NBO analysis was performed due to its beneficial information in the changes of fullerene electronic system properties for all the anion states in the selected computational level method. The changes of the calculated IR spectra patterns have discussed due to the charge changes of C60n (n = 0 to −6) and on the basis of the structural distortions role (John-Teller effect). A linear equation was obtained by using graph theory (GT) between the energy of HOMO and/or LUMO orbitals and load amount of anions. The DOS spectrums and HOMO and LUMO orbitals in the different discussed spin states show that all fullerene anions have different spin behavior and magnetic specifications, in which anions −2 and −3 anions of C60 act as the spin channels.

Structural distortion induced ferromagnetism in two-dimensional metal-free graphitic-C3N4 nanosheets.

With the assistance of innovative approaches driven by nanotechnology, engineering 2D materials into designed architectures or desired structures could tailor the electronic structure into an appropriate energy band structure, tuning the properties of the materials to be a predictable manner. Here we systematically studied the role that the structural distortion plays in the magnetism by taking two-dimensional metal-free graphitic-C3N4 as an example. Through the controllable structural distortion engineering introduced by post-heat-treatment in the experiment, the ferromagnetism is observed in graphitic-C3N4 nanosheets, which benefits from the electronic structural deformation, showing intriguing structural distortion-dependent ferromagnetism. This study not only offers new insight into the in-depth understanding of the structural distortion effect on the magnetism, but also provides a new way for searching and designing new magnetic materials.

Tuning the Néel Temperature of Hexagonal Ferrites by Structural Distortion.

To tune the magnetic properties of hexagonal ferrites, a family of magnetoelectric multiferroic materials, by atomic-scale structural engineering, we studied the effect of structural distortion on the magnetic ordering temperature (T_{N}) in these materials. Using the symmetry analysis, we show that unlike most antiferromagnetic rare-earth transition-metal perovskites, a larger structural distortion leads to a higher T_{N} in hexagonal ferrites and manganites, because the K_{3} structural distortion induces the three-dimensional magnetic ordering, which is forbidden in the undistorted structure by symmetry. We also revealed a near-linear relation between T_{N} and the tolerance factor and a power-law relation between T_{N} and the K_{3} distortion amplitude. Following the analysis, a record-high T_{N} (185K) among hexagonal ferrites was predicted in hexagonal ScFeO_{3} and experimentally verified in epitaxially stabilized films. These results add to the paradigm of spin-lattice coupling in antiferromagnetic oxides and suggests further tunability of hexagonal ferrites if more lattice distortion can be achieved.

“DistorX” program for analysis of structural distortions affecting X-ray diffraction patterns

For the purposes of research on strongly correlated electronic systems (SCES), a computer program, DistorX (Distortion or X-ray diffraction patterns), was created. The program is an interactive Jupyter notebook for simulating the effects of structural distortions on X-ray diffraction patterns. The program is designed to be universal, in that it may be successfully applied to a variety of structures. In previous reports, a structural transition from a cubic phase of Yb3Rh4Sn13 – type to the superlattice variant was been observed at 160 K for a series of skutterudite-related Ce3M4Sn13 compounds, where M = Co, Ru or Rh. In this work, we use a specialized build of DistorX to simulate the low-temperature X-ray diffraction patterns of a distorted unit cell. The method described here obtains simulated XRD patterns from the atomic positions and permits investigation of crystal structure without imposed symmetry operations. We further indicate the crystallographic plane in which the distortion occurs, and explain the possible origin of CDW in these materials.

Influence of cation doping (Li+, Na+, K+) on photocatalytic activity of MIL-53(Fe)

The work highlights the influence of cation doping on photocatalytic activity of MIL-53(Fe) in artificial light. The effect of intrinsic factors, viz. band-gap energy, electron conductivity and surface area, influencing photocatalysis was studied in detail w.r.t. the degradation of brilliant green dye both in the presence and in the absence of H2O2 as an electron scavenger. Reduction in band gap after cation doping was observed for Li+ (1.81 eV), Na+ (1.875 eV) and K+ (1.89 eV) when compared with the parent MIL-53(Fe) (1.95 eV). However, the photoactivity of the synthesized materials was in the following progression: MIL-53(Fe)(81.6%) > Li-MIL-53(Fe)(27.6%) > Na-MIL-53(Fe)(25.77%) > K-MIL-53(Fe) (24.18%). Effect of structural distortions in the octahedral backbone of MIL-53(Fe) due to cation doping was inferred as the plausible reason behind such behavior. The degree of electron delocalization and specific surface areas were found to be in the following progression: Na-MIL-53(Fe) (very low, 7.72 m2 g−1) < K-MIL-53(Fe) (low, 28.38 m2 g−1) < Li-MIL-53(Fe) (moderate, 42.14 m2 g−1) < MIL-53(Fe) (high, 51.71 m2 g−1), indicating that the intrinsic material properties have combinatorial influence on photocatalytic activity.

Effect of structural distortion on the electronic band structure of NaOsO3 studied within density functional theory and a three-orbital model

Effects of the structural distortion associated with the $\rm OsO_6$ octahedral rotation and tilting on the electronic band structure and magnetic anisotropy energy for the $5d^3$ compound NaOsO$_3$ are investigated using the density functional theory (DFT) and within a three-orbital model. Comparison of the essential features of the DFT band structures with the three-orbital model for both the undistorted and distorted structures provides insight into the orbital and directional asymmetry in the electron hopping terms resulting from the structural distortion. The orbital mixing terms obtained in the transformed hopping Hamiltonian resulting from the octahedral rotations are shown to account for the fine features in the DFT band structure. Staggered magnetization and the magnetic character of states near the Fermi energy indicate weak coupling behavior.

Effects of structural distortion on magnetic properties of CexY3−xFe5O12

This paper reports on preparation of the garnet compounds CexY3−xFe5O12 (x = 0.0, 0.1, 0.2, 0.3, 0.4, 0.5 and 0.6) by sol-gel method. The structural and magnetic properties were characterized by means of X-ray diffraction (XRD), scanning electron microscope (SEM), fourier transform infrared spectra (FTIR), alternative gradient field magnetometer (AGFM) and Mössbauer measurement. The XRD analysis indicated that all the samples have garnet structure. Also, the SEM images revealed that the particle sizes are in the range of 65 nm and has no noticeable effect by Ce3+ ion substitution. The a-d superexchange interactions is found to play an important role in the saturation magnetization of the sample. The variation of coercivity is governed by the change of anisotropy. The hyperfine parameters were discussed based on fitting the Mössbauer spectra with four sextets corresponding to distorted and undistorted tetrahedral and octahedral sites.

Study of structural and conduction properties of Co3O4-doped corundum-type Mg4Ta2O9ceramics

The effects of structural distortion on charge transport, crystal defects and relaxor response are studied for corundum-type (Mg1−xCox)4Ta2O9(x= 0–0.5) ceramics.