Undistorted Structure Research Articles

Catalytic pyrolysis of fish waste oil using ZSM-5 catalyst for the production of renewable biofuel

This present study enlightens the eco-friendly green synthesis of ZSM-5 from natural clay montmorillonite, and its proper incorporation with ‘Ni’. Nickle (Ni) was wet impregnated onto HZSM-5 and the resulting catalyst was characterized by various techniques including XRD, BET, N2 Sorption Studies, TPD, SEM and TEM techniques. The SEM images revealed the uniform distribution of Ni over HZSM-5 zeolite catalyst and the XRD results indicated the undistorted crystalline structure of HZSM-5 even after impregnation of Ni. The latter part of the work concentrates on the strength of the catalyst in cracking oil derived from discarded fish parts. Discarded fish waste was pyrolyzed to obtain the fish oil, which was then used for cracking studies. The fish oil was efficiently converted (99% conversion) by Ni/ZSM5 (50 wt %) and yielded 70% liquid fractions, which formed gasoline (78.6%), kerosene (12.3%) and diesel (9.1%). The research is a complete parcel to examine the working potential of the produced biofuel in pre-existing engines. The quality of gasoline fraction was tested according to ASTM standards, which showed that the heating value was slightly lower compared to fossil gasoline. The torque and brake fuel consumption were also examined and it indicated that the fish oil derived gasoline fuel may need to be mixed with the commercial gasoline to optimize its performance.

Face deblurring based on regularized structure and enhanced texture information

Image deblurring is an essential problem in computer vision. Due to highly structured and special facial components (e.g. eyes), most general image deblurring methods and face deblurring methods failed to yield more explicit structure and facial details, resulting in too smooth, uncoordinated and distorted face structure. Considering the unique face texture and sufficient facial details, we present an effective face deblurring network by exploiting more regularized structure and enhanced texture information (RSETNet). We first incorporate the face parsing network with fine-tuning to obtain more accurate face structure, and we present the feature adaptive denormalization (FAD) to regularize the facial structure as a condition of auxiliary to generate more harmonious and undistorted face structure. Meanwhile, to improve the generated facial texture information, we propose a new Laplace depth-wise separable convolution (LDConv) and multi-patch discriminator. Compared with existing methods, our face deblurring method could restore face structure more accurately and with more facial details. Experiments on two public face datasets have demonstrated the effectiveness of our proposed methods in terms of qualitative and quantitative indicators.

On the possibility of low-symmetry structures in Zr2CoSi

First principles calculation shows that the energy difference between the cubic L21 and Hg2CuTi inverse structure for Zr2CoSi is 1.454 eV/f.u., indicating than the L21 structure is more stable. The density of states and band structure of Zr2CoSi in the L21 structure do not support the half–metallic state. Moreover, van Hove singularity involving partially occupied Co 3d eg states occurs in minority states at the Fermi level and the Zr2CoSi in L21 structure satisfies the preconditions to exhibit a cubic-to-tetragonal instability according to a band-Jahn-Teller mechanism. The computed total energy landscapes of the system shows that there are two tetragonal and one orthorhombic distorted phases of Zr2CoSi that are energetically favored over the undistorted cubic L21 structure. However, only one tetragonal phase of Zr2CoSi with c/a > 1.0 fulfills necessary and sufficient elastic stability conditions for both GGA and LDA exchange–correlation potentials.

Understanding structural distortions in hybrid layered perovskites with the n = 1 Ruddlesden-Popper structure.

A symmetry mode analysis yields 47 symmetrically distinct patterns of octahedral tilting in hybrid organic-inorganic layered perovskites that adopt the n = 1 Ruddlesden-Popper (RP) structure. The crystal structures of compounds belonging to this family are compared with the predictions of the symmetry analysis. Approximately 88% of the 140 unique structures have symmetries that agree with those expected based on octahedral tilting alone, while the remaining compounds have additional structural features that further lower the symmetry, such as asymmetric packing of bulky organic cations, distortions of metal-centered octahedra or a shift of the inorganic layers that deviates from the a/2 + b/2 shift associated with the RP structure. The structures of real compounds are heterogeneously distributed amongst the various tilt systems, with only 9 of the 47 tilt systems represented. No examples of in-phase ψ-tilts about the a and/or b axes of the undistorted parent structure were found, while at the other extreme ∼66% of the known structures possess a combination of out-of-phase φ-tilts about the a and/or b axes and θ-tilts (rotations) about the c axis. The latter combination leads to favorable hydrogen bonding interactions that accommodate the chemically inequivalent halide ions within the inorganic layers. In some compounds, primarily those that contain either Pb2+ or Sn2+, favorable hydrogen bonding interactions can also be achieved by distortions of the octahedra in combination with θ-tilts.

Magnetic structure of the swedenborgite compound CaBaMn2Fe2O7 derived by powder neutron diffraction and Mössbauer spectroscopy

We present a study combining neutron diffraction and $^{57}\mathrm{Fe}$ M\"ossbauer spectroscopy on a powder sample of ${\mathrm{CaBaMn}}_{2}{\mathrm{Fe}}_{2}{\mathrm{O}}_{7}$ belonging to the large family of swedenborgite compounds. The undistorted hexagonal crystal structure (space group $P{6}_{3}mc$) is preserved down to low temperatures, and all employed techniques reveal a transition into a magnetically long-range ordered phase at ${T}_{\mathrm{N}}$ = 205 K. The magnetic Bragg peak intensities from the powder diffraction patterns together with a symmetry analysis of the employed models unambiguously reveal the classical $\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3}$ magnetic structure on a hexagonal lattice with propagation vector $\mathbf{q}=(\frac{1}{3}\phantom{\rule{0.28em}{0ex}}\frac{1}{3}\phantom{\rule{0.28em}{0ex}}0)$. The nuclear Bragg peak intensities allowed the statistical distribution of Fe and Mn ions on both trigonal and kagome sites of the complex swedenborgite structure to be analyzed which was considered to explain the complex shape of the M\"ossbauer spectra.

Competing magnetic and nonmagnetic states in monolayer VSe2 with charge density wave

The field of two-dimensional ferromagnets has been reinvigorated by the discovery of ${\mathrm{VSe}}_{2}$ monolayer grown on van der Waals substrates, which is reported to be ferromagnetic with a Curie point higher than 330 K. However, the ferromagnetic and nonmagnetic states of pristine monolayer ${\mathrm{VSe}}_{2}$ are highly debated. Here, employing density functional theory, Wannier function calculations, and the band unfolding method, we explore the electronic structure of monolayer ${\mathrm{VSe}}_{2}$ with a $\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{7}$ charge density wave (CDW). Certain qualitative aspects of the calculated unfolded band dispersion and unfolded Fermi surface of monolayer ${\mathrm{VSe}}_{2}$ with the $\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{7}$ CDW in the nonmagnetic state agree well with previous angle-resolved photoemission spectroscopy results, albeit with uncertainty about whether these experiments probed single or multiple domains. Specifically, we find that an isolated CDW domain naturally induces a strong breaking of the threefold symmetry of the electronic structure. In addition we find that, relative to the undistorted structure, the CDW structure shows a strong competition between nonmagnetic and various magnetic states, with an energy difference less than 5 meV/f.u. For the CDW structure in the antiferromagnetic state, the band dispersions and Fermi surface are similar to those in the nonmagnetic state, while the unfolded bands of the ferromagnetic CDW state display a sizable exchange splitting. These results indicate the possibility of various antiferromagnetic fluctuations in ${\mathrm{VSe}}_{2}$ to coexist and compete with ferromagnetic order and the experimentally reported CDW order. Our calculations build insights for exploring the interplay between magnetism and CDW behaviors more generally in transition metal dichalcogenides.

Metal-insulator transition and anomalous lattice parameters changes in Ru-doped VO2

${\mathrm{VO}}_{2}$, of interest for decades due to both its phenomenology and its potential applications, has a monoclinic distortion of the rutile crystal structure at ambient temperature that is coupled to its metal-insulator transition. In contrast, ${\mathrm{RuO}}_{2}$ has three electrons more per formula unit, is a metallic conductor, and has an undistorted rutile structure. Here, we report a systematic study of Ru-doped ${\mathrm{VO}}_{2}$ (${\mathrm{V}}_{1\text{\ensuremath{-}}x}{\mathrm{Ru}}_{x}{\mathrm{O}}_{2}, 0.01\ensuremath{\le}x\ensuremath{\le}0.9)$, generally characterizing its crystal structure, magnetic and electronic properties, and heat capacity. The composition-dependent Wilson ratio is determined. We find that an unusually high Ru doping value (80%, $x=0.8$) is required to achieve a metallic state in ${\mathrm{V}}_{1\text{\ensuremath{-}}x}{\mathrm{Ru}}_{x}{\mathrm{O}}_{2}$. No superconductivity was observed down to 0.1 K in the metallic materials. We propose a possible understanding for how the insulating state can exist in ${\mathrm{V}}_{1\text{\ensuremath{-}}x}{\mathrm{Ru}}_{x}{\mathrm{O}}_{2}$ at high Ru contents.

Competition between Ta-Ta and Te-Te bonding leading to the commensurate charge density wave in TaTe4

The origin of the charge density wave in ${\mathrm{TaTe}}_{4}$ is discussed on the basis of a first-principles density functional theory analysis of the Fermi surface, electron-hole response function, phonon band structure of the average structure, and structural optimization of the modulated phase. Analysis of the band structure and Fermi surface of the average structure clearly proves that despite the presence of $\mathrm{Ta}{\mathrm{Te}}_{4}$ chains in the crystal structure, $\mathrm{Ta}{\mathrm{Te}}_{4}$ is in fact a 3D material as far as the electronic structure near the Fermi level is concerned. A Fermi surface nesting mechanism is dismissed as the origin of the $2a\ifmmode\times\else\texttimes\fi{}2a\ifmmode\times\else\texttimes\fi{}3c$ structural modulation. The optimized $2a\ifmmode\times\else\texttimes\fi{}2a\ifmmode\times\else\texttimes\fi{}3c$ structure, which is found to be the more stable modulation in agreement with the experimental observations, can be obtained directly from a soft-phonon mode computed for the undistorted structure. Our results suggest that the driving force for the distortion is the maximization of Ta-Ta metal-metal bonding subject to inducing the minimum bonding decrease in the Te sublattice.

Effect of static local distortions vs. dynamic motions on the stability and band gaps of cubic oxide and halide perovskites

Ternary ABX3 perovskites made of corner-sharing BX6 octahedra have long featured prominently in solid-state chemistry and condensed matter physics. Still, the joint understanding of their two main subgroups—halides and oxides—has not been fully developed. Indeed, unlike the case in simpler compounds having a single, robust repeated motif (“monomorphous”), certain cubic perovskites can manifest a non-thermal (= intrinsic) distribution of local motifs (“polymorphous networks”). Such static deformations can include positional degrees of freedom (e.g., atomic displacements and octahedral tilting) or magnetic moment degrees of freedom in paramagnets. Unlike thermal motion, such static distortions do not time-average to zero, being an expression of the intrinsic symmetry breaking preference of the chemical bonding. The present study compares electronic structure features of oxide and halide perovskites starting from the static polymorphous distribution of motifs described by Density Functional Theory (DFT) minimization of the internal energy, continuing to finite temperature thermal disorder modeled via finite temperature DFT molecular dynamics. We find that (i) different oxide vs. halide ABX3 compounds adopt different energy-lowering symmetry-breaking modes. The calculated pair distribution function (PDF) of SrTiO3 from the first-principles agrees with recently measured PDF. (ii) In both oxides and halides, such static distortions lead to band gap blueshifts with respect to undistorted cubic Pm-3m structure. (iii) For oxide perovskites, high-temperature molecular dynamics simulations initiated from the statically distorted polymorphous structures reveal that the thermally-induced distortions can lead to a band gap redshift. (iv) In contrast, for cubic halide perovskite CsPbI3, both the intrinsic distortions and the thermal distortions contribute in tandem to band gap blueshift, the former, intrinsic effect being dominant. (v) In the oxide SrTiO3 and CaTiO3 (but not in halide) perovskites, octahedral tilting leads to the emergence of a distinct Γ–Γ direct band gap component as a secondary valley minimum to the well-known indirect R–Γ gap. Understanding such intrinsic vs. thermal effects on oxide vs. halide perovskites holds the potential for designing target electronic properties.

Symmetry mode analysis of distorted polar/nonpolar structures in A -site ordered SmBaMn2O6 perovskite

We present a comprehensive structural study of the charge-orbital ordering and magnetic phase transitions observed in the $A$-site ordered $\mathrm{SmBa}{\mathrm{Mn}}_{2}{\mathrm{O}}_{6}$ perovskite combining synchrotron radiation x-ray powder diffraction and symmetry-adapted modes analysis. In $\mathrm{SmBa}{\mathrm{Mn}}_{2}{\mathrm{O}}_{6}$, successive phase transitions in charge, spin, and lattice degrees of freedom take place with decreasing temperature at ${T}_{\mathrm{CO}1}\ensuremath{\approx}380\phantom{\rule{0.16em}{0ex}}\mathrm{K}, {T}_{\mathrm{CO}2}\ensuremath{\approx}190\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, and ${T}_{\mathrm{N}}\ensuremath{\approx}250\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. The main difference between the two charge-ordered phases concerns the stacking sequence along the $c$ axis, which is double for the high temperature charge-ordered phase and has led to controversy in the literature. We show that both charge-ordered phases are pseudosymmetric with respect to the ideal undistorted tetragonal structure of $A$-site ordered $R\mathrm{Ba}{\mathrm{Mn}}_{2}{\mathrm{O}}_{6}$ perovskites and lead to two nonequivalent Mn sites. However, the charge segregation stabilizes at about $0.35{e}^{\ensuremath{-}}$ in the low temperature charge-ordered phase, clearly below the nominal separation of one charge unit between ${\mathrm{Mn}}^{3+}$ and ${\mathrm{Mn}}^{4+}$ and undergoes a prominent increase in the high temperature charge-ordered phase when warming above $\ensuremath{\approx}250\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. The two Mn sites are anisotropic in both charge-ordered phases but the analysis of the active modes discloses that only the low temperature charge-ordered phase displays a Jahn-Teller-like distortion for one of the Mn sites. In addition, this low temperature charge-ordered phase has polar symmetry compatible with ferroelectricity along the $a$ axis.

Coexistence of Superconductivity and Charge Density Waves in Tantalum Disulfide: Experiment and Theory

The coexistence of charge density wave (CDW) and superconductivity in tantalum disulfide (2H-TaS_{2}) at low temperature is boosted by applying hydrostatic pressures to study both vibrational and magnetic transport properties. Around P_{c}, we observe a superconducting dome with a maximum superconducting transition temperature T_{c}=9.1 K. First-principles calculations of the electronic structure predict that, under ambient conditions, the undistorted structure is characterized by a phonon instability at finite momentum close to the experimental CDW wave vector. Upon compression, this instability is found to disappear, indicating the suppression of CDW order. The calculations reveal an electronic topological transition (ETT), which occurs before the suppression of the phonon instability, suggesting that the ETT alone is not directly causing the structural change in the system. The temperature dependence of the first vortex penetration field has been experimentally obtained by two independent methods. While a d wave and single-gap BCS prediction cannot describe the lower critical field H_{c1} data, the temperature dependence of the H_{c1} can be well described by a single-gap anisotropic s-wave order parameter.

Improved Structural Properties in Homogeneously Doped Sm0.4Ce0.6O2-δ Epitaxial Thin Films: High Doping Effect on the Electronic Bands.

The study of ionic materials on nanometer scale is of great relevance for efficient miniaturized devices for energy applications. The epitaxial growth of thin films can be a valid route to tune the properties of the materials and thus obtain new degrees of freedom in materials design. High crystal quality SmxCe1-xO2-δ films are here reported at a high doping level up to x = 0.4, thanks to the good lattice matching with the (110) oriented NdGaO3 substrate. X-ray diffraction and transmission electron microscopy demonstrate the ordered structural quality and absence of Sm segregation at the macroscopic and atomic level, respectively. Therefore, in epitaxial thin films, the homogeneous doping can be obtained even with the high dopant content not always approachable in bulk form, getting even an improvement of the structural properties. In situ spectroscopic measurements by X-ray photoemission and X-ray absorption show the O 2p band shift toward the Fermi level, which can favor the oxygen exchange and vacancy formation on the surface when the Sm doping is increased to x = 0.4. X-ray absorption spectroscopy also confirms the absence of ordered oxygen vacancy clusters and further reveals that the 5d eg and t2g states are well separated by the crystal field in the undistorted local structure even in the case of a high doping level up to x = 0.4.

Four Lead-free Layered Double Perovskites with the n = 1 Ruddlesden-Popper Structure.

Herein we report the synthesis, structure, and band gaps of four layered halide double perovskites, i.e., BA2Cu0.5In0.5Cl4, BA2Ag0.5In0.5Cl4, BA2Ag0.5Sb0.5Cl4, and BA2Ag0.5Sb0.5Br4 [BA = butylammonium = CH3(CH2)3NH3+], each of which has the n = 1 Ruddlesden-Popper structure. In addition, the crystal structure of BA2Ag0.5Bi0.5Br4 is revisited and that of BA2PbCl4 is reported for the first time. Only BA2Ag0.5Sb0.5Cl4 has the tetragonal I4/mmm symmetry of the undistorted Ruddlesden-Popper structure. The other five compounds have orthorhombic structures due to tilts of the octahedra and orientational ordering of the butylammonium groups. As the lateral dimensions of the inorganic layer decrease, the c/a ratio increases due to decreased interdigitation of the alkyl ends of the butylammonium cations. This structural feature may help to explain the increased stability of the bromide phases with respect to the chloride phases. There are features in the diffraction patterns of BA2Ag0.5Bi0.5Br4 and BA2Cu0.5In0.5Cl4 that suggest ordering of octahedral cations within the layers, but in those compounds there appears to be a high concentration of stacking faults between layers that limits long-range, three-dimensional ordering of cations. In the other cases the scattering powers of the cations (Ag/Sb and Ag/In) are too similar to say anything definitive about cation ordering. The band gaps of these compounds range from 2.65 to 4.27 eV, with the bromide compositions possessing smaller band gaps than the chlorides. The band gaps of layered BA2M0.5M'0.5X4 compositions studied here are roughly 0.5-0.8 eV larger than analogous Cs2MM'X6 cubic double perovskites due to a combination of dimensional reduction (3D → 2D), distortions of the octahedral environment around the M/M' ions, and octahedral tilting distortions.

Monolayer Modification of VTe2 and Its Charge Density Wave.

Interlayer interactions in layered transition metal dichalcogenides are known to be important for describing their electronic properties. Here, we demonstrate that the absence of interlayer coupling in monolayer VTe2 also causes their structural modification from a distorted 1T' structure in bulk and multilayer samples to a hexagonal 1T structure in the monolayer. X-ray photoemission spectroscopy indicates that this structural transition is associated with electron transfer from the vanadium d bands to the tellurium atoms for the monolayer. This charge transfer may reduce the in-plane d orbital hybridization and thus favor the undistorted 1T structure. Phonon-dispersion calculations show that, in contrast to the 1T' structure, the 1T structure exhibits imaginary phonon modes that lead to a charge density wave (CDW) instability, which is also observed by low-temperature scanning tunneling microscopy as a 4 × 4 periodic lattice distortion. Thus, this work demonstrates a novel CDW material, whose properties are tuned by interlayer interactions.

Direct femtosecond-laser writing of optical-range nanoscale metagratings/metacouplers on diamond surfaces

A natural diamond surface was nanopatterned in a subablative scanning mode by multiple tightly focused 515-nm, 300-fs laser pulses, resulting in regular surface metagratings (nanoripples) with a subwavelength period of Λ ∼ 100 nm, an almost undistorted crystalline structure, free from sp2-carbon, oriented perpendicular to the laser polarization, and stackable both along and normal to their stripes. In the visible-near-IR ranges (400–800 nm), these metagratings demonstrate the pronounced resonant antireflective effect in the wavelength range near ∼4Λ, while the overall specular reflectance/transmittance is considerably diminished with respect to both the simulation results and the reference diamond surface, indicating strong (∼50%) coupling/scattering of light inside the crystal. Our modeling indicates the laser wavelength-dependent plasmon-based tunability of metagrating periods, implying the corresponding tunability of their spectral response.

Optical control of vibrational coherence triggered by an ultrafast phase transition

Femtosecond time-resolved x-ray diffraction is employed to study the dynamics of the periodic lattice distortion (PLD) associated with the charge-density-wave (CDW) in K0.3MoO3. Using a multi-pulse scheme we show the ability to extend the lifetime of coherent oscillations of the PLD about the undistorted structure through re-excitation of the electronic states. This suggests that it is possible to enter a regime where the symmetry of the potential energy landscape corresponds to the high symmetry phase but the scattering pathways that lead to the damping of coherent dynamics are still controllable by altering the electronic state population. The demonstrated control over the coherence time offers new routes for manipulation of coherent lattice states.

Quadrupolar superexchange interactions, multipolar order, and magnetic phase transition in UO2

The origin of noncollinear magnetic order in ${\mathrm{UO}}_{2}$ is studied by an ab initio dynamical-mean-field-theory framework in conjunction with a linear-response approach for evaluating intersite superexchange interactions between U $5{f}^{2}$ shells. The calculated quadrupole-quadruple superexchange interactions are found to unambiguously resolve the frustration of face-centered-cubic U sublattice toward stabilization of the experimentally observed noncollinear 3k-magnetic order. Therefore, the exotic 3k-antiferromagnetic order in ${\mathrm{UO}}_{2}$ can be accounted for by a purely electronic exchange mechanism acting in the undistorted cubic lattice structure. The quadrupolar short-range order above magnetic ordering temperature ${T}_{N}$ is found to qualitatively differ from the long-range order below ${T}_{N}$.

Temperature dependence of the C13 hyperfine structure of the negatively charged nitrogen-vacancy center in diamond

The nitrogen-vacancy (NV) center is a well utilized system for quantum technology, in particular quantum sensing and microscopy. Fully employing the NV center's capabilities for metrology requires a strong understanding of the behavior of the NV center with respect to changing temperature. Here, we probe the NV electronic spin density as the surrounding crystal temperature changes from 10 K to 700 K by examining its $^{13}$C hyperfine interactions. These results are corroborated with \textit{ab initio} calculations and demonstrate that the change in hyperfine interaction is small and dominated by a change in the hybridization of the orbitals constituting the spin density. Thus indicating that the defect and local crystal geometry is returning towards an undistorted structure at higher temperature.

2 × 2 charge density wave in single-layer TiTe2

A density functional theory study concerning the origin of the recently reported charge density wave (CDW) instability in single-layer TiTe is reported. It is shown that, whereas calculations employing the semi-local functional PBE favor the undistorted structure, the hybrid functional HSE06 correctly predicts a distortion. The study suggests that the magnitude of the semi-metallic overlap between the valence band top at and the conduction band bottom at is a key factor controlling the tendency towards the distortion. It is also shown that tensile strain stabilizes a CDW, and we suggest that this fact could be further used to induce the instability in double-layers of TiTe, which in the absence of strain remain undistorted in the experiment. The driving force for the CDW instability seems to be the same phonon mediated mechanism acting for single-layer TiSe, although in single-layer TiTe the driving force is smaller, and the semimetallic character is kept below the transition temperature.

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.