Orthorhombic Distortion Research Articles

Topological magnetic structures in MnGe: Neutron diffraction and symmetry analysis

From new neutron powder diffraction experiments on the chiral cubic ($P2{_1}3$) magnet manganese germanide MnGe, we analyse all of the possible crystal symmetry-allowed magnetic superstructures that are determined successfully from the data. The incommensurate propagation vectors $k$ of the magnetic structure are found to be aligned with the [100] cubic axes, and correspond to a magnetic periodicity of about 30 $\r{A}$ at 1.8 K. Several maximal crystallographic symmetry magnetic structures are found to fit the data equally well and are presented. These include topologically non-trivial magnetic hedgehog and "skyrmion'' structures in multi-$k$ cubic or orthorhombic 3+3 and orthorhombic 3+2 dimensional magnetic superspace groups respectively, with either potentially responsible for topological Hall effect. The presence of orthorhombic distortions in the space group $P2_12_12_1$ caused by the transition to the magnetically ordered state does not favour the cubic magnetic hedgehog structure, and leave both orthorhombic hedgehog and "skyrmion'' models as equal candidates for the magnetic structures. We also report on a new combined mechanochemical and solid-state chemical route to synthesise MnGe at ambient pressures and moderate temperatures, and compare with samples obtained by the traditional high pressure synthesis.

Interplay between structural changes, surface states and quantum confinement effects in semiconducting Mg2Si and Ca2Si thin films.

Ab initio techniques have been used to investigate structural changes in semiconducting Mg2Si and Ca2Si thin films (from 17 nm down to 0.2 nm corresponding to the 2D structure) along with band-gap variations due to quantum confinement. Cubic Mg2Si(111) thin films being dynamically stable at thicknesses (d) larger than 0.3 nm displayed an indirect band gap, the reduction of which with increasing d could be reasonably well described by the simple effective mass approximation. Only 2D Mg2Si has a unique structure because of the orthorhombic distortion and the direct band gap. Since the surface energy of cubic Ca2Si(111) films was lower with respect to any surface of the orthorhombic phase, which is the ground state for the Ca2Si bulk, the metastable in-bulk cubic phase in the form of thin films turned out to be preferable in total energy than any orthorhombic Ca2Si thin film for d < 3 nm. Sizable structural distortion and the appearance of surface states in the gap region of Ca2Si thin films with d < 3 nm could be the reason for an odd dependence of the band-gap variation on d.

Orthorhombic distortion and rectangular skyrmion crystal in a centrosymmetric tetragonal host

We theoretically investigate the stability of a rectangular skyrmion crystal without fourfold rotational symmetry under an orthorhombic distortion in centrosymmetric tetragonal magnets. The results are obtained by numerically simulated annealing for an effective spin model with competing interactions in momentum space and magnetic anisotropy. By constructing the low-temperature phase diagram while changing the interaction ratio arising from the orthorhombic distortion, we find that the rectangular skyrmion crystal remains stable in an external magnetic field against distortion. We show that the degree of fourfold rotational symmetry tends to recover when the magnetic field is increased. The relevance to the skyrmion-hosting material EuAl4 is also discussed.

Soft-Phonon and Charge-Density-Wave Formation in Nematic BaNi2As2

We use diffuse and inelastic x-ray scattering to study the formation of an incommensurate charge-density-wave (I-CDW) in BaNi_{2}As_{2}, a candidate system for charge-driven electronic nematicity. Intense diffuse scattering is observed around the modulation vector of the I-CDW, Q_{I-CDW}. It is already visible at room temperature and collapses into superstructure reflections in the long-range ordered state where a small orthorhombic distortion occurs. A clear dip in the dispersion of a low-energy transverse optical phonon mode is observed around Q_{I-CDW}. The phonon continuously softens upon cooling, ultimately driving the transition to the I-CDW state. The transverse character of the soft-phonon branch elucidates the complex pattern of the I-CDW satellites observed in the current and earlier studies and settles the debated unidirectional nature of the I-CDW. The phonon instability and its reciprocal space position are well captured by our abinitio calculations. These, however, indicate that neither Fermi surface nesting, nor enhanced momentum-dependent electron-phonon coupling can account for the I-CDW formation, demonstrating its unconventional nature.

Magnetic and crystal structure of the antiferromagnetic skyrmion candidate GdSb0.71Te1.22

GdSb0.46Te1.48, a nonsymmorphic Dirac semimetal with Dirac nodes at the Fermi level, has a rich magnetic phase diagram with one of the phases predicted to be an antiferromagnetic skyrmion state. In the current work, we investigate GdSb0.71Te1.22 through bulk magnetization measurements, single-crystal, and powder synchrotron X-ray diffraction, as well as single-crystal hot-neutron diffraction. We resolve a weak orthorhombic distortion with respect to the tetragonal structure and charge density wave (CDW) satellites due to incommensurate modulations of the crystal structure. At 2 K the magnetic structure is modulated with two propagation vectors, kI = (0.45 0 0.45) and kII = (0.4 0 0), with all their arms visible. While kI persists up to the transition to the paramagnetic state at TN = 11.9 K, kII disappears above an intermediate magnetic transition at T1 = 5 K. Whereas magnetic field applied along the c-axis has only a weak effect on the intensity of antiferromagnetic reflections, it is effective in inducing an additional ferromagnetic component on Gd atoms. We refine possible magnetic structures of GdSb0.71Te1.22 and discuss the possibility of hosting magnetic textures with non-trivial 3D+ 2 topologies in the GdSb1−xTe1+x series.

Giant Magnetic In-Plane Anisotropy and Competing Instabilities in Na3Co2SbO6

We report magnetometry data obtained on twin-free single crystals of Na3Co2SbO6, which is considered a candidate material for realizing the Kitaev honeycomb model for quantum spin liquids. Contrary to a common belief that such materials can be modeled with the symmetries of an ideal honeycomb lattice, our data reveal a pronounced two-fold symmetry and in-plane anisotropy of over 200%, despite the honeycomb layer's tiny orthorhombic distortion of less than 0.2%. We further use magnetic neutron diffraction to elucidate a rich variety of field-induced phases observed in the magnetometry. These phases manifest themselves in the paramagnetic state as diffuse scattering signals associated with competing ferro- and antiferromagnetic instabilities, consistent with a theory that also predicts a quantum spin liquid phase nearby. Our results call for theoretical understanding of the observed in-plane anisotropy, and render Na3Co2SbO6 a promising ground for finding exotic quantum phases by targeted external tuning.

Deciphering the Nature of Temperature-Induced Phases of MAPbBr3 by Ab Initio Molecular Dynamics

We present an ab initio molecular dynamics study of the temperature-induced phases of methylammonium lead bromide (MAPbBr3). We confirm that the low-temperature phase is not ferroelectric and rule out the presence of any overall polarization arising from the motion of the individual sublattices. Our simulations at room temperature resulted in a cubic Pm3̅m phase with no discernible local orthorhombic distortions. At low temperatures, such distortions are shown to originate from octahedral scissoring modes, but they vanish at room temperature. The predicted timescales of MA motion agree very well with experimental estimates, establishing dynamic disordering of the molecular dipoles over several orientational minima at room temperature. We also identify the key modes of the inorganic and organic sublattices that are coupled at all temperatures, mainly through the N–H···Br hydrogen bonds. Estimated lifetimes of the H bonds correlate well with MA dynamics, indicating a strong connection between these two aspects of organic–inorganic hybrid perovskites. We also confirm that, in addition to the disordering of MA orientations, the transition to the cubic phase is also associated with displacive characteristics arising from both MA and Br ions in the lattice.

Orthorhombic distortion-induced anatase-like optoelectronic properties of rutile TiO2

Titanium dioxide (TiO2) is an excellent photocatalyst and transparent conducting oxide. It has two major crystal structures: rutile and anatase. Anatase TiO2 is valuable from an industrial point of view because it typically displays better photocatalytic and electronic transport properties than rutile TiO2. To further extract the functional properties of TiO2, understanding the correlation between the electronic structure and the crystal structure is essential. Because the electronic structure strongly depends on the crystal structure, introducing lattice distortion to rutile TiO2 should effectively modulate its electronic structure. Here, we show that Nb-doped rutile TiO2 epitaxial films on (11¯00) α-Al2O3 substrates exhibit anatase-like optoelectronic properties due to orthorhombic lattice distortions and experimentally observe the change in the electronic structure. Reducing the film thickness increases the orthorhombic distortion ratio (b/a) up to 3.4%. As b/a increases, the carrier effective mass decreases from 35 to 3 m0 (m0: electron mass) and the optical bandgap significantly increases. The present observations provide insight into regulating the TiO2 physical properties and should be beneficial for designing TiO2-based photocatalysts and transparent conducting electrodes.

The structure, magnetic and dielectric properties of the CaBaCo4-xNixO7 (0 ≤ x ≤ 0.1) compounds

We report the structure, magnetic and dielectric behaviors of the polar magnet CaBaCo4-xNixO7 (0 ≤ x ≤ 0.1). The Rietveld refinements results for polycrystalline samples shows that the orthorhombic distortion gradually decreases with the increase of Ni2+ content. Different from the undoped sample, two antiferromagnetic phases and a ferrimagnetic phase can be observed in M-T curves for Ni-doped samples. For x = 0.06, the antiferromagnetic phase around 79K competes with another antiferromagnetic phase that appears around 62K, giving a magnetic plateau between 62K and 79K. With increasing Ni-doping, the ferrimagnetic phase transition peak moves to lower temperature and it fades gradually, while the antiferromagnetic phase transition peak around 79K shifts to higher temperature. The magnetization behaviors could be attributed to the enhanced antiferromagnetic interactions and the reorientation of the adjacent spins of cobalt ions due to the introduction of Ni2+ doping. As a result, the Ni-doped samples present stronger magnetoelectric coupling than the undoped sample, as well as an enhanced polarization.

Possible helimagnetic order in Co4+-containing perovskites Sr1−xCaxCoO3

We systematically synthesized perovskite-type oxides Sr1−xCaxCoO3 containing unusually high valence Co4+ ions by a high pressure technique and investigated the effect of systematic lattice change on the magnetic and electronic properties. As the Ca content x exceeds about 0.6, the structure changes from cubic to orthorhombic, which is supported by the first-principles calculations of enthalpy. Upon the orthorhombic distortion, the ground state remains to be apparently ferromagnetic, with a slight drop of the Curie temperature. Importantly, the compounds with x larger than 0.8 show antiferromagnetic behavior, with positive Weiss temperatures and nonlinear magnetization curves at the lowest temperature, implying that the ground state is non-collinear antiferromagnetic or helimagnetic. Considering the incoherent metallic behavior and the suppression of the electronic specific heat at the high x region, the possible emergence of a helimagnetic state in Sr1−xCaxCoO3 is discussed in terms of the bandwidth narrowing and the double-exchange mechanism with the negative charge transfer energy, as well as the spin frustration, owing to the next-nearest neighbor interaction.

Ni/(R2O3,CaO) Nanocomposites Produced by the Exsolution of R1.5Ca0.5NiO4 Nickelates (R = Nd, Sm, Eu): Rare Earth Effect on the Catalytic Performance in the Dry Reforming and Partial Oxidation of Methane

In order to clarify the role of R2O3 in the metal-oxide catalysts derived from complex oxide precursors, a series of R1.5Ca0.5NiO4 (R = Nd, Sm, Eu) complex oxides was obtained. A significant systematic increase in the orthorhombic distortion of the R1.5Ca0.5NiO4 structure (K2NiF4 type, Cmce) from Nd to Eu correlates with a corresponding decrease in their ionic radii. A reduction of R1.5Ca0.5NiO4 in the Ar/H2 gas mixture at 800 °C causes a formation of dense agglomerates of CaO and R2O3 coated with spherical 25–30 nm particles of Ni metal. The size of metal particles and oxide agglomerates is similar in all Ni/(R2O3,CaO) composites in the study. Their morphology is rather similar to the products of redox exsolution obtained by the partial reduction of complex oxides. All obtained composites demonstrated a significant catalytic activity in the dry reforming (DRM) and partial oxidation (POM) of methane at 700–800 °C. A systematic decrease in the DRM catalytic activity of composites from Nd to Eu could be attributed to the basicity reduction of R2O3 components of the composite catalysts. The maximum CH4 conversion in POM reaction was observed for Ni/(Sm2O3,CaO), while the maximum selectivity was demonstrated by Nd2O3-based composite. The possible reasons for the observed difference are discussed.

Loop currents in AV3Sb5 kagome metals: Multipolar and toroidal magnetic orders

Experiments in the recently discovered vanadium-based kagome metals have suggested that their charge-ordered state displays not only bond distortions, characteristic of a ``real" charge density-wave (rCDW), but also time-reversal symmetry-breaking, typical of loop currents described by an ``imaginary" charge density-wave (iCDW). Here, we combine density-functional theory, group-theory, and phenomenological modeling to investigate the complex charge-ordered states that arise from interactions between the low-energy van Hove singularities present in the electronic structure of $A$V$_3$Sb$_5$. We find two broad classes of mixed iCDW-rCDW configurations: triple-$\mathbf{Q}$ iCDW, triple-$\mathbf{Q}$ rCDW order, dubbed $3\mathbf{Q}$-$3\mathbf{Q}$, and double-$\mathbf{Q}$ iCDW, single-$\mathbf{Q}$ rCDW order, dubbed $2\mathbf{Q}$-$1\mathbf{Q}$. Moreover, we identify seven different types of iCDW order, stemming from the different vanadium-orbital and kagome-sublattice structures of the two pairs of van Hove singularities present above and below the Fermi level. While the $2\mathbf{Q}$-$1\mathbf{Q}$ states trigger an orthorhombic distortion that breaks the threefold rotational symmetry of the kagome lattice, the $3\mathbf{Q}$-$3\mathbf{Q}$ states induce various types of subsidiary uniform magnetic orders, from conventional ferromagnetism to magnetic octupolar, magnetic toroidal, and even magnetic monopolar order. We show that these exotic orders display unique magneto-striction, magneto-electric, and magneto-electric-striction properties that can be probed experimentally to identify which iCDW state is realized in these compounds. We briefly discuss the impact of an out-of-plane modulation of the charge order and the interplay between these complex charge-ordered states and superconductivity.

Spray-flame-synthesized Sr- and Fe-substituted LaCoO3 perovskite nanoparticles with enhanced OER activities

La1–xSrxCoO3 (x = 0, 0.1, 0.2) and LaCo0.8Fe0.2O3 perovskite nanoparticles were synthesized by spray-flame synthesis, and their electrocatalytic water oxidation activity was evaluated in the prepared state. Highly crystalline, rhombohedrally distorted cubic structures of the cobaltite perovskites were confirmed by STEM and XRD analyses. The decreased JT distortions were observed in the cobaltite perovskite structure upon substitution with Sr, while orthorhombic distortions in Fe-substituted perovskites could explain the changes in their Raman spectra. Debye–Waller factors and coordination numbers from fitted EXAFS data indicate more disordered crystalline structures upon Sr substitution and a lower Co–O coordination number at 20 at% Sr. Thermal characterization of the catalysts by STA coupled with QMS shows higher mass losses in Sr-substituted catalysts, and these results were associated with a higher concentration of carbonate species in these catalysts, which was also confirmed by XPS measurements. Both Fe and Sr substitution lead to higher catalytic OER activity of the cobaltite perovskites with lower overpotentials of about 30–50 mV. The cobaltite perovskite catalyst substituted with 20 at% Sr exhibited the highest OER activity and stable electrocatalytic performance at moderate conditions.

Lattice distortion inducing exciton splitting and coherent quantum beating in CsPbI3 perovskite quantum dots.

Anisotropic exchange splitting in semiconductor quantum dots results in bright-exciton fine-structure splitting important for quantum information processing. Direct measurement of fine-structure splitting usually requires single/few quantum dots at liquid-helium temperature because of its sensitivity to quantum dot size and shape, whereas measuring and controlling fine-structure splitting at an ensemble level seem to be impossible unless all the dots are made to be nearly identical. Here we report strong bright-exciton fine-structure splitting up to 1.6 meV in solution-processed CsPbI3 perovskite quantum dots, manifested as quantum beats in ensemble-level transient absorption at liquid-nitrogen to room temperature. The splitting is robust to quantum dot size and shape heterogeneity, and increases with decreasing temperature, pointing towards a mechanism associated with orthorhombic distortion of the perovskite lattice. Effective-mass-approximation calculations reveal an intrinsic 'fine-structure gap' that agrees well with the observed fine-structure splitting. This gap stems from an avoided crossing of bright excitons confined in orthorhombically distorted quantum dots that are bounded by the pseudocubic {100} family of planes.

Effect of sintering temperature on the structural properties for CaBi4Ti4O15 compound

Polycrystalline CaBi4Ti4O15 (CBT) ceramics were successfully prepared by traditional ceramic technique. The X-ray diffraction spectrum of produced powder was studied at different temperatures (650 C, 850 C, and 1050C. The sintering temperature is very effective factor to obtained high crystallinity with very pure for CBT compound. The product structures analyzed using X-ray diffraction, it confirms, high purity polycrystalline phase formation revealed the existence of bismuth layered perovskite phase Aurivillius orthorhombic crystal structure at 1050ᶹ C. The produced samples' A field emission scanning electron microscope was used to analyze the morphologies (FESEM). It shows crystallites particles in the range about 149-349 nm for the product sintered at1050ᶹ C, it shows a dense microstructure with the presence of large orthorhombic distortion. The dielectric constant and dielectric loss were measured at room temperature as a function of frequency. The dielectric constant of CBT increases in sintering temperature. Moreover, the samples sintered at 1050 ᶹC exhibits good dielectric properties and lower value of tan .

Enhanced s⁎-wave component of the superconducting gap in overdoped HTSC cuprates with orthorhombic distortion

In this work, the concentration and temperature dependence of the superconducting gap of the HTSC cuprate in the orthorhombic phase are obtained within the framework of the Hubbard model taking into account the exchange mechanism of pairing. To elucidate the reasons for the unexpectedly large s⁎-wave component of the superconducting gap in the overdoped regime, the electronic structure of low-energy excitations and the k-dependent contributions of pair states to the gap components were investigated. It is shown that the s⁎-wave component growth results from the shallowing of the electron pocket in the region of the s⁎-wave component maximum.

Nonlinear deformation and elasticity of BCC refractory metals and alloys

Application of isotropic pressure or uniaxial strain alters the elastic properties of materials; sufficiently large strains can drive structural transformations. Linear elasticity describes stability against infinitesimal strains, while nonlinear elasticity describes the response to finite deformations. It was previously shown that uniaxial strain along [100] drives refractory metals and alloys towards mechanical instabilities. These include an extensional instability, and a symmetry-breaking orthorhombic distortion caused by a Jahn-Teller-Peierls instability that splays the cubic lattice vectors. Here we analyze these transitions in depth. Eigenvalues and eigenvectors of the Wallace tensor identify and classify linear instabilities in the presence of strain. We show that both instabilities are discontinuous, leading to discrete jumps in the lattice parameters. We provide physical intuition for the instabilities by analyzing the changes in first-principles energy, stress, bond lengths, and angles upon application of strain. Electronic band structure calculations show differential occupation of bonding and antibonding orbitals, driven by the changing bond lengths and leading to the structural transformations. Strain thresholds for these instabilities depend on the valence electron count.

Thickness and temperature dependence of the atomic-scale structure of SrRuO3 thin films

The temperature-dependent layer-resolved structure of 3 to 44 unit cell thick SrRuO3 (SRO) films grown on Nb-doped SrTiO3 substrates is investigated using a combination of high-resolution synchrotron x-ray diffraction and high-resolution electron microscopy to understand the role that structural distortions play in suppressing ferromagnetism in ultra-thin SRO films. The oxygen octahedral tilts and rotations and Sr displacements characteristic of the bulk orthorhombic phase are found to be strongly dependent on temperature, the film thickness, and the distance away from the film–substrate interface. For thicknesses, t, above the critical thickness for ferromagnetism (t &amp;gt; 3 uc), the orthorhombic distortions decrease with increasing temperature above TC. Below TC, the structure of the films remains constant due to the magneto-structural coupling observed in bulk SRO. The orthorhombic distortions are found to be suppressed in the 2–3 interfacial layers due to structural coupling with the SrTiO3 substrate and correlate with the critical thickness for ferromagnetism in uncapped SRO films.

Role of nematicity in controlling spin fluctuations and superconducting Tc in bulk FeSe

FeSe undergoes a transition from a tetragonal to a slightly orthorhombic phase at 90\,K, and becomes a superconductor below 8\,K. The orthorhombic phase is sometimes called a nematic phase because quantum oscillation, neutron, and other measurements detect a significant asymmetry in $x$ and $y$. How nematicity affects superconductivity has recently become a matter of intense speculation. Here we employ an advanced \emph{ab-initio} Green's function description of superconductivity and show that bulk tetragonal FeSe would, in principle, superconduct with almost the same T$_{c}$ as the nematic phase. The mechanism driving the observed nematicity is not yet understood. Since the present theory underestimates nematicity, we simulate the full nematic asymmetry by artificially enhancing the orthorhombic distortion. For benchmarking, we compare theoretical spin susceptibilities against experimentally observed data over all energies and relevant momenta. When the orthorhombic distortion is adjusted to correlate with observed nematicity in spin susceptibility, the enhanced nematicity causes spectral weight redistribution in the Fe-3d$_{xz}$ and d$_{yz}$ orbitals, but it leads to at most 10-15$\%$ increment in T$_{c}$. This is because the d$_{xy}$ orbital always remains the most strongly correlated and provides most of the source of the superconducting glue. Nematicity suppresses the density of states at Fermi level; nevertheless T$_{c}$ increases, in contradiction to both BCS and BEC theories. We show how the increase is connected to the structure of the particle-particle vertex. Our results suggest while nematicity may be intrinsic property of bulk FeSe, is not the primary force driving the superconducting pairing.

Formula omitted] perovskites as electrode materials for SOFCs

In this work, the Pechini method was used to synthesize La0.7−xLnxCa0.3MnO3,(Ln=ProrSm)-La1−xLnxCa0.3MnO3 type perovskites, evaluating the effect of the type of cation and composition on the structural, morphological and textural properties. The use of similar rare earth cations was studied to promote weak bonds between the reactive surface and adsorbed oxygen species that could facilitate the oxygen reduction reaction and thus improve electrochemical performance of SOFC devices. To achieve this goal, different compositions of Pr or Sm ions (x = 0.1, 0.3, 0.5 or 0.6) were used for the partial substitution of lanthanum at the A site. The results indicated that the substitution of La by Pr or Sm did not modify the original orthorhombic perovskite structure of Ca-doped lanthanum manganites. However, a shift in the main reflections can be obtained as the content of both cations increases due to cell distortion. Rietveld refinement confirms that the crystal structure belongs to the Pnma space group with a large distortion along the b-axis but no secondary phase formation. The compensated charge neutrality of the Mn3+/Mn4+ ratio influences the octahedral sites of MnO6 and cell volume. Orthorhombic distortion (c2<a<b) occurs through Jahn-Teller mechanism promoting a hole-doped system for electrical conductivity. The adsorption-desorption isotherms reveal that in any composition of Pr, a mesoporous isotherm (type IV) is obtained. In contrast, the isotherms changed from micro- to meso-porous features depending on the amount of Sm substitution at the A-site. All the prepared samples showed a soft granular structure with agglomerates ranging between 200 and 300 nm with well-interconnected pores. Comparing the La substitution by Pr or Sm, it was found that Sm can form perovskites that can better promote oxygen vacancies and triple boundary phase formation, which is essential in SOFC devices.