Range Order Research Articles

Magnetic nature of wolframite MgReO4

Rhenium oxides belonging to the family AReO4 where A is a metal cation, exhibit interesting electronic and magnetic properties. In this study we have utilized the muon spin rotation/relaxation (µ +SR) technique to study the magnetic properties of the MgReO4 compound. To the best of our knowledge, this is the first investigation reported on this interesting material, that is stabilized in a wolframite crystal structure using a special high-pressure synthesis technique. Bulk magnetic studies show the onset of an antiferromagnetic (AF) long range order, or a possible singlet spin state at T C1 ≈ 90 K, with a subtle second high-temperature transition at T C2 ≈ 280 K. Both transitions are also confirmed by heat capacity (Cp ) measurements. From our µ +SR measurements, it is clear that the sample enters an AF order below T C1 = T N ≈ 85 K. We find no evidence of magnetic signal above T N, which indicates that T C2 is likely linked to a structural transition. Further, via sensitive zero field (ZF) µ +SR measurements we find evidence of a spin reorientation at T Cant ≈ 65 K. This points towards a transition from a collinear AF into a canted AF order at low temperature, which is proposed to be driven by competing magnetic interactions.

Quasi-localized charge approximation approach for the nonlinear structures in strongly coupled Yukawa systems

Strongly coupled systems occupying the transitional range between the Wigner crystal and fluid phases are the most dynamic constituents of the nature. Highly localized but strongly interacting elements in this phase possess enough thermal energy to trigger the transition between a variety of short to long range order phases. Nonlinear excitations are often carriers of proliferating structural modifications in the strongly coupled Yukawa systems. Well represented by laboratory dusty plasmas, these systems show explicit propagation of nonlinear shocks and solitary structures both in experiments and first principles simulations. The shorter scale length contributions remain absent at strong screening in the present approximate models, which nevertheless prescribe nonlinear solitary solutions that consequently lose their coherence in a numerical evolution of the system under the special implementation of a quasi-localized charge approximation (QLCA) formulation. The stable coherent structures self-consistently emerge following an initial transient in the numerical evolution that adapts QLCA approach to spatiotemporal domain for accessing the nonlinear excitations in the strong screening limit. The present κ∼1 limit of the existing Yukawa fluid models to show agreement with the experiment and molecular dynamical simulations has, therefore, been overcome, and the coherent nonlinear excitations have become characterizable up to κ∼2.7, before they become computationally challenging in the present implementation.

MIDDLE RANGE ORDER IN GLASSES OF La2O3–Nb2O5–B2O3 AND BaO–Nb2O5–P2O5 SYSTEMS ACCORDING TO X-RAY TOTAL SCATTERING

We have obtained the X-ray total scattering patterns at wavelength 0.20735 ? and corresponding to them radial distribution functions (RDF) of electron density for glasses in the La2O3–Nb2O5–B2O3 and BaO–Nb2O5–P2O5 systems. The RDF demonstrate separate peaks at distances below ~1 nm which points on the middle range ordering in the considered glasses. The consideration of RDF for BaO–Nb2O5–P2O5 glasses system with high concentration of BaO show that the middle range order is caused by the polyhedra chains of ideal BaO crystal, and these chains does not belong to any low-index crystal plane. For La2O3–Nb2O5–B2O3 glasses system the polyhedra chains are observed which resemble those in the LaB3O6 crystal, with polyhedra bound through borate groups. For all considered glasses the niobium polyhedra does not form extended chains.

Universality of Replica-Symmetry Breaking in the Transverse Field Sherrington–Kirkpatrick Model

The existence theorem for replica-symmetry breaking (RSB) in the transverse field Sherrington-Kirkpatrick (SK) model is extended to the model with a general random exchange interactions. The relation between the expectation value of the exchange interaction energy and the Duhamel correlation function of spin operators can be obtained by an approximate integration by parts for general random interactions. In addition to the Falk-Bruch inequality, these explicit evaluations enable us to prove that the variance of overlap between two replica spin operators does not vanish under sufficiently weak transverse field in sufficiently low temperature. The absence of the ferromagnetic long range order is also shown to distinguish RSB from the $\mathbb Z_2$-symmetry breaking.

Nucleation kinetics model for primary crystallization in Al-Y-Fe metallic glass.

The high density of aluminum nanocrystals (>1021m-3) that develop during the primary crystallization in Al-based metallic glasses indicates a high nucleation rate (∼1018m-3s-1). Several studies have been advanced to account for the primary crystallization behavior, but none have been developed to completely describe the reaction kinetics. Recently, structural analysis by fluctuation electron microscopy has demonstrated the presence of the Al-like medium range order (MRO) regions as a spatial heterogeneity in as-spun Al88Y7Fe5 metallic glass that is representative for the class of Al-based amorphous alloys that develop Al nanocrystals during primary crystallization. From the structural characterization, an MRO seeded nucleation configuration is established, whereby the Al nanocrystals are catalyzed by the MRO core to decrease the nucleation barrier. The MRO seeded nucleation model and the kinetic data from the delay time (τ) measurement provide a full accounting of the evolution of the Al nanocrystal density (Nv) during the primary crystallization under isothermal annealing treatments. Moreover, the calculated values of the steady state nucleation rates (Jss) predicted by the nucleation model agree with the experimental results. Moreover, the model satisfies constraints on the structural, thermodynamic, and kinetic parameters, such as the critical nucleus size, the interface energy, and the volume-free energy driving force that are essential for a fully self-consistent nucleation kinetics analysis. The nucleation kinetics model can be applied more broadly to materials that are characterized by the presence of spatial heterogeneities.

Sol-gel based synthesis to explore structure, magnetic and optical properties of double perovskite Y2FeCrO6 nanoparticles

In this present investigation, nanoparticles of B-site disordered Y2FeCrO6 (YFCO) double perovskite have been successfully synthesized for the first time by optimizing synthesis steps and temperatures of facile sol-gel technique to investigate their structural, magnetic, and optical properties. The Rietveld refinement of the X-ray diffraction pattern of YFCO nanoparticles revealed a single-phase orthorhombic structure with pbnm space group. The average size of the nanoparticles is around 67±15 nm determined by both field emission scanning electron microscopy and transmission electron microscopy imaging. The existence of mixed valence states of Fe and Cr ions was confirmed by X-ray photoelectron spectroscopy which is an indication of the absence of proper B site long range ordering. The temperature dependent magnetization curves demonstrated negative magnetization of this double perovskite material with compensation temperature at around 170 K. The field-dependent magnetic hysteresis loops exhibited the coexistence of weak ferromagnetic and antiferromagnetic domains in YFCO nanoparticles. The exchange bias effect was observed below Néel temperature which is tunable by a cooling magnetic field. The UV–visible spectroscopy ensured that YFCO nanoparticles have a direct band gap of ∼ 1.90 eV, which was further confirmed by steady-state photoluminescence spectroscopy. This B-site disordered YFCO might enhance interest on basic fundamental research on disordered rare-earth and transition metal-based perovskite systems and can be used for spintronics as well as photocatalytic applications because of its favorable magnetic and optical properties.

Effect of the alkali vs iron ratio on glass transition temperature and vibrational properties of synthetic basalt-like glasses

Volcanic eruptions generate huge amounts of material with a wide range of compositions and therefore different physicochemical properties. We present a combined Raman and calorimetric study carried out on four synthetic basaltic glasses with different alkali vs iron ratio which spans the typical compositions of basalts on Earth. Differential scanning calorimetry shows that changes of this ratio modify the glass transition interval whereas Raman spectra allow to gain insight about the structure of the glass in the microscopic and macroscopic range. Indeed, our Raman analysis is extended from the high frequency region, characterized by the molecular peaks, to the very low frequency region where glasses exhibit the boson peak. Spectra show a variation of the non-bridging oxygens number that affects the medium range order of the glass and the network interconnections. In the considered substitution interval, the boson peak shape is conserved while its position shift upwards. This means that increasing the alkali vs iron content, the elastic medium hardens but it does not change nature. This study emphasizes the importance of considering the full-range spectra when analysing multicomponent or natural systems with small chemical variations.

Role of cholesterol in pulmonary surfactant at ambient and physiological temperatures.

Pulmonary surfactant is the mixture of lipids and proteins that lowers surface tension in the lungs. The material forms a thin film on the aqueous layer that lines the alveolar air-sacks. When compressed by the shrinking alveolar surface area during exhalation, the surfactant film achieves very low surface tensions. Our studies used monolayers of DPPC with variable amounts of cholesterol as a simple model of the alveolar film. A widely held supposition contends that, relative to the multilamellar vesicles secreted by the type II pneumocytes, the alveolar film is greatly enriched in DPPC. We have recently published that structural changes by which cholesterol causes faster collapse of DPPC monolayers and determined the structure of calf lung surfactant extract (CLSE) and DPPC/cholesterol mixed films with Xchol = 0.0-0.4. This work is an extension of this effort, but at physiological temperature of 37⁰C. Here, we compare the structure of CLSE and DPPC/cholesterol mixtures with varying Xchol at ambient and physiological temperatures. We have used grazing incidence X-ray off-specular scattering (GIXOS) and grazing incidence X-ray diffraction (GIXD) to obtain the detailed structural data of the interfacial films. GIXOS measurements showed that CLSE remains a single monolayer at surface pressures from 15mN/m to 45 mN/m, contrary to popular predictions that pulmonary surfactant must form a multilayer at low surface pressures. Both CLSE and DPPC/cholesterol films at 37C produced a single diffraction peak, indicating little change in the lattice-parameters of the ordered film. Predictably, with heating both CLSE and DPPC/cholesterol films showed the significant broadening of the GIXD peak and the dramatic decrease in its maximum scattered intensity, indicating a radical loss of the long range order among the hydrocarbon chains and the extensive conversion of ordered to disordered structures.

Multiple Dielectric Relaxations in CaMn2O4 Polycrystalline Compound

The dielectric response of the CaMn2O4 polycrystalline compound has been reported in this paper. The orthorhombic room-temperature structure (space group Pbcm) is confirmed by powder X-ray diffraction (XRD). The temperature dependent magnetization M(T) data reveal that antiferromagnetic ordering below T N = 220 K. Further decreasing the temperature magnetization shows increasing trend of canted spin structure. Dielectric anomalies are clearly observed at the onset of the magnetic ordering (∼220 K) and also at the onset of the canted structure (50 K) in CaMn2O4. Such type of dielectric anomaly around long range order and canted spin transition will correlate the influence of underlying magnetic structure with electric ordering.

A combined study on structural, magnetic and specific heat on double perovskite iridates Ln2CoIrO6 [Ln = Pr, Nd

We report rich magnetic behavior for Co–Ir based double perovskites consisting of different rare earth cations Pr and Nd: Pr2CoIrO6 (PCIO) and Nd2CoIrO6 (NCIO). Both oxides show an antisite disorder of 10% and a ferrimagnetic transition, T around 96 K and 98 K respectively. The long range magnetic ordering is arising from the canted antiferromagnetic ordering between the Co2+ and Ir4+ ions. A prominent peak around 27 K in magnetization data of NCIO indicates that the total moment of Nd ion is antiferromagnetically coupled to the Co–Ir sublattice. The long range order of the Nd sublattice is corroborated by the evidence of an anomaly in specific heat at very low temperature. The compounds exhibit a maximum change of magnetic entropy of 0.57 (0.48) J kg.K−1 at T in a magnetic field of 5 T. The strong spin–orbit coupling in 5d states of Ir and cation disorder lead to the Mott insulating phase as found from the analysis of temperature dependent resistivity. These unique behaviors suggest an interesting interplay between localized Pr/Nd-4f, itinerant Co-3d and Ir-5d electrons.

Models of configurationally-complex alloys made simple

We present a Python package for the efficient generation of special quasi-random structures (SQS) for atomic-scale calculations of disordered systems. Both, a Monte-Carlo approach or a systematic enumeration of structures can be used to carry out optimizations to ensure the best optimal configuration is found for given cell size and composition. We present a measure of randomness based on Warren-Cowley short-range order parameters allowing for fast analysis of atomic structures. Hence, optimal structures are found in a reasonable time for several dozens or even hundreds of atoms. Both SQS optimizations and analysis of structures can be carried out via a command-line interface or a Python API. Additional features, such as optimization towards partial ordering or independent sublattices allow the generation of atomistic models of modern complex materials. Moreover, hybrid parallelization, as well as distribution of vacancies, are supported. The output data format is compatible with ase, pymatgen and pyiron packages to be easily embeddable in complex simulation workflows. Program summaryProgram title: sqsgeneratorCPC Library link to program files:https://doi.org/10.17632/m2sb3wzcvc.1Developer's repository link:https://github.com/dgehringer/sqsgeneratorLicensing provisions: MITProgramming language: Python, C++Supplementary material:https://sqsgenerator.readthedocs.ioNature of problem: Many technological relevant materials, exhibit a crystalline disorder. Within atomistic modelling approaches such as Density Functional Theory (DFT) or Molecular Dynamics, disorder is modelled with a cell containing a (small) finite set of atoms. Such an atomic configuration is usually found by enumerating structures. However, since configurational space is growing exponentially efficient tools are needed to sample it properly.Solution method: Efficient quantification of disorder using a generalization of Warren-Cowley Short Range Order (WC-SRO) parameters [1,2]. By either a Monte-Carlo approach or systematic enumeration, optimal structures can be found. The software is distributed as a Python package offering a command line interface. Core parts are written in C++ and exhibit shared (OpenMP) and distributed (MPI) memory parallelism. For embedding into complex simulation workflows, the tool exposes a Python API to integrate into popular packages such as ase [3], pymatgen [4] or pyiron [5].

Phase formation in the Ni-enriched zone below the surface oxide on NiTi

Upon exposure to oxygen, NiTi forms a Ti-rich surface oxide layer, and mass balance requires that a Ni-enriched zone forms below the oxide. Both the surface oxide layer and the Ni-enriched zone are discussed to affect key properties of NiTi as material for minimally invasive medical application or for actuators, e.g., the release of Ni and the resistance to initiation and propagation of cracks when exploiting the shape memory effect/pseudoelasticity. However, owing to the small extension of the Ni-enriched zone of a few nanometers, little is known about its crystallinity, phase and evolution of composition.We study the formation of the surface layers during annealing at ∼500 °C for up to 10 min, as routinely applied for shape-setting. With an approach employing nano beam electron diffraction in a transmission electron microscope and an in-house software package for phase analysis it is shown for the first time that phase transformations in the Ni-enriched zone, specifically from NiTi to Ni4Ti3, occur already after 2 min of annealing, much earlier than documented in the literature. Furthermore, the phase transformation to the thermodynamically stable Ni3Ti in the Ni-enriched zone is shown to be mediated by metastable Ni4Ti3. In contrast to Ni4Ti3 precipitates in NiTi bulk, the morphology of the crystalline phases is granular. Considering the swift formation of Ni-rich phases below the surface oxide and the observation of a band-like diffraction contrast in the TEM images originating from crystal interfaces, a phase transformation mechanism in the Ni-enriched zone is suggested consisting of short range order rearrangements of atoms.

Pressure-Induced Structural Transformations and Electronic Transitions in TeO2 Glass by Raman Spectroscopy.

TeO2 glass has been studied by Raman spectroscopy up to the record pressure of 70 GPa. The boson peak frequency ωb exhibits a decrease of the ∂ωb/∂P slope at 5-6 GPa and saturates above 30 GPa with a practically constant value up to 70 GPa. Experiment and theory indicate that pressures up to 20 GPa induce the transformation of single Te-O-Te bridges to double Te-O2-Te bridges, leading to a more compact structure, while Raman activity developing at higher pressures around 580 cm-1 signals the increase of Te coordination from 4- to 6-fold. Natural bond orbital analysis shows that double Te-O2-Te bridges favor the s → d transition and promote the increase of Te coordination through d2sp3 hybridization. This transition leads to the formation of TeO6 octahedra, in strict difference with crystalline TeO2 at the same pressure range, and to the development of a 3D network that freezes the medium range order.

Medium Range Ordering in liquid Al-based alloys: towards a machine learning approach of solidification

Ab initio molecular dynamic simulations of liquid Al93Cr7 and Al83Zn10Cr7 alloy have evidenced the presence of an icosahedral short range order (iSRO) which develops into icosahedral medium range order (iMRO) as the melt is undercooled. This atomic arrangement accounts for the presence of Dynamic Heterogeneities characterized by Al fast-dynamics regions and Cr-rich slow-dynamics regions. Characterisation of the medium range order was carried out by a direct connectivity approach. However, given the small size of the simulation (256 atoms), such characterisation remains partial. In order to better describe both iMRO formation and more dilute alloys closer to industrial compositions, a new modelling strategy has been initiated to allow in the long term for large-scale atomic-level simulations. Molecular Dynamics (MD) of million to billion atoms may indeed lead to meaningful results. Exploitation of such large amounts of MD-generated big data can be carried out by means of Machine Learning (ML) tools which provide relevant and powerful analysis methods. An unsupervised ML approach based on topological descriptors using persistent homology concepts is proposed to reveal the structural features of atomic arrangements without a priori knowledge on the studied system. This approach has been applied so far to pure Al melts. Both translational and orientational orderings are thus evidenced together with nucleation pathways, whose revealed features are beyond the hypotheses of the Classical Nucleation Theory.

Short range order in disordered spinels and the impact on cation vacancy transport

Spinels can exhibit high levels of cation disorder under irradiation. We show that short range order in spinels manifests as chains of nearest neighbor coordinated antisites, which modifies defect transport depending on the spinel chemistry and degree of disorder.

An ab initio study of the static, dynamic and electronic properties of some liquid 5d transition metals near melting

We report a study on the static and dynamic properties of several liquid 5d transition metals at thermodynamic conditions near their respective melting points. This is performed by resorting to ab initio molecular dynamics simulations in the framework of the density functional theory. Results are presented for the static structure factors and pair distribution functions; moreover, the local short range order in the liquid metal is also analized. As for the dynamical properties, both single-particle and collective properties are evaluated. The dynamical structure shows the propagating density fluctuations, and the respective dispersion relation is obtained. Results are also obtained for the longitudinal and transverse current spectral functions along with the associated dispersion of collective excitations. For some metals, we found the existence of two branches of transverse collective excitations in the region around the main peak of the structure factor. Finally, several transport coefficients are also calculated.

Origin of structural and dynamic heterogeneity in thymol and coumarin-based hydrophobic deep eutectic solvents as revealed by molecular dynamics.

Hydrophobic deep eutectic solvents (HDESs) have recently emerged as a class of water-immiscible solvents with greener starting materials and inherent hydrophobic character, opening the gates to various new promising applications. Herein, we have carried out all-atom molecular dynamics simulations to comprehend the bulk phase structural organization and dynamic behavior of thymol and coumarin-based HDESs at two molar ratios of the constituent components. The simulated X-ray and neutron scattering structure functions (S(q)s) indicate a prepeak signifying that these HDESs possess nanoscale heterogeneity or intermediate range ordering. The decomposition of the total S(q)s based on polarity reveals that clustering of the polar group present in thymol and coumarin leads to the presence of the prepeak which also has small contributions from the apolar-apolar correlations. The intermolecular hydrogen bonding network between thymol-coumarin and thymol-thymol mainly guides the arrangement of the HDESs. We find a stronger hydrogen bond between the carbonyl oxygen of coumarin and the hydroxyl hydrogen of thymol, marked by the longer hydrogen bond lifetime. In contrast, the shorter lifetime of the hydrogen bond between the hydroxyl oxygen and the hydroxyl hydrogen of thymol suggests a weaker hydrogen bonding. On changing the thymol : coumarin molar ratio from 1 : 1 to 2 : 1, the average lifetimes of both the hydrogen bonds decrease, suggesting stronger hydrogen bonds in the 1 : 1 HDES. The translational dynamics of thymol and coumarin become faster in the 2 : 1 thymol : coumarin HDES. A slightly stronger caging effect is observed for coumarin in comparison to thymol molecules. From the analysis of the non-Gaussian parameter, we observe the presence of heterogeneity in the translational displacements of thymol and coumarin molecules. Furthermore, the computed self-van Hove correlation functions reveal that thymol and coumarin molecules cover more distances than the ideal diffusive displacements, confirming the presence of dynamic heterogeneity.

Selecting optimal R6TX2 intermetallics (R = Gd, Tb, Dy; T = Mn, Fe, Co, Ni; X = Sb, Te) for magnetic refrigeration.

A complete experimental study of the physical properties playing a relevant role in the magnetic refrigeration application (structural, magnetic, magnetocaloric and thermal) has been performed over nine selected Fe2P-type R6TX2 (R = Gd, Tb, Dy; T = Mn, Fe, Co, Ni; X = Sb, Te) intermetallic compounds, to work close to room temperature. Two magnetic phase transitions are observed for these materials: a paramagnetic to ferromagnetic transition in the range of 182-282 K and a spin reorientation transition in the range of 26-76 K. As a consequence, two peaks related to a direct magnetocaloric effect (DMCE) appear with the magnetic entropy change, generating a wide table-like plateau region in between both peaks, which is required to improve the efficiency of refrigerators following an Ericsson cycle. The highest magnetic entropy peak value for μ0ΔH = 5 T is found for Tb2Dy4FeSb2, with 7.72 J kg-1 K-1 around 182 K. For the same applied field the other compounds show moderate values around room temperature (2.88-4.53 J kg-1 K-1). However, the superposition of the two peaks results in huge refrigerant capacity values, up to RCFWHM(5 T) = 1103.04 J kg-1 in the case of Tb2Dy4FeSb2. The thermal diffusivity, thermal effusivity, thermal conductivity and specific heat capacity have been measured at room temperature, and the temperature dependence of the former has been obtained around the relevant magnetic phase transition region, with values in the range of 1.3-2.3 mm2 s-1, which are good for magnetic refrigerators at high working frequencies. The study is completed with a rigorous critical behavior analysis of the second order PM-FM transition. The critical exponent γ points to long range order interactions, in general, while β values are in the range of 0.59-0.90, indicating a deviation from theoretical models as a reflection of the magnetic complexity in these compounds. The critical exponents have been used to confirm the scaling relations of magnetocaloric properties, and the scaling of refrigerant capacity (RC) values in materials exhibiting two magnetic phase transitions is addressed, concluding that for a correct scaling of RC the magnetic entropy change peak must be considered symmetric. The role of each atom in the properties of the compounds is discussed.

Glassy and liquid Sb2S3: insight into the structure and dynamics of a promising functional material

High optical and electric contrast between the SET (crystalline) and RESET (amorphous) logic states is related to the different short and intermediate range order in orthorhombic and vitreous Sb2S3.

In situ observation of medium range ordering and crystallization of amorphous TiO2 ultrathin films grown by atomic layer deposition

The evolution of medium range ordering (MRO) and crystallization behavior of amorphous TiO2 films grown by atomic layer deposition (ALD) were studied using in situ four-dimensional scanning transmission electron microscopy. The films remain fully amorphous when grown at 120 °C or below, but they start showing crystallization of anatase phases when grown at 140 °C or above. The degree of MRO increases as a function of temperature and maximizes at 140 °C when crystallization starts to occur, which suggests that crystallization prerequires the development of nanoscale MRO that serves as the site of nucleation. In situ annealing of amorphous TiO2 films grown at 80 °C shows enhancement of MRO but limited number of nucleation, which suggests that post-annealing develops only a small portion of MRO into crystal nuclei. The MRO regions that do not develop into crystals undergo structural relaxation instead, which provides insights into the critical size and degree of ordering and the stability of certain MRO types at different temperatures. In addition, crystallographic defects were observed within crystal phases, which likely negate corrosion resistance of the film. Our result highlights the importance of understanding and controlling MRO for optimizing ALD-grown amorphous films for next-generation functional devices and renewable energy applications.