Order-disorder Transformation Research Articles

Partially Oxidized Purely Organic Zwitterionic Neutral Radical Conductor: Multi-step Phase Transitions and Crossover Caused by Intra- and Intermolecular Electronic Interactions.

Formation of a partially charge-transfer or partially oxidized/reduced state has been one of the most important requirements for the development of highly conducting molecular materials, such as organic metals and superconductors. This requirement has been fulfilled by combining appropriate electron-donor and acceptor molecules to construct multi-component molecular complexes/salts, such as (TTF+0.59)(TCNQ-0.59) and (BEDT-TTF+0.5)2X-, where TTF = tetrathiafulvalene, TCNQ = tetracyanoquinodimethane, BEDT-TTF = bis(ethylenedithio)tetrathiafulvalene, and X = monovalent inorganic anion. Here, we propose a methodology to fulfill this requirement by a single neutral molecule; namely, we have connected two TTF+0.5-type partially oxidized π-skeletons through a boron anion to design a purely organic zwitterionic neutral radical {[(PDT-TTF-Cat)2]+B-}•. This molecule was successfully obtained as air-stable crystals containing solvent tetrahydrofuran (THF) molecules. Measurements of electrical resistivity, magnetic susceptibility, and X-ray diffraction reveal that the partially oxidized state is certainly formed, which enables realization of a 3/4-filled electron band. Furthermore, this system has intramolecular charge degrees of freedom, attributable to the two TTF+0.5 π-skeletons introduced into the molecule. The resulting interplay of intra- and intermolecular charge degrees of freedom (or simply, intra- and intermolecular electronic interactions) has led to multi-step phase transitions and crossover, providing unique strongly correlated electron properties, such as the formation of a three-dimensional charge-ordered dimer-Mott insulating state and its melting triggered by disorder-order transformation of the co-crystallized solvent THF molecules at low temperatures.

Influence of La Addition on Order–Disorder Transformation and Deformation Softening Behavior of Fe–6.5 wt % Si Alloy

Influence of La Addition on Order–Disorder Transformation and Deformation Softening Behavior of Fe–6.5 wt % Si Alloy

Study of solid state transformations and their effect on mechanical response in the stoichiometric Ni2Cr using in-situ synchrotron radiation diffraction experiments

A complete microstructural characterization of the stoichiometric Ni2Cr alloy after different ageing times in which different degrees of order are attained has been carried out. The study of order-disorder transformation has been done using synchrotron radiation diffraction monitoring eventual changes in high-energy X-ray diffraction (HEXRD) patterns every six seconds in the course of differential scanning calorimetry DSC experiments. According to acquired data, the onset of the order-disorder transformation occurs at 630 °C, temperature higher than that reported in previous works. In addition, the appearance of fine carbide precipitation has also been found inside the grains and at grain boundaries. Both ordering of the stoichiometric Ni2Cr and carbide precipitation seems to be responsible for the strengthening of the alloy.

The effects of discontinuous precipitation and ordering on the age-hardening in Au-20Ag-30Cu (wt.%) alloy

Discontinuous precipitation (DP) and ordering have been typically considered to influence the mechanical properties of many alloy systems during the aging process. In the present study, the microstructural evolution and the effect of discontinuous precipitation and ordering on the age-hardening behavior in Au-20Ag-30Cu (wt.%) ternary alloy were investigated. Au-20Ag-30Cu (wt.%) alloy is found to be typically age-hardened. Thermodynamic calculation forecasts the discontinuous precipitation forming Cu-rich α1 phase and Ag-rich α2 phase, companied by the disorder-order transformation forming AuCu3 phase with L12 structure for this Au-Ag-Cu alloy. The DP is experimentally observed to form at grain boundaries consisting of alternative α1 and α2 two-phase lamellae. And the L12-type AuCu3 ordered phase, which presents nanometric spherical particle, forms at grain interior and the lamellar α1 phase of DP. The DP and L12-type AuCu3 nanoprecipitates both grow with the elevated aging temperature. The DP at grain boundaries is found to deleteriously affect the overall age-hardening with different deformation mechanisms between these two lamellar phases of DP. High-density dislocations are observed in the lamellar α1 phase of DP around the hardness indentation, which indicates that the interior L12-type AuCu3 nanoprecipitates in α1 phase can effectively impede the movement of dislocations to boost the age-hardening. The overall age-hardening is determined by the competitive effect of the DP and L12-type AuCu3 nanoprecipitates. This study offers theoretical foundation for controlling the microstructures and optimizing the mechanical properties of Au-Ag-Cu alloys.

Activation energy of phase transformations at high-temperature synthesis of tungsten carbide by electrothermal explosion under pressure

The effect of activation energy on phase transformations (transitions) in the W–C system during the synthesis induced by an external heat source was investigated by electrothermal explosion (ETE) under pressure. The ETE technology combines self-propagating high-temperature synthesis (SHS) with additional sample heating by Joule heat – electric current passing through the synthesized mixture, and it makes it possible to determine the chemical reaction rate that is highly susceptible to external impacts such as pressure, concentration, sample shape, any film present on combustion products, etc. The chemical reaction rate, i.e. external source current, may be controlled by changing the activation energy. The study was conducted in the following conditions: temperature Т = 293÷3700 K; carbon concentration of 49.8–50.2 at.%; quasi-static compression at P = 96 MPa; external source voltage and current density V = 10 V, I = 20 МА/m2, respectively; samples 8 mm in diameter weighing 6 g. The Т–τ thermogram of the W–C system was used to determine the following parameters: four stages of the synthesis process, temperatures of special points of phase transformations, temperature boundaries of phases and process activation energy. Thermograms of intermediate states are presented as isothermal plateaus of phase transformations. The analysis of experimental results and the physical representation of the process make it possible to assert that temperature plateau parameters are the effective value of activation energy for synthesis mode maintenance. Each of the 4 W–C mixture synthesis stages is described. Pre-explosion stage I – sample heating in the temperature range of Т = 293÷563К, endothermic reaction, effective activation energy for synthesis mode maintenance Q = 2.96 kJ, and taking into account 1-mole mass Еа = 111.6 kJ/mol. Low-temperature (563–1190 К) stage II – ignition, Q = 5.46 kJ, Еа = 109.2 kJ/mol. High-temperature stage (III) in the range of Т = 1190÷2695К, order–disorder transformation, Q = 14.25 kJ, Еа = 424 kJ/mol. Finally, Stage IV occurs in the range of Т = 2695÷3695К, Q = 14.31 kJ, Еа = 143.2 kJ/mol. It was shown that the limiting stage with the highest activation energy is the melting process.

MnO2 Polymorphs as Cathode Materials for Rechargeable Ca‐Ion Batteries

AbstractCalcium‐ion batteries (CIBs) have attracted extensive interest for energy storage due to the high operating potentials and the abundant resources of calcium. However, the application of CIBs has been limited by the absence of high‐voltage and high‐performance cathode materials. Herein, for the first time, four kinds of MnO2 polymorphs (α, β, γ, and δ‐phase) as cathode materials for CIBs are reported, and the δ‐MnO2 exhibits the highest electrochemical performance, which is mainly due to the higher Ca‐ion diffusivity of δ‐MnO2. The excellent rate capability obtained herein outperforms those of previously reported for all oxides as cathode materials for CIBs. Furthermore, the Ca‐ion storage mechanism of δ‐MnO2 based on the reversible order–disorder transformation is demonstrated by the in situ/ex situ characterizations. In addition, the capacity decay mechanism of δ‐MnO2 during the cycling process is confirmed as the dissolution of Mn and the optimization strategy is proposed. These results demonstrate that the δ‐MnO2 is a promising cathode material for CIBs, providing new opportunity for the development of high‐performance CIBs.

Biotropic liquid crystal phase transformations in cellulose-producing bacterial communities

Biological functionality is often enabled by a fascinating variety of physical phenomena that emerge from orientational order of building blocks, a defining property of nematic liquid crystals that is also pervasive in nature. Out-of-equilibrium, "living" analogs of these technological materials are found in biological embodiments ranging from myelin sheath of neurons to extracellular matrices of bacterial biofilms and cuticles of beetles. However, physical underpinnings behind manifestations of orientational order in biological systems often remain unexplored. For example, while nematiclike birefringent domains of biofilms are found in many bacterial systems, the physics behind their formation is rarely known. Here, using cellulose-synthesizing Acetobacter xylinum bacteria, we reveal how biological activity leads to orientational ordering in fluid and gel analogs of these soft matter systems, both in water and on solid agar, with a topological defect found between the domains. Furthermore, the nutrient feeding direction plays a role like that of rubbing of confining surfaces in conventional liquid crystals, turning polydomain organization within the biofilms into a birefringent monocrystal-like order of both the extracellular matrix and the rod-like bacteria within it. We probe evolution of scalar orientational order parameters of cellulose nanofibers and bacteria associated with fluid-gel and isotropic-nematic transformations, showing how highly ordered active nematic fluids and gels evolve with time during biological-activity-driven, disorder-order transformation. With fluid and soft-gel nematics observed in a certain range of biological activity, this mesophase-exhibiting system is dubbed "biotropic," analogously to thermotropic nematics that exhibit solely orientational order within a temperature range, promising technological and fundamental-science applications.

Suppressing disorder-order phase transition of Gd2Zr2O7 pyrochlore by Dy3+ doping and their impact on luminescence

The functionality of the Gd2Zr2O7 matrix is dictated by ordering at the cationic lattice (pyrochlore or defective fluorite) and associated local manifestations. In this work, we have synthesized nano and bulk undoped Gd2Zr2O7 (GZO) and Gd2Zr2O7:Dy3+ (GZOD) by molten salt synthesis adopting a crystal engineering approach and investigated their impact on photoluminescence properties. DFT calculations along with the higher intensity of yellow emission in the bulk GZOD confirm that the presence of Dy3+ is energetically favorable in defect fluorite structure compared to ordered pyrochlore counterpart. Increase/adjustments in the Zr–O bond distances in Dy3+ surroundings facilitate the release of local strain/stress in defective fluorite structure, which is not feasible in the perfectly ordered pyrochlore structure. Overall, the preference of defective fluorite structure by the GZO NCs, GZOD NCs, and bulk GZOD has been attributed to inherent flexibility in local structure with varying coordination numbers and polyhedral arrangements that assist in reducing the strain arising due to smaller particle size and/or by Dy3+ doping. It is believed that the present work provides simplistic pathways to control the order-disorder transformation, and in turn, the functionality of the Gd2Zr2O7 matrix for better hosting of nuclear waste and phosphor centers.

Intermetallic compounds of the Cr–Mn system investigated using in situ powder neutron diffraction: The reported order-disorder transformation of the σ phase elucidated

Cr–Mn system is very important for many applications. In the present work, the reported intermetallic compounds have been studied using in situ high temperature neutron powder diffraction. For the first time, site occupancies in a σ phase are obtained as a function of temperature by Rietveld analysis. They are compared with calculated data obtained from DFT. The previously reported order-disorder transition in the σ phase around 1000 °C is shown to be in fact a congruent phase transformation to the bcc Cr–Mn solid solution. This solid solution is probably continuous between pure Cr and δ-Mn. The second reported intermetallic α’ phase (Cr2Mn3) phase is shown to be in fact a ternary nitride of the Cr–Mn–N system that should not appear in the binary phase diagram. The present work clarifies and explains all the features of the previous literature on this system and brings major revisions to the phase diagram. General conclusions are drawn concerning mistakes that can be done in the evaluations of phase diagrams.

Breaking with the Principles of Coreduction to Form Stoichiometric Intermetallic PdCu Nanoparticles.

Intermetallic nanoparticles (NPs) have shown enhanced catalytic properties as compared to their disordered alloy counterparts. To advance their use in green energy, it is crucial to understand what controls the formation of intermetallic NPs over alloy structures. By carefully selecting the additives used in NP synthesis, it is here shown that monodisperse, intermetallic PdCu NPs can be synthesized in a controllable manner. Introducing the additives iron(III) chloride and ascorbic acid, both morphological and structural control can be achieved. Combined, these additives provide a synergetic effect resulting in precursor reduction and defect-free growth; ultimately leading to monodisperse, single-crystalline, intermetallic PdCu NPs. Using in situ X-ray total scattering, a hitherto unknown transformation pathway is reported that diverges from the commonly reported coreduction disorder-order transformation. A Cu-rich structure initially forms, which upon the incorporation of Pd(0) and atomic ordering forms intermetallic PdCu NPs. These findings underpin that formation of stoichiometric intermetallic NPs is not limited by standard reduction potential matching and coreduction mechanisms, but is instead driven by changes in the local chemistry. Ultimately, using the local chemistry as a handle to tune the NP structure might open new opportunities to expand the library of intermetallic NPs by exploiting synthesis by design.

Volume effects on microstructures and magnetic properties of MnGa thin films during order-disorder transformation

In this study, L10 MnGa thin films were grown on an amorphous glass substrate, and the relationship between the volume effect and the magnetic properties of thin films as a function of the film thickness was investigated. An amorphous thin film and weak magnetic properties were observed at the thickness of 10 nm. With the increase in thickness, the magnetization (M10 kOe) and coercivity (Hc⊥) also increased. The maximum M10 kOe and Hc⊥ values were 106.3 ± 5.7 emu/cm3 and 8.54 kOe in a 200 nm-thick sample, respectively. X-ray diffraction indicated that the order-disorder transformation occurred when the thickness was greater than 25 nm. Plan-view transition electron microscopy (TEM) images indicated the emergence of larger grains and blurred grain boundaries when the thickness was increased beyond 25 nm, while cross-sectional TEM images suggested that the appearance of nuclei promote the growth of columnar MnGa grain structures. Energy dispersion spectroscopy analysis indicated that the composition of MnGa was Mn-rich (or Ga-poor) in the nuclei at the initial growth state and then gradually changed to Mn50Ga50 as the thickness increased. In addition, the effects of the base pressure of the chamber on the phase and magnetic properties of the MnGa films were also investigated.

Probing the Short-Range Ordering of Ion Irradiated Gd2Ti2-yZryO7 (0.0 ≤ y ≤ 2.0) Pyrochlore under Electronic Stopping Regime

A series of Gd2Ti2-yZryO7 (0.0 ≤ y ≤ 2.0) solid solutions has been synthesized by a solid-state process using a three-stage heating protocol. Raman spectroscopy was used to investigate the structural modification with an increasing Zr content in pristine samples. Short-range ordering or local disorder around cationic sites was investigated in irradiated samples with 120 MeV Au+9 ions by Raman spectroscopy. Phase transformation was observed in Zr-rich pyrochlore compositions due to the anion disordering around the cationic sites in pristine samples. Moreover, SHI irradiation-driven phase transformation was observed dominantly in Zr-rich compositions due to cation disordering. Like XRD analysis in the previous study, the present study provides information about the higher efficacy of Zr-rich composition towards SHI-induced disordered transformation with very little amorphization. Since Raman spectroscopy is able to observe the polyhedral structure of AO8 and BO6 even at nanoscale also, therefore, this study shows relatively smaller SHI induced disordered ion tracks as compared to XRD analysis. Thus, the short-range ordering becomes important to investigate the local order-disorder transformation on SHI irradiation.

A study of order-disorder phase transformation in Ag50Pd50 alloy

In this study, the structure of Ag50Pd50 alloy was investigated with temperature dependent in-situ X-ray diffraction, residual resistometry, and differential scanning calorimetry (DSC) as well as differential thermal analysis (DTA) techniques. Isochronal annealing experiment was performed to determine the impact of ordering on the residual electrical resistivity. The residual resistivity curve shows a sharp minimum near 250 °C indicating the on-set of ordering. The in-situ X-ray diffraction data taken for a well annealed sample shows no super lattice reflections over the investigated temperature range but the lattice parameter determined from this data shows an abrupt decrease in its value at 250 °C showing an order-disorder phase transformation (ODPT). The diffraction pattern of a sample annealed at 230 °C shows splitting of the high angle 511 fundamental reflection due to the tetragonal distortion. The peak intensity and c/a ratios are found to be 1:1.94 and 1.0028, respectively. A similar phase change was also observed during a DSC experiment around 250 °C on this alloy. The existence of these anomalies may be attributed to L1o - FCC type order-disorder phase transformation at this temperature.

Intense luminescence in nanostructured germanate glass

The phosphors free of active dopants have been attracted much attention due to their interesting optical properties and potentials for construction of rare-earth free functional materials. In this letter, we report the construction of phosphor free of active dopants via nanocrystallization of glass. In a typical example, a single-phase Zn2GeO4 phosphor is fabricated via controllable crystallization of the germinate glass. Intense green luminescence with the central wavelength at 523 nm can be realized in nanostructured glass and it is estimated to be ∼150 times higher than that of the as-made glass. The physical mechanism is associated with the formation of Zn related interstitial defects during the disorder-order phase transformation in glass. The progress in this work indicates that the avenue of glass crystallization provides an effective strategy for the development of novel active photonic materials free of active dopants.

Large dielectric switch effects induced by an order-disorder transformation in cyclopropylamine perchlorate crystals.

Solid-state crystals with two distinct dielectric states can be a physical practice in binary-based technologies. A large dielectric switch effect up to 103 caused by an order-disorder structural phase transition is found in cyclopropylamine perchlorate (CPA-ClO4) crystals at temperatures around 230 K (Tc) and 220 K (T'c). Large dielectric switch effects here can be compared to that of the famous ceramic oxide dielectrics. As far as we know, this is the highest dielectric switch effect in simple organic salt crystals and organic-metal compounds so far. If the phase transition temperature can be adjusted by molecular manipulation, one of the most promising candidates for technological applications may emerge in the future.

Two-step thermotropic phase transition and dielectric relaxation in 1D supramolecular lead iodide perovskite [NH4@18-crown ether]PbI3.

The supramolecular lead iodide perovskite crystals, {[NH4(18-crown-6)]PbI3}∞ (1), (18-crown-6 = 1,4,7,10,13,16-hexaoxacyclooctadecane), was successfully achieved by a facile solvent evaporation strategy using a DMF solution containing equal molar quantities of PbI2, NH4I and 18-crown-6. The supramolecular perovskite was characterized by microanalysis for C, H and N elements, thermogravimetric (TG) analysis, differential scanning calorimetry (DSC), powder X-ray diffraction (PXRD) and single crystal X-ray diffraction techniques. DSC measurements demonstrated that 1 experiences a two-step thermotropic phase transition around 333 K and 383 K, respectively. The phase transition is relevant to the disorder-order transformation of the 18-crown-6 molecule at ∼333 K, while both breaking-symmetry and ordered-disordered transformation of the 18-crown-6 molecule occurred at ∼383 K. In addition, the sharp change of the PbI6 coordination octahedron distortion degree plays a synergistic role in the two-step phase transition. The dielectric relaxation occurs above 243 K in 1 and is mainly attributed to the displacement of the NH4+ ions relative to the ring of the 18-crown-6 molecule and {PbI3}∞ chain induced by an AC electrical field.

Persistence of variant selection in red gold alloys

Recently, a new thermally activated distortion with amplification (TADA) effect has been reported in red gold alloys caused by the A1 → L10 phase transformation. The macroscopic amplification is due to the persistence of variant selection nucleated under stress but growing in stress-free condition. In this work, we show that the TADA effect can generate a compressive force whose absolute value exceeds 40 MPa in bending and uniaxial loading. Complementary EBSD analyses indicate that the compressive force has no influence on the variant selection initiated under tension. This result confirms that the A1 → L10 disorder-order phase transformation is diffusive, thermodynamically of 1st order, displacive and crystallographically continuous. The TADA force may be exploited for actuators.

Acceleration of B2/L21 order-disorder transformation in Ni2MnAl Heusler alloys by in-magnetic-field annealing

Ferromagnetic L21 phase of Ni2MnAl with high order degree is difficult to synthesize because of low L21/B2 order-disorder transformation temperature. It is found that magnetic field accelerated the transformation from the antiferromagnetic disordered B2 to ordered L21 phase. Magnetization increased by in-magnetic-field annealing at 673 K, which indicated the acceleration of the ordering to L21 phase. Magnetic field effects on the B2/L21 order-disorder transformation were discussed based on magnetic-field-induced change of transformation temperature, enhancement of degree of order at annealing temperature, and reduction of critical radius of L21 nuclei. Growth kinetics of L21 phase were evaluated by Johnson-Mehl-Avrami equation, which indicated that 15-T annealing accelerated the growth of L21 phase 5.7 times faster than zero-field annealing. Our results showed that magnetic field can contribute to shortening the annealing time for synthesis of ferromagnetic L21-Ni2MnAl alloy.

Neutral 1D Perovskite-Type ABX3 Phase Transition Material with a Narrowband Emission and Semiconductor Property.

Cyclic organic amines are emerging as excellent building blocks to assemble organic-inorganic hybrid phase transition materials due to their flexible cyclic structure. Here, we have assembled a 1D organic-inorganic hybrid dielectric material C5 H6 NOPbBr3 (1) by alloying the cyclic organic amine 3-hydroxypyridine. 1 displays a remarkable switchable dielectric response induced by an order-disorder transformation of the organic moiety, this transformation behaviour is confirmed by DSC and Hirshfeld surface measurements. More interestingly, 1 has a narrowband emission (FWHM=4.64 nm) at 590 nm; FWHM is a major quality figure for narrowband photodetectors. In addition, 1 exhibits semiconducting properties with an indirect bandgap of 2.78 eV by the analysis of the UV-Vis absorption results.

Atomic scale insights into the rapid crystallization and precipitation behaviors in FeCu binary alloys

Molecular dynamics (MD) simulations are employed to probe the crystallization and precipitation behaviors in FeCu alloys. The alloys will undergo liquid phase separation at high temperature, after which α-Fe nucleates preferentially, then metastable Cu solid solutions in BCC lattice (ε` phase) nucleate and transform into FCC structure (ε-Cu) via martensite phase transformation immediately. Reversible order-disorder transformation behavior is observed during time-consuming crystallization process of ε-Cu. Interestingly, the ε`-Cu with only approximately three atomic layers in thickness exists stably near BCC/FCC α/ε interface at room temperature, following the Kurdjumov–Sachs (K–S) orientation relationship. The lower nucleation energy and interface energy are the reasons for the existence of abnormal ε`-Cu at the interface. It is also found that the composition and cooling rate play key roles on the final phase structure. The alloys can be fully crystallized upon cooling down at a rate of 0.1 K/ps.