Solid-state Phase Transition Research Articles

On the mechanism of solid-state phase transitions in molecular crystals - the role of cooperative motion in (quasi)racemic linear amino acids.

During single-crystal-to-single-crystal (SCSC) phase transitions, a polymorph of a compound can transform to a more stable form while remaining in the solid state. By understanding the mechanism of these transitions, strategies can be developed to control this phenomenon. This is particularly important in the pharmaceutical industry, but also relevant for other industries such as the food and agrochemical industries. Although extensive literature exists on SCSC phase transitions in inorganic crystals, it is unclear whether their classications and mechanisms translate to molecular crystals, with weaker interactions and more steric hindrance. A comparitive study of SCSC phase transitions in aliphatic linear-chain amino acid crystals, both racemates and quasi-racemates, is presented. A total of 34 transitions are considered and most are classified according to their structural change during the transition. Transitions without torsional changes show very different characteristics, such as transition temperature, enthalpy and free energy, compared with transitions that involve torsional changes. These differences can be rationalized using classical nucleation theory and in terms of a difference in mechanism; torsional changes occur in a molecule-by-molecule fashion, whereas transitions without torsional changes involve cooperative motion with multiple molecules at the same time.

Microstructure and formation mechanism of WC coating on tungsten fabricated by interstitial carburization: A multiscale investigation

We fabricated WC coatings on tungsten using the interstitial carburization technique. High-carbon steel was selected as the carbon source to supply interstitial carbon atoms and was hot-pressed with tungsten at 1100 °C. The interstitial carbon atoms diffused into tungsten, generating a completely nonporous carbide coating with a carbide phase content of approximately 100%. The coating consists of 96% (vol%) WC and 4% W2C dispersed at the coating–substrate interface. This microstructure is formed by a two-step solid-state phase transition from W to W2C to WC, with crystallographic orientation relationships of [1 1 1](101¯)W//[112¯0](0001)W2C//[112¯0](0001)WC. The WC grains are block-shaped and contain a high density of (101¯0) stacking faults generated via collective shifts of atoms along the 1/2 [12¯11] vector. The coating thickness is proportional to the square root of the diffusion time and reaches 6.5 μm after 10 h hot-pressing. The surface microhardness and cross-sectional nanohardness of the coating reach 2300 HV and 40 GPa, respectively, showing remarkable improvements compared to those of the uncoated tungsten (500 HV and 8 GPa). In response to a scratch, the coating shows strong adhesion and excellent cooperative deformation with the substrate. We expect that interstitial carburization can also be applied to various W-based alloys.

Reversible Solid-State Phase Transitions between Au-P Complexes Accompanied by Switchable Fluorescence.

Six complexes {(3-bdppmapy)(AuCl)2}n (1-6; 3-bdppmapy = N,N'-bis(diphenylphosphanylmethyl)-3-aminopyridine and tht = tetrahydrothiophene) were simultaneously formed by the reaction of Au(tht)Cl and 3-bdppmapy in CH2Cl2 followed by infusion with hexane. Complexes 4-6 could be produced independently by volatilizing solvent in air, solid-state heating, or solvothermal reaction. The PPh2-Au-Cl moieties extended in different directions, forming Au-Au and Au-Au-Au interactions. Complex 4 could be converted to 5 by heating to 130 °C, with the cleavage of one Au-Au bond, while 5 reverted back to 4 upon exposure to CH2Cl2 vapor over 11 h. This solid-state phase transition could be recycled and was accompanied by a change in solid-state fluorescence, without obvious intensity decay over five cycles. The reason for both the phase transition and difference in photoluminescence is related to the different numbers and strengths of aurophilic interactions in each complex that could be modeled by density functional theory calculations.

Topological polymorphism and temperature-driven topotactic transitions of metal–organic coordination polymers

A facile crystal-to-crystal solid-state phase transition between a low-temperature phase and a high temperature phase changes the MOF topology and involves a significant rearrangement of bulky organic ligands.

Влияние гамма-облучения на фазовые переходы в политетрафторэтилене, допированном диоксидом кремния растительного происхождения

Abstract The effect of gamma irradiation and doping with silicon dioxide on the first-order solid-state phase transition in polytetrafluoroethylene is studied by the differential scanning calorimetry method. A quantitative analysis of the profiles of the heat capacity peaks is carried out on the basis of the theory of diffuse phase transitions. It is shown that the elementary volume of the phase transformation depends on the γ irradiation and the concentration of silicon dioxide.

Формирование регулярных слоистых структур при твердотельных фазовых переходах с изменением концентрации

The self-oscillation dynamic of an interface is calculated in the framework of the one-dimensional capillary-wave model. The parameters of a periodic layered impurity microstructure are obtained. This microstructure arises as a result of a solid-state phase transition with a change in concentration that occurs when a moving temperature gradient is applied.

Temperature-independent thermal radiation

Thermal emission is the process by which all objects at nonzero temperatures emit light and is well described by the Planck, Kirchhoff, and Stefan-Boltzmann laws. For most solids, the thermally emitted power increases monotonically with temperature in a one-to-one relationship that enables applications such as infrared imaging and noncontact thermometry. Here, we demonstrated ultrathin thermal emitters that violate this one-to-one relationship via the use of samarium nickel oxide (SmNiO3), a strongly correlated quantum material that undergoes a fully reversible, temperature-driven solid-state phase transition. The smooth and hysteresis-free nature of this unique insulator-to-metal phase transition enabled us to engineer the temperature dependence of emissivity to precisely cancel out the intrinsic blackbody profile described by the Stefan-Boltzmann law, for both heating and cooling. Our design results in temperature-independent thermally emitted power within the long-wave atmospheric transparency window (wavelengths of 8 to 14 µm), across a broad temperature range of ∼30 °C, centered around ∼120 °C. The ability to decouple temperature and thermal emission opens a gateway for controlling the visibility of objects to infrared cameras and, more broadly, opportunities for quantum materials in controlling heat transfer.

Luminescent Organogels Formed by Ionic Self‐Assembly of AIE‐Active Phospholes

AbstractA straightforward synthesis of molecular gelators [1][CnBn] and [1][bisCnBn] (n=12, 16) based on ionic interactions between Aggregation‐Induced Emission (AIE)‐active anionic phosphole and functionalized imidazolium [CnBn] and [bisCnBn] is described. These new molecular gelators can form luminescent organogels in various solvents. The luminescent fibers were characterized by laser scanning confocal microscopy. Also, a 30‐fold increase in luminescence quantum yield was observed between the diluted solution (0.6 %) and the xerogel (20 %) of [1][C12Bn], thus highlighting the AIE effect. In these systems, the luminescence can be used to probe the dynamics of the aggregation upon solid‐state phase transition and gelation. Solid‐state investigations have also allowed the full characterization of the thermal behaviour of these ionic complexes and the extraction of some packing features.

Fast simulation of temperature and phase transitions in directed energy deposition additive manufacturing

In this contribution, a simplified macroscopic and semi-analytical thermal analysis of directed energy deposition (DED) is presented to obtain computationally efficient simulations of the entire process. Solidification and solid-state phase transitions are taken into account. The model is derived for laser metal powder directed energy deposition, although it can be simply adapted for other focused thermal energy (e.g., electron beam, or plasma arc). The gas flow used for carrying the powder significantly influences cooling conditions, which is included in the model. The proposed simulation strategy applies to multilayer composites with a wide range of shapes in the horizontal plane and arbitrary laser scanning strategies (continuous way, back and forth, etc.). The proposed work provides a simple tool to study the influence of most process parameters, design in situ experiments and in turn develop optimization loops to reach material requirements and specific microstructures. In situ pyrometer measurements have been compared to the model, and good agreement has been observed with 2.6% error in average. The model is used to demonstrate the effect of various process parameters for a simple cylindrical geometry and a more complex auxetic cell.

Heat Effect Due to Monoclinic-to-Orthorhombic Phase Transition in Ultrahigh Molecular Weight Polyethylene

We analyze heat effects associated with the solid-state phase transition in ultrahigh molecular weight polyethylene from the unstable monoclinic phase into thermodynamically stable orthorhombic phase. The model proposed here for the structure of supramolecular formations of monoclinic phase does not contradict to earlier X-ray diffraction data for this polymer.

Conditions of Formation of Doping Superstructures at Phase Transitions

This work is aimed at general analytical representation of the conditions necessary for generation of layered periodic doping superstructures resulting from the solid-state phase transition accompanied by variation in admixture concentration. Using the capillary-wave model, the explicit expression for the region of temperature gradient and averaged interface velocity, inside which the interface velocity transits to the regime of periodic oscillations, is obtained. The formula for the spatial period of the doping superstructure whose appearance is caused by these oscillations is derived.

Broadband transparent optical phase change materials for high-performance nonvolatile photonics

Optical phase change materials (O-PCMs), a unique group of materials featuring exceptional optical property contrast upon a solid-state phase transition, have found widespread adoption in photonic applications such as switches, routers and reconfigurable meta-optics. Current O-PCMs, such as Ge–Sb–Te (GST), exhibit large contrast of both refractive index (Δn) and optical loss (Δk), simultaneously. The coupling of both optical properties fundamentally limits the performance of many applications. Here we introduce a new class of O-PCMs based on Ge–Sb–Se–Te (GSST) which breaks this traditional coupling. The optimized alloy, Ge2Sb2Se4Te1, combines broadband transparency (1–18.5 μm), large optical contrast (Δn = 2.0), and significantly improved glass forming ability, enabling an entirely new range of infrared and thermal photonic devices. We further demonstrate nonvolatile integrated optical switches with record low loss and large contrast ratio and an electrically-addressed spatial light modulator pixel, thereby validating its promise as a material for scalable nonvolatile photonics.

Lattice hardening due to vacancy diffusion in (GeTe)mSb2Te3 alloys

GeTe-Sb2Te3 alloys have been widely studied for use in rewritable media, and in recent years, they have emerged as excellent thermoelectric materials, with reports of zT>2 for Ge-rich compositions. GeTe-Sb2Te3 alloys exhibit a solid-state phase transition from a layered structure with rhombohedral symmetry to a cubic rocksalt structure, which plays an important role in their thermoelectric behavior. Here, we investigate the impact of the phase transition on the thermal expansion and elastic moduli of (GeTe)17Sb2Te3 using high-temperature X-ray diffraction and resonant ultrasound spectroscopy. The high-temperature elastic moduli of GeTe, Sb2Te3, and Bi2Te3 were also measured for comparison. While it is typical for materials to soften with increasing temperature due to thermal expansion, our study reveals anomalous hardening of the elastic moduli in (GeTe)17Sb2Te3 at temperatures below the phase transition, followed by further hardening at the transition temperature. In contrast, the elastic moduli of GeTe, Sb2Te3, and Bi2Te3 soften with increasing temperature. We attribute the anomalous hardening of (GeTe)17Sb2Te3 to the gradual vacancy diffusion accompanying the transition from a layered to a cubic structure. The stiffening elastic moduli lead to increasing speed of sound, which impacts the lattice thermal conductivity by flattening the temperature dependence.

Electronic behaviors during martensitic transformations in all-d-metal Heusler alloys

For solid-state phase transitions, the alterations of electronic structure driven by the band Jahn–Teller effect would play an essential role in the structural phase transitions and in switching the resistivity or magnetization states for potential applications. However, this evolution of the electronic structure and electronic transport during the martensitic transformations (MT) still lacks comprehensive investigations, especially in magnetic martensitic materials studied in recent years. In this work, we report a study on the electronic behaviors during the MT in a kind of all-d-metal Ni50−xFexMn35Ti15 Heusler magnetic shape memory alloys, by combining x-ray diffraction, calorimetric, magnetic, transport measurements and calculations. Based on the magnetic MTs, the system shows large magnetocaloric effect and magnetoresistance. In the whole temperature range, the system is dominated by hole carriers in both parent and martensite phases. A sharp increase in carrier concentration is observed across the transformations. Meanwhile, the mobility of holes is depressed due to the lattice distortion. A picture of the characteristics of MTs has been proposed for general understanding and clues of the potential spintronic applications based on the magnetostructural phase transitions.

Multi-level model describing phase transformations of polycrystalline materials under thermo-mechanical impacts

The problem of constructing the multilevel physical model of inelastic deformation in steels allowing to take into consideration diffusionless solid-state phase (martensitic) transitions is considered. The model structure includes three scale levels with the closed system of equations offered for them. Explicit internal variables reflecting the evolution of the material structure (both the defect structure and the grain one) are introduced at the lower scale levels of the model. The distinctive feature of the developed model is consideration of the lower scale level in such a way that a homogeneous element of this level completely turns into a new phase at a high speed (relative to the kinematic quasi-static loading), that is close to the speed of sound in the crystal medium. Based on the principles of classical thermodynamics the phase transformation criterion is written. According to this criterion, the choice of a transformational system under the martensitic transition is made. The algorithm of the model is developed and its realization features are described in connection with the high-rate restructuring of the face-centered cubic lattice to the body-centered tetragonal one. The result of this restructuring is a severe change in the physic-mechanical properties of the material.

Influence of Open-Porous System on the Solid-State Phase Transition in 1-Octadecene

The solid-state phase transition in the silica gel–1-octadecene system has been studied at various temperatures with the help of differential scanning calorimetry and FTIR spectroscopy. Silica gels KSK-2.5, KSS-4, and Silica Gel 60 with different surface parameters are used. The temperature of the solid-state phase transition in the silica gel–1-octadecene system is shown to linearly depend on the ratio between the pore surface area and the pore volume. The energy characteristics of 1-octadecene nanocrystals in the surface layer are calculated in the framework of a thermodynamical model of crystal melting in pores.

Polymorphic transition and nucleation pathway of barium dititanate (BaTi2O5) during crystallization from undercooled liquid

The nucleation pathway plays an important role in vitrification, preparation of glass-ceramic composites and synthesis of metastable materials. In this paper, we studied the nucleation pathway of a novel ferroelectric BaTi2O5 (BT2) during crystallization from undercooled liquid by aerodynamic levitation (ADL) containerless processing and structural analysis. An interesting polymorphic transition of BT2 regulated by the undercooling was observed during the crystallization process: the ferroelectric monoclinic phase (γ-BT2) was fabricated at low undercoolings and the paraelectric orthorhombic metastable phase (β-BT2) was obtained from hypercooled liquid. This polymorphic transition phenomenon corresponds to a non-classical nucleation pathway: metastable β-BT2 preferentially nucleates from undercooled melt and γ-BT2 is generated from β phase by solid-state phase transition. The two-step nucleation pathway stems from the structural heredity between the undercooled liquid and crystals. A stronger structural homology exists between the undercooled melt and β-BT2 than γ-BT2 based on diffraction data and atomic configurations analysis. This structural homology coupled with nucleation barrier calculation was used to elucidate the non-classical nucleation pathway of BT2 crystallization: the similarity of the structural unit (Ti-O polyhedra) between the undercooled liquid and the metastable β-BT2 reduces the nucleation barrier and contributes to the preferential precipitation of β-like clusters. This work reveals the formation route of BT2 from cooling melt, which not only benefits the synthesis and application of this novel functional material but also provides a guideline of the crystallization process of titanates from melt at atomic level.

Electric-Field-Controlled Thermal Switch in Ferroelectric Materials Using First-Principles Calculations and Domain-Wall Engineering

Ferroelectric materials attract attention as potential solid-state thermal switches, due to various phenomena near their solid-state phase transition. In principle these phenomena could also be triggered by an external electric field, though recent experiments on thin films have shown only weak effects. The authors explore this issue by solving the linearized Boltzmann equation with first-principles force constants, to study the thermal conductivity of PbTiO${}_{3}$ as a function of electric field. The results emphasize the need for large-domain samples and lower temperatures, to have the best chance at large voltage-controlled thermal switching in this class of materials.

C6N2H18][SbI5]: A Lead-free Hybrid Halide Semiconductor with Exceptional Dielectric Relaxation.

[C6N2H18][SbI5] (1), a novel metal halide semiconductor with dielectric relaxation behavior, has been successfully synthesized, in which the cavities between the one-dimensional [SbI5] n2- polyanions are occupied by 2-methyl-1,5-pentanediammonium (2-MPDA) cations. 1 undergoes a reversible solid-state phase transition at TC = 192.7 K and shows a step-like dielectric anomaly. Interestingly, above TC, distinct dielectric dispersion in a wide temperature range is also witnessed. Remarkably, the solid state UV-vis diffuse reflectance spectrum of 1 exhibits a slightly gentler absorption edge at about 650 nm; that is, 1 adopts an indirect band gap with 1.92 electron volts. The combined narrow band gap, strong photoconductivity effect, and excellent dielectric relaxation might shed light on the exploitation of lead-free hybrid metal halide molecular materials with promising application prospects in thermoresponsive relaxation-type dielectric materials and photovoltaic conversion devices.

Temperature-Dependent Structural Properties, Phase Transition Behavior, and Dynamic Properties of a Benzene Derivative in the Solid State

We report the solid-state structural properties and phase transition behavior of 1,4-dibromo-2,3,5,6-tetramethylbenzene, demonstrating that this material undergoes an order–disorder phase transition below ambient temperature (at ca. 154 K on cooling and ca. 160 K on heating). In both the high-temperature and low-temperature phases, the crystal structure is based on π-stacking of the molecules. In the crystal structure of the high-temperature phase, the bromine occupancy in each substituent site is ca. 1/3 and the methyl group occupancy in each substituent site is ca. 2/3, consistent with statistical orientational disorder of the molecule between six distinct orientations. Natural-abundance solid-state 2H NMR spectroscopy confirms that, at ambient temperature, this disorder is dynamic via rapid molecular reorientation about an axis perpendicular to the aromatic ring. In the low-temperature phase, the bromine and methyl substituents occupy preferred sites within the crystal structure, with the distribution of site occupancies becoming progressively more ordered on decreasing temperature.