Polymorphic Transition Research Articles

Влияние низкотемпературного отжига неупорядоченных плёнок Ge10Se90 на их оптические свойства

In the present paper, the effect of low-temperature annealing on the linear optical properties of disordered thin films (~350 nm) synthesized from Ge10Se90 glass by vacuum thermal evaporation on quartz substrates is reported. Based on the experimental transmission spectra of films in the field of optical transparency before and after annealing below and at the glass transition temperature Tg = (3612) K (313 and 363 K, respectively), the main optical characteristics (dispersion dependences of the refractive index and extinction coefficient, optical gap energy, the Tautz parameter B1/2, etc.) were determined using the Swanepoel and Tautz methods. It is revealed that the edge of optical absorption in the annealed films is caused by indirect optical transitions. Using the Wemple-DiDomenico single oscillator model, the Urbach energy EU, the energy of one oscillator E0, the scattering energy Ed, the linear susceptibility  and the static refractive index n0 were calculated. At the same time, a higher value of the Ed/E0 ratio of the film annealed at Tg indicates its greater nonlinear susceptibility. The X-ray diffraction studies have confirmed the amorphous nature of the initial films and the appearance of Se nanocrystallites in them after annealing of a certain polymorphic modification depending on the temperature and time of annealing. It was found that annealing both below Tg and at Tg initiates red shift in the optical absorption edge, an increase in EU and , and a decrease in the thickness of the film and the parameter B1/2. It is shown that the annealing of identical films is accompanied by a change in their phase composition with the appearance of either monoclinic beta-Se8 nanocrystallites or the mixture of beta-Se8 and trigonal t-Se. From the standpoint of the concept of the polymerpolymorphoid structure of glass and a glass-forming liquid, it is due to the mutual transformation of polymorphoids of various polymorphic modifications because of a change in their concentration ratio depending on the annealing temperature relative to Tg. The resulting films can be used in optoelectronics as nonlinear optical elements or media for radiation limiters.

Copper- and manganese-based layered hybrid organic–inorganic compounds with polymorphic transitions as energy storage materials

The bis(dodecylammonium) tetrachlorocuprate(ii) and bis(dodecylammonium) tetrachloromanganate(ii) show promise for passively cooling electronics due to their excellent thermal properties, including low thermal hysteresis and good stability.

Oscillatory phase transition induced structural extension during iron oxide reduction

Probing the molecular-level redox behavior and mechanism of oxides is essential to developing innovative applications in different fields such as catalysis, but remains a challenge for the scientific community. Using in-situ transmission electron microscopy, here we provide an overall reduction view of single-crystalline α-Fe2O3 with different characteristics from the conventional wisdom of oxide reduction. Specifically, the formation of epitaxial nanoislands with concomitant oscillatory phase transitions (α-Fe2O3→defective γ-Fe2O3→α-Fe2O3) at the subsurface is observed during reduction. The dynamic equilibrium of lattice oxygen at the surface and the limited oxygen replenishment from the deep layer drive the α-Fe2O3→defective γ-Fe2O3 transformation in the subsurface, while the polymorphic transition (defective γ-Fe2O3→α-Fe2O3) spontaneously occurs under heating conditions. Such oscillatory phase transition is accompanied by the release of asymmetric stress, inducing the extension of epitaxial nanoislands. Our work highlights the complexity of reduction by providing an integral picture of oxide reduction, which contributes to the understanding of the site evolution of oxide-based catalysts in their working state.

Effect of radiation-induced vacancy saturation on the first-order phase transformation in nanoparticles: insights from a model.

By employing a model of nanomaterials with polymorphic phase transitions and using a thermodynamic approach to describe the effects of vacancy saturation, irradiation dose, powder dispersion, and surface energies, we demonstrate the possibility of radiation-induced phase transitions and the zones of radiation stability for nanoparticles. We utilize nanoparticles exhibiting transitions from the body-centered cubic α phase to the face-centered cubic β phase, and the reverse transition from β phase to α phase, as a model system for first-order phase transformations. We incorporate nucleation through the appearance and growth of the nucleus of a new phase, resulting in the formation of a two-phase α+β system, and we highlight the importance of accounting for nucleation. Our model study reveals that very small α-phase particles are unstable (while very small β-phase particles are stable) because of surface effects. There is an intermediate zone of sizes and parameters where radiation-induced defects become important so that the α-phase particle is unstable without irradiation but becomes stable under irradiation. For large sizes and low temperatures, the α→β transformation cannot occur regardless of irradiation because of bulk driving forces; initially, α-phase particles are stable, whereas the β-phase particles are unstable. In some cases, nucleation requires a large additional energy change, resulting in a low probability of phase change fluctuations. This behavior is confirmed by calculations for iron particles under irradiation. Substances characterized by high vacancy migration energy, small diffusion coefficients of defects, and low temperatures of first-order phase transitions can serve as suitable candidates for radiation-induced phase transitions in nanosystems. Ceramic nanomaterials, which possess high vacancy migration energy, will have their behavior significantly influenced by radiation doses. In contrast, most metals exhibit small vacancy migration energy and demonstrate better resistance to irradiation, making them recommended candidates for nuclear materials.

Ionic conduction in ammonia functionalised closo-dodecaborates MB12H11NH3 (M = Li and Na).

Metal hydroborates and their derivatives have been receiving attention as potential solid-state ion conductors for battery applications owing to their impressive electrochemical and mechanical characteristics. However, to date only a fraction of these compounds has been investigated as solid-state electrolytes. Here, MB12H11NH3 (M = Li and Na) hydroborates are synthesized and investigated as electrolyte materials for all-solid-state batteries. The room temperature α-NaB12H11NH3 was structurally solved in P212121 (a = 7.1972(3) Å, b = 9.9225(4) Å, and c = 14.5556(5) Å). It shows a polymorphic structural transition near 140 °C to cubic Fm3̄m. LiB12H11NH3 and NaB12H11NH3 exhibit cationic conductivities of σ(Li+) = 3.0 × 10-4 S cm-1 and σ(Na+) = 1.2 × 10-4 S cm-1 at 200 °C. Hydration is found to improve ionic conductivity of the hydroborates. It is presumed that modest ionic conductivities could be due to a lack of significant re-orientational dynamics in the crystal structure resulting from the presence of the bulky -NH3 group in the anion.

Probing capping mechanisms and polymer matrix loading of biogenic vaterite CaCO3-Ag hybrid through X-ray photoelectron spectroscopy (XPS).

Despite extensive research in the literature, the synthesis of silver nanoparticles (AgNPs) via capping mechanisms remains incompletely understood. This study employs a mechanistic approach to unravel the underlying molecular interactions driving the capping process of biogenic vaterite CaCO3-Ag and explores their interactions with different polymer matrices. X-ray photoelectron spectroscopy (XPS) was used to reveal the capping mechanisms, surface composition alterations, and vaterite polymorph transitions. The oxidation states of AgNPs exhibited distinct changes under different capping agents. The Ag3d spin-orbit splitting profiles revealed the coexistence of Ag+ and Ag0 within CaCO3-Ag, with a significant presence of Ag0 when poly(sodium 4-styrene sulfonate) was employed as the capping agent. Conversely, the use of carboxy methyl cellulose as the capping agent resulted in Ag+ dominance. XPS analysis illuminated the transformation of CaCO3 polymorphs from calcite to vaterite structure, which remained stable following embedding within polymer matrices. Integrating CaCO3-Ag microspheres into polymer matrices and investigating their surface characteristics represents a strategic step toward tailoring material properties for potential applications in active packaging and biomedicine.

Acoustic shock wave-induced dynamic recrystallization of amino acids: a case study on l-serine

Herein, we report the impact of acoustic shock waves on l-serine powder samples and found significant structural and surface modifications under shocked conditions without undergoing polymorphic transition.

Zinc-induced polycrystalline transformation of high-entropy fluorides and derived regulatory mechanisms for bifunctional oxygen electrocatalysis

Zinc-induced polymorphic transition of high-entropy fluorides to enhance efficient bifunctional oxygen electrocatalysis.

Direct in situ measurement of polymorphic transition temperatures under thermo-mechanochemical conditions.

For the first time, we directly measured the onset and completion temperatures of polymorphic transitions under thermo-mechanochemical conditions by simultaneous in situ synchrotron powder X-ray diffraction and temperature monitoring. We determined the thermo-mechanochemical polymorphic transition temperature in 1-adamantyl-1-diamantyl ether to be 31 °C lower than the transition temperature determined by DSC. Our findings highlight the uniqueness of thermo-mechanochemical conditions, with potential applications in polymorph screening.

In Ukrainian

The results of theoretical modeling of the spatial structure and electronic absorption spectra of isolated small particles of polymorphic modifications of lead monoxide - litharge and massicot, which are characterized by the presence of hypercoordinated atoms, have been analyzed. The optical properties of many oligomers of lead oxide are systematically considered in the work. The results of quantum chemical calculations by the methods of electron density functional theory (DFT and TDDFT) on the spatial structure and energy characteristics, as well as the electronic absorption spectra of the considered models, are discussed. Theoretical results are compared with available experimental data. The calculation method used and the proposed cluster models with or without attached (chemisorbed) water molecules allow us to reproduce the spatial structure and energy characteristics of polymorphic modifications of lead oxide at a semi-quantitative level. It has been found that the addition of water molecules to models for α-lead oxide nanoparticles contributes to their stabilization. The calculated values of the cohesion energy for litharge nanoparticles are greater than those for massicot ones, which corresponds to the experimental data for lead monoxide crystals. The calculated electronic spectra of the litharge nanoparticle models are characterized by absorption bands which lie in the region that corresponds to the red color, and those of massicot relate to yellow; the corresponding values of the width of the energy gap for the litharge models are smaller than for the massicot ones, which is consistent with the experimental data for crystals. The results of calculations show that the presence of hydroxyl groups in nanoparticle models leads to a hypsochromic shift of absorption maxima, so that with a sufficient number of such groups, these species may lose their color.

Polymorphic phase transition in CoCrNi medium-entropy alloy under impact loadings

Polymorphic phase transition in metallic materials under high pressure is a critical aspect of dynamic properties and has been attracting a great interest. Despite the extensive researches have been made on understanding of this phase transition in traditional single-principal element alloys, little is known about the phase transition in recently emergent multi-principal medium and high entropy alloys, especially compressed under high strain rates. In this work, based on molecular dynamic simulations, three impact loading strategies with distinct loading paths, such as single-shock, double-shock and ramp-wave loading are carried out on the single crystalline CoCrNi medium-entropy alloy (MEA) to investigate the phase transition under high strain-rate compression. Careful characterizations show that the phase transition of CoCrNi MEA is loading-path dependent, as evidenced by the significant differences in macroscopic pressure evolution and microscopic structural phase transition among the samples under various thermodynamic paths. An intriguing pressure “overshoot” is found and demonstrated as the characteristic of the critical structural phase transition from face-centered cubic (FCC) structure to hexagonal-close-packed (HCP) structure mediated by body-centered cubic (BCC) like clusters. We show that such loading-path dependence is attributed to the strain rate and temperature rise in the loading process, which control the evolution of microstructure and deformation field. The inherent correlation between the atomistic process of phase transition and loading strategies results in polymorphic phase transition under high strain rates. These findings shed new light on the nature of impact phase transition of multi-principal alloys.

Spatiotemporal Control of Polymorphic Phase Transition of Glycine Crystals by Three-Dimensional Femtosecond Laser Ablation Processing.

Spatiotemporal control of the polymorphic phase transition of glycine crystals was demonstrated by three-dimensional (3D) processing with a focused femtosecond laser pulse as an external stimulus. We found that the transition from a metastable form (β-form) to more stable ones (α- or γ-form) could be triggered from the irradiated area of not only the surface but also inside of glycine crystals. This 3D processing with a focused femtosecond laser pulse enabled us to precisely monitor the transition dynamics from a targeted position to the entire part of crystals. The systematic study with the space-selective phase transition method revealed that the phase transition inside of glycine crystals was significantly slower (e.g., ∼50 times) than that at the crystal surface, which indicates the crucial role of water molecules in air on the phase transition dynamics. We foresee that this laser method can be a practical tool for monitoring spatiotemporal dynamics of phase transition.

New Polymorph of β-Cyclodextrin with a Higher Bioavailability

A new polymorph of anhydrous β-cyclodextrin (polymorph III) was obtained and characterized for the first time using powder X-ray diffraction, infrared spectroscopy, and thermal analysis. The solution enthalpy and time of dissolution in water were determined using solution calorimetry for this polymorph and compared with those of the dried commercial form of β-cyclodextrin (polymorph I), its amorphous form, and 2-hydroxypropyl-β-cyclodextrin. The specific heat capacities of polymorphs I and III were determined using differential scanning calorimetry across a wide range of temperatures, providing enthalpy and Gibbs energy values for the polymorphic transition at 298 K. The affinities of polymorph III and 2-hydroxypropyl-β-cyclodextrin for water were characterized by determining their hydration isotherms, which provided values of hydration Gibbs energy. Being energy-rich, the new-found polymorph of β-cyclodextrin has a significantly higher dissolution rate and an increased affinity for water compared with the dried commercial form of β-cyclodextrin. These properties render the new polymorph promising in industrial applications for guest inclusion in aqueous solutions and pastes, and may be a desirable alternative for water-soluble β-cyclodextrin derivatives.

Regulated and Gradual Polymorph Transition By Thermal and Plasma Treatment on Transition Metal Dichalcogenides

Transition metal dichalcogenides (TMDs) have two different phase structure which exhibit different properties. Generally, it can be classified into 1T (metallic) and 2H (semiconductor) phase structures according to the relative atomic direction of the TMDs. Currently, the utilization of two-dimensional (2D) materials is limited due to the lack of an appropriate phase control method of TMDs. There are several phase control methods reported so far (post-treatment chemical process, lattice deformation of physical crystals due to ion collision, phase control of materials through strain, etc.), but the disadvantages of requiring additional processes and low yield are limited for practical usage of those method. In this research, we conducted a study on gradual phase control of TMDs according to substrate temperature (150, 200, 250, 300, and 350°C) while other plasma parameters were kept constant (power, pressure, processing time, and gas ratio). By controlling the phase, the versatility of TMDs is dramatically improved. 1T-TMDs basal plane plays a crucial role as catalytically active site to enhance electrochemical properties more than 2H-TMDs. 1T-TMDs used as a hydrogen generation catalyst has been proposed as an alternative to noble metals (e.g., Pt, Ir, Ru). This is because 1T-TMDs have a metallic property and thus have dense catalytically active sites as edge and basal plane, which can be used as an electrochemical catalyst. However, the problem of (1T-TMDs) manufactured in the conventional way is that the unstable properties at room temperature of them. During the low-temperature plasma process, ion bombardment onto the substrate induces a grain boundary of several nano sizes to expand the active site per area of the surface crystal structure resulting in high catalytic activity. In addition, the possibility of large-scale production was presented on large-scale wafer synthesis, and the original phase of 1T-TMDs was preserved after the 1,000 HER cycle to prove robustness and durability. On the other hand, in the case of (2H-TMDs), instead of showing low catalytic activity, the existence of band gap makes them favorable for semiconductor device applications such as photodiode, transistor, and neuromorphic computing. In this study, by using the sputtered MoS2 and WS2 to control the ratio of different polymorph, memristive switching (RS) is investigated. Similarly, by ion bombardment onto substrates in a low-temperature plasma process, a few nano-sized particle boundaries were induced, creating a defect on the material surface. An experiment was conducted in which filaments are mimicking artificial neural networks and then ruptured under electrical bias. However, the reproducibility of filament formation for 2H-TMDs was relatively low, and the formed filament was relatively unstable. Therefore, it was intended to form a kind of 'road' to solve this problem. Here, the road was using a 1T-TMDs structure with high catalytic activity. The mixed phase of TMDs consist of a different phase (1T and 2H) structure, which depends on their process temperature. Diversity of process temperature results in various phase ratio into the material structure. Gradual phase change from 1T to 2H at various temperature (150, 200, 250, 300, and 350 °C) confirmed by X-ray photoelectron spectroscopy. By utilizing the mixed phase of TMDs, the metal filaments were formed along the 1T structure within 2H structure, which showing uniform RS behavior. Higher reproducibility was obtained for mixed phase structure than the 2H-TMDs and secured stability. In conclusion, by presenting a research direction on how to control the TMDs’ phases, this study can increase the utilization of the 2D material while adjusting different phases together, rather than single usage of the 2D material phases such as 1T- and 2H-TMDs.Acknowledgement: This work was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education 2022R1A6A3A13063381 and 2022R1A3B1078163). And this work was also supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (No.2022R1A4A1031182).

Polymorphic phase transitions in triglycerides and their mixtures studied by SAXS/WAXS techniques: In bulk and in emulsions

Triacylglycerols (TAGs) exhibit a monotropic polymorphism, forming three main polymorphic forms upon crystallization: α, β’ and β. The distinct physicochemical properties of these polymorphs, such as melting temperature, subcell lattice structure, mass density, etc., significantly impact the appearance, texture, and long-term stability of a wide range products in the food and cosmetics industries. Additionally, TAGs are also of special interest in the field of controlled drug delivery and sustained release in pharmaceuticals, being a key material in the preparation of solid lipid nanoparticles. The present article outlines our current understanding of TAG phase behavior in both bulk and emulsified systems. While our primary focus are investigations involving monoacid TAGs and their mixtures, we also include illustrative examples with natural TAG oils, highlighting the knowledge transfer from simple to intricate systems. Special attention is given to recent discoveries via X-ray scattering techniques. The main factors influencing TAG polymorphism are discussed, revealing that a higher occurrence of structural defects in the TAG structure always accelerates the rate of the α → β polymorphic transformation. Diverse approaches can be employed based on the specific system: incorporating foreign molecules or solid particles into bulk TAGs, reducing drop size in dispersed systems, or using surfactants that remain fluid during TAG particle crystallization, ensuring the necessary molecular mobility for the polymorphic transformation. Furthermore, we showcase the role of TAG polymorphism on a recently discovered phenomenon: the creation of nanoparticles as small as 20 nm from initial coarse emulsions without any mechanical energy input. This analysis underscores how the broader understanding of the TAG polymorphism can be effectively applied to comprehend and control previously unexplored processes of notable practical importance.

Observation of a temperature dependent anomaly in the UV translucency of milk useful for UV-C preservation techniques

Milk fat globules and casein micelles are the dispersed particles of milk that are responsible for its typical white turbid appearance and usually make it difficult to treat with modern ultraviolet light (UV) preservation techniques. The translucency of milk depends largely on the refractive indices of the dispersed particles, which are directly affected by temperature changes, as incorporated triglycerides can crystallize, melt or transition into other polymorphs. These structural changes have a significant effect on the scattering properties and thus on the UV light propagation in milk, especially by milk fat globules. In this study, a temporary minimum in the optical density of milk was observed within UV wavelength at 14 °C when heating the milk from 6 to 40 °C. This anomaly is consistent with structural changes detected by a distinct endothermic peak at 14 °C using differential scanning calorimetry. Apparently, the optical density anomaly between 10 and 20 °C disappears when the polymorphic transition already has proceeded through previous isothermal equilibration. Thus, melting of equilibrated triglycerides may not affect the RI of milk fat globules at ca. 14 °C as much as melt-mediated polymorphic transitioning. An increased efficiency of UV-C preservation (254 nm) at the translucency optimum was demonstrated by temperature-dependent microbial inactivation experiments.

Системи CeO2—Ln2O3 (Ln = лантаноїди ітрієвої підгрупи, Y2O3

Based on the analysis of literature data from the experimentally constructed phase diagrams of CeO2—Ln2O3 systems (Ln = Tb—Lu), as well as the temperatures of polymorphic transformations of oxides of rare earth elements (REEs), approximate phase diagrams of the indicated system were constructed in the entire range of temperatures and concentrations. Cerium dioxide crystallizes in the cubic type of fluorite crystal lattice and has no polymorphic transformations in the entire temperature range until melting at 2400 ºC. The solubility based on CeO2 is higher than based on other lanthanide oxides. The components of the systems show complete solubility below solidus with the formation of solid solutions with a structure of the fluorite-type F. The appearance below the solidus regions of solid solutions based on polymorphic modifications of lanthanide oxides H, A, B and C with different width leads to the formation of cascades of peritectoid transformations. During the transition in the Tb—Lu series, their temperatures gradually increase, which is associated with an increase in the temperatures of polymorphic transformations of Tb—Lu oxides. Hypothetical phase diagrams of the CeO2—Ln2O3 series (Ln = Tb—Lu, Y) with adjustments according to such regularities of interaction during the transition from Tb to Lu. Experimental verification of the phase diagrams structure of the considered systems is possible when conducting experiments using increased pressures of oxygen-containing media, or by thermodynamic calculations. Keywords: REE oxides, Y2O3, CeO2, polymorphic transformation of REE, phase diagrams.

Similarity between Shock-Induced Polymorphic Transitions in the Silica System

Similarity between Shock-Induced Polymorphic Transitions in the Silica System

The use of polymorphic state modifiers in solid lipid microparticles: The role of structural modifications on drug release performance

This study investigates the correlation between the structural and release properties of solid lipid microparticles (MPs) of tristearin containing 5 % w/w of four different liquid additives used as crystal modifiers: isopropyl myristate (IM), ethyl oleate (EO), oleic acid (OA) and medium chain triglycerides (MCT). All additives accelerated the conversion of the unstable α-form of tristearin, formed after the MPs manufacturing, to the stable β-polymorph and the transformation was completed within 24 h (for IM and EO) or 48 h (for OA and MCT). The kinetic of polymorphic transition at 25 °C was investigated by simultaneous synchrotron SAXS/WAXS and DSC analysis after melting and subsequent cooling of the lipid mixture. After crystallization in the α-phase, additives accelerate the solid-solid phase transformation to β-tristearin. SAXS data showed that two types of structural modifications occurred on MPs during storage: compaction of the crystal packing (slight decrease in lamellar thickness) and crystal growth (increased number of stacked lipid lamellae). The release behavior of a model hydrophilic drug (caffeine) at two different amounts (15 % and 30 %) from MPs was studied in water and biorelevant media simulated the gastric and intestinal environment. It was particularly significant that the introduction of IM, EO and MCT were able to prolong the drug release in water, passing from a diffusion-based Higuchi kinetics to a perfect zero-order kinetic. Moreover, the overall release profiles were higher in biorelevant media, where erosion/digestion of MPs was observed. After 6 months, a moderate but statistically significant change in release profile was observed for the MPs with IM and EO, which can be correlated with the time-dependent structural alterations (i.e. larger average crystallite size) of these formulations; while MPs with OA or MCT displayed stable release profiles. These findings help to understand the correlation between release behavior, polymorphism and supramolecular‐level structural modification of lipid formulations containing crystal modifiers.

Thermal and structural behavior of mango (Mangifera indica L) kernel fat from three Ivorian varieties

AbstractMango (Mangifera indica L.) seeds is an interesting source of fat, rich in stearic (St) and oleic (O) acid with three major triacylglycerols (TAG): StOSt, StOO, and StLSt. The kernel fat content and quality depend however on the varieties and their origin. The objective of this study was to investigate the crystallization and polymorphic behavior of mango kernel fat extracted from three selected Ivorian varieties which differ in terms of TAG composition: Kent (KT), Djakoumankoun (DN) and Brooks (BR). The isothermal crystallization behavior was examined at 15 and 20°C by pulsed nuclear magnetic resonance, differential scanning calorimetry, x‐ray diffraction (XRD) and polarized light microscope. Under static conditions, DN crystallized faster, followed by BR, then KT. At 15°C, an unusual evolution was observed for both DN and BR, which can be explained by a melt‐mediated polymorphic transition from α into more stable forms. Using a differential scanning calorimetry (DSC) stop‐and‐return technique, different crystallization behaviors were also observed. Isothermal XRD experiments confirmed that the kinetics of the polymorphic transformation was different within the three samples, even if the three fats were β‐3L‐tending. At 15°C, KT transformed from liquid state to stable form without passing through α‐form, while DN and BR crystallized first into α‐from which transformed further to β′, then into stable β‐form. At 20°C, DN and BR crystallized directly from liquid to β′, which later transformed into β‐form while KT did not crystallize under the same conditions. The huge differences observed result from the differences in the TAG profiles, mainly in the StOO and StOSt content.