Crystal Transition Research Articles

Tough, Antifreezing, and Piezoelectric Organohydrogel as a Flexible Wearable Sensor for Human-Machine Interaction.

Piezoelectric hydrogel sensors are becoming increasingly popular for wearable sensing applications due to their high sensitivity, self-powered performance, and simple preparation process. However, conventional piezoelectric hydrogels lack antifreezing properties and are thus confronted with the liability of rupture in low temperatures owing to the use of water as the dispersion medium. Herein, a kind of piezoelectric organohydrogel that integrates piezoelectricity, low-temperature tolerance, mechanical robustness, and stable electrical performance is reported by using poly(vinylidene fluoride) (PVDF), acrylonitrile (AN), acrylamide (AAm), p-styrenesulfonate (NaSS), glycerol, and zinc chloride. In detail, the dipolar interaction of the PVDF chain with the PAN chain facilitates the crystal phase transition of PVDF from the α to β phase, which endows the organohydrogels with a high piezoelectric constant d33 of 35 pC/N. In addition, the organohydrogels are highly ductile and can withstand significant tensile and compressive forces through the synergy of the dipolar interaction and amide hydrogen bonding. Besides, by incorporating glycerol and zinc chloride, the growth of ice crystals is inhibited, allowing the organohydrogels to maintain stable flexibility and sensitivity even at -20 °C. The real-time monitoring of the pulse signal for up to 2 min indicates that the gel sensor has stable sensitivity. It is believed that our organohydrogels will have good prospects in future wearable electronics.

Influence of the length of end-group double bond substituents on the temperature change of phase transition in liquid crystals

ABSTRACT A series of nematic liquid crystal (LC) monomers, containing a reactive group in the lateral substituent, were designed and synthesised through an organic synthesis reaction. Considerable changes were made to the properties of the liquid monomers by varying the terminal alkyl chain lengths. The molecular structures of the intermediates and the LC monomers were characterised by flourier infrared absorption spectrometry and proton nuclear magnetic resonance (1H NMR) spectroscopy, leading to the identification of the target product. The phase transition temperatures of the LC monomers were observed and analysed using differential scanning calorimetry (DSC) and polarising optical microscopy (POM) with a hot stage. The two-way thermotropic liquid crystalline behaviour was observed during the heating process, providing insight into the effect of substituent changes on the phase transition temperature.

Spectroscopy of monolayer and multilayer tungsten disulfide under high pressure

Recently exfoliated monolayer and multilayered transition metal dichalcogenides have gathered significant interest based on their tunable bandgap and extremely high carrier mobility. We have investigated the Raman and photoluminescence spectra of monolayer and multilayer WS2 as a function of pressure. The Raman-inactive mode B1u, which is activated by structural disorder, was revealed at 6.7 GPa in monolayers, at 8.0 GPa in bilayers, and at 13.7 GPa in multilayers, respectively. With the enhancement of pressure-induced interlayer interaction, the crystal phase transition due to layer sliding like 2Hc to 2Ha occurs at 14.8 and 18.7 GPa in bilayers and multilayers, as evidenced by the split of E12g and B1u. The electronic phase transition of the monolayer is supposed to be a direct K-K bandgap changing to an indirect Λ-K bandgap at 2.6 GPa. These observations contribute to a better understanding of the impact of interlayer interactions on the modulation of WS2 energy bands and structure, as well as fundamental studies of two-dimensional layered materials, which can inform the development of device applications.

Effect of glycerol incorporation on the liquid crystal structure of sucrose fatty acid ester in aqueous solution

A series of homogeneous samples with high sugar-surfactant content (sucrose fatty acid ester/water and sucrose fatty acid ester/glycerol/water systems) were prepared with well-controlled processes and investigated by DSC, polarized microscopy, X-ray scattering and rheology. Meanwhile the accuracy of the preparation process is confirmed gas chromatography. The polarized microscopy indicates that the ordering degree of the network structure composed of the liquid crystals may be reduced by the glycerol addition. The results of DSC revealed that 50% glycerol solution could significantly inhibit the crystallization of sucrose fatty acid ester and resulted in a significant widening of the temperature interval for the transformation of the liquid crystal phase into the gel phase. In order to further evaluate the effect of glycerol molecules versus water molecules on the liquid crystal structure of sucrose fatty acid ester, we comparatively analyzed the X-ray scattering data from two perspectives: the mass fraction and molar fraction. It was found that 50% glycerol solution could inhibit the crystallization of sucrose fatty acid ester through changing the water-head group interaction of surfactant, facilitates the integration of sucrose fatty acid ester with different alkyl chain length and may reduce the regularity of liquid crystal, both from the perspective of surfactant mass and molar fraction. Furthermore, the addition of 50% glycerol (w/w) does not significantly impede the liquid crystal formation of sucrose fatty acid ester in aqueous solution. However, from the perspective of surfactant mass fraction, glycerol incorporation could reduce the effective cross-sectional areas of the hydrophilic part of sucrose fatty acid ester in aqueous solution and induce the phase transition of hexagonal liquid crystals to aggregates with less positive curvature, while an opposite tendency could be obtained from the perspective of surfactant molar fraction. The conclusions are also confirmed by subsequent rheological test. Moreover, the results of rheological measurements reveals that the interactions among cylindrical units of the hexagonal liquid crystalline phase become weaker and the surfactant molecules aggregate more loosely in the cylindrical unit with the glycerol incorporation. Moreover, the above-mentioned influence of glycerol on the liquid crystal structure of sucrose fatty acid ester in aqueous solution would be similar but more prominent from the perspective of the molar fraction.

Poisson's Ratio of Glasses, Ceramics, and Crystals.

Poisson's ratio is the fundamental metric used to discuss the performance of any material when strained elastically. However, the methods of the determination of Poisson's ratio are not yet discussed well. The first purpose of this paper is to introduce the five kinds of typical experimental methods to measure Poisson's ratio of glasses, ceramics, and crystals. The second purpose is to discuss the experimental results on the variation of Poisson's ratio by composition, temperature, and pressure reviewed for various glasses, ceramics, and crystals, which are not yet reviewed. For example, in oxide glasses, the number of bridging oxygen atoms per glass-forming cation provides a straightforward estimation of network crosslinking using Poisson's ratio. In the structural-phase transition of crystals, Poisson's ratio shows remarkable temperature-dependence in the vicinity of a phase-transition temperature. The mechanism of these variations is discussed from physical and chemical points of view. The first-principles calculation of Poisson's ratio in the newly hypothesized compounds is also described, and its pressure-induced ductile-brittle transition is discussed.

Crystal symmetry transition and its influence on optical, dielectric, and ferroelectric properties in (1-x)Na0.5Bi0.5TiO3-xSrTiO3 system

Ferroelectric perovskites materials are the most promising materials because of their multifunctional applications as well as the ongoing fundamental research on them. The present study investigates the correlation between the crystal structure and various physical properties such as dielectric, ferroelectric, and optical properties of Sr2+ modified Sodium bismuth titanate Na0.5Bi0.5−xSrxTiO3 (x = 0.00, 0.02, 0.04, and 0.06) synthesized by solid-state reaction method. Rietveld refinement of the X-ray diffraction pattern reveals that the system undergoes a crystal symmetry transition from a two-phase crystal symmetries system (rhombohedral (R3c) and tetragonal (P4bm)) to a three-phase crystal symmetries system (rhombohedral (R3c), tetragonal (P4bm) and Cubic (Pm3̅m)) with the rise in Sr2+ concentration in Na0.5Bi0.5−xSrxTiO3 which is the foremost reason behind the variation of physical properties of the system. The dielectric constant and maximum polarization increase with the Sr2+ concentration up to 2% and it decreases with the further increase in Sr concentration due to the distortion in crystal structure induced by the introduction of cubic phase (Paraelectric) in the system. The maximum value of the dielectric constant is estimated to be 3629 at a low frequency on the Na0.50Bi0.48Sr0.02TiO3 system. The optical band gap (Eg) of Na0.5Bi0.5−xSrxTiO3 (x = 0.00, 0.02, 0.04, and 0.06) ceramics are found to vary between 3.202 eV to 3.172 eV which indicates the semiconducting nature of prepared samples but urbach energy shows an opposite behaviour i.e. increases with Sr concentration.

Transition of iPB-1 with low molecular weight crystallized from solutions.

The dimension or entanglement of polymer chains is crucial to chain dynamics, and the polymer chains can be diluted and disentangled in a solution by reducing the interaction with each other. This letter addresses the role of solution concentration in the crystal transition of isotactic polybutene-1 (iPB-1) with low molecular weight crystallized from solution. It turned out that the transition of iPB-1 obtained from the higher concentration of the solution is faster than that obtained from the lower one. In addition, the disparity in the transition between the early stage and later stage for samples prepared from solutions with and without stirring was characterized. DSC and X-ray experiments revealed that initial temperatures for the solution-crystallized iPB-1 to melt and crystallize into form II dominated the transition. The role of entropy in the transition was proposed as a primary factor influencing the iPB-1 transition. Increasing the concentration of the solution or stirring during crystallization increases the supersaturation for iPB-1 to crystalize from the solution and form less stable crystals. The less stable iPB-1 crystals cause the formation of form II at lower temperatures during heating. Therefore, the lower entropy in amorphous regions resulted in an enhanced propensity for the helical conformation with a lower entropy and consequently accelerated the crystal transition.

Possibilities and limitations of convolutional neural network machine learning architectures in the characterisation of achiral orthogonal smectic liquid crystals.

Machine learning is becoming a valuable tool in the characterisation and property prediction of liquid crystals. It is thus worthwhile to be aware of the possibilities but also the limitations of current machine learning algorithms. In this study we investigated a phase sequence of isotropic - fluid smecticA - hexatic smectic B - soft crystal CrE - crystalline. This is a sequence of transitions between orthogonal phases, which are expected to be difficult to distinguish, because of only minute changes in order. As expected, strong first order transitions such as the liquid to liquid crystal transition and the crystallisation can be distinguished with high accuracy. It is shown that also the hexatic SmB to soft crystal CrE transition is clearly characterised, which represents the transition from short- to long-range order. Limitations of convolutional neural networks can be observed for the fluid to hexatic SmA to SmB transition, where both phases exhibit short-range ordering.

Room-temperature spin reorientation transition and low-temperature significant magnetothermal effect in Mn-doped DyFeO3 single crystals

Rare-earth orthoferrites (RFeO3) promise in spin storage and spin sensing, but the majority of the magnetic phase transition and intriguing properties occur at low temperatures. Searching for higher-temperature or even room-temperature spintronics devices has been a critical challenge in the development of spintronics. We doped Mn ions into Fe sites of DyFeO3 in 10%–50 % ratios to coexist antiferromagnetic and ferromagnetic coupling in the ab plane and elevate their spin reorientation transition (SRT) temperatures. The structural, magnetic, and magnetothermal properties of the Mn-doped DyFeO3 single-crystal system are investigated. With increasing doping ratio, the Néel temperature progressively falls, while the SRT temperature (TSR) continuously rises to the room temperature of 307 K with a 40 % doping ratio, making it promising for room-temperature spin storage devices. When the ratio reaches 50 % (half doping), the magnetic configuration of Dy sublattices changes and a significant magnetothermal effect is observed. Because of low-field metamagnetic phase transition, the magnetic entropy change along the c-axis approaches 10.41 J/kg·K for the half doping crystal at 5 K (0 J/kg·K for DyFeO3, 0.93 J/kg·K for orthorhombic DyMnO3). The large adiabatic temperature change contributed by magnon indicates the exceptional refrigeration efficiency in a direct way. Therefore, it is possible to convert DyFeO3 into a room-temperature spin storage device material or a magnetic refrigeration contender at low temperatures.

Low field controllable continuous spin switching in the thulium-ytterbium single crystal.

We report the spin reorientation transition (SRT) and the low field controllable continuous spin switching (SSW) of the Tm0.75Yb0.25FeO3 (TYFO) single crystal in this study. The SRT, characterized by the transition from Γ2(Fx, Cy, Gz)-Γ4(Gx, Ay, Fz), occurs within the temperature range of 20-27 K. Under an external magnetic field of 50 Oe, the SSW occurs along the c-axis at approximately 98 K due to the reversal of Tm3+ magnetic moment induced by the magnetic coupling change between Tm3+ and Fe3+, transitioning from a parallel to an antiparallel alignment. Notably, a continuous SSW is observed along the a-axis at low temperatures, which has not been previously reported in rare earth orthoferrites. This unique behavior can be easily manipulated by low magnetic fields within the temperature range of 2-20 K. Both the spin reorientation transition and spin switching phenomena in the TYFO single crystal arise from interactions between rare earth ions and iron ions and can be effectively regulated by applied low magnetic fields, making it a promising material for low-field spin devices.

Anisotropic crystal growth in blue phase I transitioned from a uniformly oriented cholesteric phase.

Phase transitions in blue phase liquid crystals (BPLCs) have attracted significant attention from the technical perspective of BP orientation control and the theoretical perspective of analogs with crystalline solids. The phase transition phenomena in BPs significantly depend on the phase state before the transition. In this study, we focused on the cholesteric (Ch)-BPI phase transition and found that BPI crystals exhibited a square shape upon the phase transition from the uniformly oriented Ch phase to the BPI phase. This square crystal shape was common across three BPLC materials with different elastic constants, and the shape reflected the crystal axis. The in-plane crystal orientation correlates with the easy axis on the substrate surface, suggesting that the [011] axis tends to coincide with the easy axis. However, the easy axis has little effect on the crystal growth rate. Furthermore, scratches on the substrate surface promoted nucleation. Based on this behavior, it was demonstrated that the nucleation position and density could be controlled by intentionally disturbing the Ch orientation by locally changing the easy axis using photoalignment. This study focused on the anisotropic crystal growth of BPI, providing interesting insights into LC phase transitions and soft matter crystal growth. In addition, it offers techniques for the fabrication of large BPI crystals, contributing to the enhanced electrical and optical performances of BPLC devices.

Orientational transitions of discotic columnar liquid crystals in cylindrical pores.

Confined in a cylindrical pore with homeotropic anchoring condition, the hexagonal columnar phase of discotic liquid crystals can form a "log-pile" configuration, in which the columns are perpendicular to the long axis of the pore. However, the {100} planes of the hexagonal lattice can orient either parallel (termed (100)‖ orientation) or perpendicular ((100)⊥) to pore axis. Here we experimentally show that the (100)‖ orientation is found in narrower cylindrical pores, and the (100)‖-(100)⊥ transition can be controlled by engineering the structure of the molecules. The (100)‖ orientation is destroyed in asymmetric discotics hepta(heptenyloxy)triphenylene (SATO7); replacing the oxygen linkage in hexa(hexyloxy)triphenylene (HATO6) by sulphur (HATS6) improves the (100)‖ orientation in small pores; adding a perfluorooctyl end to each alkyl chain of HATO6 (HATO6F8) moves the (100)‖-(100)⊥ transition to larger pores. We have provided a semi-quantitative explanation of the experimental observations, and discussed them in the context of previous findings on related materials in a wider pore size range from 60 nm to 100 μm. This allows us to produce a comprehensive picture of confined columnar liquid crystals whose applications critically depend on our ability to align them.

High entropy effect on thermoelectric properties of the nonequilibrium cubic phase of AgBiSe2-2xSxTex with x = 0-0.6.

Silver bismuth diselenide (AgBiSe2) has attracted much attention as an efficient thermoelectric material owing to its low thermal conductivity. However, AgBiSe2 exhibits multiple crystal structural transitions with temperature, and high thermoelectric performance was realized only in the high-temperature cubic phase. We previously reported the stabilization of the cubic phase in AgBiSe2-2xSxTex with x = 0.6-0.8 at room temperature using a high-entropy-alloy (HEA) approach. In this paper, we succeeded in stabilizing the cubic phase in AgBiSe2-2xSxTex with x = 0-0.6 using an ice-quenching method and investigated the HE effect on thermoelectric properties below room temperature to avoid the emergence of a hexagonal phase above room temperature. Cubic AgBiSe2-2xSxTex exhibited n-type conductivity at 10-300 K. The increase in electrical conductivity is due to a combined effect of increased charge density and electron mobility, depending on the sample. It is evident that charge carrier density strongly increases in the x = 0.4 sample while the mobility remains roughly the same compared to the x = 0.0 sample. In x = 0.2 and 0.6 samples, electronic conductivity increases primarily due to enhanced mobility. S and Te substitutions induced a variation in the band structure, resulting in carrier mobility enhancement. Furthermore, thermal conductivity showed reduction tendency with increasing amounts of S and Te due to the enhancement of phonon scattering. Simultaneous electronic conductivity increase and thermal conductivity reduction resulted in the systematic improvement of ZT values for HE-type cubic AgBiSe2-2xSxTex.

Comprehensively understanding the distinct properties of crystal structure, phase transition and dielectrics in [M(crown)n]ReO4 (M+ = Na+, K+, NH4+; crown = 15-crown-5, 18-crown-6; n = 1 or 2)

Supramolecular salts exhibit distinct crystal structures, phase transitions and dielectric properties, stemming from the compatibility between M+ (Na+, K+ and NH4+) radii and crown-ether ring sizes.

In English

The article discusses the use of KDP ferroelectric crystals (phosphates and arsenates of potassium, rubidium, cesium) and their deuterated analogues in various industries, including the creation of electro-optical devices and as hydrogen sorbents. The paper describes the physical properties of KDP crystals, changes in their properties near the phase transition temperature, as well as methods for obtaining KDP nanocrystals and their application in biomedicine. The paper also states that the phase transition in KDP crystals occurs near room temperature and manifests itself in a change in their physical properties, such as dielectric constant, optical properties, and heat capacity. In addition, approaching the phase transition temperature causes a change in the crystal lattice parameters, which can lead to the appearance of anomalous effects. The structure of the unit cell of potassium dihydrogen phosphate (KH2PO4) is considered. The plots of the temperature dependence of the order parameter of spontaneous polarization and the plots of the temperature dependence of the configurational heat capacity of the crystal in the phase transition region are calculated, and the plots of the temperature of the inverse and direct dielectric susceptibility are calculated. Graphs of the order parameter, which characterizes the degree of spontaneous polarization for different temperatures, depending on the strength of the external electric field, are also calculated.

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.

Ductile–brittle transition and ductile-regime removal mechanisms in micro- and nanoscale machining of ZnS crystals

The ductile–brittle transition and ductile-regime removal mechanisms of ZnS crystal, a typical soft-brittle infrared optical material, have remained unclear at the micro- and nanoscale. In this context, this study modeled the energies consumed in two machining modes, namely ductile deformation and brittle fracture, during the ultraprecision cutting of ZnS crystal. Subsequently, based on the transition point of the energy mode, the critical undeformed chip thickness (CUCT) for the ductile–brittle transition of the material was determined. Additionally, the effect of tool parameters on CUCT was quantitatively analyzed, and the correctness of the model was verified by scratch experiments. Moreover, the cutting parameters that affect the maximum undeformed chip thickness (MUCT) were analyzed. By comparing the MUCT and CUCT, the cutting conditions to realize ductile-regime machining were predicted. The correctness of the ductile-regime machining conditions was also verified through surface quality characterization after face-cutting experiments. The MUCT can be controlled to be less than the CUCT by using a cutting tool with a sufficiently large tool rake angle, a nose radius and a reduced feed rate, thus realizing the ductile-regime removal of the material. The surface morphology, chip morphology, and surface roughness of machined ZnS crystal confirmed the validity of the model. Using a diamond cutting tool with a large cutting edge angle (–25°) and nose radius (1.2 mm) as well as a low feed rate (0.5 μm/rev) and depth of cut (3 μm), the generation of cracks and pits was effectively inhibited, resulting in a smooth surface with a surface roughness of 1.60 nm. Overall, the findings of this study provide important insights into the superfinishing of soft–brittle materials such as ZnS crystal.

Aquatic Functional Liquid Crystals: Design, Functionalization, and Molecular Simulation.

Aquatic functional liquid crystals, which are ordered molecular assemblies that work in water environment, are described in this review. Aquatic functional liquid crystals are liquid-crystalline (LC) materials interacting water molecules or aquatic environment. They include aquatic lyotropic liquid crystals and LC based materials that have aquatic interfaces, for example, nanoporous water treatment membranes that are solids preserving LC order. They can remove ions and viruses with nano- and subnano-porous structures. Columnar, smectic, bicontinuous LC structures are used for fabrication of these 1D, 2D, 3D materials. Design and functionalization of aquatic LC sensors based on aqueous/LC interfaces are also described. The ordering transitions of liquid crystals induced by molecular recognition at the aqueous interfaces provide distinct optical responses. Molecular orientation and dynamic behavior of these aquatic functional LC materials are studied by molecular dynamics simulations. The molecular interactions of LC materials and water are key of these investigations. New insights into aquatic functional LC materials contribute to the fields of environment, healthcare, and biotechnology.

Structural geometry and molecular dynamics by nuclear magnetic resonance of organic-inorganic perovskite [NH3(CH2)6NH3]CuBr4 crystal near phase transition temperature

The structural, thermal, and physical properties of newly developed lead-free hybrid perovskite solar cells are discussed for [NH3(CH2)6NH3]CuBr4. The hybrid material undergoes a phase transition at 269 K (TC), and is thermodynamically stable until approximately 472 K. The crystal structure determined by single-crystal X-ray diffraction is monoclinic and triclinic above and below TC, respectively. The 1H and 13C nuclear magnetic resonance spectra, recorded as a function of the temperature, revealed considerable changes in the 1H chemical shifts of NH3 and the 13C chemical shifts of the carbon atoms close to N as a result of the structural phase transition. The large difference in the activation energy, obtained from the 13C spin-lattice relaxation time above and below TC, also appears to be related to energy transfer changes in 13C. The thermal stabilities of [NH3(CH2)nNH3]CuBr4 (n = 3, 4, and 6) were compared by varying the number of methylene units, and the temperature with 2 % weight loss was shown to decrease as the number of methylene units increases. This in-depth analysis of the physical properties is expected to promote the development and application of lead-free hybrid perovskite solar cells.

Recent progress on solid-state cross-polarization (CP)/magic angle spinning (MAS) NMR measurements: a tool for investigating structural changes in stimuli-responsive organic compounds

Abstract Over the years, cross-polarization/magic angle sample spinning (CP/MAS) NMR has proven to be valuable for obtaining detailed structural information in solid-state materials. A notable advantage of this technique is that it does not require a well-organized arrangement in the sample such as single crystals. Consequently, even materials with low crystallinity such as amorphous substances can be analyzed to obtain structural information. In recent years, research on externally stimuli-responsive molecules utilizing crystal phase transitions has attracted attention. Certain aryl disilane and oligosilane compounds exhibit external stimulus responsiveness even in the crystalline state due to the hindered rotation of the Si–Si σ bond and sparse crystal packing. Understanding the structural alternations linked to changes in physical properties is crucial for investigation of the responsiveness. High-resolution CP/MAS NMR spectra are particularly suitable for analyzing crystalline polymorphs, crystal phase transitions, and solid-state conformations. In this report, we present the results of natural abundance 13C, 29Si, and 15N CP/MAS NMR experiments conducted on externally stimuli-responsive disilane-bridged molecules. These measurements provide information on the structure before and after crystal phase transitions, as well as the amorphous state. A comprehensive analysis of our findings is essential for elucidating alternations in the structure and arrangement of crystalline materials under external stimuli.