Crystal Phase Transition Research Articles

Effects of alkylthio groups on phase transitions of organic molecules and liquid crystals: a comparative study with alkyl and alkoxy groups

The effects of the alkylthio groups on the phase transition behavior of organic liquid crystal molecules were examined by comparing them with the effects of alkyl and alkoxy groups.

Probing a microviscosity change at the nematic-isotropic liquid crystal phase transition by a ratiometric flapping fluorophore.

Understanding the microviscosity of soft condensed matter is important to clarify the mechanisms of chemical, physical or biological events occurring at the nanoscale. Here, we report that flapping fluorophores (FLAP) can serve as microviscosity probes capable of detecting small changes. By the ratiometric fluorescence analysis, one of the FLAP probes detects a macroscopic viscosity change of a few cP, occurring at the thermal phase transition of a nematic liquid crystal. We discuss the impact of the chemical structure on the detection capability, and the orientation of the FLAP molecules in the ground and excited states. This work contributes to experimentally providing a molecular picture of liquid crystals, which are often viewed as a continuum.

A Study of Phase Transitions of Molecular Crystals by Means of Thermal Conductivity Measurements

A Study of Phase Transitions of Molecular Crystals by Means of Thermal Conductivity Measurements

Design of continuously deformable topological phononic structures and their application to elastic waveguides

We design a topological phononic crystal in a thin plate, aiming at the development of an efficient elastic waveguide based on the concept of band topology in phonon dispersion. We adopt a snowflake-like structure for the crystal unit cell and search for the optimal structure that exhibits the topological phase transition of the three-dimensional phononic crystal by changing the unit cell structure. The bandgap width can be adjusted by varying the length of the branch of the snow side, and a topological phase transition occurs at the unit cell structure with three-fold rotational symmetry. Elastic waveguides based on edge modes appearing at interfaces between crystals with different band topologies are designed, and their transmission efficiency is evaluated numerically and experimentally. The results demonstrate robustness of the elastic wave propagation in thin plates. Moreover, we experimentally estimate a robustness of the topologically protected propagating states against structural inhomogeneities. The results obtained in this study pave the way for practical development of new surface acoustic wave technologies for high-frequency communication devices.

Topological Phase Transition of Photonic Crystals Based on Electrostatic Interaction Model

Topological Phase Transition of Photonic Crystals Based on Electrostatic Interaction Model

Two Pressure Cells for Quasielastic and Inelastic Neutron Scatterings

Two clamp pressure cells for QENS and INS have been developed. One is a hybrid CuBe/NiCrAl cell which is for relative high pressure up to 2.5 GPa and another one is made from high strength aluminium alloy (mesolite NA723) with pressure up to 0.5 GPa. The sample volume is 0.3 mL and 1 mL, respectively. The pressure cells have been thoroughly calibrated and tested. In addition, the contribution to phonon density of states from the pressure cells has been evaluated. Measurements of the phonon density of states for two perfluorocarbon polymer liquids FOMBLIN oil and Fluorinert have indicated that they are suitable to serve as the low background pressure transmission media for high pressure INS experiments. The applications of the pressure cells for INS and QENS are demonstrated by studies of pressure-induced phase transition of plastic crystals.

Crystal Structure, Phase Transitions and Raman Scattering in the New Non-Centrosymmetric [(C3h7)4n]3bi3br12 Compound

Crystal Structure, Phase Transitions and Raman Scattering in the New Non-Centrosymmetric [(C3h7)4n]3bi3br12 Compound

Crystal structure, phase transition, vibrational spectra, and microwave dielectric characteristics of Ta5+ ionic substituted Co0.5Ti0.5NbO4 ceramics

In the present research, structural representation, phase transition, and dielectric characteristics of Ta5+ ionic substituted Co0.5Ti0.5(Nb1-xTax)O4 (x = 0–1) ceramics were thoroughly investigated. Combined with the changes of diffraction peaks in the X-ray diffraction spectra, such as (002) and (101) crystalline planes near 20°, and the evolutionary trend of the strongest peak in the Raman spectra, a phase transition from rutile to trirutile structure at x = 0.6 was confirmed, suggesting the emergence of structural evolution. Thus, the structural evolution showed a significant impact on the dielectric characteristics, as evidenced by the weakening of the dielectric polarizability with the increasing amount of Ta5+ ion, which led to a reduction in the dielectric constant; the dielectric loss is also closely related to the packing fraction (PF) arising from the structural changes. Additionally, infrared and THz time-domain spectroscopy revealed that the dielectric losses in the microwave band were originated from the oscillatory absorption of structural phonons and supported that the effects of structural evolution on the intrinsic dielectric properties due to the introduction of Ta5+ ion.

Magnetic bistable organic ionic plastic crystal with room temperature ion conductivity comparable to NASICON and superionic conduction in a broad temperature window

A radical salt experiences crystal–crystal and crystal–plastic crystal phase transitions with magnetic bistability and negative thermal expansion, high room-temperature ion-conduction in crystal phase and superionic conduction in plastic crystal phase.

Atomic scale modulation strategies and crystal phase transition of flower-like CoAl layered double hydroxides for supercapacitors

CoAl LDH and its derivatives with a flower-like morphology were successfully synthesized via some phase transition routes.

Investigation of the synergistic effect of melt-extension and nanofiller on the crystal-crystal phase transition from form II to I of isotactic polybutene-1.

New questions and conjectures are raised on the crystal-crystal phase transition of isotactic polybutene-1 (iPB-1) containing nanofiller in the flow field. In this work, we investigate the phase transition from flow-induced oriented form II to I in iPB-1 blends with multi-walled carbon nanotubes (MWCNTs) with a homemade two-drum extensional rheometer combined with in situ wide-angle X-ray diffraction (WAXD) measurements. The MWCNTs show a limited promoting effect on the phase transition kinetics under quiescent conditions, while the phase transition kinetic is highly accelerated with the impose of melt-extension. When the loading extension strain is 0.5 or 2.0, the half time of phase transition (t1/2) is shortened from tens of hours to a few hours, depending on the melt-extension strain and the MWCNTs content in iPB-1. When the extension strain increases to 3.5, t1/2 decreases to about 30 min, which is independent of the MWCNTs content in all iPB-1 blends except in blends with MWCNTs content of 1%, where the phase transition rate in the middle and late stages is restrained. It's speculated that flow-induced molecular orientation or shish-kebab morphology affects the internal stress or stress transfer. The addition of a nanofiller enlarged the effect of melt-extension through strengthening the localized intensity of flow field. In general, the combination of nanofiller and melt-extension can obviously promote the phase transition kinetics.

Tuning of Optical Behavior in Monolayer and Bilayer Molybdenum Disulfide Using Hydrostatic Pressure.

Researchers have shown great interest in two-dimensional crystals recently, because of their thickness-dependent electronic and optical properties. We have investigated the Raman and photoluminescence spectra of free-standing monolayer and bilayer MoS2, as a function of pressure. As the enforcement of layer interaction, an electronic and a crystal phase transition were revealed at ∼6 GPa and ∼16 GPa, respectively, in bilayer MoS2, while no phase transition in the monolayer is observed. The electronic phase transition at ∼6 GPa is supposed to be a direct interband changing to an indirect Λ-K interband transition, and the new structure shown at ∼16 GPa is not metallized and supposed to be a transformation from stacking faults due to layer sliding like 2Hc to 2Ha. The different pressure-induced features of monolayer MoS2, compared with bilayer MoS2, can help to get a better understanding about the importance of interlayer interaction on modifying the optical properties of MoS2 and other fundamental understanding of 2D materials.

Temperature‐Reliant Dynamic Properties and Elasto‐Plastic to Plastic Crystal (Rotator) Phase Transition in a Metal Oxyacid Salt

AbstractAlthough, dynamic crystals are attractive for use in many technologies, molecular level mechanisms of various solid‐state dynamic processes and their interdependence, remain poorly understood. Here, we report a rare example of a dynamic crystal (1), involving a heavy transition metal, rhenium, with an initial two‐face elasticity (within ≈1 % strain), followed by elasto‐plastic deformation, at room temperature. Further, these crystals transform to a rotator (plastic) crystal phase at ≈105 °C, displaying exceptional malleability. Qualitative and quantitative mechanical tests, X‐ray diffraction, μ‐Raman and polarized light microscopy experiments reveal that the elasto‐plastic deformation involves both partial molecular rotations and slip, while malleability in the rotator phase is facilitated by reorientational motions and increased symmetry (slip planes). Our work, connecting the plastically bendable (1D or 2D) crystals with the rotator phases (3D), is important for designing multi‐functional dynamic crystals.

Temperature-Reliant Dynamic Properties and Elasto-Plastic to Plastic Crystal (Rotator) Phase Transition in a Metal Oxyacid Salt.

Although, dynamic crystals are attractive for use in many technologies, molecular level mechanisms of various solid-state dynamic processes and their interdependence, remain poorly understood. Here, we report a rare example of a dynamic crystal (1), involving a heavy transition metal, rhenium, with an initial two-face elasticity (within ≈1 % strain), followed by elasto-plastic deformation, at room temperature. Further, these crystals transform to a rotator (plastic) crystal phase at ≈105 °C, displaying exceptional malleability. Qualitative and quantitative mechanical tests, X-ray diffraction, μ-Raman and polarized light microscopy experiments reveal that the elasto-plastic deformation involves both partial molecular rotations and slip, while malleability in the rotator phase is facilitated by reorientational motions and increased symmetry (slip planes). Our work, connecting the plastically bendable (1D or 2D) crystals with the rotator phases (3D), is important for designing multi-functional dynamic crystals.

The effect of ferromagnetism on the CO activation over FCC crystal phase transition metal catalysts: Insights from DFT calculations

In order to understand the relationship between the ferromagnetism and catalytic performance, three pathways of CO activation over transition metal catalysts (Co, Ni, Cu, Rh, Pd, Ag, Ir, Pt, and Au) have been studied by using density functional theory (DFT) calculations. It is shown that Co and Ni metal catalysts have better catalytic performance for CO activation than others. The density of states (DOS) and spin density plots reveal that Co and Ni metal possess ferromagnetism. So it is concluded that the catalytic activity connects with the metal ferromagnetism. To further confirm this deduction, the magnetic moments and Bader charge have been calculated when small molecules (CO, CHO, and COH) adsorbed on the Co and Ni metal surface, and the results shows that the electron transfer between the small molecules and metal catalysts happens on the unpaired d-orbital electrons (nd) of the metal, which changes the magnetic moments of Co and Ni metal. Meanwhile, electron transfer also changes the catalytic performance of Co and Ni metal catalysts. Compared with the CO, the electron transfer of CHO and COH is more, the magnetic moments of metal changes greatly, and the carbon-oxygen bonds of CHO and COH are easier to break. So the magnetic moments of metal catalysts can be used as an indicator to select different metal catalysts.

The mechanism of plasticity and phase transition in iron single crystals under cylindrically divergent shock loading

Cylindrically divergent shock loading significantly influences the plasticity and phase transition of iron. In this study, the mechanisms of plasticity and α→ε phase transition are investigated using large-scale nonequilibrium molecular dynamics simulation. We find that iron undergoes a local bcc–hcp–bcc–hcp (bcc: body-centered cubic; hcp: hexagonal close-packed) phase transition under cylindrically divergent shock loading with cylinder axis lying in [001] and [110] directions. A bcc lattice reorientation driven by resolved shear stress occurs during the hcp–bcc reverse transition and results in bcc twins, which transform into hcp twin structures with an unexpected c-axis, as compared with planar shock. Moreover, we analyze the coupling behavior between plasticity and phase transition and find that collective movement and rearrangement of atoms induced by plastic slip on the shuffle plane play a key role in the activation of phase transition, which significantly differs from that of planar shock.

Revealing the crystallization kinetics and phase transitions in Mg65Zn30Ca5 metallic glass by nanocalorimetry

As a potential candidate for biomedical applications, Mg65Zn30Ca5 (at%) metallic glass has attracted considerable attention. To guide the development of MgZnCa alloys with improved mechanical and biomedical properties, it is necessary to understand the phase transitions of this metallic glass. In this study, controlling phase transitions in Mg65Zn30Ca5 metallic glass is realized by nanocalorimetry. The underlying thermodynamic and kinetic features at heating rates ranging from 102 to 105 K s−1 are therefore revealed. Upon rapid heating, an intermediate amorphous phase is detected. This metastable phase facilitates the formation of the Mg7Zn3 crystalline phase with a remarkably reduced reaction temperature and activation energy. With the combination of nanocalorimetry and structural characterization, the continuous heating transformation (CHT) diagram for Mg65Zn30Ca5 metallic glass is plotted, and crystallization trajectories under various heating conditions are distinguished. Meanwhile, the crystallization kinetics covering seven orders of magnitude is estimated, giving the growth rate of the MgZn crystalline phase in the undercooled liquid with temperatures ranging from Tg to Tm. Accordingly, the maximum crystal growth rate and corresponding temperature are estimated at umax= 0.49 m s−1 and Tu,max= 563 K, respectively. With the decrease in undercooling, the transition from kinetics-controlled crystallization to thermodynamics-controlled crystallization is illustrated, causing divergence of the estimated growth rate from the experimental results.

Dynamic shock wave induced order-disorder type phase transition of sodium nitrate single crystal

In this letter, we demonstrate the dynamic shock wave induced order-disorder type phase transition exhibited by sodium nitrate single crystal and the results have been evaluated by diffraction, spectroscopic and microscopic techniques. The changeover of order into the disordered phase of NaNO3 crystal at shocked conditions is due to the occurrence of rotational disorder of planer anions (NO3).

Creation of topological charges by the spontaneous symmetry breaking phase transition in azo dye-doped nematic liquid crystals

Liquid crystals are anisotropic fluids with long-range directional order. They can be easily used to create topological defects, but creating a controlled topological defect is difficult and involves many tasks and complex processes. However, in our study, we were able to easily generate disclinations in the symmetry-breaking boundaries of an azo dye-doped nematic liquid crystal cell owing to photoisomerization and symmetry-breaking isotropic-nematic phase transition. The method proposed here marks the starting point for the easier control of topological defects in liquid crystals.

Phase transition and domain formation in ferroaxial crystals

The ferroaxial order, which is characterized by a rotational structural distortion in a crystal, has been recently proposed as one of ferroic orders. Though the domain formation is a characteristic feature in ferroic materials, there has been little study done concerning that for the ferroaxial order. Here, we investigate ferroaxial domains that are formed through a ferroaxial transition in two representative ferroaxial materials, ${\mathrm{NiTiO}}_{3}$ and $\mathrm{RbFe}{(\mathrm{Mo}{\mathrm{O}}_{4})}_{2}$. We spatially resolve their domain structures using an optical method based on electric-field-induced optical rotation, that is, electrogyration (EG). In ${\mathrm{NiTiO}}_{3}$, multidomains are constructed when crystals undergo a ferroaxial transition and the domain size depends on the cooling rate around the transition temperature. Furthermore, the ferroaxial domain structure obtained by the EG measurement is well matched with that by scanning x-ray diffraction. $\mathrm{RbFe}{(\mathrm{Mo}{\mathrm{O}}_{4})}_{2}$ also exhibits multidomain states in which domain patterns are different each time a crystal undergoes a ferroaxial transition. In addition, the temperature dependence of the EG signal well obeys that of the order parameter of a first-order phase transition. These results ensure the effectiveness of the EG effect to elucidate the nature of ferroaxial order.