Polymorphic Phase Transition Research Articles

Domain reorientation and electric-field-induced phase transition in (K,Na)(Nb,Sb)O3-(Bi,Na)ZrO3 piezoceramics

Polymorphic phase transition boundary (PPB) strategy is effective in designing high-performance piezoelectric materials in (K0.5Na0.5)NbO3 (KNN)-based systems. However, the underlying mechanism of large piezoelectric response in KNN-based is still blurry and thus needs to be explored properly. In this paper, it is found that the domain reorientation becomes easier and more sufficient in the KNN-based ceramics with two-phase coexistence. Moreover, in situ transmission electron microscope experiments disclose the occurrence of a new intermediate phase under an external electric filed, which could relieve the internal stress generated by the domain reorientation and then facilitate the further domain switching along the electric field direction. The enhanced domain reorientation contributes to the high piezoelectric response in PPB region (d33 = 438 pC/N, kp = 48%, d33* = 520 p.m./V at 1.5 kV/mm). Our work provides an in-depth understanding of the contribution of the domain switching and electric-filed-induced phase transition to piezoelectric properties in KNN-based ceramics, which shall benefit the design of new lead-free high-performance piezoelectric materials.

Hexagonal Phase Formation and Crystalline Structural Transition in Long-Spaced Aliphatic Polyesters with Side Groups.

Side substitution is an effective method for the chemical modification and functionalization of linear polyesters. The presence of side groups can have a profound effect on the crystalline structure and phase transition of semicrystalline polyesters. Herein, we synthesized the long-spaced polyesters with -OH and -CH3 side groups and various methylene segment lengths and studied the effects of the side groups on the crystal polymorph and phase transition of substituted polyesters. The substituted polyesters grow in the thermally stable phase (form I) at a higher temperature. However, the polyesters crystallize in a metastable hexagonal phase (form II) with trans chain conformation at a lower temperature. The metastable form II transforms into the more stable form I during long-time annealing or upon heating; this phase transition is accompanied by chain tilting and crystal lamellar thickening. This study has elucidated the critical role of side groups in the polymorphic crystallization and phase transition of linear polyesters.

Defect Dipole‐Induced Fatigue Strain Recoverability in Lead‐Free Piezoelectric Ceramics

AbstractPiezoelectric ceramics have garnered extensive utilization in high‐precision actuators, where the magnitude of electric field‐induced strain and fatigue resistance play crucial roles in actuation applications. Herein, an innovative strategy based on defect dipoles is proposed to form a defect‐engineered polymorphic phase transition and achieve a giant piezoelectric strain coefficient of 3080 pm V−1 in Li/Sr‐doped (K0.5Na0.5)NbO3 lead‐free piezoceramics. The mechanism responsible for the enhanced strain performance lies in the optimized strain compatibility between the refined stripe domains and the nanosized domains. Additionally, the results demonstrate that the material is able to recover from fatigue‐induced strain depletion under the stimulation of bipolar electric fields. This property can be phenomenologically explained by the rigid ion model, in which the application of reversal electric fields can facilitate the restoration of defect dipoles, thereby greatly contributing to strain recoverability. This study establishes a close correlation between the unipolar strain properties and the inherent flexibility of defect dipoles and provides new insight into the design of high‐reliability, large‐stroke piezoceramics.

Influence of Mn-doping on the structure, high-temperature dielectric, and conductive properties of NaNbO3 ceramics

Single phase NaNb1−xMnxO3 (NMn) with x = 0.00−0.06 ceramics were synthesized using solid state reaction technique at 1150 °C sintering temperature. The Rietveld refinement of XRD patterns of NMn ceramics confirms the orthorhombic phase symmetry with Pbma space group. The field emission scanning electron microscopy (FE-SEM) reveals that grains become denser and larger with doping. The dielectric constant (ϵ′) and tangent loss (tan δ) study has been done at room temperature to 500 °C over a wide frequency in both cooling and heating mode. The anomaly at high temperature region in temperature dependent dielectric constant (ϵ′) plot in both thermal cycle attributes to the P(AFE)-R(AFE) polymorphic phase transition. The relaxor nature of the studied samples strongly depends on dopant concentration and applied field. The impedance analysis reveals negative temperature coefficient of resistance (NTCR) behaviour of material. The Nyquist study suggests the non-Debye characteristic of the dielectric relaxation in NMn ceramics.

Solid-to-solid polymorphic phase transitions in two isostructural Bi(III) complexes with 1-phenylethyl-N-ethylthiosemicarbazide and halogens

Two isostructural (in room temperature) complexes of Bi(III) with halogens and sulfur ligands have been investigated in terms of the solid-to-solid phase transitions indicated by temperature. Both chloride and bromide (X) complexes of the general formula (µ2-X)-(BiX2L2)2 exhibit some phase transitions between 100 and 333 K, which, apart from the numerous similarities, show significant differences, which have been noted and analyzed in detail in this paper by using different techniques, i.e., powder and single crystal diffraction or DSC. The obtained results have also been collated with those obtained for solid solutions of both complexes.

Reversible Thermally Driven Phase Change of Layered In2Se3 for Integrated Photonics.

Two-dimensional In2Se3, an unconventional phase-change material, has drawn considerable attention for polymorphic phase transitions and electronic device applications. However, its reversible thermally driven phase transitions and potential use in photonic devices have yet to be explored. In this study, we observe the thermally driven reversible phase transitions between α and β' phases with the assistance of local strain from surface wrinkles and ripples, as well as reversible phase changes within the β phase family. These transitions lead to changes in the refractive index and other optoelectronic properties with minimal optical loss at telecommunication bands, which are crucial in integrated photonic applications such as postfabrication phase trimming. Additionally, multilayer β'-In2Se3 working as a transparent microheater proves to be a viable option for efficient thermo-optic modulation. This prototype design for layered In2Se3 offers immense potential for integrated photonics and paves the way for multilevel, nonvolatile optical memory applications.

Temperature Induced Monoclinic to Orthorhombic Phase Transition in Protonated ZSM-5 Zeolites with Different Si/Al Ratios: An In-Situ Synchrotron X-ray Powder Diffraction Study

ZSM-5 zeolite is the synthetic counterpart to mutinaite. After thermal activation of the as-synthesized form, the symmetry of the ZSM-5 zeolite is lowered to the monoclinic P21/n. ZSM-5 then undergoes a polymorphic displacive phase transition from the monoclinic P21/n to the orthorhombic Pnma, Pn21a or P212121 space groups, which occurs upon heating. This phase transition can be influenced by factors such as the type and amount of sorbate molecules present in the zeolite channels. ZSM-5 has many applications, including as a catalyst or sorbent in various industries, where high thermal stability is required. In this study, four ZSM-5 zeolites with different Si/Al ratios were investigated by synchrotron X-ray powder diffraction at both room temperature and high temperature conditions to determine the effects of chemical composition on the structural response of the zeolite lattice. The results showed that the ZSM-5 zeolites retained their crystallinity and structural features throughout the thermal treatment, indicating that they could be used as effective acid catalysts. Distortions in the zeolite framework can occur after TPA+ decomposition and thermal activation, affecting thermal regeneration and efficiency. The charge balance in ZSM-5 is achieved by the formation of Brønsted acid sites, and variations in bonding geometries are influenced by the initial Si/Al ratio.

Equilibrium Melting Temperature of the Hexagonal Crystals of Polybutene-1 and Its Copolymer

TheLamellar structures of form I in polymorphic isotactic polybutene-1 (iPB-1) and forms I and I′ in butene-1/ethylene random copolymer (iPB-1/C2) were investigated by studying their melting behavior using the combination of small-angle X-ray scattering (SAXS) and fast scanning chip calorimetry (FSC) techniques. Hexagonal form I was transformed from tetragonal form II obtained by crystallization of a homogeneous melt of iPB-1 and iPB-1/C2. Hexagonal form I′ was crystallized directly from a heterogeneous melt of iPB-1/C2. Despite the same hexagonal crystallographic structure, forms I and I′ have obviously distinct melting behaviors due to different ways of crystal formation. Well-defined linear dependences of the melting temperature against the reciprocal lamellar thickness were observed for forms I and I′ with frozen chain mobility in crystals. The slopes of the thus obtained melting lines for forms I and I′ are different. Their extrapolations to imaginary infinitely large lamellar thickness provide the equilibrium melting temperature Tm∞ for forms I and I′, which is associated with the stability of the lamellar crystals. In spite of crystal defects in form I′, a higher Tm∞ is observed for this form in iPB-1/C2 than Tm∞ for form I in both iPB-1 and iPB-1/C2. The difference in Tm∞ indicates a less perfect structure of form I crystal lamellae that is caused by the fragmentation of form II crystal lamellae during the polymorphic phase transition from form II to I. The lamellae broke into crystal blocks along the lamellar direction. The smaller lateral sizes of the fragmented lamellae clearly induce a decrease in the stability of crystals, as expressed in a lower Tm∞ for form I.

Characterization of yttrium-doped γ-glycine crystal for nonlinear optical properties, antibacterial, and anticancer activities

The slow evaporation method was employed to grow γ-glycine nonlinear optical crystal at ambient temperature. Analyses of powder X-ray diffractions confirm the crystal is in a γ-phase. Our experiment investigated how the additive impacted the optical transparency of the glycine crystal using a linear optical transmittance analysis. In order to determine whether the title compound had polymorphic phase transitions and heat stability, a DSC study was conducted. The SHG productivity of the grown material is 3.83 times larger than the reference (KDP) crystal. There is evidence that these crystals are cytotoxic, indicating that they might serve as anticancer agents.

Peculiar dielectric properties of (1-x)(K,Na)(Nb,Sb)O3−x(Bi,Na)ZrO3 ceramics

In spite of the significance from both scientific and technological viewpoints, systematical studies on the dielectric spectra of (K,Na)NbO3-based lead-free ceramics particularly in the poled state had been scarcely performed so far. In this study, the compositional series of (1-x)(K0.48Na0.52)(Nb0.96Sb0.04)O3−x(Bi0.50Na0.50)ZrO3 ceramics were chosen representatively to shed more light on the characters of dielectric properties. All the ceramics in the unpoled state show dielectric spectra of large dispersion with the relaxational characteristic frequency higher than the measured upper limit of 60 MHz at room temperature. Additional resonance peaks due to the electromechanical coupling of piezoelectric effect appear and are accompanied by a large step-like reduction of dielectric permittivity ε′ in the dielectric spectra after poling. An abnormal phenomenon that poling induces the increase of low-frequency ε′ occurs in those ceramics with x ≥ 0.04 at room temperature, and is confirmed to associate closely with the orthorhombic-tetragonal and rhombohedral-orthorhombic polymorphic phase transitions. Further study of domain structure indicates that the origin of this abnormal phenomenon should be ascribed to the enhancement of domain-wall mobility because of the large reduction of domain-wall density due to the poling.

Near-zero thermal quenching of upconversion green emission implemented with the polymorphic phase transition

Thermally stable upconversion luminescence is highly expected for applications in high-temperature environments. Herein, the mechanism of the phase transition inducing near-zero thermal quenching in upconversion green light is reported. The phosphor Ba3Gd0.78Yb0.2Er0·02Al2O7.5 is synthesized by the conventional high-temperature solid-state method and quenched in air during cooling. In-situ XRD and DSC curves demonstrate the polymorphic phase transition in the subsequent heating. The temperature-dependent upconversion luminescence exhibits stable green emission instead of thermal quenching when the material is excited by a 980 nm near-infrared laser. The more intense green emission resulting from the low symmetry of the low-temperature phase and its increase in the phase content with temperature are responsible for the thermal stability in the luminescence of the material. This research provides an approach to utilizing phase transformation to obtain stable emissions in upconversion phosphors.

Ag8GeS6(Se6)–Ag8GeTe6 SYSTEMS: PHASE RELATIONS, SYNTHESIS, AND CHARACTERIZATION OF SOLID SOLUTIONS

Phase equilibria in the Ag8GeS6-Ag8GeTe6 and Ag8GeSe6-Ag8GeTe6 systems were studied using differential thermal analysis and powder X-ray diffraction technique. The T-x phase diagrams were constructed. It has been found that both systems are partially quasi-binary and are characterized by the formation of continuous series of substitutional solid solutions between Ag8GeTe6 and high-temperature cubic modifications of Ag8GeS6 and Ag8GeSe6. When solid solutions are formed, the temperatures of polymorphic phase transitions of these compounds decrease. This leads to the stabilization of the ion-conducting cubic phase in the 40 mol% Ag8GeTe6 range of compositions at room temperature and below. The regions of homogeneity based on Ag8GeS6 and Ag8GeSe6 are 15 and 10 mol.%, respectively. The crystal lattice parameters of the identified solid solutions were calculated from the XRD data, and their linear dependence on the composition is shown

Extensive Polymorphism in the Molecular Ferroelectric 18-Crown-6 Oxonium Tetrachloro-Gallium(III).

The materials property of ferroelectricity is intimately linked with symmetry-changing phase transitions. Characterizing such transitions is therefore essential for understanding molecular ferroelectrics. In this paper, we explore the temperature and thermal history dependence of polymorphic phase transitions in the multiaxial molecular ferroelectric 18-crown-6 oxonium tetrachloro-gallium(III). We have solved the structures of two previously suggested polymorphs (D and Y) ab initio from high-temperature powder diffraction data. We also report the structure of a new polymorph (X) using low-temperature powder diffraction data and identify a fifth (W) that can form on cooling. These polymorphs can be related using two distinct group-subgroup trees. Structure types A-C observed in this and related compounds can be derived from high-temperature polymorph D by group-subgroup relationships. The X and Y polymorphs can be described as child structures of a hypothetical polymorph Z using a molecular rotational distortion mode description. The ferroelectric properties of the various polymorphs can be rationalized based on our structural findings.

Positive charge-mediated phase modulation of MoTe2 synthesized by molecular beam epitaxy

Polymorphic phase transition between semi-conducting 2H and semi-metallic 1T′ in MoTe2 has garnered significant interest due to its wide applicability for memory device. Recently, it has been reported that charge doping is a prospective method for facilitating practical application of phase transition. In particular, the positively doped system with lower energy barrier between two phases is more advantageous than the negatively doped system. However, although various methods for negative charge-mediated phase transition have been developed, study on positive charge-mediated phase transition is scarce due to the requirement of high charge density. Herein, we report positive charge-mediated phase modulation in MoTe2 synthesized by molecular beam epitaxy. The structural phase of MoTe2 is significantly influenced by the stoichiometric ratio of molybdenum to tellurium, and the tellurium adatoms make 1T' phase more stable than 2H phase. Photoelectron spectroscopy reveals that positive charge doping by electron transfer to tellurium adatoms is crucial in the determination of phase. Finally, based on the energy difference and energy barrier between two phases, the doping effects during and after synthesis are verified. This study on the structure modulation of MoTe2 provides physical insight for phase transition as well as a basis for the development of nanoscale electronic devices.

Phase Transitions, Mechanical and Dynamic Stability, and Electronic Properties of SnO Polymorphs under High Pressure

The structures, phase transition, mechanical properties, dynamic stability, and electronic properties of SnO polymorphs (α, γ, herzenbergite, B1, and B2 phases) under pressure have been studied using the first‐principles calculations. The obtained structural parameters are in agreement with the available data. According to the enthalpy–pressure curves of SnO, the pressure‐induced phase transitions are verified to be from α to γ at 0.94 GPa and from B1 to B2 at 49.1 GPa, respectively. The calculated elastic constants indicate that B2 and γ are unstable under zero pressure. With increasing pressure, there is stability–instability transition for B1, whose transition pressure is 20.6 GPa, respectively. Moreover, a range of mechanical stability is from 5.5 to 10.8 for α phase and from 3.4 to 14.3 GPa for γ phase. The corresponding elastic modulus has been analyzed under pressures. With regard to elastic anisotropy, a variety of methods are taken to analyze it and the causes of various anisotropic characters are explained. Phonon dispersions show the same dynamic stability of SnO with the mechanical stability. In addition, the density of states and charge density all reflect that the interactions between Sn and O elements are enhanced with the increasing pressure.

Tunable phase structure in NaNbO3 ceramics by grain-size effect, electric field and heat treatment

Large polarization and strain change during antiferroelectric - ferroelectric phase transition under electric field is the foundation for realizing excellent electrical properties in antiferroelectric ceramics, therefore, the adjustment of antiferroelectricity and clarification of the corresponding mechanism is the foundation for controlling electrical properties. NaNbO3 is the most complex perovskite system showing multiple antiferroelectric phases in a wide temperature range, in which the antiferroelectricity shows obvious instability with changing external and internal conditions, namely the antiferroelectric phase can be adjusted by grain-size effect, electric field and heat treatment. According to the systematical study in terms of the Rietveld refinement of synchrotron XRD and Raman, NaNbO3 exhibits a ferrielectric P21ma structure at room temperature, the ferroelectric component of which increases with decreasing grain size. Two antiferroelectric tetragonal phases exist around Curie temperature TC before the entrance of antiferroelectric R phase zone, while an antiferroelectric monoclinic phase, which can be maintained to room temperature by annealing treatment, acts as the bridge for the depolarization of the poled NaNbO3 with ferroelectric Q phase. A detailed phase diagram mainly focused on the antiferroelectric phase zones of NaNbO3 is plotted, which gives a clear understanding about the polymorphic phase transitions under different conditions. The results concluded in this work would give a clear guidance for designing high-performance NaNbO3-based lead-free ceramics from the point of structure.

Dicopper(II) complexes with N3,N4-bridging 1-alkyltetrazoles: Synthesis, crystal structure, magnetic and thermodynamic properties

1-R-Tetrazoles (R ​= ​Et, i-Pr, t-Bu) were found to react with copper(II) tetrafluoroborate hexahydrate and copper(II) nitrate tetrahydrate giving the complexes [Cu2L8(H2O)2]X4, where L ​= ​1-R-tetrazole, X ​= ​BF4− or NO3−. Single crystal X-ray analysis showed that the complexes comprised dinuclear cations [Cu2L8(H2O)2]4+, with six tetrazole ligands showing monodentate N4-coordination, and two ones providing N3,N4 bridges between the copper(II) cations; water molecules complete the distorted octahedral coordination of metal ions. Magnetic susceptibility measurements revealed that the copper(II) ions were very weakly antiferromagnetically coupled. Сomplex [Сu2L8(H2O)2](BF4)4 (L ​= ​1-isopropyltetrazole) was found to exhibit a temperature induced reversible single-crystal to single-crystal polymorphic phase transition. For this complex, adiabatic calorimetry was used to find the standard thermodynamic characteristics of this transition as well as the standard thermodynamic functions of the compound in the temperature range of 80–370 ​K. Additionally, mononuclear complex of the composition [CuL6](BF4)2 (L ​= ​1-ethyltetrazole) was obtained during the investigation and structurally characterized.

Polymorphic protein phase transitions driven by surface anisotropy.

Phase transitions of proteins are strongly influenced by surface chemical modifications or mutations. Human γD-crystallin (HGD) single-mutants have been extensively studied because they are associated with the onset of juvenile cataract. However, they have also provided a rich library of molecules to examine how specific inter-protein interactions direct protein assembly, providing new insights and valuable experimental data for coarse-grained patchy-particle models. Here, we demonstrate that the addition of new inter-protein interactions by mutagenesis is additive and increases the number and variety of condensed phases formed by proteins. When double mutations incorporating two specific single point mutations are made, the properties of both single mutations are retained in addition to the formation of a new condensed phase. We find that the HGD double-mutant P23VC110M self-assembles into spherical particles with retrograde solubility, orthorhombic crystals, and needle/plate shape crystals, while retaining the ability to undergo liquid-liquid phase separation. This rich polymorphism is only partially predicted by the experimental data on the constituent single mutants. We also report a previously un-characterized amorphous protein particle, with unique properties that differ from those of protein spherulites, protein particulates previously described. The particles we observe are amorphous, reversible with temperature, tens of microns in size, and perfectly spherical. When they are grown on pristine surfaces, they appear to form by homogeneous nucleation, making them unique, and we believe a new form of protein condensate. This work highlights the challenges in predicting protein behavior, which has frustrated rational assembly and crystallization but also provides rich data to develop new coarse-grained models to explain the observed polymorphism.

Investigation of FT-IR and impedance spectroscopy of nanocomposite PVDF-HFP based polymer electrolytes

The fabrication of nano TiO2 polymer electrolyte of PVDF-HFP: Mg (ClO4)2 polymer matrix deploying solution casting technique. An effect of nano TiO2 with PVDF-HFP: Mg(ClO4)2 was studied by FTIR, and Electrochemical impedance spectroscopy. The FT-IR results revealed the dramatic changes in α, β, γ and δ polymorphous phase transition of –CF-CF- on PVDF-HFP polymer shows fair miscibility and interaction between Mg2+ ion and PVDF-HFP and nano TiO2; when nano TiO2 added to PVDF-HFP: Mg solid polymer electrolyte. The impedance spectroscopy results of PVDF-HFP: Mg(ClO4)2 with nano-TiO2 included revealed that bulk resistance decreased with increasing TiO2 nano filler concentration up to a specific optimum concentration, and then rose with additional increases in TiO2 nano filler concentration. The optimum conductivity was obtained for the PVDF-HFP:Mg(ClO4)2:TiO2(nano) solid polymer electrolyte in the 100:40:12 ratio, and the observed value is in the order of 10−3 Scm−1.

Synthesis and Characterization of Biodegradable Polymers Based on Glucose Derivatives.

Syntheses of two new monomers, namely the glucose derivatives 2,3,4,6-tetra-O-acetyl-1 methacryloyl-glucopyranose (MGlc) and 2,3,4,6 tetra-O-acetyl-1-acryloylglucopyranose (AGlc), are presented. Their chemical structures were determined by the FTIR, 1H and 13C NMR spectroscopies, the single-crystal X-ray analysis, supported by the powder X-ray diffraction, and the DSC analyses. Molecules of both monomers exist in the β-anomeric form in the solid state. The variable temperature X-ray diffraction studies, supported by the DSC analyses, revealed AGlc's propensity for polymorphism and temperature-induced phase transitions. MGlc and AGlc crystallised from methanol were polymerized or copolymerized with methyl methacrylate and N-vinylpyrrolidone. The biodegradabilities of polymers as well as thermal and optical properties were studied. The results show that some properties of the obtained homopolymers and copolymers resemble those of PMMA. The main difference is that the AGlc and MGlc homopolymers are biodegradable while PMMA is not. The ternary copolymers, i.e., MGlc/AGlc-MMA-NVP lose more than 10% of their weight after six months.