Sequence Of Phase Transitions Research Articles

Manipulation of Supramolecular Chirality in Bicontinuous Networks of Bent-Shaped Polycatenar Dimers.

Bicontinuous cubic liquid crystalline (LC) phases are of particular interest due their possible applications in electronic devices and special supramolecular chirality. Herein, we report the design and synthesis of first examples of achiral bent-shaped polycatenar dimers, capable of displaying mirror symmetry breaking in their cubic and isotropic liquid phases. The molecules have a taper-shaped 3,4,5-trialkoxybenzoate segment connected to rod-like building unit terminated with one terminal flexible chain. The two segments were connected using an aliphatic spacer with seven methylene units to induce bending of the whole structure. Investigated by the small-angle X-ray scattering (SAXS), a double network achiral cubic phase Cub/Ia3 d, which is a meso-structure, and a chiral triple network cubic phase Cub/I23[*] are formed. The molecules self-assemble into molecular helices and progress along the networks. Interestingly, different linking groups such as ester or azo linkages and core fluorination lead to distinct local helicity, resulting in an alkyl chain volume dependent phase transition sequence Ia3d(L)-I23*-Ia3d(S). The re-entry of Iad phase and loss of supramolecular chirality is attributed to the delicate influence of steric effect at the mono-substitute end and interhelix interaction. Besides, aromatic core fluorination was proved to be a successful tool stabilizing the cubic phases in these dimers.

Molecular Dynamics, Dielectric Properties, and Textures of Protonated and Selectively Deuterated 4'-Pentyl-4-biphenylcarbonitrile Liquid Crystal.

Using liquid crystals in near-infrared applications suffers from effects related to processes like parasitic absorption and high sensitivity to UV-light exposure. One way of managing these disadvantages is to use deuterated systems. The combined 1H and 2H nuclear magnetic resonance relaxometry method (FFC NMR), dielectric spectroscopy (DS), optical microscopy (POM), and differential scanning calorimetry (DSC) approach was applied to investigate the influence of selective deuteration on the molecular dynamics, thermal properties, self-organization, and electric-field responsiveness to a 4'-pentyl-4-biphenylcarbonitrile (5CB) liquid crystal. The NMR relaxation dispersion (NMRD) profiles were analyzed using theoretical models for the description of dynamics processes in different mesophases. Obtained optical textures of selectively deuterated 5CB showed the occurrence of the domain structure close to the I/N phase transition. The dielectric measurements showed a substantial difference in switching fields between fully protonated/deuterated 5CB and selectively deuterated molecules. The DSC thermograms showed a more complex phase transition sequence for partially deuterated 5CB with respect to fully protonated/deuterated molecules.

Structural Phase Transitions and Metallization in Zirconium Disulfide (ZrS2) under High Pressure.

The high-pressure study on zirconium disulfide (ZrS2) has been conducted up to ∼33.9 GPa by using in-situ synchrotron X-ray diffraction at room temperature and electrical transport measurements, aiming to investigate the pressure-induced structural phase transitions and the metallization of the material. The experiments indicated that a progressive structural evolution occurs as the sequence below: hexagonal (space group (SG): P3̅m1) → monoclinic (SG: P21/m) started at 2.8 GPa → orthorhombic (SG: Immm) started at 9.9 GPa → tetragonal (SG: I4/mmm) started at 30.4 GPa. The lattice parameters and bulk modulus of the phases at high pressure are calculated. The decompression results demonstrate the coexistence of ZrS2 stability in monoclinic (SG: P21/m) and orthorhombic (SG: Immm) structures. The electrical transport results demonstrate a significant enhancement in the conductivity of ZrS2 at 8 GPa, and metallization occurs at ∼29.8 GPa. Combined with XRD results, the metallization is coincident with the phase transition to the tetragonal (SG: I4/mmm) structure. Our study presents a more precise phase transition sequence, calculates the cell parameters and bulk modulus of ZrS2 under high-pressure conditions based on experimental results, and confirms that metallization is induced by the structural phase transition. These findings provide an experimental basis for the further utilization of transition metal sulfides (TMDs) under high pressure.

Establishing universality in entropy change and critical behavior for magnetocaloric effect in Si-substituted Mn50Ni40Sn10 inverse Heusler alloy

Magnetic refrigeration working based on the magnetocaloric effect can be the perfect replacement of the conventional gas compression-based refrigeration technology and reduces its harmful effects on the environment. The boundary between a first-order and a second-order phase transition would be where the perfect magnetocaloric material would be found. Therefore, establishing the sequence of phase transitions clearly is essential for the characterization of other phase change materials and for applied magnetocaloric research. A quantitative fingerprint of second-order thermomagnetic phase transitions is reported here in Si-substituted high content Mn-based inverse Heusler alloy systems, which are found to be crystallized in cubic structures. The second-order nature of the phase transition has been confirmed from the Arrott plot analysis and a correlation between magnetocaloric effect and local exponent is established. Using the Arrott plot, the critical exponents are evaluated employing different techniques such as modified Arrott plot, Kouvel–Fisher method, and critical isotherm. Their values are found to be in great agreement with each other and follow the mean-field model signifying the presence of long-range ordering in the materials. The high value of isothermal magnetic entropy change and the reversibility justifies the suitability of the reported materials in the practical application as magnetic refrigerants.

In situ synchrotron X-ray diffraction study: Phase evolution in transition zone of TiAl/Ti2AlNb dual alloy fabricated by laser-directed energy deposition

The phase evolution in the transition zone (TZ) of TiAl/Ti2AlNb dual alloy during laser-directed energy deposition (L-DED) process was investigated using in situ synchrotron radiation X-ray diffraction. The as-solidified microstructure of the TZ forms through the following phase transitions: Liquid→ Liquid + β/B2 →β/B2→ β/B2 + α2. Thermal cycles promote the β/B2 to α2 phase transition with the α2 phase precipitates from the (011) plane of the β/B2 matrix during this transition. The phase transition sequence of the TZ during the first thermal cycle is: β/B2 + α2 → β/B2 → β/B2 + α2. The second thermal cycle leads to a partial transformation of β/B2 phase to α2 phase. The TZ experiences no phase transition from the third thermal cycle. This study provides a comprehensive understanding of the phase formation mechanism in the TZ of L-DEDed TiAl/Ti2AlNb dual alloys.

Anomalous Landau Level Gaps Near Magnetic Transitions in Monolayer WSe2

First-order phase transitions produce abrupt changes to the character of both ground and excited electronic states. Here we conduct electronic compressibility measurements to map the spin phase diagram and Landau level (LL) energies of monolayer WSe2 in a magnetic field. We resolve a sequence of first-order phase transitions between completely spin-polarized LLs and states with LLs of both spins. Unexpectedly, the LL gaps are roughly constant over a wide range of magnetic fields below the transitions, which we show reflects spin-polarized ground states with opposite spin excitations. These transitions also extend into compressible regimes, with a sawtooth boundary between full and partial spin polarization. We link these observations to the important influence of LL filling on the exchange energy beyond a smooth density-dependent contribution. Our results show that WSe2 realizes a unique hierarchy of energy scales where such effects induce reentrant magnetic phase transitions tuned by density and magnetic field. Published by the American Physical Society 2024

Stabilizing inverse ringwoodite with defects, and a possible origin for the 560-km seismic discontinuity

Ringwoodite is an important mineral in the mantle transition zone, and its cationic disorder can profoundly affect its physicochemical properties, but there is currently much controversy about this disorder. In this study, we investigate the cation disorder states of pure Mg2SiO4-ringwoodite and defective ringwoodite under mantle transition zone conditions through DFT calculations and thermodynamic models. Two stable endmembers are seen, one with normal ringwoodite structure and the other with inverted structure (its Si atoms and half of its Mg atoms have swapped sites). Our results indicate that pure ringwoodite does not invert (swap Mg and Si cations) under normal mantle temperatures but the introduction of a Si-excess, Mg-deficient defect induces a swap at normal mantle temperatures and this swap is likely induced by a wide range of defects including water. Thus, in the presence of such a defect or similar defects the olivine phase transition sequence may then go from olivine to wadsleyite to inverse ringwoodite, and then normal ringwoodite. We calculate the seismic properties of normal and inverse ringwoodite and find significantly slower wave speeds in inverted ringwoodite. Due to this difference the presence of inverse ringwoodite may provide a potential explanation for the discontinuous interface of seismic waves at the depth of ∼560 km.

Revealing the phase transition scenario in antiferroelectric thin films by x-ray diffuse scattering

There is no consensus among researchers regarding how phase transitions occur in antiferroelectric (AFE) epitaxial heterostructures, in particular, in heterostructures based on model AFE lead zirconate. The questions about the number of phase transitions in such films and by what mechanism they occur remain controversial. This paper presents a look at the phase transition scenario in two types of epitaxial heterostructures: PbZrO3/Ba[La–Sn]O3/MgO (001) thin films with thicknesses from 25 to 1000 nm and PbZrO3/SrRuO3/SrTiO3 (001) thin film 100 nm thick using the diffuse x-ray scattering in the grazing incidence setup. We register the characteristic butterfly-shaped diffuse scattering (DS) intensity distribution in the HK pseudocubic planes, which corresponds to the anisotropic ferroelectric soft mode. No incommensurate soft mode was observed in the cuts of reciprocal space parallel to the film surface by diffuse scattering. We reproduce the shape of DS distribution at different temperatures by the model based on the dielectric stiffness and the electric polarization correlation tensor in the cubic approximation. Such modeling allows not only to characterize the DS parameters from the challengingly low signal-to-background data set, but also to extract experimentally the sensitivity of the materials with respect to inhomogeneous polarization. While the observed temperature evolution of DS is consistent with the dielectric measurements, the correlation between the DS and the phase transition sequence observed by superstructure reflections is yet to be understood better.

From Insulator to Superconductor: A Series of Pressure-Driven Transitions in Quasi-One-Dimensional TiS3 Nanoribbons.

Transition metal trichalcogenides (TMTCs) offer remarkable opportunities for tuning electronic states through modifications in chemical composition, temperature, and pressure. Despite considerable interest in TMTCs, there remain significant knowledge gaps concerning the evolution of their electronic properties under compression. In this study, we employ experimental and theoretical approaches to comprehensively explore the high-pressure behavior of the electronic properties of TiS3, a quasi-one-dimensional (Q1D) semiconductor, across various temperature ranges. Through high-pressure electrical resistance and magnetic measurements at elevated pressures, we uncover a distinctive sequence of phase transitions within TiS3, encompassing a transformation from an insulating state at ambient pressure to the emergence of an incipient superconducting state above 70 GPa. Our findings provide compelling evidence that superconductivity at low temperatures of ∼2.9 K is a fundamental characteristic of TiS3, shedding new light on the intriguing high-pressure electronic properties of TiS3 and underscoring the broader implications of our discoveries for TMTCs in general.

Phenomenological analysis of positive and negative electrocaloric effects in Rochelle salt

Ferroelectrics exhibiting both positive and negative electrocaloric effects stand out as promising materials for achieving high-efficiency solid-state refrigeration. The phase transition is acknowledged as a crucial factor in designing such materials. This study investigates the electrocaloric behavior and its correlation with phase transitions in Rochelle salt single crystal using Landau thermodynamic theory. Analysis of free energy density, ferroelectric, and dielectric data uncovers a complex temperature-dependent phase transition sequence of paraelectric–ferroelectric–paraelectric. This imparts Rochelle salt single crystal with intriguing field-induced phase transition behaviors and electrocaloric responses. Pronounced positive and negative electrocaloric effects are, respectively, observed near the high-temperature and low-temperature Curie point. The opposite sign of entropy differences between the ferroelectric and the two paraelectric phases is identified as the origin of the difference in electrocaloric response. These findings not only enhance our understanding of the electrocaloric effect but also provide a design solution for materials with the coexistence of positive and negative electrocaloric effects.

Decoding the oxidation mechanism of Zircaloy-4 via in situ synchrotron X-ray diffraction and computational elucidation

Comprehensive understanding of the oxidation behavior of Zr alloys, a vital cladding material in nuclear power plants, is essential for developing improved materials and enhancing the safety and performance of nuclear systems. Herein, the bulk oxidation behavior in ambient air atmosphere of Zircaloy-4 were revisited through in situ synchrotron X-ray diffraction and DFT computation. The results reveal the phase transition sequences at a representative 900 °C oxidation temperature: hcp Zr recrystallizes rapidly and then transforms into bcc Zr while reaching the α+β transus, followed by speedy appearance of textured tetragonal (t-)ZrO2, monoclinic (m-)ZrO2 and ZrN that consume oxygen and nitrogen. Continuous air ingress favors t-ZrO2 and m-ZrO2 dominance, accompanied by re-oxidation of ZrN into t-ZrO2 due to its low thermodynamic stability revealed by experiment-informed DFT calculation and low oxygen activity at the oxide-metal interface. Ensemble-averaged lattice volume expansions during phase transitions have been quantified. This expansion-induced compressive stress promotes the presence of a significant fraction of t-ZrO2 at elevated temperature that eventually transforms into m-ZrO2 during cooling.

Size-Dependent High-Pressure Behavior of Pure and Eu3+-Doped Y2O3 Nanoparticles: Insights from Experimental and Theoretical Investigations.

We report a joint high-pressure experimental and theoretical study of the structural, vibrational, and photoluminescent properties of pure and Eu3+-doped cubic Y2O3 nanoparticles with two very different average particle sizes. We compare the results of synchrotron X-ray diffraction, Raman scattering, and photoluminescence measurements in nanoparticles with ab initio density-functional simulations in bulk material with the aim to understand the influence of the average particle size on the properties of pure and doped Y2O3 nanoparticles under compression. We observe that the high-pressure phase behavior of Y2O3 nanoparticles depends on the average particle size, but in a different way to that previously reported. Nanoparticles with an average particle size of ~37 nm show the same pressure-induced phase transition sequence on upstroke and downstroke as the bulk sample; however, nanoparticles with an average particle size of ~6 nm undergo an irreversible pressure-induced amorphization above 16 GPa that is completed above 24 GPa. On downstroke, 6 nm nanoparticles likely consist of an amorphous phase.

Synthesis and Characterization of the Electrical and Energy Storage Properties of New Barium Titanate - Europium Titanate Solid Solution

In the present work, the investigation of the structural, dielectric, ferroelectric and energy storage properties of polycrystalline samples of (1-x)BaTiO3 – xEuTiO3 (BT-ET) solid solution materials prepared by conventional solid-state reaction method has been carried out. X-ray diffraction analyses have revealed the formation of a single-phase tetragonal structure with the P4mm space group. Furthermore, the temperature dependence of the dielectric constants marked the presence of the following sequence of structural phase transitions, namely from rhombohedral to orthorhombic, from orthorhombic to tetragonal and from tetragonal to cubic phase transitions with the increase of temperature. A diffuse phase transition has been observed from the dielectric permittivity peak and has been analyzed using the modified Curie-Weiss law and the degree of diffuseness is found to be in the range of 1.16 - 1.76 due to the existence of different states of polarization. A phase diagram has been established based on the temperature-dependent dielectric permittivity studies. The room temperature ferroelectric properties point to a coercive field larger than that of BaTiO3 ceramics. Broad dielectric peaks associated with diffuse phase transition appear in a wide temperature range (40 - 100 °C) with the highest dielectric permittivity ε’ = 6498 found in the x = 0.05 ceramics. Moreover, an improved energy storage performance is obtained in the 0.95BaTiO3-0.05EuTiO3 ceramic with a recoverable energy density of Wrec = 0.797 J/cm3, coupled with an efficiency η = 73% at 170 kV/cm and the highest storage energy density of Wst = 1.571 J/cm3 is obtained for 0.90BaTiO3-0.10EuTiO3, which is superior to other lead-free BT-based ceramics. All these features demonstrate that the BT-ET ceramics can be widely used in energy storage applications and also in high-temperature multilayer capacitors for automotive, aerospace and related industries.

Raman spectroscopic insights into the transition to the post-post-perovskite phase in NaZnF3

The phase transition sequence of perovskite materials is an important subject in numerous fields ranging from geophysics to material science. The fluoride NaZnF3, adopting the distorted perovskite (Pv) structure (GdFeO3-type), is a model system for studying such transitions. Up to date compression experiments on this compound, limited to pressures of 40 GPa, have identified a perovskite to post-perovskite (pPv, CaIrO3-type structure) transition to occur above 11 GPa followed by yet another transition commencing at 25 GPa and leading to a post-post-perovskite (ppPv) phase whose structure is yet to be identified. Here we present high-pressure Raman spectroscopic study aided by Density Functional Theory (DFT) calculations to explore the phase transition sequence of NaZnF3 at pressures reaching 70 GPa. We confirm that NaZnF3 undergoes a gradual phase transition between the GdFeO3 and CaIrO3 phases starting at 12 GPa. Our results also indicate that above 31 GPa, the CaIrO3 polymorph slowly transforms to the ppPv phase which we unambiguously confirm to adopt the metastable Sb2S3-type structure despite the ground state structure above 23 GPa being La2S3. The transition into Sb2S3-type structure type instead of the La2S3 polytype is most probably due to kinetic effects which also hinder the predicted decomposition of NaZnF3.

Sequence of Structural and Magnetic Phase Transitions in (NO)Mn6(NO3)13.

New nitrosonium manganese(II) nitrate, (NO)Mn6(NO3)13, has been synthesized and structurally characterized. In the temperature range of 45-298 K, the crystal is hexagonal (centrosymmetric sp. gr. P63/m). Mn2+ ions are assembled into tubes along axis c with both NO3- filling and coating. The nitrosonium cation is located in the framework cavity and is disordered by a 3-fold axis. At the temperature TS1 = 190 K, a structural phase transition related to the libration of the intertube NO3 group and a small variation of Mn polyhedron is observed. Moreover, the anomalies in physical properties of (NO)Mn6(NO3)13 allow suggesting that ordering of NO+ units occurs at low temperatures. The antiferromagnetic ordering in this compound is preceded by the formation of a short-range correlation regime at about 25 K and takes place in two steps at TN1 = 12.0 K and TN2 = 8.4 K.

Phase transitions of choline dihydrogen phosphate: A vibrational spectroscopy and periodic DFT study.

Choline dihydrogen phosphate, [Chol][H2PO4], is a proton-conducting ionic plastic crystal exhibiting a complicated sequence of phase transitions. Here, we address the argument in the literature around the thermal properties of [Chol][H2PO4] using Raman and infrared microspectroscopy. The known structure of the low-temperature crystal, which contains the anti-conformer of [Chol]+ and hydrogen-bonded dimers of anions, was used to do periodic density functional theory calculations of the vibrational frequencies. Raman spectra indicate that the solid-solid transition at 20 °C is linked to a conformational change to the gauche [Chol] conformer with a concurrent local rearrangement of the anions. The distinct bands of lattice modes in the low-frequency range of the Raman spectra vanish at the 20 °C transition. Given the ease with which metastable crystals can be produced, Raman mappings demonstrate that a sample of [Chol][H2PO4] at ambient temperature can contain a combination of anti- and gauche conformers. Heating to 120 °C causes continuous changes in the local environment of anions rather than melting as suggested by a recent calorimetric investigation of [Chol][H2PO4]. The monotonic change in vibrational spectra is consistent with earlier observations of a very small entropy of fusion and no abrupt jump in the temperature dependence of ionic conductivity along the phase transitions of [Chol][H2PO4].

Crystal structure analysis of the adduct 1:1 hexamethylenetetramine and succinic acid (HMT−C4) as a function of the temperature

Co−crystals of hexamethylenetetramine [N4(CH2)6, (HMT)] and dicarboxylic acids of general formula HOOC(CH2)n−2COOH with 4 ≤ n ≤ 14 (Cn), called HMT−Cn, exhibits structural properties strongly dependent on the length of the dicarboxylic acid chain, showing a wide variety of thermotropic phase transition sequence. This study focuses on describing the structural features of the HMT−C4 (1:1) adduct, a co-crystal of HMT and succinic acid (C4), exhibiting a reversible first-order phase transition at 222(4)K, as evidenced by differential scanning calorimetry (DSC) experiments. In contrast to a prior investigations , which did not identify this phase transition, our comprehensive temperature-dependent analysis, incorporating single crystal X-ray diffraction and DSC experiments, reveals a reversible structural change in the HMT−C4 (1:1) adduct structure upon cooling and heating. The crystal structures of the different observed phases are discussed in detail, emphasizing the role of O−H···N intermolecular hydrogen bonds in maintaining the connectivity between HMT and C4 molecules. The observed phase transition involves doubling the unit cell volume and significant reorientation of the −COOH groups at the end of the C4 chain at around 222(4)K. Our findings underscore the importance of additional temperature-dependent experiments, such as DSC, in avoiding erroneous structure modeling due to symmetry changes during temperature variations. The preservation of the acidic character of the C4 dicarboxylic chain throughout the temperature range adds additional insights to the understanding of the structural behavior of all the HMT−Cn (1:1) adducts.

Compressed Sr superconducting transition temperature up to 17.65 K.

Due to the simplicity of their composition, the study of the superconducting properties of elemental substances holds significant importance for understanding the mechanisms of high-temperature superconductivity. This work involves simulated calculations to investigate the phase transition sequence and superconducting properties of Sr under pressure. The stability range of the Sr-IV phase C2/c was redefined, determining that it can extend up to 150 GPa, and the phase transition sequence of Sr under high pressure was studied. It was discovered that the d-electrons in the Sr-IV phase significantly contribute to the Fermi surface, a phenomenon closely related to the Van Hove singularity (VHS) near the saddle points. The increase in Tc of Sr under pressure is attributed to phonon softening and strong coupling resulting from the gradual overlap of VHS with the Fermi level, which is associated with the incomplete saturation of s-d electron transfer. Ultimately, the Tc of Sr reaches 17.65 K at 150 GPa, with a λ value of 1.26. This strong EPC is contributed by the interaction between d-electrons and medium-high-frequency phonons. This study extends new pathways for investigating the superconductivity of high-pressure phases of Sr and provides new insights for the theoretical study of elemental superconductors.

Modeling ferroelectric phase transitions with graph convolutional neural networks

Ferroelectric materials are widely used in functional devices, however, it has been a long-standing issue to achieve convenient and accurate theoretical modeling of them. Herein, a noval approach to modeling ferroelectric materials is proposed by using graph convolutional neural networks (GCNs). In this approach, the potential energy surface of ferroelectric materials is described by GCNs, which then serves as a calculator to conduct large-scale molecular dynamics simulations. Given atomic positions, the well-trained GCN model can provide accurate predictions of the potential energy and atomic forces, with an accuracy reaching up to 1 meV per atom. The accuracy of GCNs is comparable to that of <i>ab inito</i> calculations, while the computing speed is faster than that of <i>ab inito</i> calculations by a few orders. Benefiting from the high accuracy and fast prediction of the GCN model, we further combine it with molecular dynamics simulations to investigate two representative ferroelectric materials—bulk GeTe and CsSnI<sub>3</sub>, and successfully produce their temperature-dependent structural phase transitions, which are in good agreement with the experimental observations. For GeTe, we observe an unusual negative thermal expansion around the region of its ferroelectric phase transition, which has been reported in previous experiments. For CsSnI<sub>3</sub>, we correctly obtain the octahedron tilting patterns associated with its phase transition sequence. These results demonstrate the accuracy and reliability of GCNs in the modeling of potential energy surfaces for ferroelectric materials, thus providing a universal approach for investigating them theoretically.

Kinetics of hydrogen absorption/desorption in the Zr-2.5Nb alloy.

The Zr-2.5Nb alloy is a typical pressure tube material in heavy water nuclear reactors, and an increase of hydrogen isotope content in the alloy during service can pose major safety risks; hot vacuum extraction-mass spectrometry is an efficient method for evaluating hydrogen isotope concentrations in the Zr-2.5Nb alloy. This work investigates the kinetics and thermodynamic properties of deuterium (D) absorption and desorption of the Zr-2.5Nb alloy using the constant volume adsorption method and the hot vacuum extraction method. In addition to the previously reported volume contraction model, it was observed that at 600 °C and above, the reaction between D2 and Zr-2.5Nb is dominated by diffusion, while the reaction is predominantly influenced by surface adsorption and dissociation below 600 °C. Phase transition sequence of Zr-2.5Nb deuterides during non-isothermal desorption was established using quantitatively calibrated thermal desorption spectra combined with the phase transition process of deuteride decomposition. These results can provide important references for optimizing the process parameters of the hot vacuum extraction-mass spectrometry method.