Structural Transition Temperature Research Articles

Exploring the Possibility of Thermally Assisted Creation and Annihilation of Anti‐Frenkel Defects in a Multiferroic Oxide for Tuning Interfacial Ferroelectricity

AbstractLattice defects such as oxygen vacancies, interstitials, and their complexes are present in crystalline oxide materials. In particular, anti‐Frenkel defects, which refer to charge‐neutral anion vacancy‐interstitial pairs, are strongly coupled with ferroelectric and dielectric properties as electric dipoles. However, in order to observe their macroscopic manifestation, delicate defect controls are required to the extent that electronic and ionic charges are almost completely suppressed. Here, the thermal cycle dependence of dielectric and piezoelectric properties is scrutinized in the strain‐driven morphotropic phase boundaries of multiferroic La‐substituted BiFeO3 thin films. Electrochemical impedance spectroscopy provides the Warburg feature that is considered evidence of the ionic origin. The observations are discussed based on anti‐Frenkel defects that are created or annihilated reversibly by thermal cycles through high‐temperature structural phase transition temperature or magnetic Néel temperature. The defect dipoles are spontaneously aligned by the flexoelectric effect in the phase boundaries inducing a metastable interfacial ferroelectric phase. The findings offer useful insight into defect dipoles.

Impact of the electrospinning synthesis route on the structural and electrocatalytic features of the LSCF (La0.6Sr0.4Co0.2Fe0.8O3–δ) perovskite for application in solid oxide fuel cells

In-house electrospun La0.6Sr0.4Co0.2Fe0.8O3–δ (LSCF) nanofibers have been tested through synchrotron x-ray diffraction and electrochemical impedance spectroscopy (EIS) in the 823–1173 K range, namely in the operating window of intermediate-temperature solid oxide fuel cells. Identical tests have been carried out on commercial LSCF powders, as a control sample. The results demonstrate that the electrospinning manufacturing procedure influences the crystalline properties of the perovskite. The rhombohedral structure (R), stable at room temperature, is retained by nanofibers throughout the whole temperature range, while a rhombohedral to cubic transition (R→C) is detected in powders at ⁓1023 K as a discontinuity in the rhombohedral angle α, accompanied by an abrupt change in oxygen occupation and microstrain. EIS data have a single trend in the nanofibers Arrhenius plot, and two different ones in powders, separated by a discontinuity at the structural transition temperature. Thus, a striking parallel is demonstrated between the variation with temperature of crystallographic features and electrochemical performance. Interestingly, this parallel is found for both nanofiber and granular electrodes. This opens up questions and new perspectives in attributing activation energies derived from EIS tests of LSCF materials to electrochemical processes and/or crystal structure variations.

Synthesis, crystal structure of Ca6Sb2O11 from experiments and DFT-electron structure calculation

This article reports a new perovskite compound Ca6Sb2O11. The X-ray powder diffraction technique was used to analyze its crystal structure. The results show that Ca6Sb2O11 has a tetragonal structure with the I4/m space group, and the lattice parameters are a = b = 5.658 (1) Å, c = 7.984 (1) Å, α = β = γ = 90°. It exhibits a typical A4B'2B″2X12-x anoxic multi-perovskite structure with rock-salt ordering of the B-site cations. The compound's chemical composition and valence states were characterized through scanning electron microscopy-energy dispersion spectrum (SEM-EDS) and X-ray photoelectron spectroscopy (XPS). The presence of oxygen vacancies was confirmed by electron paramagnetic resonance (EPR) spectroscopy. A chemical and structure transition temperature and the full melting temperature of Ca6Sb2O11 were measured, with values of 1515 ± 3 K and 1720 ± 3 K, respectively. The density functional theory (DFT)-electron structure calculation results show that Ca6Sb2O11 is an indirect wide-band gap (∼4.47 eV) semiconductor material, which agrees well with the experimental value (∼4.38 eV) obtained through diffuse reflection spectroscopy (DRS). Moreover, the top of the valence band and bottom of the conduction band are mainly contributed to by O-2p and Ca-3d states, respectively.

Structural phase transitions in multicomponent La0.2Nd0.2Sm0.2Gd0.2RE50.2NbO4 (RE5 = Ho, Y, Tb, Eu, Pr) oxides

AbstractIn this work, the influence of compositional complexity on the structural and thermal properties of multicomponent rare‐earth ortho‐niobates from the La0.2Nd0.2Sm0.2Gd0.2RE50.2NbO4 (RE5 = Ho, Y, Tb, Eu, Pr) series was investigated. Based on X‐ray powder diffraction studies using synchrotron radiation, it was found that all tested materials were pure single‐phase compositions and showed stability in the monoclinic I2/c crystal structure at room temperature. High‐temperature X‐ray powder diffraction studies and dilatometry studies confirmed the presence of a structural phase transition between low‐temperature (I2/c) and high‐temperature (I41/a) polymorphs. The structural phase transition temperatures are between 676°C and 701°C. Interestingly, despite their compositional complexity, the structural phase transition temperature behaves similarly to conventional ortho‐niobates, that is, it depends on the radius of the A‐cation; that is, as the ionic radius increases, the phase transition temperature decreases. The transition has been categorized as a second‐order phase transition based on the observed relationship between the Landau order parameter and spontaneous strain. The coexistence of the tetragonal and monoclinic phases has been seen in all compositions around the temperature of the structural phase transition. The presence of two orientation states in the monoclinic structure leads to the so‐called spontaneous strain, which consists of longitudinal (u) and shear (v) strain components. The values of these strains at 300°C range between 2.42 and 2.58 × 10−2 for longitudinal, 2.98 and 3.04 × 10−2 for shear, and 5.46 and 5.57 × 10−2 for scalar spontaneous strain. It was found that the spontaneous strain in each of the materials under test was very little impacted by the variation in the complexity of the A sublattice's composition. In addition, thermal expansion coefficients of both polymorphs were determined, which range from 12.7 × 10−6 K−1 to 13.2 × 10−6 K−1 for the monoclinic structure and 9.7 × 10−6 K−1 to 9.9 × 10−6 K−1 for the tetragonal one.

The effects of high-pressure annealing on magnetostructural transitions and magnetoresponsive properties in stoichiometric MnCoGe

In this study, phase transitions (structural and magnetic) and associated magnetocaloric properties of stoichiometric MnCoGe have been investigated as a function of annealing pressure. Metastable phases were generated by annealing at 800 °C followed by rapid cooling under pressures up to 6.0 GPa. The x-ray diffraction results reveal that the crystal cell volume of the metastable phases continuously decreases with increasing thermal processing pressure, leading to a decrease in the structural transition temperature. The magnetic and structural transitions merge and form a first-order magnetostructural transition between the ferromagnetic orthorhombic and paramagnetic hexagonal phases over a broad temperature range (>80 K) spanning room temperature, yielding considerable magnetic entropy changes. These findings demonstrate the utility of thermal processing under high pressure, i.e., high-pressure annealing, to control the magnetostructural transitions and associated magnetocaloric properties of MnCoGe without altering its chemical composition.

Fine tuning of Mn/Co vacancies for optimized magnetocaloric performance in MnCoGe alloys

As a family of rare-earth-free magnetocaloric materials, MnCoGe-based alloys have attracted intensive attention in the past few years. In this study, we reveal that the deficiency of either Mn or Co reduces the valence electron concentration of MnCoGe system, thereby lowering its structural transition temperature (Ttr) by pushing the Fermi level away from the Mn-Mn antibonding electronic states. Moreover, Ttr is found to be more sensitive to Co content. By systematically analyzing the temperature dependent magnetization plots and Arrott curves, it is proven that the magnetic and structural degrees of freedom are coupled in the Mn0.97CoGe alloy, resulting in a first-order magnetic phase transition. On the contrary, MnCo0.98Ge undergoes a second-order transition. As a consequence, the Mn0.97CoGe alloy exhibits a maximum magnetic entropy change of 27.6 J kg−1K−1 under a magnetic field of 5 T near the Curie temperature of 265 K, corresponding to a refrigeration capacity of 212.1 J kg−1. Our work demonstrates Mn/Co vacant MnCoGe alloy as a robust candidate of magnetic substance for room temperature magnetocaloric refrigeration.

Effect of isothermal aging on Poly(N-isopropylacrylamide): Correlation between chemical structure and phase transition temperature property

Poly(N-isopropylacrylamide) (PNIPAm) is a stimuli-responsive polymer with wide applications as a scaffold in biomedical applications. This study presents the PNIPAm's thermoresponsive behavior during storage and its stability in potential pharmaceutical formulations as a drug delivery excipient. Therefore, the turbidity curves of linear PNIPAm aqueous solutions are investigated before and after its thermal oxidation under air at 120°C and 130°C. First, PNIPAm aqueous phase separation diagram is determined for unaged polymer and then compared to thermally aged samples at two distinct points (1 and 10%(w/w)). After isothermal aging, the transition temperature has increased with a shift up to 4°C. The correlation between PNIPAm's chemical structure and phase transition temperature properties is illustrated in terms of chemical modifications, macromolecular changes as well as the polar/non-polar groups balance. It is observed that the water affinity and glass transition temperature (Tg) have increased at the macromolecular level. In addition, chains scissions are highlighted and seem to explain the shift of transition temperature. Chemical changes have revealed the presence of supplementary polar groups induced by aging. This multiscale study suggests a possible mechanism for the partial thermal oxidation based on all the observations.

Influence of Chemical Composition and Structure on the Cooperative Fluctuation in Supercooled Glass-Forming Liquids.

The kinetics of the glass transition and the characteristic size of the fluctuating spatio-temporal domains in supercooled glass-forming liquids, i.e., the Cooperatively Rearranging Regions (CRR), were measured upon cooling over a broad range of cooling rates using Differential Scanning Calorimetry (DSC) and chip-based Fast Scanning Calorimetry (FSC). The investigations were conducted on a selection of fragile glass formers (fragility indices between 80 and 140), with a large variance in the atomic or molecular structure but comparable thermal glass transition temperatures Tg, with the aim of evaluating the influence of chemical composition and structure on the CRR size and the associated temperature fluctuation. The selected materials are two polymers (poly(vinyl acetate) (PVAc), poly(lactic acid) (PLA)) as well as the simplest chalcogenide glass-former (selenium). It turned out that the CRR size plotted against the reduced temperature T/Tg follows the same trend, irrespective of the considered glass-former.

Graphene conductance modulation through controlled molecular release in a hybrid coordination polymer/graphene field-effect transistor

Coordination polymers, including metal-organic frameworks (MOFs), present a wide range of functionalities ranging from sensing to energy storage. Despite this, many coordination polymers are typically insulating which prevents their implementation into nanoscale electronic devices. Besides, the hybridization of MOFs with other conducting materials, like graphene, is rather unexplored. Here we introduce the grafting of a soft non-porous coordination polymer, the 1·2CH3CN, on a single-layer graphene to form a hybrid 1·2CH3CN/graphene field effect transistor. Electron transport measurements show that the sharp structural transformations in 1·2CH3CN, triggered at two specific temperatures, are detected in graphene as two sharp increments in the electrical current. Gate spectroscopy measurements show that the origin of the electrical modulation is an abrupt p-doping due to the sudden release of acetonitrile right at the structural transition temperatures. Reciprocally, the sensitivity of graphene to external perturbations allows to detect additional transformations in the coordination polymer not observed in direct transport measurements across crystals. Finally, the results have been reproduced in in-situ grown 1·2CH3CN nanoscale films on CVDG FETs. This simple method optimizes the contact area between graphene and 1·2CH3CN and facilitates the integration of coordination polymers with van der Waals materials and electronic devices. Further, a selective doping at specific temperatures could be obtained by modifying the nature of the interstitial molecule in 1.

The varying nature of the martensitic phase transition and associated entropy changes in a Co and Cr doped pseudo-ternary Heusler alloy system

We have investigated the magnetostructural phase transitions and the associated entropy changes in a system of three Heusler alloys, namely: Ni2Mn0.55Cr0.45Ga, Ni2Mn0.55Co0.10Cr0.35Ga, and Ni2.15Mn0.75Cr0.1Ga. All three alloys exhibited first-order martensitic phase transition. An inter-martensitic phase transition raised the magnetic and structural phase transition temperatures, and partially decoupled the said transitions. Furthermore, field magnetization measurements demonstrated that a significant reduction in magnetic hysteresis is achieved in the Ni2Mn0.55Co0.10Cr0.35Ga compound, resulting in improved effective refrigerant capacity values. The largest magnetic entropy change of −23.5 J/kg K and refrigeration capacity of 97.7 J/kg were observed in Ni2.15Mn0.75Cr0.1Ga. This observation of mitigated deficiencies in materials experiencing a first-order phase transition is relevant towards their application in magnetic refrigeration.

Structural Evolution and Mechanical Behavior of Ytterbia Doped Hafnia Biphasic Ceramics under Annealing at 1500 °C

HfO2 has become a promising thermal barrier coating material due to its similarity in structure and chemical properties with ZrO2 and its higher phase structure transition temperature. However, the fracture toughness of HfO2 is not ideal, greatly limiting its application. In this report, we find a special sandwich structure of ceramics, comprising a cubic (C) phase /monoclinic (M) phase/cubic (C) phase. The microstructural evolution and mechanical properties of these ceramics were investigated under annealing at 1500 °C. The results indicate that, with an increase in annealing duration, there was a gradual augmentation in the proportion of the monoclinic (M) phase and the fracture toughness increased from 2.18 MPa·m0.5 to 2.83 MPa·m0.5 after 48 h of annealing, which is higher than many potential TBC materials. The residual compressive stress present in the M phases during the progression of crack propagation served to facilitate the bridging and deflection of cracks. As such, this process led to the alleviation of stress concentration at the crack tip, ultimately enhancing the toughening effect.

Controlling phase transitions in MnNiGe using thermal quenching and hydrostatic pressure

Abstract The phase transitions in MnNiGe compounds were explored by manipulating the heat treatment conditions and through hydrostatic pressure application. As the quenching temperature increased, both the first-order martensitic structural transition temperatures and magnetic transition temperatures decreased relative to those in the slowly-cooled samples. When the samples were quenched from 1200 ℃, the first-order martensitic structural transition temperature lowered by more than 200 K. The structural transitions also shifted to lower temperature with the application of hydrostatic pressure during measurement. Temperature-dependent X-ray diffraction results reveal
that the changes of the cell parameters resulting from the structural transitions are nearly identical for all samples regardless of the extensive variation in their structural transition temperatures. In addition, neutron scattering measurements confirm the magnetic structure transition between simple and cycloidal spiral magnetic structures.

Synthesize and Characterization of Crystalline Structure and Phase Transition Temperature of Ce Substituted Ba- TiO3 Ceramics

Characterization and synthesize of Ba1-xCexTiO3 (x=0.01mol.) annealed at three different temperatures (1100oC, 1200 oC, 1300 oC) have been studied. For preparing powder of (x = 0.01 moles), Sol- Gel synthesis was carried out according to the stoichiometric conditions. The powder obtained palletized at 1.5 ton /mm2 pressure into a 10 mm dia. Pellet using PVA (polyvinyl alcohol) as binder. The pellets were then annealed at 1100oC for 1 hour. Then silver paste was applied on pellet by fully coating the one side and applying a single dot on other side, for making the pellet as capacitor. The XRD pattern reveals that the lattice parameter increases with the Ce doping and c/a ratio decreased which shows that tetragonality decreases. In all investigated samples it is assumed that the phase transition analysis, after doping Ce in BaTiO3 the Curie temperature decreases.

Microstructure and mechanical properties of phase change cloud concrete stone cementitious composites

In this study, a novel kind of phase-change cementitious composites with cloud concrete stone (CCS) as aggregate were prepared and studied. The CCS was synthesized with fly ash, slag, and other construction waste through water heat and autoclaving. The phase-change paraffin was absorbed into the CCS using the vacuum impregnation technique, and then the epoxy resin combined with cement powder was applied to the aggregate surface to prevent the leakage of phase-change materials (PCMs). The composite aggregate with phase change function was, thus, fabricated. A series of experiments were carried out to study and evaluate the molecular structure, physical phase, and phase transition temperature of encapsulated composite aggregate with the means of Fourier infrared spectroscopy (FT-IR), X-ray diffraction (XRD), and differential scanning calorimetry (DSC). The PCM-cement composites were prepared by replacing coarse aggregates with the composite aggregate in proportions of 25%, 50%, 75%, and 100% in volume. To explore the strength degradation in view of the microscopic perspective, scanning electron microscopy (SEM), XRD, and Image-Pro-Plus (IPP) were used. The experimental results revealed that the effect of the micropores of CCS on the melting temperature of the phase-change aggregates (24.84 °C) is higher than that of the paraffin (22.91 °C). The mechanical experiments results demonstrated that the compressive strength and splitting strength losses in the PCM-cement composite with replacement ratios of 75% were 24% and 39%, respectively. The XRD results revealed that with the incorporation of PCM, the calcium silicate hydrate (C-S-H) in the hydration products decreased and the Ca/Si value in C-S-H decreased from 9.05 to 2.13, which is one of the reasons for the strength decline of the PCM-cement composites. The cracks in the interfacial transition zone between the aggregate and matrix became large with increasing admixture, and the pore characteristics gradually became more apparent. The strength of the PCM-cement composite decreased with the increase in the average porosity, pore size, and perimeter of the matrix material.

A strategy for enhancing luminescence thermometry via Ta5+-triggered K0.5Na0.5NbO3: Er3+ transparent ceramics

Luminescence thermometry is a reliable approach for remote thermal sensing, and extensive studies have been devoted to designing a luminescence thermometer with heightened thermal sensitivity. Herein, we report a promising luminescence thermometric material, Ta5+-substituted K0.5Na0.5NbO3:0.003Er3+ transparent ferroelectric ceramics. The temperature sensing sensitivity is significantly improved by adjusting the concentration of Ta5+ in the material. Specifically, utilizing the fluorescence intensity ratio from the 2H11/2 and 4S3/2 thermally coupled states of Er3+ as a detecting signal within the temperature range of 273–543 K, an optimal maximum absolute sensitivity of 0.0058 K–1 and relative sensitivity of 0.0158 K–1 are achieved for K0.5Na0.5NbO3: 0.65Ta5+/0.003Er3+. Simultaneously, as the concentration of Ta5+ increase, a unique evolution of structural phase transitions is observed from orthorhombic to tetragonal and then to cubic. This is accompanied by an improvement in luminescence temperature sensing properties, and the best sensitivity is demonstrated in the cubic-phase region. Intriguingly, a huge change in infrared luminescence properties as a function of temperature is found around the structure transition temperature of the samples. These results indicate a promising potential for achieving highly sensitive thermometry or monitoring phase structure transitions through luminescence thermometry behavior in the K0.5Na0.5NbO3 host.

Study of Elastic and Structural Properties of BaFe2As2 Ultrathin Film Using Picosecond Ultrasonics.

We obtain the through-thickness elastic stiffness coefficient (C33) in nominal 9 nm and 60 nm BaFe2As2 (Ba-122) thin films by using picosecond ultrasonics. Particularly, we reveal the increase in elastic stiffness as film thickness decreases from bulk value down to 9 nm, which we attribute to the increase in intrinsic strain near the film-substrate interface. Our density functional theory (DFT) calculations reproduce the observed acoustic oscillation frequencies well. In addition, temperature dependence of longitudinal acoustic (LA) phonon mode frequency for 9 nm Ba-122 thin film is reported. The frequency change is attributed to the change in Ba-122 orthorhombicity (a-b)/(a+b). This conclusion can be corroborated by our previous ultrafast ellipticity measurements in 9 nm Ba-122 thin film, which exhibit strong temperature dependence and indicate the structural phase transition temperature Ts.

Effect of Ga Partial Substitution on the Structural and Magnetic Properties of Heusler Ni–Mn–Ga Alloys

Abstract—The effect of doping with Cu and Zn atoms on the magnetic and structural properties of the cubic and tetragonal phase of the Heusler Ni2Mn1.125Ga0.875 alloy has been investigated in the framework of density functional theory. Partial substitution of Ga atoms by Cu and Zn atoms leads to a decrease in the energy difference between the ferromagnetic and ferrimagnetic states of the cubic phase and an increase in the energy barrier between the cubic and tetragonal phase. The latter observation indirectly points to the increase in the structural transition temperature observed experimentally. Within the framework of calculations of exchange interaction parameters from first principles and Monte Carlo modeling of temperature dependences of magnetization, a phase T–x diagram is constructed. It is shown that the Curie temperature of the tetragonal phase decreases significantly compared to the Curie temperature of the cubic phase with increasing Cu and Zn content due to an increase in the inter-sublattice antiferromagnetic interaction between Mn atoms.

Entropy change reversibility in MnNi1−x Co x Ge0.97Al0.03 near the triple point

The nature of the phase transition has been studied in MnNi1−x Co x Ge0.97Al0.03 (x= 0.20–0.50) through magnetization, differential scanning calorimetry and x-ray diffraction measurements; and the associated reversibility in the magnetocaloric effect has been examined. A small amount of Al substitution for Ge can lower the structural phase transition temperature, resulting in a coupled first-order magnetostructural transition (MST) from a ferromagnetic orthorhombic to a paramagnetic hexagonal phase in MnNi1−x Co x Ge0.97Al0.03. Interestingly, a composition-dependent triple point (TP) has been detected in the studied system, where the first-order MST is split into an additional phase boundary at higher temperature with a second-order transition character. The critical-field-value of the field-induced MST decreases with increasing Co concentration and disappears at the TP (x= 0.37) resembling most field-sensitive MST among the studied compositions. An increase of the hexagonal lattice parameter a hex near the TP indicates a lattice softening associated with an enhancement of the vibrational amplitude in the Ni/Co site. The lattice softening leads to a larger field-induced structural entropy change (structural entropy change≫ magnetic entropy change, for this class of materials) with the application of a lower field, which results in a larger reversibility of the low-field entropy change (|ΔS rev| = 6.9 J kg−1 K for Δμ 0 H = 2 T) at the TP.

Effect of rapid solidification and post-processing on microstructure, magnetic and structural transition temperatures and magnetic properties in Ni50Mn29Ga21 magnetic shape-memory alloy

This study investigated the effect of rapid solidification and post-processing on microstructure, composition and magneto-structural properties of Ni50Mn29Ga21 (at.%) magnetic shape memory alloy (MSMA). Samples with varying cooling rates (1.1 ∙ 102 to 3.2 ∙ 103K/s) were prepared using arc melting for a comparative study in as-solidified condition and after two different heat treatments. A notable change from columnar-dendritic to cellular-dendritic solidification and a decrease in solidification length scale was observed at more rapid solidification. All samples showed Mn enrichment and corresponding Ga depletion at the sub-grain boundaries. The fine-scale solidification structure, chemical heterogeneity and lack of ordering resulted in broad martensitic transformations. At more rapid solidification, a decrease in saturation magnetization and abnormal slope changes in the hysteresis loop were observed. Heat-treatment resulted in grain growth, significant reduction in transformation breadth, and increase in magnetization saturation. Rapidly solidified samples with an estimated characteristic cooling rate of 1.1 ∙ 103K/s most closely resembled microstructures and magneto-structural properties from laser directed energy deposition (L-DED) processing. The results of this study help to understand cooling rate effects in L-DED additive manufactured Ni-Mn-Ga and provide post-processing routes that will result in microstructures and properties that resemble those of functional MSMA.

Ultrasonic investigation of the elastic properties of the approximant GdCd6

We have investigated complementarily the magneto-elastic coupling in the approximant GdCd6 by measuring the temperature and magnetic field dependence of the longitudinal elastic constant C11. A pronounced elastic softening toward the phase transition temperature Ts= 156 K was observed in the temperature dependence of the C11, suggesting the presence of a structural phase transition. Furthermore, anomalous elastic behavior was observed in the temperature dependence of the C11 below 20 K, closely related to the already known consecutive magnetic phase transitions. The H–T magnetic phase diagram deduced from elastic anomalies appearing at the different phase transitions are consistent with our previous data of the other independent elastic constants CE= (C11−C12)/2 and C44 for a cubic crystal. We argue the origin of the structural phase transition and low temperature elastic property including a possible origin of such a rich variety of magnetic phases.