Energetics Of Phase Transition Research Articles

Energetics of phase transitions in homogeneous antiferromagnets

Spatially uniform static phases in an antiferromagnet (AFM) at 0 K accessible by varying an external magnetic field along the anisotropy axis are investigated. Using the macrospin model, the energy contributions are comprised of the external field, effective anisotropy, and spatially homogeneous AFM exchange. The critical energy configurations are fully cataloged, and local energy concavity is used to identify stable states. Relative energy levels are taken into account to classify phase transitions. Phase diagrams for energetic stability and phase transitions are provided in terms of the strength of the applied field and the ratio between anisotropy and the AFM exchange. Two nonstandard critical energy states are identified as energy saddle points so are not stable but function as energy barriers between multiple stable states. The results determine thermal switching rates and suggest interesting AFM textures and solitons.

Physically driven enhancement of the stability of Bi2O3-based ionic conductors via grain boundary engineering

Fast oxygen-ion conductors for use as electrolyte materials have been sought for energy conversion and storage. Bi2O3-based ionic conductors that exhibit the highest known oxygen-ion conductivities have received attention for use in next-generation solid electrolytes. However, at intermediate temperatures below ~600 °C, their conductivities degrade rapidly owing to a cubic-to-rhombohedral phase transformation. Here, we demonstrate that physical manipulation of the grain structure can be used to preserve the superior ionic conductivity of Bi2O3. To investigate the effects of microstructural control on stability, epitaxial and nanopolycrystalline model films of Er0.25Bi0.75O1.5 were fabricated by pulsed laser deposition. Interestingly, in situ impedance and ex situ XRD analyses showed that the grain boundary-free epitaxial film significantly improved the stability of the cubic phase, while severe degradation was observed in the conductivity of its polycrystalline counterpart. Consistently, the cation interdiffusion coefficient measured by the Boltzmann–Matano method was much lower for the epitaxial thin film compared to the polycrystalline thin film. Furthermore, first-principles calculations revealed that the presence of grain boundaries triggered the structural resemblance between cubic and rhombohedral phases, as evidenced by radial distribution functions. Additionally, phase transition energetics predicted that the thermodynamic stability of the cubic phase with respect to the rhombohedral counterpart is reduced near grain boundaries. Thus, these findings provide novel insights into the development of highly durable superionic conductors via microstructural engineering.

Nanoscale Control of Oxygen Defects and Metal–Insulator Transition in Epitaxial Vanadium Dioxides

Strongly correlated vanadium dioxide (VO2) is one of the most promising materials that exhibits a temperature-driven, metal-insulator transition (MIT) near room temperature. The ability to manipulate the MIT at nanoscale offers both insight into understanding the energetics of phase transition and a promising potential for nanoelectronic devices. In this work, we study nanoscale electrochemical modifications of the MIT in epitaxial VO2 thin films using a combined approach with scanning probe microscopy (SPM) and theoretical calculations. We find that applying electric voltages of different polarity through an SPM tip locally changes the contact potential difference and conductivity on the surface of VO2 by modulating the oxygen stoichiometry. We observed nearly 2 orders of magnitude change in resistance between positive and negative biased-tip written areas of the film, demonstrating the electric field modulated MIT behavior at the nanoscale. Density functional theory calculations, benchmarked against more accurate many-body quantum Monte Carlo calculations, provide information on the formation energetics of oxygen defects that can be further manipulated by strain. This study highlights the crucial role of oxygen vacancies in controlling the MIT in epitaxial VO2 thin films, useful for developing advanced electronic and iontronic devices.

Plane-wave pseudopotential implementation and performance of SCAN meta-GGA exchange-correlation functional for extended systems

We present the implementation and performance of the strongly constrained and appropriately normed, SCAN, meta-GGA exchange-correlation (XC) approximation in the planewave-pseudopotential (PW-PP) formalism using the Troullier-Martins pseudopotential scheme. We studied its performance by applying the PW-PP implementation to several practical applications of interest in condensed matter sciences: (a) crystalline silicon and germanium, (b) martensitic phase transition energetics of phosphorene, and (c) a single water molecule physisorption on a graphene sheet. Given the much-improved accuracy over the GGA functionals and its relatively low computational cost compared to hybrid XC functionals, the SCAN functional is highly promising for various practical applications of density functional theory calculations for condensed matter systems. At same time, the SCAN meta-GGA functional appears to require more careful attention to numerical details. The meta-GGA functional shows more significant dependence on the fast Fourier transform grid, which is used for evaluating the XC potential in real space in the PW-PP formalism, than other more conventional GGA functionals do. Additionally, using pseudopotentials that are generated at a different/lower level of XC approximation could introduce noticeable errors in calculating some properties such as phase transition energetics.

Laser-Induced Melting of Co-C Eutectic Cells as a New Research Tool

A new laser-based technique to examine heat transfer and energetics of phase transitions in metal–carbon fixed points and potentially to improve the quality of phase transitions in furnaces with poor uniformity is reported. Being reproducible below 0.1 K, metal–carbon fixed points are increasingly used as reference standards for the calibration of thermocouples and radiation thermometers. At NMIA, the Co–C eutectic point is used for the calibration of thermocouples, with the fixed point traceable to the International Temperature Scale (ITS-90) using radiation thermometry. For thermocouple use, these cells are deep inside a high-uniformity furnace, easily obtaining excellent melting plateaus. However, when used with radiation thermometers, the essential large viewing cone to the crucible restricts the furnace depth and introduces large heat losses from the front furnace zone, affecting the quality of the phase transition. Short \(\hbox {CO}_{2}\) laser bursts have been used to illuminate the cavity of a conventional Co–C fixed-point cell during various points in its melting phase transition. The laser is employed to partially melt the metal at the rear of the crucible providing a liquid–solid interface close to the region being observed by the reference pyrometer. As the laser power is known, a quantitative estimate of \((166 \pm 30) \hbox { J}{\cdot }\hbox {g}^{-1}\) can be made for the Co–C latent heat of fusion. Using a single laser pulse during a furnace-induced melt, a plateau up to 8 min is observed before the crucible resumes a characteristic conventional melt curve. Although this plateau is satisfyingly flat, well within 100 mK, it is observed that the plateau is laser energy dependent and elevates from the conventional melt “inflection-point” value.

Use of thermodynamic coupling between antibody–antigen binding and phospholipid acyl chain phase transition energetics to predict immunoliposome targeting affinity

Thermodynamic analysis of ligand-target binding has been a useful tool for dissecting the nature of the binding mechanism and, therefore, potentially can provide valuable information regarding the utility of targeted formulations. Based on a consistent coupling of antibody–antigen binding and gel–liquid crystal transition energetics observed for antibody–phosphatidylethanolamine (Ab–PE) conjugates, we hypothesized that the thermodynamic parameters and the affinity for antigen of the Ab–PE conjugates could be effectively predicted once the corresponding information for the unconjugated antibody is determined. This hypothesis has now been tested in nine different antibody-targeted echogenic liposome (ELIP) preparations, where antibody is conjugated to dipalmitoylphosphatidylethanolamine (DPPE) head groups through a thioether linkage. Predictions were satisfactory (affinity not significantly different from the population of values found) in five cases (55.6%), but the affinity of the unconjugated antibody was not significantly different from the population of values found in six cases (66.7%), indicating that the affinities of the conjugated antibody tended not to deviate appreciably from those of the free antibody. While knowledge of the affinities of free antibodies may be sufficient to judge their suitability as targeting agents, thermodynamic analysis may still provide valuable information regarding their usefulness for specific applications.

Structure/Property Relationships for the Thermotropic Behavior of Lysine-Based Amphiphiles: from Hexagonal to Smectic Phases

Amino acid-derived gemini surfactants arise as a potentially good alternative to the more conventional lipid and synthetic catanionic systems in view of their enhanced interfacial properties, increased chemical stability, and low toxicity. The presence of an amino acid as the polar headgroup allows toxicity reduction, with the simultaneous increase of biodegradability. For these compounds, the establishment of structure/function relationships from the assessment of their basic aggregation properties is therefore of the utmost interest, e.g., in the design of operative self-assembled systems (e.g., liposomes, nanotubes, etc). In this context, the study of the thermal phase behavior of the dry surfactants is a natural, straightforward first step, the more so as thermotropic liquid crystals are also relevant for practical applications. In this work, several lysine-based amphiphiles with a gemini-like configuration have been synthesized, with the amino acid side chain as the spacer group. The molecules are either esters (neutral, with C6-C12 even chains) or sodium carboxylates (anionic, with C6-C12 even chains). Upon increasing the temperature, different crystalline (cr) and liquid-crystalline (lc) phases have been detected and the corresponding thermodynamic and structural parameters determined by a combination of differential scanning calorimetry, polarizing light microscopy and small-angle X-ray scattering. The phase behavior of the amphiphiles is highly dependent on both the chain length and the presence of charge on the headgroup, with significant differences occurring within and between each group of molecules. The C6 and C8 esters form reverse hexagonal cr and lc phases, while C10 and C12 self-assemble into smectic cr and lc structures, with C10 showing also a reverse hexagonal lc phase prior to isotropization. All the carboxylate derivatives form smectic lc phases at high enough temperature prior to isotropization. The rationalization of the phase behavior and phase transition energetics of the compounds has been put forth on the basis of the intermolecular interactions at stake (van der Waals, H-bonding, electrostatic, and packing) and the molecular shape of the amphiphile.

Thermodynamic measurements of submilligram bulk samples using a membrane-based “calorimeter on a chip”

Calorimetry offers a direct measurement of thermodynamic properties of materials, including information on the energetics of phase transitions. Many materials can only be prepared in thin film or small crystal (submilligram) form, negating the use of traditional bulk techniques. The use of micromachined, membrane-based calorimeters for submilligram bulk samples is detailed here. Numerical simulations of the heat flow for this use have been performed. These simulations describe the limits to which this calorimetric technique can be applied to the realm of small crystals (1-1000 microg). Experimental results confirm the feasibility of this application over a temperature range from 2 to 300 K. Limits on sample thermal conductivity as it relates to the application of the lumped and distributed tau 2 models are explored. For a typical sample size, the simulations yield 2.5% absolute accuracy for the heat capacity of a sample with thermal conductivity as low as 2 x 10(-5) W/cm K at 20 K, assuming a strong thermal link to the device. Silver paint is used to attach (both thermally and physically) the small samples; its heat capacity and reproducibility are discussed. Measurements taken of a submilligram single crystal of cobalt oxide (CoO) compare favorably to the results of a bulk calorimetric technique on a larger sample.

Energetics of phase transitions in BaO through DFT calculations with norm-conserving pseudopotentials: LDA vs. GGA results

We present DFT calculations for barium monoxide (BaO) in its four different allotropic forms, in order to study the energetics of structural phase transitions in this system. We find that LDA is unable to determine the correct sequence of structural transitions, while GGA reproduces it correctly. Results for the energetics of the system are presented along with transition pressures at 0 K, in reasonable agreement with ultrasoft pseudopotential calculations. Our study shows that the use of GGA is fundamental for an adequate description of the energetics of phase transitions in BaO.

Phase transition energetics and thermodynamic properties of ferroelectric PbTiO3

Phase transition energetics and thermodynamic properties of ferroelectric PbTiO3

The energetics of phase transitions in the system Na 1+ XZr 2Si XP 3- XO 12, 1.9 ⩽ X ⩽ 2.5

The temperatures and enthalpies of phase transitions in the system Na 1+ X Zr 2Si XP 3- X O 12 with 1.9 ⩽ X ⩽ 2.5, have been determined by differential scanning calorimetry. Second-order phase transitions were observed in all samples except for X = 2.5. The transition temperatures decreased systematically with X, varying from approximately 423 K for X = 1.9 to 375 K for X = 2.3. The enthalpies of transition followed a similar trend, 2.11 kJ mol −1 for X = 1.9 and 0.52 kJ mol −1 for X = 2.3.

Transformation Kinetics for Crystalline, Quasicrystalline, and Glassy Alloys in the Binary Al100-xMnx and Pseudobinary Al84-ySiyMn16Systems

AbstractThe condensed state phase transition energetics and kinetics are reported for specific compositions within the Al-Si-Mn system which demonstrate crystal → crystal, quasicrystal→ crystal, and glass.→crystal structural transformations. Our observations, and their interpretation, for the solid state crystalline AI(Mn)→Al+Al6 Mn reaction and the crystallization of glassy Al54Si 30Mn16 are consistent with the anticipated mode for these transformations. The nature of the quasicrystal→ crystal transformation, consistent with our observations, is an enigma.