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Low Temperature Thermodynamic Study of the Stable and Metastable Phases of Silver
LOW TEMPERATURE THERMODYNAMIC STUDY OF THE STABLE AND METASTABLE PHASES OF SILVER
XB Li,YQ Xie,YZ Nie,HJ Peng,
X.B.,Li,Y.Q.,Xie,Y.Z.,Nie,H.J.,Peng
金属学报(英文版) , 2007,
Abstract: The low temperature thermodynamics of the stable phase of silver was assessed by the polynomial model and Debye model from the experimental data available in the literature. By means of the constrained non-linear least squares curve fitting arithmetic, two sets of parameter values were determined. The expression of the thermodynamic functions cp(T) and G(T)-H (298.15K) at 0-298.15K were presented. Only the low temperature thermodynamics of the metastable phase of silver can be extrapolated by the Debye model. The expression of the thermodynamic functions cp(T) at 0-298.15K was presented.
Stable and metastable phases in the atomic limit of the extended Hubbard model with intersite density-density interactions  [PDF]
Konrad Kapcia,Stanis？aw Robaszkiewicz
Physics , 2011,
Abstract: We have studied a simple effective model of charge ordered insulators. The tight binding Hamiltonian consists of the effective on-site interaction U and the intersite density-density interaction Wij (both: nearest-neighbor and next-nearest-neighbor). In the analysis of the phase diagrams and thermodynamic properties of this model we have adopted the variational approach, which treats the on-site interaction term exactly and the intersite interactions within the mean-field approximation. Our investigations of the general case (as a function of the electron concentration) have shown that the system exhibits various critical behaviors including among others bicritical, tricritical, critical-end and isolated critical points. In this report we concentrate on the metastable phases and transitions between them. One finds that the first- and second order transitions between metastable phases can exist in the system. These transitions occur in the neighborhood of first as well as second order transitions between stable phases. For the case of on-site attraction the regions of metastable homogeneous phases occurrence inside the ranges of phase separated states stability have been also determined.
Nucleation of a stable solid from melt in the presence of multiple metastable intermediate phases: Wetting, Ostwald step rule and vanishing polymorphs  [PDF]
Mantu Santra,Rakesh S. Singh,Biman Bagchi
Physics , 2013,
Abstract: In many systems, nucleation of a stable solid may occur in the presence of other (often more than one) metastable phases. These may be polymorphic solids or even liquid phases. In such cases, nucleation of the solid phase from the melt may be facilitated by the metastable phase because the latter can "wet" the interface between the parent and the daughter phases, even though there may be no signature of the existence of metastable phase in the thermodynamic properties of the parent liquid and the stable solid phase. Straightforward application of classical nucleation theory (CNT) is flawed here as it overestimates the nucleation barrier since surface tension is overestimated (by neglecting the metastable phases of intermediate order) while the thermodynamic free energy gap between daughter and parent phases remains unchanged. In this work we discuss a density functional theory (DFT) based statistical mechanical approach to explore and quantify such facilitation. We construct a simple order parameter dependent free energy surface that we then use in DFT to calculate (i) the order parameter profile, (ii) the overall nucleation free energy barrier and (iii) the surface tension between the parent liquid and the metastable solid and also parent liquid and stable solid phases. The theory indeed finds that the nucleation free energy barrier can decrease significantly in the presence of wetting. This approach can provide a microscopic explanation of Ostwald step rule and the well-known phenomenon of "disappearing polymorphs" that depends on temperature and other thermodynamic conditions. Theory reveals a diverse scenario for phase transformation kinetics some of which may be explored via modern nanoscopic synthetic methods.
Thermodynamic phase-field model for microstructure with multiple components and phases: the possibility of metastable phases  [PDF]
Daniel A. Cogswell,W. Craig Carter
Physics , 2010, DOI: 10.1103/PhysRevE.83.061602
Abstract: A diffuse-interface model for microstructure with an arbitrary number of components and phases was developed from basic thermodynamic and kinetic principles and formalized within a variational framework. The model includes a composition gradient energy to capture solute trapping, and is therefore suited for studying phenomena where the width of the interface plays an important role. Derivation of the inhomogeneous free energy functional from a Taylor expansion of homogeneous free energy reveals how the interfacial properties of each component and phase may be specified under a mass constraint. A diffusion potential for components was defined away from the dilute solution limit, and a multi-obstacle barrier function was used to constrain phase fractions. The model was used to simulate solidification via nucleation, premelting at phase boundaries and triple junctions, the intrinsic instability of small particles, and solutal melting resulting from differing diffusivities in solid and liquid. The shape of metastable free energy surfaces is found to play an important role in microstructure evolution and may explain why some systems premelt at phase boundaries and phase triple junctions while others do not.
Diffusive Evolution of Stable and Metastable Phases I: Local Dynamics of Interfaces  [PDF]
R. M. L. Evans,M. E. Cates
Physics , 1997, DOI: 10.1103/PhysRevE.56.5738
Abstract: We find analytical solutions to the Cahn-Hilliard equation for the dynamics of an interface in a system with a conserved order parameter (Model B). We show that, although steady-state solutions of Model B are unphysical in the far-field, they shed light on the local dynamics of an interface. Exact solutions are given for a particular class of order-parameter potentials, and an expandable integral equation is derived for the general case. As well as revealing some generic properties of interfaces moving under condensation or evaporation, the formalism is used to investigate two distinct modes of interface propagation in systems with a metastable potential well. Given a sufficient transient increase in the flux of material onto a condensation nucleus, the normal motion of the interface can be disrupted by interfacial unbinding, leading to growth of a macroscopic amount of a metastable phase.
Nucleation and Formation of Stable and Metastable Phases in Undercooled AI-Li Melts
Shenglong DAI Guifu YU Minggao YAN Institute of Aeronautical Materials,Beijing,China,
材料科学技术学报 , 1993,
Abstract: A modified nucleation and competitive growth kinetic theory considering unsteady nucleation andeffect of composition in dilute binary alloys was suggested,based on which the selections of δ.δ'and α phases in undercooled Al-Li melts were summarized by both isothermal and continuous cool-ing treatment methods.
Metastable phases and "metastable" phase diagrams  [PDF]
V. V. Brazhkin
Physics , 2006, DOI: 10.1088/0953-8984/18/42/010
Abstract: The work discusses specifics of phase transitions for metastable states of substances. The objects of condensed media physics are primarily equilibrium states of substances with metastable phases viewed as an exception, while the overwhelming majority of organic substances investigated in chemistry are metastable. It turns out that at normal pressure many of simple molecular compounds based on light elements (these include: most hydrocarbons; nitrogen oxides, hydrates, and carbides; carbon oxide (CO); alcohols, glycerin etc) are metastable substances too, i.e. they do not match the Gibbs' free energy minimum for a given chemical composition. At moderate temperatures and pressures, the phase transitions for given metastable phases throughout the entire experimentally accessible time range are reversible with the equilibrium thermodynamics laws obeyed. At sufficiently high pressures (1-10 GPa), most of molecular phases irreversibly transform to more energy efficient polymerized phases, both stable and metastable. These transformations are not consistent with the equality of the Gibbs' free energies between the phases before and after the transition, i.e. they are not phase transitions in "classical" meaning. The resulting polymeric phases at normal pressure can exist at temperatures above the melting one for the initial metastable molecular phase. Striking examples of such polymers are polyethylene and a polymerized modification of CO. Many of energy-intermediate polymeric phases can apparently be synthesized by the "classical" chemistry techniques at normal pressure.
Diffusive Evolution of Stable and Metastable Phases II: Theory of Non-Equilibrium Behaviour in Colloid-Polymer Mixtures  [PDF]
R. M. L. Evans,W. C. K. Poon
Physics , 1997, DOI: 10.1103/PhysRevE.56.5748
Abstract: By analytically solving some simple models of phase-ordering kinetics, we suggest a mechanism for the onset of non-equilibrium behaviour in colloid-polymer mixtures. These mixtures can function as models of atomic systems; their physics therefore impinges on many areas of thermodynamics and phase-ordering. An exact solution is found for the motion of a single, planar interface separating a growing phase of uniform high density from a supersaturated low density phase, whose diffusive depletion drives the interfacial motion. In addition, an approximate solution is found for the one-dimensional evolution of two interfaces, separated by a slab of a metastable phase at intermediate density. The theory predicts a critical supersaturation of the low-density phase, above which the two interfaces become unbound and the metastable phase grows ad infinitum. The growth of the stable phase is suppressed in this regime.
Electronic structure and physical properties of stable and metastable phases in YN: density-functional theory calculations
PengFei Guan,ChongYu Wang,Tao Yu
Chinese Science Bulletin , 2008, DOI: 10.1007/s11434-008-0414-2
Abstract: Local density functional is investigated using the full-potential linearized augmented plane wave (FP-LAPW) method for YN in the hexagonal structure and the rocksalt structure and for hexagonal structures linking a layered hexagonal phase with wurtzite structure along a homogeneous strain transition path. It is found that the wurtzite YN is unstable and the layered hexagonal phase, labeled as h o, in which atoms are approximately fivefold coordinated, is metastable, and the rocksalt ScN is stable. The electronic structure, the physical properties of the intermediate structures and the energy band structure along the transition are presented. It is noticeable that the study of ScN provides an opportunity to apply this kind of material (in wurtzite[h]-derived phase).
Unconventional phase selection in high-driven systems: A complex metastable structure prevails over simple stable phases  [PDF]
Zhuo Ye,Feng Zhang,Yang Sun,Manh Cuong Nguyen,Mikhail I. Mendelev,Ryan T. Ott,Eun Soo Park,Matthew Besser,Matthew J. Kramer,Ze-Jun Ding,Cai-Zhuang Wang,Kai-Ming Ho
Physics , 2015,
Abstract: Phase selection in deeply undercooled liquids and devitrified glasses during heating involves complex interplay between the barriers to nucleation and the ability for these nuclei to grow. During the devitrification of glassy alloys, complicated metastable structures often precipitate instead of simpler, more stable compounds. Here, we access this unconventional type of phase selections by investigating an Al-10%Sm system, where a complicated cubic structure first precipitates with a large lattice parameter of 1.4 nm. We not only solve the structure of this "big cubic" phase containing ~140 atoms but establish an explicit interconnection between the structural orderings of the amorphous alloy and the cubic phase, which provides a low-barrier nucleation pathway at low temperatures. The surprising rapid growth of the crystal is attributed to its high tolerance to point defects, which minimize the short-scale atomic rearrangements to form the crystal. Our study suggests a new scenario of devitrification, where phase transformation proceeds initially without partitioning through a complex intermediate crystal phase.
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