Metastable Lattice Research Articles

Metastable lattice of droplets in phase separating polymer blends.

Phase separation in a polymer mixture with an off-critical composition is described by a Ginzburg-Landau Hamiltonian that contains both cubic and quartic terms in the deviation of composition from its mean value in the homogeneous phase. Our analysis suggests that when a blend is brought in the vicinity of the spinodal, the initial homogeneous phase becomes unstable against the formation of a metastable lattice of spherical droplets whose lifetime diverges in the limit of infinite molecular weight. The composition of the droplets approaches that of the background phase and their size diverges with the approach to the critical point, but the composition contrast is enhanced and droplet radii become comparable to polymer dimensions, away from criticality. The connection between our predictions and the results of recent neutron scattering experiments is discussed, and new experiments that could probe the proposed droplet lattice are proposed.

Modulation of the nucleation rate preexponential in a low-temperature Ising system.

A metastable lattice gas with nearest-neighbor interactions and continuous-time dynamics is studied using a generalized Becker-Döring approach in the multidimensional space of cluster configurations. The preexponential of the metastable-state lifetime (inverse of nucleation rate) is found to exhibit distinct peaks at integer values of the inverse supersaturation. Peaks are unobservable (infinitely narrow) in the strict limit T-->0, but become detectable and eventually dominate at higher temperatures.

Hundred years of precipitation hardening

About 100 years ago, precipitation hardening was discovered and the first structural aluminum alloys with considerable strength were developed. Our present understanding of the phenomenon in based on the application of dislocation theory as well as reaction kinetics, especially the role of metastable phases and lattice defects in nucleation from supersaturated solids. The prerequisites for optimum precipitation hardening can be precisely defined. However, the progress in the development of new alloys has been slow in the recent years. The original alloy is still in use today.

Metastable Superstructures in RuSr2Gd1.4Ce0.6Cu2O10-δ?Superconductor Based on TEM Observation at Room Temperature

ABSTRACTThree types of metastable modulations in Ru-based superconductor RuSr2Gd1.4Ce0.6Cu2O10-δ are observed by electron diffraction at room temperature and reported in this paper. The modulations are sensitive to the irradiation of the electron beam and the sample storage time. Having the tetragonal symmetry (I4/mmm) with a=0.384nm, c=2.864nm, the structure of RuSr2Gd1.4Ce0.6Cu2O10-δ?resembles that of YBa2Cu3O7-δ by inserting a fluorite type Gd1.4Ce0.6O2 layer instead of the Y layer and Ru ions residing in the Cu(1) site. In this compound, superconductivity is confined to the CuO2 layer while magnetism stems from the RuO2 layer. The metastable modulations display the interaction between the two layers from the structural point of view. The results suggest that although, superconductivity and magnetism are decoupled from each other in the unconventional superconductor, the effect may be limited by metastable lattice modulation.

Equilibration and Dynamic Phase Transitions of a Driven Vortex Lattice

We report on the observation of two types of current driven transitions in metastable vortex lattices. The metastable states, which are missed in usual slow transport measurements, are detected with a fast transport technique in the vortex lattice of undoped 2H-NbSe2. The transitions are seen by following the evolution of these states when driven by a current. At low currents we observe an equilibration transition from a metastable to a stable state, followed by a dynamic crystallization transition at high currents.

The light-induced metastable lattice expansion in hydrogenated amorphous silicon

A new insight of the light induced metastable lattice volume change is presented for a-Si:H prepared by plasma enhanced chemical vapor deposition (PECVD). The laser optical-lever bending method is demonstrated for detecting a volume change of film semiconductors. The observed light-induced expansion is persistent after light soaking and is recovered by thermal annealing at 200°C for 2 h. The light exposure time dependence of the lattice expansion was the same dependence of the photoconductivity and light-induced defect density. A correlation between the light-induced expansion and the photolitic changes is obtained. The experimentally obtained light-induced expansions normalized by the initial volume, Δ V/ V, are the order of 10 −6–10 −5, which is two orders smaller than for chalcogenide glasses such as a-As 2S 3. The Δ V/ V has a maximum at deposition temperatures between 200°C and 300°C. The Δ V/ V is a maximum at hydrogen dilution ratio with silane gas, r=∼12, but the microcrystalline silicon at r=39 has a negligible small Δ V/ V. It is demonstrated that the light-induced expansion is from the structure of a-Si:H.

Density functional for hard sphere crystals: A fundamental measure approach

A new free energy density functional for hard spheres is presented, along the lines of the fundamental measure theory, which reproduces the Percus-Yevick equation of state and direct correlation function for the fluid, with a tensor weighted density. The functional, based on the zero-dimension limit, is exact for any one-dimensional density distribution of the spheres. The application to the hard sphere crystals gives excellent results, solving all of the qualitative problems of previous density functional approximations, including the unit cell anisotropy in the fcc lattice and the description of the metastable bcc lattice.

The Trace-Component Trapping Effect: Experimental Evidence, Theoretical Interpretation, and Geochemical Applications

Experimental data indicating increase of crystal-melt (fluid) partition coefficients in the range of microconcentrations of trace elements are reviewed and analyzed in detail. This concentration dependence of partition coefficients has been referred to as either deviations from Henry’s law or the trace-component trapping effect. A critical review of a variety of models proposed to explain this phenomenon is also given. It is shown that the most reasonable and developed of these models relate changes in trace element partition coefficient at low concentrations to interactions between the trace element ions and metastable lattice defects (i.e., linear and planar defects) at low temperatures or intrinsic point defects of thermal origin at higher temperatures. The mechanism of interaction between trace element substituent atoms and intrinsic defects is considered in detail, with particular consideration given to the creation of pair associates, coupled substitutions, and the influence of other impurities on the trace element dissolution. The models developed are fit to the available experimental data to provide descriptions of the dependence of partition coefficients on composition and to estimate the concentrations and free energies of formation of the intrinsic defects (i.e., vacancies and interstitial atoms) in a matrix crystal. Some probable geochemical applications and manifestations of the trapping effect are discussed. This leads to the conclusion that there is an urgent need for further consideration of the problem.

Photoinduced metastable lattice dilation of CdF2:In crystals

A novel technique for measuring differential lattice dilation by scanning tunneling microscope is proposed. The method was used to measure the metastable lattice dilation caused by a deep-shallow phototransformation of bistable In impurities in CdF2 crystals. A total linear crystal shrinkage of 1.8×10−6 results from a partial counterbalancing of the lattice contraction associated with the photoionized deep localized In state by a lattice expansion caused by weakly bound electrons at the shallower hydrogenlike state of the bistable In donors.

Early nitriding stage of evaporated-Ti thin films by N-ion implantation

The early growth stage of epitaxial titanium nitride (TiN) films, formed by implanting nitrogen ions (N2+) with 62 keV into 100-nm-thick evaporated-Ti films, was studied by transmission electron microscopy, Rutherford backscattering spectrometry and elastic recoil detection analysis. Evaporated-Ti films spontaneously absorb hydrogen (H) from the interior of the NaCl substrate, and then TiHx partially grows in addition to the hcp-Ti. The implantation of N into evaporated-Ti films expands the hcp-Ti lattice and reduces the H concentration in the evaporated-Ti film. The former induces the hcp–fcc transformation and then leads to the growth of (001)-oriented TiNy by the occupation of N in octahedral (O) sites in the fcc-Ti sublattice. The latter induces contraction of the fcc-Ti sublattice by the escape of H from (110)-oriented TiHx and then leads to the growth of (110)-oriented TiNy by the occupation in O sites of the H-escaped metastable fcc-Ti lattice by N. The nitriding mechanism of epitaxial Ti thin films is discussed.

The magnetic field and energy of an Abrikosov vortex in an anisotropic London superconductor

The behavior of a straight Abrikosov vortex in an anisotropic uniaxial London superconductor is studied. Analytical expressions are derived that approximately describe the magnetic field in three regions: the asymptotic region, where the distance r from the vortex line is greater than λΓ (λ is the London length and Γ is the anisotropy constant), the intermediate region λ<r<λΓ, and the region r<λ. It is found that in the intermediate region with high anisotropy the component of the magnetic field along the vortex line changes sign for a certain interval of angles between the vortex line and the anisotropy axis. Because of this the interaction of parallel vortices whose plane is parallel to the anisotropy axis has a minimum and a maximum. This means that numerous metastable vortex lattices can exist. Additional terms in the vortex self-energy are obtained, and although they are smaller than the leading logarithmic term, they display a different dependence on the angle between the vortex line and the anisotropy axis.

Dynamic instability of microtubules: Monte Carlo simulation and application to different types of microtubule lattice

Dynamic instability is the term used to describe the transition of an individual microtubule, apparently at random, between extended periods of slow growth and brief periods of rapid shortening. The typical sawtooth growth and shortening transition behavior has been successfully simulated numerically for the 13-protofilament microtubule A-lattice by a lateral cap model (Bayley, P. M., M. J. Schilstra, and S. R. Martin. 1990. J. Cell Sci. 95:33-48). This kinetic model is now extended systematically to other related lattice geometries, namely the 13-protofilament B-lattice and the 14-protofilament A-lattice, which contain structural "seams". The treatment requires the assignment of the free energies of specific protein-protein interactions in terms of the basic microtubule lattice. It is seen that dynamic instability is not restricted to the helically symmetric 13-protofilament A-lattice but is potentially a feature of all A- and B-lattices, irrespective of protofilament number. The advantages of this general energetic approach are that it allows a consistent treatment to be made for both ends of any microtubule lattice. Important features are the predominance of longitudinal interactions between tubulin molecules within the same protofilament and the implication of a relatively favorable interaction of tubulin-GDP with the growing microtubule end. For the three lattices specifically considered, the treatment predicts the dependence of the transition behavior upon tubulin concentration as a cooperative process, in good agreement with recent experimental observations. The model rationalizes the dynamic properties in terms of a metastable microtubule lattice of tubulin-GDP, stabilized by the kinetic process of tubulin-GTP addition. It provides a quantitative basis for the consideration of in vitro microtubule behaviour under both steady-state and non-steady-state conditions, for comparison with experimental data on the dilution-induced disassembly of microtubules. Similarly, the effects of small tubulin-binding molecules such as GDP and nonhydrolyzable GTP analogues are readily treated. An extension of the model allows a detailed quantitative examination of possible modes of substoichiometric action of a number of antimitotic drugs relevant to cancer chemotherapy.

Conformational transformations in the protein lattice of phage P22 procapsids

During the packaging of double-stranded DNA by bacterial viruses, the precursor procapsid loses its internal core of scaffolding protein and undergoes a substantial expansion to form the mature virion. Here we show that upon heating, purified P22 procapsids release their scaffolding protein subunits, and the coat protein lattice expands in the absence of any other cellular or viral components. Following these processes by differential scanning calorimetry revealed four different transitions that correlated with structural transitions in the coat protein shells. Exit of scaffolding protein from the procapsid occurred reversibly and just above physiological temperature. Expansion of the procapsid lattice, which was exothermic, occurred after the release of scaffolding protein. Partial denaturation of coat subunits within the intact shell structure was detected prior to the major endothermic event. This major endotherm occurred above 80 degrees C and represents particle breakage and irreversible coat protein denaturation. The results indicate that the coat subunits are designed to form a metastable precursor lattice, which appears to be separated from the mature lattice by a kinetic barrier.

Metastable layered chalcogenides 1T-MoS 2, 2M-WS 2 and [formula omitted]: Electrochemical study on their intercalation reactions

The formation of hydrated layered sulfides K x(H 2O) y[MoS 2] from the metastable host lattices 1T-MoS 2, 2M-WS 2 and 1 T-Mo 1 2 W 1 2 S 2 has been investigated by galvanostatic electrochemical reduction and by cyclovoltammetry. Essential results are (i) the formation of an unusually large number of intermediate phases, (ii) high resolution cyclovoltammograms of solid state processes, (iii) excellent reversibility of the reaction upon cycling. The appearance of four to five intermediate phases in a narrow stoichiometry range has been interpreted on grounds of structural instabilities and ordering processes in the interlayer electrolyte phase.

Magnetic flux penetration to the layered superconductors in tilted magnetic field

The authors consider the vortex penetration into layered superconductors in the framework of the anisotropic London theory. Possible structures of flux lattices are investigated and the possibility of different metastable vortex lattices is pointed out. The magnetization curves have a specific two-maxima shape and the absolute maximum may not correspond to HC1.

Metastable lattice stabilities for the elements

Lattice stabilities for the metastable FCC (Al), BCC (A2) and CPH (A3) allotropes of 43 elements have been evaluated. The results are based on (1) Assessed stable phase data; (2) Phase boundary extrapolations from binary alloy, and elemental pressure-temperature, phase diagrams; (3) A relationship between the entropy of fusion, crystal structure and melting point; (4) Stacking fault energies; (5) Periodic and group trends and (6) First principle electronic energy calculations. Qualitative trends proposed by previous thermochemical evaluations for the transition metals are to a large extent confirmed. However, the evaluated energy differences between the different crystal allotropes are substantially higher and can be closer in magnitude to those predicted by ab-initio electron energy calculations, although particular discrepancies, for example concerning Cr(FCC), still remain. Many of the changes proposed here arise from the reassessment of stable phase data, particularly with respect to recently measured heats of fusion of the high melting point elements.

Long-period crystal lattices in thin Bi-Mn alloy films prepared by successive and simultaneous vacuum depositions

Thin Bi-Mn alloy films were investigated by transmission high resolution electron microscopy and diffraction. The films were prepared both by successive and simultaneous vacuum deposition of Bi and Mn metals. Both types of film were then annealed slightly below the melting point of Bi (271°C) for 50 to 200 h. In the composite double layer films prepared by successive deposition, a long-period tetragonal lattice with a = 17.26 Å, c = 9.97 Å, two kinds of hexagonal lattices, one with a = 8.66 Å, c = 5.32 Å and the other with a = 8.54 Å, c = 15.91 Å, and a peculiar structure which may be a precursor stage to the tetragonal lattice were found. In the simultaneously deposited films, a metastable hexagonal lattice with a = 19.97 Å, c = 4.49 Å was produced as a crystallization product of non-crystalline films. It will thus be emphasized that many new long-period crystal lattices are formed by these two methods of thin alloy film preparation.

The synergetic phenomena in photoexcited semiconductors

The recombination-stimulated aftereffects are considered of metastable lattice defect creation as well as of defect-impurity complex structural transformations, which may result in the selfinhibition of their recombination activity. It is obtained that within similar semiconductors cooperative processes develop, resulting in the self-organization of an inhomogeneous spatialtemporal distribution of free carriers and defect-impurity complex concentration. As a consequence within the homogeneous bulk of an uniformly photoexcited semiconductor spontaneous dynamic potential barrier formation arises. It is established that photogenerated electron-hole pairs being separated with similar dynamic barriers results in the appearance of a Synergetic photovoltaic effect, i.e. direct or alternating photovoltage and photocurrent formation within the bulk of an uniformly photoexcited homogeneous semiconductor specimen. [Russian Text Ignored].

Stabilization of bcc Co via epitaxial growth on GaAs.

A new metastable phase of cobalt---body-centered cubic---has been synthesized by use of molecular-beam epitaxial growth to stabilize the crystal structure. Large (1\ifmmode\times\else\texttimes\fi{}1 ${\mathrm{cm}}^{2}$) films have been grown on (110) GaAs and characterized by high-energy electron diffraction, Auger analysis, x-ray diffraction, magnetometry, spin-polarized photoemission, and ferromagnetic resonance. The results indicate a ferromagnetic material very similar to $\ensuremath{\alpha}$-Fe both electronically and magnetically, with a metastable lattice constant ${a}_{0}=2.827$ \AA{}. This technique provides a general approach for the realizing of metastable phases.

Kinetics of β-phase formation in 1- O-alkylglycerols

The rate of β-phase formation in the ether lipids 1- O-alkylglycerols have been investigated at various temperatures. The concentrations of the phases vs. time in 1- O-hexadecylglycerol (C 16G) were measured using automatic X-ray powder diffraction peak area measurements. In 1- O-decylglycerol (C 16G) the rate was estimated using the heat evolved during the transition. At least two factors are important for the low transition rate. At higher temperatures the rate appears to be limited by a low probability of β crystallite formation (nucleation). As the temperature is decreased, crystallite formation probably increases. A second factor involves an activation of the metastable lattice. The activation process results in a lower rate at decreased temperatures. The two factors together give the highest transition rate at the α ↔ sub-α-phase transition temperature.