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Showing papers on "Phase transition published in 1974"


Journal ArticleDOI
TL;DR: In this article, the physical properties of nematic, cholesteric, and smectic liquid crystals are discussed and a wide variety of phenomena in liquid crystals, including elastic distortions, disclinations, flow properties, fluctuations, light scattering, wave propagation, nuclear magnetic resonance, effects of magnetic and electric fields, electrohydrodynamics, and optical properties.
Abstract: This review discusses the physical properties of nematic, cholesteric, and smectic liquid crystals. Molecular theories of the liquid crystal phases are discussed and the molecular field theories of the phase transitions between the various liquid crystal phases are presented. The elastic theory and hydrodynamics of liquid crystals is developed. A wide variety of phenomena in liquid crystals, including elastic distortions, disclinations, flow properties, fluctuations, light scattering, wave propagation, nuclear magnetic resonance, effects of magnetic and electric fields, electrohydrodynamics, and optical properties, is discussed.

978 citations


Journal ArticleDOI
TL;DR: The superconducting phase transition is predicted to be weakly first order, because of effects of the intrinsic fluctuating magnetic field, according to a Wilson-Fisher $\ensuremath{\epsilon}$expansion analysis, as well as a generalized mean-field calculation appropriate to a type-I superconductor.
Abstract: The superconducting phase transition is predicted to be weakly first order, because of effects of the intrinsic fluctuating magnetic field, according to a Wilson-Fisher $\ensuremath{\epsilon}$-expansion analysis, as well as a generalized mean-field calculation appropriate to a type-I superconductor. Similar results hold for the phase transition from a smectic-$A$ to a nematic liquid crystal.

653 citations


Journal ArticleDOI
TL;DR: The effects of pH and mono- and divalent cations on the phase transition of lecithin, cephalin, phosphatidylserine, andosphatidic acid bilayers are studied to parallels current theories on nerve excitation and sensory transduction where cation-induced structural changes in biomembranes are invoked.
Abstract: Ordered → fluid phase transitions in bilayers of charged lipids are accompanied by a decrease in electrostatic free energy mainly as a result of bilayer expansion. For uniform charge distribution the Gouy-Chapman theory of the electrical double layer predicts a decrease of the transition temperature with increasing charge density. We studied the effects of pH and mono- and divalent cations on the phase transition of lecithin, cephalin, phosphatidylserine, and phosphatidic acid bilayers. Phosphatidic acid with two ionizable protons was selected for a systematic investigation. A change in pH from 7 to 9 increases the charge per polar group from one to two elementary charges. This lowers the transition temperature by about 20°C in agreement with the theory. In this pH region rather small changes in pH suffice to induce the phase transition at constant temperature. Divalent cations (Mg++ and Ca++) increase the transition temperature by charge neutralization and thus can be used to induce the fluid → ordered transition at constant temperature. In contrast, monovalent cations (Li+, Na+, K+) lower the transition temperature, or fluidize the bilayer structure at a given temperature. Rather small changes in ionic environment can induce gross alterations in bilayer structure; divalent and monovalent cations have antagonistic effects. This result parallels current theories on nerve excitation and sensory transduction where cation-induced structural changes in biomembranes are invoked.

499 citations


Journal ArticleDOI
TL;DR: Differential scanning calorimetry has been applied to the study of some simple model biomembranes and the endothermic phase transitions which they exhibit when transforming from the gel to liquid crystal phase have been studied, showing that clusters of gel and liquid crystalline lipids can coexist within this temperature range.

409 citations


Journal ArticleDOI
TL;DR: In this article, the phase transition temperature and Curie point decrease monotonically with increasing SrTiO3 concentration, and an intermediate antiferroelectric phase has been observed in the composition range of 0 ⩽ χ⩽ 0.5 of this system.
Abstract: Phase transitions in the solid solution Sr χ(Na 0.5Bi 0.5)l-χTiO3 have been studied by dielectric, pyroelectric and D-E characteristic measurements. An intermediate antiferroelectric phase has been observed in the composition range of 0 ⩽ χ ⩽ 0.5 of this system. These compositions undergo a ferroelectric-antiferroelectric-paraelectric sequence of transitions at elevated temperatures. The antiferroelectric phase transition temperature and Curie point decrease monotonically with increasing SrTiO3 concentration.

399 citations


Journal ArticleDOI
E. Pytte1
TL;DR: In this article, the fermion representation is used to describe the spin system, and the Hartree-Fock approximation is treated in the Fermi wave vector with the underlying lattice which have arisen in connection with the conventional Peierls transition.
Abstract: Dimerization phase transitions for $S=\frac{1}{2}$ linear antiferromagnetic chains is investigated by analogy with Peirels transitions in linear conducting chains. This analogy is very close when the fermion representation is used to describe the spin system, and the fermion interactions are treated in the Hartree-Fock approximation. Since the Fermi wave vector can be varied continuously by a magnetic field, the linear antiferromagnetic chain may be a convenient system for studying questions concerning. the commensurability of the Fermi wave vector with the underlying lattice which have arisen in connection with the conventional Peierls transition.

278 citations


Journal ArticleDOI
TL;DR: In this paper, the phase transition at 130°C is caused by condensation of the M 3 mode at the M point of the cubic Brillouin zone, while the one at 88°C results from condensing of the doubly degenerate R 25 -like mode (Z 9 mode) at the Z point of a tetragonal Brilloupin zone.
Abstract: Structural phase transitions in perovskite-type CsPbBr 3 have been investigated by neutron diffraction method. Phase transitions occur at 88°C and 130°C, which are respectively second and first order. The phase transition at 130°C is caused by condensation of the M 3 mode at the M point of the cubic Brillouin zone, while the one at 88°C results from condensation of the doubly degenerate R 25 -like mode ( Z 9 mode) at the Z point of the tetragonal Brillouin zone. Group theoretical considerations based on these results reveal that the crystal trans-forms from cubic perovskite structure (O h 1 - P m 3 m ) to tetragonal D 4h 5 - P 4/ m b m at 130°C and further to orthorhombic D 2h 16 - P m b n at 88°C. Possible atomic displacements induced at the phase transitions are obtained from the eigenvectors of the condensing modes.

204 citations


Journal ArticleDOI
TL;DR: In this article, an analogy between transitions in unstable systems and equilibrium phase transitions is developed in detail and the properties of a reacting system near an instability are investigated and relations between these critical exponents are defined for those properties of the unstable systems which are singular at the transition points.
Abstract: The properties of a reacting system near an instability are investigated and the analogy between transitions in unstable systems and equilibrium phase transitions is developed in detail. The set of macroscopic steady state rate equations plays the role of an equation of state. The bifurcation points of this set are analogous to transition and critical points of equilibrium phase transitions. Hard transitions of unstable systems correspond to first order and soft transitions to second and higher order phase transitions. Critical exponents are defined for those properties of the unstable systems which are singular at the transition points, and relations between these critical exponents are investigated. Critical fluctuations are studied with stochastic analogs of the macroscopic rate equations. Both master and Langevin equations are considered and lead to the following conclusions: When a transition or a critical point is approached (a) the amplitude of fluctuations grows; (b) the lifetime of these fluctuations becomes longer; and (c) the spatial correlation length increases. Our approximations are similar to those made in mean field theories of phase transitions and our results are thus ``classical.'' However the critical exponents are not necessarily numerically identical to the Landau‐Ginzburg exponents since they depend on the particular nonlinear system.

187 citations


Book ChapterDOI
TL;DR: In this paper, the authors studied the statistical mechanics of hydrogen dissolved in metals and proposed a model based on the assumption that the dominant attractive interaction between the protons in the metal is of an elastic nature.
Abstract: We study the statistical mechanics of hydrogen dissolved in metals. The underlying model is based on the assumption that the dominant attractive interaction between the protons in the metal is of an elastic nature. In the first part of the paper we review some general properties of the elastic interaction. We then discuss the importance of boundary conditions for the form of the elastic interaction, which turns out to be of the Curie-Weiss type with macroscopic range. In the second part we investigate the a-a' (‘gas-liquid’) phase transition in the hydrogen lattice fluid. The long-range part of the elastic interaction is treated in mean field approximation. In the canonical ensemble as opposed to the grand canonical ensemble one finds no co-existing phases near the critical point. Instead there is a continuous transition which changes into a first-order transition at tricritical points. In the temperature-density region which normally corresponds to the two-phase co-existence region the hydrogen ...

174 citations


Journal ArticleDOI
TL;DR: In this article, a simple geometrical model was proposed to model the atomic displacements at the transition from α-to β-quartz and the eigenvector of the soft mode at the Γ-point are related to a single (order-) parameter.

164 citations


Journal ArticleDOI
TL;DR: In this paper, the viscous properties of poly(para-benzamide) (PBA) solutions in dimethyl acetamide, which undergo a transition from an isotopic to an anisotropic (liquid-crystal) state at a definite concentration C*, were investigated.
Abstract: A study has been made of the viscous properties of poly(para-benzamide) (PBA) solutions in dimethyl acetamide, which undergo a transition from an isotopic to an anisotropic (liquid-crystal) state at a definite concentration C*. The polymer solutions behave in many respects (as regards the concentration and temperature dependence of viscosity, etc.) like solutions of low molecular weight compounds forming a liquid crystal phase, although the transitions are less pronounced in the polymer solutions owing to their polydispersity. It is shown that the viscometric method, being extremely sensitive to C*, is convenient for determining phase diagrams of anisotropic polymer solutions. The values of C* as related to the molecular weight of PBA have been determined, and a general criterion for transition from isotropic to anisotropic solutions established; the latter has the form (CM)* ≈ 1.3 × 105 at 20°C. This criterion is in line with the condition for the formation of the liquid-crystal structure in a dispersion of rodlike particles as proposed by Flory. Generalized concentration dependences of viscosity have been plotted by reducing concentration to C* and viscosity, to the maximum viscosity at the phase transition point. In investigating the flow properties of PBA solutions we revealed the existence of a yield point in the range of low shear stresses, and an intersection of the flow curves of solutions of different concentration at high shear stresses, which excludes a generalized representation of the flow curves in reduced ordinary-type coordinates.

Journal ArticleDOI
TL;DR: In this article, the diffuse, paraelectric/ferroelectric, phase transition of Potassium Strontium Niobate is discussed and several possible causes of the diffuse phase transition (DPT) are discussed.
Abstract: The diffuse, paraelectric/ferroelectric, phase transition of Potassium Strontium Niobate is discussed. It is found that the Curie point is strongly dependent on the K and Sr content. Several possible causes of the diffuse phase transition (DPT) are discussed. An empirical relationship describing the behaviour of the inverse dielectric constant in the transition region, is presented. A statistical analysis of the DPT is described in terms of microscopic composition variations within the crystal arising from non-stoichiometry. Plots of the polar dielectric constant and spontaneous polarization against temperature, derived from this model, compare well with the experimentally measured quantities.

Journal ArticleDOI
TL;DR: In this paper, the authors suggest that the interaction of interstitial cation defects with the strain field they induce is the main cause of the phase transition to the cation disordered state of the superionic conductor.
Abstract: We suggest that it is the interaction of interstitial cation defects with the strain field they induce which is primarily responsible for the phase transition to the cation disordered state of the superionic conductor. This idea is developed in terms of a simple model that yields an analytic solution for the phase transition and relates the necessary strength of the interaction to anomalously large free volumes of cation interstitial formation.

Journal ArticleDOI
TL;DR: In this paper, a generalized thermodynamic potential is defined for a horizontally unbounded liquid layer on both sides of and at the B\'enard point by using my potential and applying various approximations.
Abstract: Hydrodynamic fluctuations of a horizontal liquid layer heated from below are considered in the vicinity of the point where convection sets in because of buoyancy. It is assumed that convection occurs in the form of nearly two-dimensional rolls. Close to the instability, the hydrodynamics (described in the Boussinesq approximation) is simplified considerably by the appearance of a slow mode which dominates the motion of all hydrodynamic variables. It is described by a slowly varying complex amplitude whose absolute value and phase describe the strength and the position of the convection rolls, respectively. Generalizing previous work by several authors, an approximate equation of motion is derived, which is satisfied by the slow variable. New in this analysis is the inclusion of fluctuating terms, which leads to a Langevin equation. The fluctuations are shown to satisfy a detailed-balance principle. Consequently, a generalized thermodynamic potential can be defined, which was discussed briefly in an earlier paper. It depends as a functional on the slow variable, which thereby assumes the role of an order parameter of the transition. I give a further evaluation of the hydrodynamic fluctuations for a horizontally unbounded liquid layer on both sides of and at the B\'enard point by using my potential and applying various approximations. For strictly two-dimensional flows (i.e., independent of one horizontal coordinate) I calculate the time-independent steady-state properties (coherence lengths) without any further approximation by relying on published numerical data obtained for one-dimensional Ginzburg-Landau fields. Dynamic steady-state properties (coherence times) for that case and fluctuations in the three-dimensional case are calculated in a quasilinear approximation which reproduces the time-independent results for two-dimensional flows reasonably well. In the purely heat-conducting region my results contain some earlier results of Zaitsev and Shliomis in lowest order. Large and long-lived fluctuations of velocity and temperature are shown to appear at the critical wave number as the liquid is brought near the convection instability. They are due to the random appearance and disappearance of convection cells. Their size and lifetime at the B\'enard point are only limited by nonlinear coupling of the critical modes to other passive modes. In the heat-convection region, the coupling to passive modes stabilizes the amplitude of the convection cells; only slow fluctuations of the positions of the rolls remain (for unbounded layers) and destroy the long-range order of the one-dimensional roll lattice, in agreement with well-known general theorems. If the B\'enard point is approached from this side, the stabilizing influence of the passive modes decreases and is efficient only for the large fluctuations at the B\'enard point. Approached from either side, the B\'enard point resembles the critical point of a Landau phase transition. The width of the region around the B\'enard point where the Landau description breaks down is calculated and found to be unobservably small in realistic liquids. An experimental check of these results, though very tedious, seems possible and very worthwhile.

Journal ArticleDOI
TL;DR: The phase behavior of bovine brain sphingomyelin in water has been determined by polarizing light microscopy, differential scanning calorimetry, and X-ray diffraction.

Journal ArticleDOI
TL;DR: In this paper, it was shown that the two phases coexist between 0.49 and 0.64 in the solid PZT solution and the leverage law is obeyed.

Journal ArticleDOI
TL;DR: In this paper, it was shown that the critical properties of the two systems are the same, and therefore the phase transition from smectic A to nematic will always be at least weakly first order.

BookDOI
01 Jan 1974
TL;DR: In this paper, the Jahn-Teller Effect was used as a Mechanism for phase transition in Solid CD4 and the authors showed that it is possible to obtain a central mode at the critical point of the phase transition.
Abstract: Phase Transitions, Anharmonic Solids and Liquids.- Neutron Scattering Studies of Soft Mode Dynamics.- Slow Local Fluctuations near Tc in SrTio3 Studied by EPR.- Hydrodynamics near Phase Transitions of Second Order.- Review of Methods for Obtaining a Central Mode.- Nonlinear Fluctuations and the Central Mode.- Some Further Observations of Soft Phonon Line Shapes in KMnF3.- Critical Sound Velocity and Attenuation in KMnF3.- Raman Spectral Study of the Upper Phase Transition in KMnF3.- Soft Mode and Critical Opalescence in SrTio3.- Phonon Transport Theory and the Central Mode.- Phase Transitions in Solid CD4.- The Jahn-Teller Effect as a Mechanism for Structural Phase Transitions.- Soft Modes at First and Second Order Phase Transitions.- A Qualitative Picture of the Tricritical Point.- Tricritical Studies of ND4Cl.- Computer Simulation of a Discontinuous Phase Transition in the Two-Dimensional One-Spin-Flip Ising Model.- Continuous versus Discontinuous Phasetransitions.- Dynamics of Fluids Near the Critical Point: Light Scattering Investigations.- Neutron Scattering in Solid and Liquid H2.- Neutron Scattering by a Liquid Crystal at Temperatures Close to Its Melting Point.- X-Ray Studies of the Smectic Structure and the Solid-Smectic Transition.- High-Frequency Dynamics of Simple Solids and Liquids Near Their Melting Points.- Analysis of the Approach to the Convective Instability Point.- Melting: Theories and Recent Computer Simulations.- Volume Expansion, Thermal and Electrical Conductivities and Sound Velocities of Sodium at Temperatures Very Close to Its Melting Point.- List of Participants.

Journal ArticleDOI
TL;DR: In this paper, the authors compared the Flory lattice model for rigid, impenetrable rods with the experimental ones and concluded that molecular asymmetry alone is sufficient to produce a phase transition from a disordered to an ordered phase.
Abstract: The prediction that molecular asymmetry alone is sufficient to force a phase transition from a disordered to an ordered phase in a system of rodlike particles has been investigated using α-helical synthetic polypeptides as the source of rodlike particles. To this end the temperature–composition phase diagrams for the two component systems polybenzylglutamate (PBLG)–dimethylformamide (DMF) and polycarbobenzoxylysine (PCBL)–dimethylformamide are compared with the Flory lattice model for rigid impenetrable rods. The experimental systems exhibit and the theory predicts three distinct regions in the phase diagram: a narrow biphasic region in which isotropic and liquid crystal phases differing only slightly in composition coexist, a transition region over which solvent is increasingly excluded from the coexisting ordered phase and rods excluded from the coexisting isotropic phase, and a region where almost pure solvent coexists with a highly concentrated liquid crystal phase. A detailed comparison of the theoretical phase diagram for rigid, impenetrable rods with the experimental ones reveals discrepancies which can be attributed to the facts that the experimental rods are neither completely rigid nor impenetrable. Analysis of thermal data on PBLG–DMF indicates that the latent heat for the isotropic to liquid crystal phase transition is small and endothermic. We conclude that molecular asymmetry alone is sufficient to produce a phase transition to an ordered phase. Dynamical data show that the bulk viscosity may be considerably lower in the ordered than in the disordered phase. Although rod motion in the liquid crystal phase is correlated, electron spin resonance data suggest that individual rod motion about its mean lattice position is greater than in the isotropic solution of equivalent concentration. Additional electron spin resonance studies show that the motion of small rods and of the polymer side-chains is little affected by the presence of a high concentration of long rods, whereas the tumbling of long rods is dramatically influenced by the presence of other long rods.

Journal ArticleDOI
01 Mar 1974
TL;DR: In this paper, phase separation in aqueous monodisperse latexes of high electrolyte concentration being stabilized with nonionic surfactant is studied, in order to confirm the Kirkwood-Alder transition as its explanation.
Abstract: Phase separation in aqueous monodisperse latexes of high electrolyte concentration being stabilized with nonionic surfactant is studied, in order to confirm the Kirkwood-Alder transition as its explanation. Concentrated latexes were produced by ultrafiltration and the phase separation was detected by observing the appearance of iridescent sediment at the bottom of the containers. The phase transition starts at volume fraction of 0.39–0.49 and is completed at 0.48–0.55. These values depend upon the electrolyte concentration and the surfactant used; high electrolyte concentration and short polyoxyethylene chains give larger values. For short chain length surfactant, the results are in good agreement with Alder, Hoover and Young's [ J. Chem. Phys. 49 , 3688 (1968)] prediction that the transition starts at about 0.5 and is completed at about 0.55. The effects of the van der Waals force on the phase separation seems to be negligible. Above results, thus, substantiate the computer experiments of Alder, Hoover, and Young on the transition between fluid and solid states in hard sphere systems.

Journal ArticleDOI
TL;DR: In this paper, the critical behavior at displacive phase transitions in perovskite crystals is examined and it is shown that, under various conditions, the asymptotic critical behavior may be of Ising, $\mathrm{XY}$-model, or Heisenberg type.
Abstract: The critical behavior occurring at displacive phase transitions in anisotropically stressed perovskite crystals is examined. It is shown that, under various conditions, the asymptotic critical behavior may be of Ising, $\mathrm{XY}$-model, or Heisenberg type. The existence of a "spin-flop"-like transition (at zero stress) between two differently ordered phases is predicted. An explanation of the discrepancy between the measured exponents and those predicted theoretically is proposed, and several new experiments are suggested.

Journal ArticleDOI
TL;DR: In this article, an equilibrium phase diagram is proposed, and the relationship between structure and lattice parameter for the α-U 2 N 3 -UN 2 system is shown; the data on decomposition pressures, specific heats, and heat and free energies of formation are summarized and evaluated.

Journal ArticleDOI
TL;DR: In this paper, a model for granular superconductors consisting of an array of small superconducting particles interacting by Josephson coupling through insulating barriers is considered and the conditions for observing the phase-locking transition distinct from quasiordering within the grains are found.
Abstract: We consider a model for granular superconductors consisting of an array of small superconducting particles interacting by Josephson coupling through insulating barriers. We obtain systematically the various critical regions, critical temperature shifts, and crossover regions between zero- and three-dimensional behavior as functions of measurable sample parameters. The qualitative behavior of the system in the various regimes is analyzed and results for the specific heat and fluctuation conductivity in the Gaussian region above ${T}_{c}$ are obtained. The possibility of obtaining large critical regions is emphasized. The conditions for observing the phase-locking transition distinct from quasiordering within the grains are found. Theoretical predictions are compared with existing experimental results.

Journal ArticleDOI
TL;DR: In this article, optical birefringence, X-ray and neutron diffraction methods with single crystals were used to study the structural phase transitions of the perowskite-type layer structures of (CH3NH3)2MeCl4 with Me=Mn, Fe.

Journal ArticleDOI
TL;DR: In this article, phase transition kinetics and wave propagation in Arkansas novaculite, a fine-grained polycrystalline α quartz rock, when it is subject to high-pressure dynamic loading and relief are reported.
Abstract: This work reports on phase transition kinetics and wave propagation in Arkansas novaculite, a fine-grained polycrystalline α quartz rock, when it is subject to high-pressure dynamic loading and relief. The stress region studied is between 150 and 400 kbar, where the polymorphic phase transition from α quartz to stishovite is believed to occur. Particular emphasis was focused on the unloading behavior in the mixed phase region. High-pressure loading is provided by conventional explosive methods. Measurements of the transient flow field are obtained with in-material manganin stress gages or magnetic particle velocity gages. Results showed partial transformation to the high-density phase occurring in the shock front, the degree of transformation depending on peak driving stress. A continuing transformation rate behind the shock front is very small, i.e., at least 3 orders of magnitude slower than the initial transformation rate. Unloading from stress-volume points in the mixed phase region is observed to occur along, or close to, paths of frozen phase concentration down to approximately 80 kbar. Below this stress the data indicate a transition of the high-density phase to a lower-density phase.

Journal ArticleDOI
TL;DR: In this paper, it was shown that the Gibbs state at temperature 1/β is after interchange of space and time equal to the Schwinger functions for the vacuum or temperature zero state for the field in a periodic box of length β.
Abstract: We construct for a boson field in two-dimensional space-time with polynomial or exponential interactions and without cut-offs, the positive temperature state or the Gibbs state at temperature 1/β. We prove that at positive temperatures i.e. β<∞, there is no phase transitions and the thermodynamic limit exists and is unique for all interactions. It turns out that the Schwinger functions for the Gibbs state at temperature 1/β is after interchange of space and time equal to the Schwinger functions for the vacuum or temperature zero state for the field in a periodic box of length β, and the lowest eigenvalue for the energy of the field in a periodic box is simply related to the pressure in the Gibbs state at temperature 1/β.

Journal ArticleDOI
TL;DR: In this article, the problem of the phase transition in an interacting Bose system is investigated from a microscopic point of view, and the existence of certain poles in the self-energies and the polarization part is revealed, which offers a mechanism to separate the one-particle and collective spectra from each other at the transition point.

Journal ArticleDOI
TL;DR: In this paper, the lattice constants and atomic parameters of YMnO3 were investigated from room temperature up to 1010°C, and the crystal structure with a small hexagonal lattice cell with a = 3.61 A, c = 11.39 A, Z = 2, space group P63/mmc D4 6h was presented.
Abstract: Lattice constants and atomic parameters of YMnO3 were investigated from room temperature up to 1010°C. Lattice constant c decreases slowly with temperature, contrary to earlier publication. With rising temperature small changes of the crystal structure were observed. The paraelectric phase is probably disordered and the phase transition is rather diffuse. At 1000°C we found a new phase transition resulting in the crystal structure with a small hexagonal lattice cell with a = 3.61 A, c = 11.39 A, Z = 2, space group P63/mmc D4 6hAtomic parameters of this high temperature crystal structure are presented.

Journal ArticleDOI
TL;DR: In this paper, a super-lattice is observed by electron diffraction in 2H-TaS2 along with a discontinuity in the electrical resistivity along the layers at 75 K and a rise in the resistivity across the layers is observed below the transition.
Abstract: A discontinuity in the electrical resistivity of 2H-TaS2 along the layers is observed at 75 K. In association with this discontinuity a super-lattice is observed by electron diffraction. A rise in the resistivity across the layers is observed below the transition.

Journal ArticleDOI
TL;DR: In this paper, single crystals of potassium chloride (KCl) were impacted on quartz stress gauges at velocities ranging from 0.0363 cm/μsec, producing plane shocks with stress amplitudes between 17 and 31 kbar.
Abstract: Single crystals of potassium chloride (KCl) were impacted on quartz stress gauges at velocities ranging from 0.0363 to 0.0755 cm/μsec, producing plane shocks with stress amplitudes between 17 and 31 kbar. Stresses greater than 21 kbar are sufficient to induce a polymorphic phase transformation in KCl. For impact stresses above the phase transition point, the theory of continuum mechanics is used to relate measured stress history at the impact interface to transformation rate between the phases. Experimental results show that, when shocked above the measured dynamic transition pressure of 20.8±0.5 kbar, KCl rapidly transforms to a metastable nonequilibrium mixture of phases. Phase transformation to these initial metastable states proceeds at a rate greater than 500 μsec−1 for shock propagation along the 〈100〉 and 〈111〉 directions, based on the magnitude of initial transformation and the minimum response time of the measuring system. The degree of partial transformation is observed to depend only on impact velocity and is independent of crystallographic orientation of the KCl. The observed metastable states are consistent with the postulate that the heat of formation of the nonequilibrium high‐pressure phase is characterized by a specific entropy 3.9 × 10−6 cm2 μsec−2°K−1 higher than the equilibrium high‐pressure phase. Subsequent phase transformation proceeds at a rate that depends on crystallographic orientation of the KCl. When the shock propagates along a 〈111〉 direction, it is observed that transformation proceeds from the metastable state to completion at a rate of 25±5 μsec−1, with final states in agreement with thermodynamic equilibrium calculations. When the shock propagates along a 〈100〉 direction, transformation proceeds at a rate of less than 5 μsec−1. For the 〈100〉 orientation, data are inadequate to allow determination of whether transformation was completed during the experiment, because of uncertainty in late‐time quartz gauge response.