Formation region of amorphous thin CoZr and CuZr films prepared by cocondensation on hot substrates

The glass formation ranges of thin CoZr and CuZr films condensed on cold and hot substrates (10–800 K) are experimentally determined by X-ray diffraction. The maximum substrate temperatures allowed to condense amorphous films are only slightly below the crystallization temperatures of the amorphous phases. These experimental results are compared with predictions of the glass formation ranges from thermodynamic calculations and instability models. At low substrate temperatures, where no phase separation is possible due to kinetic restrictions, the amorphous phase is favoured in the range between theT _o lines which are the stability limits of the extended solid solutions on both sides of a metastable binary phase diagram. For high substrate temperatures, where at least one species of the binary alloy is able to move, the amorphous state can be described to be in a metastable equilibrium with the extended solid solutions neglecting the intermetallic compounds. Therefore, the single-phase amorphous state is restricted to the range between theT _ml lines which are the metastable liquidus temperatures.     

Intrinsic paramagnetic defects probe superionic phase transition in Ag1−xCuxI (x=0, 0.05, 0.15 and 0.50) nanocrystals

EPR probed the zincblende (γ) to cubic (α) AgI structural phase transition in AgI at 423 K through two intrinsic paramagnetic centers: an Ag²⁺-based hole center (signal ‘A’) and an Ag⁰-based conduction electron center (signal ‘B’) associated with AgI nanocrystallites. Sudden drops in intensity (I_PP), <g>, and ΔH_PP observed at 423 K for pure AgI nanocrystals. Addition of Cu in AgI increases the thermal stability of the cation sublattice as seen from the increase in the transition temperature from 423 K (undoped AgI) to 453 K. Abrupt jumps in the number of spins (N) and reciprocal susceptibility (1/χ) observed at increased phase transition temperatures in Cu-substituted AgI relative to that in undoped AgI reflects progressively strengthened local bonding configuration of γ-AgI structure induced by Cu.     

Investigation of phase transitions of solid ionic conductors by automatic impedance spectroscopy

The first order phase transitions of the solid ionic conductors AgI, Ag₂HgI₄, Cu₂HgI₄, and CuTeBr at 420, 320, 343, and 348 K respectively, were investigated by automatic low frequency (100 Hz to 10 MHz) impedance spectroscopy. Measurements showed all four transitions covering a rather broad interval of temperatures, i.e. 3 to 5 K with AgI and CuTeBr, and about 8 K with Ag₂HgI₄ and Cu₂HgI₄, as well as possessing a hysteresis between 2.5 K (CuTeBr) and 4.4 K (AgI).     

Structural studies of the P-T phase diagram of sodium niobate

The crystal structure of sodium niobate (NaNbO₃) has been investigated by energy-dispersive X-ray diffraction at high pressures (up to 4.3 GPa) in the temperature range 300–1050 K. At normal conditions, NaNbO₃ has an orthorhombic structure with Pbcm symmetry (antiferroelectric P phase). Upon heating, sodium niobate undergoes a series of consecutive transitions between structural modulated phases P-R-S-T(1)-T(2)-U; these transitions manifest themselves as anomalies in the temperature dependences of the positions and widths of diffraction peaks. Application of high pressure leads to a decrease in the temperatures of the structural transitions to the R, S, T(1), T(2), and U phases with different baric coefficients. A phase diagram for sodium niobate has been build in the pressure range 0–4.3 GPa and the temperature range 300–1050 K. The dependences of the unit-cell parameters and volume on pressure and temperature have been obtained. The bulk modulus and the volume coefficients of thermal expansion have been calculated for different structural modulated phases of sodium niobate. A phase transition (presumably, from the antiferroelectric orthorhombic P phase to the ferroelectric rhombohedral N phase) has been observed at high pressure (P = 1.6 GPa) and room temperature.     

Phase transitions in cerium at high pressures (up to 15 GPa) and high temperatures

Phase transitions in cerium have been studied by the electrical resistance method in the 15-GPa pressure range at high temperatures. At pressures above 10 GPa, cerium represents a mixture of stable and metastable phases, the composition of this mixture being dependent on the trajectory in the P-T plane that leads to a given point. Transformations in both stable and metastable components of the mixture proceeding rather independently display a complicated picture of phase transitions. It was assumed that only the α (fcc) and α′ (α-U) phases are stable at pressures above the well-known γ-α transition, the other phases being metastable. The proposed bow-shaped equilibrium phase diagram includes an extremely wide hysteresis region, where stable and metastable phases can coexist. The fcc α phase alone survives upon heating above 50°C at 15 GPa.     

Dendritic growth and solute trapping in rapidly solidified Cu-based alloys

Rapid solidification of binary Cu-22%Sn peritectic alloys and Cu-5%Sn-5%Ni-5%Ag quaternary alloys was accomplished by glass fluxing, drop tube and melt spinning methods. The undercooled, by glass fluxing method, Cu-22%Sn peritectic alloy was composed of α(Cu) and δ(Cu41Sn11) phases. If rapidly solidified in a drop tube, the alloy phase constitution changed from α(Cu) and δ(Cu41Sn11) phases into a single supersaturated (Cu) phase with the reducing of droplet diameter, and the maximum solubility of Sn in (Cu)...     

Superionic behavior in the xAgI–(1−x)CsAg2I3 polycrystalline system

A superionic phase behavior (with DC ionic conductivities higher than 0.01 S/cm) has been observed in xAgI–(1−x)CsAg₂I₃ (x≈0.67) polycrystalline system grown by slow evaporation using AgI and CsI powders (molar ratio Cs/Ag=0.25) as starting salts and an aqueous solution of HI as solvent. The transition from the normal-to- the superionic state is first-order with a hysteretic behavior in temperature centered at about 116 °C as reflected by thermal (DSC) and electrical conductivity measurements. This mixture is composed of CsAg₂I₃ and AgI crystalline phases and an additional amorphous AgI phase that explains the glassy-type behavior observed in the superionic phase transition.     

Texture transitions in binary mixtures of 6OBAC with compounds of its homologous series

Recently the authors have observed in compounds of the 4,n-alkyloxybenzoic acid series, with the homologous index n ranging from 6 to 9, a texture transition in the nematic range which subdivides the nematic phase in two sub-phases displaying different textures in polarised light analysis. To investigate a persistence of texture transitions in nematic phases, we prepared binary mixtures of 4,6-alkyloxybenzoic acid (6OBAC) with other members (7-, 8-, 9-, 12-, 16OBAC) of its homologous series. Binary mixtures exhibit a broadening in the temperature ranges of both smectic and nematic phases. A nematic temperature range of 75°C is observed. In the nematic phase, in spite of the microscopic disorder introduced by mixing two components, the polarised light optics analysis of the liquid crystal cells reveals a texture transition. In the case of the binary mixture of 6OBAC with 12OBAC and with 16OBAC, that is of compounds with monomers of rather different lengths, the texture transition temperature is not homogeneous in the cell, probably due to a local variation in the relative concentrations of compounds.     

Orbital ordering in e(g) orbital systems: ground states and thermodynamics of the 120° model

Orbital degrees of freedom shape many of the properties of a wide class of Mott insulating, transition metal oxides with partially filled 3d shells. Here we study orbital ordering transitions in systems where a single electron occupies the e(g) orbital doublet and the spatially highly anisotropic orbital interactions can be captured by an orbital-only model, often called the 120° model. Our analysis of both the classical and quantum limits of this model in an extended parameter space shows that the 120° model is in close proximity to several T=0 phase transitions and various competing ordered phases. We characterize the orbital order of these nearby phases and their associated thermal phase transitions by extensive numerical simulations and perturbative arguments.     

Anomalies of dielectric properties of cs2hgci4 crystals in the vicinity of phase transitions

A short review of the published experimental data on various physical properties and some new results of the measurement of dielectric and electromechanical properties of Cs₂HgCI₄ crystals in a wide temperature range are presented. The crystal is shown to reveal anomalies at seven temperatures associated with structural phase transitions and hence to have eight phases. In two temperature regions there exists a small spontaneous polarization and the anomalous dielectric permittivity is a nonlinear function of the electric biasing field. Within the phase followed by the first polar phase on cooling the crystal, the effects of thermal, electrical and mechanical memory, specific for incommensurate phases, are observable.     

Phase transitions between orthogonal and tilted hexatic phases

Tilt-driven phase transitions between hexatic smectic phases: SmF-HexB and reversed HexB-SmF have been studied in compounds belonging to two enaminoketone derivative homologue series. The tilt angle order parameter has been measured and its temperature dependence near the phase transition point has been described by applying mean-field model. For both phase sequences the tricritical points have been observed on phase transition lines in binary mixtures of respective materials having first and second order phase transitions between hexatic phases. Received 21 June 1999     

Model of grain-boundary self-diffusion in α- and β-phases of titanium and zirconium

A model of the grain-boundary self-diffusion process in metals undergoing phase transitions in the solid state is proposed. The model is based on the ideas and approaches of the theory of nonequilibrium grain boundaries. It is shown that the range of application of basic relations of this theory can be extended, and they can be used to calculate the parameters of grain-boundary self-diffusion in high-temperature and low-temperature phases of metals with phase transition. Based on the constructed model, activation energies of grainboundary self-diffusion in titanium and zirconium are calculated, and their anomalously low values in the low-temperature phase are explained. The calculated activation energies of grain-boundary self-diffusion are in good agreement with experimental data.     

Determination of phase transitions in binary systems of mesogenic compounds by optical absorption spectroscopy

The use is described of absorption spectroscopy for determining phase transitions in the mesogenic compound cholesteryl myristate (CM) and in binary mixtures of terephthalbis-butyl-aniline (TBBA) and cholesteryl myristate. The temperature dependence of the optical densities in the visible region at phase transitions is reported. The transition temperatures obtained with this method are in good agreement with the results obtained with other methods.     

Preparation and characterization of AgI nanoparticles with controlled size,morphology and crystal structure

AgI nanoparticles were prepared by solution-based routes using water-soluble anionic or cationic polyelectrolytes as capping agents. Depending on the polyelectrolytes, AgI nanoparticles with well-defined morphology, size, and phase compositions were obtained: the use of poly(sodium 4-styrenesulfonate) (PSS) resulted in AgI nano-rods of β-AgI in wurtzite structure (2H); with poly(acrylic acid sodium salt) (PAS) truncated-tetrahedron shaped γ-AgI nanoparticles (nanotetrahedra) in zinc-blende structure (3C) were obtained; by employing poly(diallyldimethylammonium chloride) (PDADMAC) plate-like AgI nanoparticles (nano-plates) consisting of unusual polytype phases of AgI (7H and 9R) were formed. Macroscopically unstable γ-AgI and 7H and 9R phases could be stabilized in the form of nanocrystalline powders. They transform reversibly into the high temperature α-AgI phase and exhibit unusually high ionic conductivity and substantially smaller transformation enthalpy values compared to the macroscopic β-AgI.     

Diffusion and Velocity Correlations of the Phase Transitions in a System of Macroscopic Rolling Spheres

We study an air-fluidized granular monolayer composed of plastic spheres which roll on a metallic grid. The air current is adjusted so that the spheres never lose contact with the grid and so that the dynamics may be regarded as pseudo two dimensional (or two dimensional, if the effects of the sphere rolling are not taken into account). We find two surprising continuous transitions, both of them displaying two coexisting phases. Moreover, in all the cases, we found the coexisting phases display a strong energy non-equipartition. In the first transition, at a weak fluidization, a glass phase coexists with a disordered fluid-like phase. In the second transition, a hexagonal crystal coexists with the fluid phase. We analyze, for these two-phase systems, the specific diffusive properties of each phase, as well as the velocity correlations. Surprisingly, we find a glass phase at a very low packing fraction and for a wide range of granular temperatures. Both phases are also characterized by strong anticorrelated velocities upon a collision. Thus, the dynamics observed for this quasi two-dimensional system unveil phase transitions with peculiar properties, very different from the predicted behavior in well-know theories for their equilibrium counterparts.     

Twisted Grain Boundary Phases in the Binary Mixtures of 3ß-Chloro-5-Cholestene and 4-N-Decyloxybenzoic Acid

From the thermodynamical, optical texture and dielectric studies of the binary mixtures of 3β-chloro-5-cholestene (ChCl) and 4-n-decyloxybenzoic acid (DOBA), the phase diagram has been drawn. It has been observed that low concentrations of ChCl (1 to 7 mol%) in DOBA induce various types of twisted grain boundary (TGB) submesophases, whereas higher concentrations induce a smectic A (SmA) mesophase. Various optical textures of the TGB phases under different conditions of molecular anchoring have been observed. Weak transitions related with TGB phases have been detected from the temperature dependence of dielectric permittivity. The observed phase diagram of ChCl-DOBA binary system is in complete conformity with the theoretically predicted mean-field phase diagram derived by Renn within the framework of the chiral Chen-Lubenski model     

Effect of Ni content on the diffusion-controlled growth of the product phases in the Cu(Ni)–Sn system

A strong influence of Ni content on the diffusion-controlled growth of the (Cu,Ni)₃Sn and (Cu,Ni)₆Sn₅ phases by coupling different Cu(Ni) alloys with Sn in the solid state is reported. The continuous increase in the thickness ratio of (Cu,Ni)₆Sn₅ to (Cu,Ni)₃Sn with the Ni content is explained by combined kinetic and thermodynamic arguments as follows: (i) The integrated interdiffusion coefficient does not change for the (Cu,Ni)₃Sn phase up to 2.5 at.% Ni and decreases drastically for 5 at.% Ni. On the other hand, there is a continuous increase in the integrated interdiffusion coefficient for (Cu,Ni)₆Sn₅ as a function of increasing Ni content. (ii) With the increase in Ni content, driving forces for the diffusion of components increase for both components in both phases but at different rates. However, the magnitude of these changes alone is not large enough to explain the high difference in the observed growth rate of the product phases because of Ni addition. (iv) Kirkendall marker experiments indicate that the Cu₆Sn₅ phase grows by diffusion of both Cu and Sn in the binary case. However, when Ni is added, the growth is by diffusion of Sn only. (v) Also, the observed grain refinement in the Cu₆Sn₅ phase with the addition of Ni suggests that the grain boundary diffusion of Sn may have an important role in the observed changes in the growth rate.     

Fast-ion conduction and flexibility of glassy networks

We observe two thresholds in the variations of electrical conductivity of dry (AgI)_{x}(AgPO3)_{1-x} solid electrolyte glasses, when the AgI additive concentration x increases to 9.5% and to 37.8%. Raman scattering complemented by calorimetric measurements confirms that these thresholds are signatures of the rigidity phase transitions at x=9.5% from a stressed rigid to an isostatically (stress-free) rigid phase, and at x=37.8% from isostatically rigid to a flexible phase. In the flexible phase, the electrical conductivity seems to increase as a power of x. This is in good agreement with the theoretical prediction based on 3D percolation.     

Phase transitions in random Potts systems and the community detection problem: spin-glass type and dynamic perspectives

Phase transitions in spin-glass type systems and, more recently, in related computational problems have gained broad interest in disparate arenas. In the current work, we focus on the “community detection” problem when cast in terms of a general Potts spin-glass type problem. As such, our results apply to rather broad Potts spin-glass type systems. Community detection describes the general problem of partitioning a complex system involving many elements into optimally decoupled “communities” of such elements. We report on phase transitions between solvable and unsolvable regimes. A solvable region may further split into “easy” and “hard” phases. Spin-glass type phase transitions appear at both low and high temperatures (or noise). Low-temperature transitions correspond to an “order by disorder” type effect wherein fluctuations render the system ordered or solvable. Separate transitions appear at higher temperatures into a disordered (or an unsolvable) phase. Different sorts of randomness lead to disparate behaviors. We illustrate the spin glass character of both transitions and report on memory effects. We further relate Potts type spin systems to mechanical analogs and suggest how chaotic-type behavior in general thermodynamic systems can indeed naturally arise in hard computational problems and spin glasses. The correspondence between the two types of transitions (spin glass and dynamic) is likely to extend across a larger spectrum of spin-glass type systems and hard computational problems. We briefly discuss potential implications of these transitions in complex many-body physical systems.