Search results for "Phase Transition"
showing 10 items of 1281 documents
Dipolar order and disorder phenomena in pure CO and dilute (CO)1−x(Ar)x mixtures physisorbed on graphite
1999
Abstract A detailed heat capacity study has been performed to characterize the properties of pure CO and (CO) 1− x (Ar) x mixture films physisorbed on the basal planes of graphite. The phase diagram of CO monolayers will be presented. At T c =5.18 K a phase transition to a novel low temperature phase is found with head-tail (dipolar) order of the molecules in an orientationally ordered herringbone structure. The phase transition is shown to belong to the 2D Ising universality class with anisotropic interactions between the molecules. Diluting CO weakly with Ar impurities leads to a dramatic destruction of the phase transition. This effect can be quantitatively described by the 2D random fie…
N2monolayers physisorbed on graphite: the herringbone transition revisited
1994
Monte Carlo simulations were undertaken of the orientational herringbone phase transition of N2 adsorbed on graphite in the complete monolayer (✓3 × ✓3) R30° structure. The non-universal aspects (c...
Crystal structure of HgGa2Se4 under compression
2013
We report on high-pressure x-ray diffraction measurements up to 17.2 GPa in mercury digallium selenide (HgGa2Se4). The equation of state and the axial compressibilities for the low-pressure tetragonal phase have been determined and compared to related compounds. HgGa2Se4 exhibits a phase transition on upstroke toward a disordered rock-salt structure beyond 17 GPa, while on downstroke it undergoes a phase transition below 2.1 GPa to a phase that could be assigned to a metastable zinc-blende structure with a total cation-vacancy disorder. Thermal annealing at low- and high-pressure shows that kinetics plays an important role on pressure-driven transitions.
Low-temperature polymorphism in tungsten trioxide powders and its dependence on mechanical treatments
1999
The polymorphism of WO3 powder samples, resulting from mild mechanical treatments and from temperature changes between 30 K and room temperature, has been investigated by using Raman spectroscopy and X-ray di⁄raction. A transition from the monoclinic (I) c-phase to the triclinic d-phase after moderate mechanical treatments has been observed for untreated powder, just what happens when the same samples are rapidly cooled to low-temperature. Evidences of the low temperature monoclinic (II) polar e-phase have been found at room temperature in samples after a stronger milling treatment. The sequence of the low-temperature phase transitions appears to be strongly dependent on the mechanical hist…
Pectin biopolymer mechanics and microstructure associated with polysaccharide phase transitions.
2019
Polysaccharide polymers like pectin can demonstrate striking and reversible changes in their physical properties depending upon relatively small changes in water content. Recent interest in using pectin polysaccharides as mesothelial sealants suggests that water content, rather than nonphysiologic changes in temperature, may be a practical approach to optimize the physical properties of the pectin biopolymers. Here, we used humidified environments to manipulate the water content of dispersed solution of pectins with a high degree of methyl esterification (high-methoxyl pectin; HMP). The gel phase transition was identified by a nonlinear increase in compression resistance at a water content …
Transition paths between phases IV, III and II of ammonium nitrate predicted from X-ray powder diffractometer and differential scanning calorimeter d…
1994
Abstract Ammonium nitrate solid phase transition paths between phases IV, III and II were explained and predicted on the basis of X-ray powder diffraction (XRD) and differential scanning calorimetry (DSC) data by applying partial least-squares regression (PLS) and principal component analysis (PCA). The samples were clustered according to their different transition paths with the PLS and PCA models, and the transition paths were predicted with PLS component clusters. The best PLS clusters were formed by a few first components. Prediction of the transition path with the PLS clusters made a semiquantitative prediction of the transition energy possible. In PCA, principal components 6 and 11, w…
Statistical and systematic errors in Monte Carlo sampling
1991
We have studied the statistical and systematic errors which arise in Monte Carlo simulations and how the magnitude of these errors depends on the size of the system being examined when a fixed amount of computer time is used. We find that, depending on the degree of self-averaging exhibited by the quantities measured, the statistical errors can increase, decrease, or stay the same as the system size is increased. The systematic underestimation of response functions due to the finite number of measurements made is also studied. We develop a scaling formalism to describe the size dependence of these errors, as well as their dependence on the “bin length” (size of the statistical sample), both…
Optimized analysis of the critical behavior in polymer mixtures from Monte Carlo simulations
1992
A complete outline is given for how to determine the critical properties of polymer mixtures with extrapolation methods similar to the Ferrenberg-Swendsen techniques recently devised for spin systems. By measuring not only averages but the whole distribution of the quantities of interest, it is possible to extrapolate the data obtained in only a few simulations nearT c over the entire critical region, thereby saving at least 90% of the computer time normally needed to locate susceptibility peaks or cumulant intersections and still getting more precise results. A complete picture of the critical properties of polymer mixtures in the thermodynamic limit is then obtained with finite-size scali…
Tuning the band gap of PbCrO4 through high-pressure: Evidence of wide-to-narrow semiconductor transitions
2014
The electronic transport properties and optical properties of lead(II) chromate (PbCrO4) have been studied at high pressure by means of resistivity, Hall-effect, and optical-absorption measurements. Band-structure first-principle calculations have been also performed. We found that the low-pressure phase is a direct band-gap semiconductor (Eg = 2.3 eV) that shows a high resistivity. At 3.5 GPa, associated to a structural phase transition, a band-gap collapse takes place, becoming Eg = 1.8 eV. At the same pressure the resistivity suddenly decreases due to an increase of the carrier concentration. In the HP phase, PbCrO4 behaves as an n-type semiconductor, with a donor level probably associat…
Phase Behavior of TmVO4 under Hydrostatic Compression: An Experimental and Theoretical Study
2020
We present a structural and optical characterization of magnetoelastic zircon-type TmVO4 at ambient pressure and under high pressure. The properties under high pressure have been determined experimentally under hydrostatic conditions and theoretically using density functional theory. By powder X-ray diffraction we show that TmVO4 undergoes a first-order irreversible phase transition to a scheelite structure above 6 GPa. We have also determined (from powder and single-crystal X-ray diffraction) the bulk moduli of both phases and found that their compressibilities are anisotropic. The band gap of TmVO4 is found to be Eg = 3.7(2) eV. Under compression the band gap opens linearly, until it unde…