Molecular dynamics simulation of the pressure-induced phase transition in BaFCl

The structural phase transition in BaFCl under high-pressure up to 30 v GPa has been studied using molecular dynamics (MD) method. It was found that BaFCl transforms from a tetragonal structure to a monoclinic structure in the upstroke process and then the tetragonal structure is recovered upon releasing the pressure. The atomistic mechanisms of the transformation have been examined using the pair-correlation functions and the coordination numbers for the lattices with or without vacancies in the MD cell. It was also demonstrated that the Cl atoms between the adjacent weakly bonded Cl layers shift in the compressed direction and move to positions with a 2-fold coordination number at a nearest-neighbour site after the transition.     

Shock metamorphism of the graphite quasimonocrystal

Abstract

Micro structure examination of graphite quasimonocrystal recovered after dynamic loading to pressure of 35-45 GPa was carried out. Only a small amount of cubic diamond and recrystalized graphite was detected. Most of the graphite (～80 vol.%) remained in initial high orientation, but transformed to fine, grained phase with crystalite size 0.1-1 microns. Relaxation time of the transformation (～ 10 ns) and the degree of the transformation (～ 70-80 vol.%) were determined by means of measurements of the electrical resistivity during loading up to 26 GPa and following computer simulation of the results. We proposed that two simultaneous processes take place at pressures higher than 20 GPa: i) relatively slow diffusive graphite to diamond transformation localized in the zones with defect structure: ii) highly oriented graphite transforms to a diamond like phase with density of about 3.2 g/cm³ at zero pressure. This, transformation has fast, martensitic kinetics and is reversible.     

Orthomodularity of Decompositions in a Categorical Setting

We provide a method to construct a type of orthomodular structure known as an orthoalgebra from the direct product decompositions of an object in a category that has finite products and whose ternary product diagrams give rise to certain pushouts. This generalizes a method to construct an orthomodular poset from the direct product decompositions of familiar mathematical structures such as non-empty sets, groups, and topological spaces, as well as a method to construct an orthomodular poset from the complementary pairs of elements of a bounded modular lattice. Mathematics Subject Classifications (2000): 06C15, 81P10, 03G12, 18A30     

Gallium and indium under high pressure

Ga and In crystallize in unusual open ground-state crystal structures. Recent experiments have discovered that Ga under high pressure transforms into a close-packed structure, while this has so far not been observed for In. Results from first principles calculations explain in a simple way this difference in behavior. We predict a so far undiscovered transition of In to a close-packed structure at extreme pressures and show that the structure determining mechanism originates from the degree of s-p mixing of the valence orbitals. Group-III elements are shown to strongly disobey the standard corresponding-state rule.     

The structure of MgO-SiO2 glasses at elevated pressure

The magnesium silicate system is an important geophysical analogue and neutron diffraction data from glasses formed in this system may also provide an initial framework for understanding the structure-dependent properties of related liquids that are important during planetary formation. Neutron diffraction data collected in situ for a single composition (38 mol% SiO(2)) magnesium silicate glass sample shows local changes in structure as pressure is increased from ambient conditions to 8.6 GPa at ambient temperature. A method for obtaining the fully corrected, total structure factor, S(Q), has been developed that allows accurate structural characterization as this weakly scattering glass sample is compressed. The measured S(Q) data indicate changes in chemical ordering with pressure and the real-space transforms show an increase in Mg-O coordination number and a distortion of the local environment around magnesium ions. We have used reverse Monte Carlo methods to compare the high pressure and ambient pressure structures and also compare the high pressure form with a more silica-poor glass (Mg(2)SiO(4)) that represents the approach to a more dense, void-free and topologically ordered structure. The Mg-O coordination number increases with pressure and we also find that the degree of continuous connectivity of Si-O bonds increases via a collapse of interstices.     

Crystal structure of simple metals at high pressures

The effects of pressure on the crystal structure of simple (or sp-) elements are analysed in terms of changes in coordination number, packing density, and interatomic distances, and general rules are established. In the polyvalent elements from groups 14–17, the covalently bonded structures tend to transform to metallic phases with a gradual increase in coordination number and packing density, a behaviour normally expected under pressure. Group 1 and 2 metallic elements, however, show a reverse trend towards structures with low packing density due to intricate changes in their electronic structure. Complex crystal structures such as host–guest and incommensurately modulated structures found in these elements are given special attention in this review in an attempt to determine their role in the observed phase-transition sequences.     

High-pressure phases of thorium and uranium compounds with the rocksalt structure

Abstract

The high-pressure crystal structures of the actinide compounds ThX and UX (X= C, N, P, S, As, Se, Sb, Te) have been studied by X-ray diffraction using synchrotron radiation, in the pressure range up to about 60 GPa Distorted fcc structures were observed for UC (27 GPa), UN (29 GPa), UP (10/28 GPa), US (10 GPa) and ThS (20 GPa). No phase transition has been observed for ThC and ThN. Compounds with As, Se, Sb all transform to the CsCl structure. ThP transform to the CsCl structure at 30 GPa. ThTe has the CsCl structure at ambient pressure and no further phase transition has been observed. UTe transforms to the CsCl structure at 9 GPa.     

Pressure-induced phase transition of crystalline and amorphous silicon and germanium at low temperatures

Abstract

X-ray diffraction has been measured for crystalline silicon, crystalline germanium, amorphous silicon and amorphous germanium at temperatures down to 100 K and pressures up to 20 GPa using a diamond anvil cell and synchrotron radiation. The structural phase transitions, including amorphization, take place in the pressure-temperature range. It has been found that the structures after the phase transitions strongly depend on the path in the pressure-temperature diagram through which the system undergoes the phase transitions. For any of the aforementioned four materials, the high-pressure phase with the p-Sn structure is quenched during a release of pressure at 100 K, and transforms into an amorphous state when heated up to around 2 GPa. The path dependence of the states is discussed in relation to the pressure dependence of the heights of the energy barriers which have to be overcome when phase transitions occur. The effect of a structural disorder on the phase transition is also discussed by comparing the experimental results for the crystalline and amorphous materials.     

Raman spectroscopic and optical absorption study of the high pressure behaviour and polymorphism in KO2

Abstract

Raman scattering, visible absorption, and optical observation studies have been made on polycrystalline potassium superoxide (KO₂) in a diamond anvil cell as a function of pressure and temperature. Three new phases are observed. With increasing pressure at 298 K, KO₂ transforms from the well known modified CaC₂ structure (Phase II), to two new phases (VII, and VIII). The transformation from III to VII occurs at about 3.2GPa. Phase VII transforms to phase VIII at about 4.4GPa. However, in some samples phase VII does not occur and phase II transforms directly into phase VIII at about 4.2 GPa. These structural transformations are indicated by marked changes in the Raman spectrum. The transitions out of phase II are also marked by a discontinuous red shift in the optical absorption edge. From optical observations we have also determined the pressure and temperature dependence of the transitions from phase II to the high temperature cubic (B1) phase I as well as from the high pressure phases VII and VIII to a new nonbirefringent phase IX. This new phase IX has the cubic B2 (CsCl) structure as is shown by our recent X-ray synchrotron experiments.     

Search for polytipic structures of gadolinium

Abstract

The trivalent rare-earth element gadolinium has been studied up to 8 GPa and 700°C by energy-dispersive synchrotron x-ray diffraction. Except for the known crystal structures of hcp, Sm-type, and dhcp, no long-period polytypes have been observed. It is found that the hcp structure transforms directly to the dhcp structure with increasing pressure at high temperatures.     

Ostwald step rule in films of metastable nanocrystalline alloys Fe-C prepared by pulsed plasma vaporization

Ferromagnetic nanocrystalline Fe(C) films were prepared by pulsed plasma vaporization. A comprehensive investigation of the structure and magnetic properties made it possible to identify the type of short-range order here and to establish the sequence of structural states occurring in these films in the process of thermal relaxation: fcc-Fe(C)→hcp-Fe(C)→bcc-Fe+C. On the basis of an analysis of the metastable phase diagrams using Ostwald’s rule, it is shown that the observed scenario of the structural transformations in these metastable nanocrystalline alloys Fe(C) is a natural phenomenon. Pis’ma Zh. éksp. Teor. Fiz. 70, No. 11, 727–732 (10 December 1999)     

A structural study of promethium metal under pressure

Abstract

The structural behaviour of Pm metal has been investigated up to 60 GPa of pressure using a Diamond Anvil Cell (DAC) and the energy dispersive X-ray diffraction technique. The room temperature/pressure structural form of Pm is dhcp and it transforms to a fcc phase by 10 GPa. This cubic phase of the metal converts by 18 GPa to a third phase, which has frequently been referred to as representing a distorted fcc structure. This latter form of Pm was retained up to 60 GPa, the maximum pressure studied, but subtle changes in the X-ray spectra between 50 and 60 GPa hinted that an additional structural change could be forthcoming at higher pressures. From the experimental data a bulk modulus (B₀) of 38 GPa and a B₀′ constant of 1.5 were calculated using the Birch equation. This modulus for Pm is in accord with the moduli reported for the neighboring lanthanide metals.     

C-directional compression of nano-graphite: a comparison between effects of uniform and non-uniform pressure

The mechanism of phase transition and evolution in graphite under uniform compression and spherical nanoindentation along the c-direction is investigated through systematical molecular dynamics simulations. Under both the loading conditions, the soft graphite phase can sustain pressure up to 16-20 GPa, beyond which it transforms into a new phase characterized by a much higher stiffness. More and more interlayer bonds will be created in the new hard phase with the increase of the pressure until an unstable state is reached. The critical pressure to produce the quenchable hard phase with a permanent sp³ bonding remaining after unloading is shown to be as high as ∼880 GPa under uniform compression, as opposed to only ∼75 GPa under nanoindentation. Therefore, application of non-uniform pressure is significantly more helpful for creating diamond-like sp³ structures in graphite by cold-compressive technique.     

Salt effect on bilayer phase transitions of dipalmitoylphosphatidylglycerol in saline water under high pressure

ABSTRACT

The phase transitions of dipalmitoylphosphatidylglycerol (DPPG) bilayer membrane at high NaCl concentrations under high pressure were investigated to construct the temperature–pressure phase diagram and to determine phase-transition properties. The constructed phase diagrams exhibited qualitative resemblance to that of the dipalmitoylphosphatidylcholine (DPPC) bilayer membrane: they showed the gel-phase polymorphism including the pressure-induced bilayer interdigitation. The phase-transition properties of the DPPG bilayer membrane changed in a salt concentration-dependent manner. We discussed the salt effect on the DPPG bilayer membrane from the variation in interactions between the polar head groups of the PG molecules.     

Substitutional alloy of Bi and Te at high pressure

Being a best known thermoelectric material and a topological insulator at ambient condition, magic bismuth telluride (Bi2Te3) under pressure transforms into several superconducting phases, whose structures remain unsolved for decades. Here, we have solved the two long-puzzling low high-pressure phases as seven- and eightfold monoclinic structures, respectively, through particle-swarm optimization technique on crystal structure prediction. Above 14.4 GPa, we experimentally discovered that Bi2Te3 unexpectedly develops into a Bi-Te substitutional alloy by adopting a body-centered cubic disordered structure stable at least up to 52.1 GPa. The continuously monoclinic distortion leads to the ultimate formation of the Bi-Te alloy, which is attributed to the Bi→Te charge transfer under pressure. Our research provides a route to find alloys made of nonmetallic elements for a variety of applications.     

High pressure properties of graphite and its intercalation compounds

The effects of applied pressure on graphite and its intercalation compounds are reviewed emphasizing the relationship between structure and transport properties. It has long been recognized that high pressure plays a crucial role in the polymorphic phase transitions of graphite, notably in the graphite-diamond transformation. More recent studies have revealed a wealth of pressure-induced phases associated with the unusual layer-stacking (‘staging’) mechanism in the intercalation compounds of graphite. The high degree of structural anisotropy associated with staging is strongly reflected in the electronic band structure and transport properties, and in the remarkable pressure dependence of the superconducting states of some of the graphite intercalation compounds. High pressure is shown to be a valuable means not only to realize new structural phases but also to improve our understanding of the fundamental behaviour of these important materials.     

The method of volumetric measurements under high pressure I. Qualitative approach

Abstract

Method of detection of the first order phase transitions under high pressure is suggested. It based on the different pressure dependences of chromel-alumel and Pt+10%Rh thermocouples. Pressure-temperature diagrams of the melts of elements, obtained by this technique, are presented.     

Pulse laser induced graphite-to-diamond phase transition:the role of quantum electronic stress

First-principles calculations show that the pulse laser induced graphite-to-diamond phase transition is related to the lattice stress generated by the excited carriers,termed as "quantum electronic stress(QES)".We found that the excited carriers in graphite generate a large anisotropic QES that increases linearly with the increasing carrier density.Using the QES as a guiding parameter,structural relaxation spontaneously transforms the graphite phase into the diamond phase,as the QES is reduced and minimized.Our results suggest that the concept of QES can be generally applied as a good measure to characterize the pulse laser induced phase transitions,in analogy to pressure induced phase transitions.     

Structure and dynamics of liquid MgO under high pressure

Microstructure, dynamics, and diffusion mechanism in liquid MgO have been studied by molecular dynamics simulation. Models consisting of 2000 atoms were constructed under a wide range of pressure and at a temperature of 3800 K. The local structure is analyzed through the coordination number distribution and topology statistics of coordination units (basic structural units) MgO _x (x=2, 3, 4, 5, 6, 7). As regards the structural dynamics, the nearest-neighbor atomic exchange among coordination units, spatially heterogeneous dynamics, clustering, and structural stability (lifetime of basic structural units) are investigated in detail. Investigation of structural dynamics allows us to gain insight into various important atomic (molecular) properties and to clarify the diffusion mechanism in liquid MgO under high pressure.