High-pressure properties of group IV clathrates

We review the investigations on clathrate materials having C, Si, Ge or Sn framework atoms in which the pressure parameter plays an important role. In this article, we studied the synthesis of clathrates, superconducting properties and those studies relative to the structural cohesivity and phase diagrams. We have tried to extensively review these subjects. Key references on other important properties of the group IV clathrates are provided in the introductory sections. As a main result of this review, we note that pressure appears as a key parameter for the elaboration of present and future clathrates in particular those exhibiting superconducting properties. We show how high-pressure research has also played an important role in the understanding of the parameters governing clathrate superconductivity. In contrast, the study of the structural evolution of group IV clathrates (bulk modulus and stability) gives abundant clues for tailoring new materials with improved mechanical properties. Finally, practically all investigations converge to point out that today hypothetical carbon clathrates are candidates for extraordinary superconducting and mechanical properties.     

Phase diagrams and dielectric studies of binary mixtures exhibiting intermediate nematic and induced smectic phases

Investigations on the phase diagrams of two bi-component mixtures of liquid crystals are presented. Whereas in one of the bi-component systems, smectic A (SmA) phase is seen to be induced, in the other case the so-called nematic “island” has been observed. The dielectric properties of magnetically oriented sample forming nematic phase and induced smectic phase in one of the mixtures, and a nematic “island” and SmA phase in the other mixture, were measured in the frequency range from 1?Hz to 10?MHz at different temperatures. The transition temperatures obtained from the polarizing microscope and differential scanning calorimeter measurements agree well with the dielectric relaxation data.     

High-pressure organic synthesis

Abstract

Some important problems of organic synthesis and ways to solve them by the use of hgh pressure are the topic of this paper. Three representative groups of organic reactions, cycloadditions, simple additions, and nucleophilic and electrophilic substitutions, illustrate the possibilities and usefulness of the high-pressure technique in organic synthesis. Moreover, applications of high-pressure methodology to syntheses of natural products are described. Examples are given of the influence of pressure on organic reactions in the following essential aspects: acceleration of the reaction rate, modification of regio-and stereochemistry, and changes in the reaction equilibrium.     

High-pressure structures of InBi predicted by particle swarm optimization algorithm

We extensively explore the high-pressure structures of InBi by using a newly developed particle swarm optimization algorithm. An orthorhombic Imma structure is discovered to be stable from 43.7 GPa to 107.9 GPa, ruling out the previously speculated cubic structure. Further increasing the pressure, we find a tetragonal P4/nmm structure which is energetically more favourable from 107.9 GPa to 200 GPa. Especially, the tetragonal P4/nmm structure is known to occur at high pressure in the structures of ZnO and MgTe. We also predict this structure to be a high-pressure structure of ZnTe. Thus the tetragonal P4/nmm structure may be a universal high-pressure structure of the Ⅱ-Ⅵ and the Ⅲ-Ⅴ compounds.     

Growth of toric domains in mesophases of oxadiazoles

In a previous paper (Phase Transitions, 80(9), 987, 2007), we discussed textures of some mesomorphic oxadiazole compounds with terminal Cl-substituent. Optical microscope investigations showed a very interesting behaviour of smectic and nematic phases: the smectic phase has fan-shaped and toric textures and the nematic phase has spherulitic domains, which disappear as the sample is further heated, the texture changing into a smooth one. Here, the focus is on the growth of toric domains from the nematic phase and on the role of defects in domain structure. We use compounds with the same structure of oxadiazoles discussed in the previous paper but with terminal Br-substituent.     

On phase diagrams and on cBN formation in the system BN-Ca3B2N4

Abstract

The phase diagrams of Ca₃B₂N₄ and of the system BN-Ca₃B₂N₄ has been studied by high pressure differential thermal analysis Ca₃B₂N₄ has 3 structural phases and melts at higher temperature. The system BN-Ca₃B₂N₄ formes an eutectic melt and cBN is synthesized by nucleation and crystal growth.     

High-pressure route to superhard boron-rich solids

Novel superhard phases are expected to be found among various high-pressure polymorphs of light element compounds. Besides diamond-like phases, the icosahedral boron-rich solids are of particular interest because they could combine high hardness with advanced electronic and phonon transport properties, lightness, high thermal and chemical stability. Here we review some recent results on high-pressure synthesis of novel boron-rich solids.     

High-pressure DTA-investigations on liquid crystals

Abstract

High pressure studies on the phase behaviour of liquid crystals up to a maximum pressure of 7 kbar were performed using a computer-assisted high-pressure differential thermal analysis apparatus that has been developed for the present experiments. T (p) - phase diagrams were determined for selected liquid crystals differing systematically in structure.

Data are presented for some phenylcyclohexanes, some bicyclo-hexanes and cyclohexylcarboxylic acid phenyl ester being substituted with different functional groups e.g. alkyl, alkoxy and/or cyano. Pressure-induced nematic and smectic phases were found, and for one substance even an “interdigitated” smectic B phase could be stabilized under pressure.     

Directed polymer melts and quantum critical phenomena

The statistical mechanics of directed linelike objects, such as directed polymers in an external field, strands of dipoles in both ferro- and electrorheological fluids, and flux lines in high-T_c superconductors, bears a close resemblance to the quantum mechanics of bosons in 2+1 dimensions. We show that single-component and binary mixture critical phenomena in these systems are in the universality class of three-dimensional uniaxial dipolar ferromagnets and ferroelectrics. Our results also apply to films of two superfluid species undergoing phase separation well below their -points near T=0. In the case of directed polymers and electrorheological fluids we analyze the effects of free ends occurring in the sample as well as a novel directionally-dependent compressibility.     

Vortex matter phase transition in the C15-Laves-phase superconductor CeRu2: comparison with Bi-2212

We present a comparative study of peak-effect in DC magnetization in single-crystal samples of CeRu₂ and Bi-2212. While the PE in Bi-2212 can be associated with the second-order phase transition from Bragg-glass to vortex-glass, the PE in CeRu₂ is anomalous and can be understood as accompanying a first-order phase transition in the vortex matter.     

High-pressure phase transitions of TiN: an ab initio constant pressure study

An ab initio constant pressure molecular dynamics technique is carried out to explore the behaviour of rock salt-structured titanium nitride (TiN) under pressure. Two successive phase transformations are successfully observed in the dynamical simulations. The first one is an isostructural phase transition accompanied by an anomalous volume compression without any symmetry breaking. The second one is a reconstructive phase transformation into a CsCl-type structure. For the first time, the previously proposed two-phase transformations for TiN are confirmed through the simulations.     

Phase diagrams of lattice systems with residual entropy

We introduce an equivalence relation on the family of ground states and generalize the Peierls and Pirogov-Sinai theory of phase transitions to systems with residual entropy. The idea consists in the replacement of the periodic ground states by equivalence classes together with an entropy factor. We apply these results to discuss the phase diagrams of diluted spin-1/2 systems.On leave of absence from the Central Research Institute for Physics, Budapest, Hungary.     

Phase transformations in amorphous fullerite C60 under high pressure and high temperature

First phase transformations of amorphous fullerite C₆₀ at high temperatures (up to 1800 K) and high pressures (up to 8 GPa) have been investigated and compared with the previous studies on the crystalline fullerite. The study was conducted using neutron diffraction and Raman spectroscopy. The amorphous fullerite was obtained by ball-milling. We have shown that under thermobaric treatment no crystallization of amorphous fullerite into С₆₀ molecular modification is observed, and it transforms into amorphous-like or crystalline graphite. A kinetic diagram of phase transformation of amorphous fullerite in temperature–pressure coordinates was constructed for the first time. Unlike in crystalline fullerite, no crystalline polymerized phases were formed under thermobaric treatment on amorphous fullerite. We found that amorphous fullerite turned out to be less resistant to thermobaric treatment, and amorphous-like or crystalline graphite were formed at lower temperatures than in crystalline fullerite.     

Bose-Fermi-Hubbard model: Pseudospin operator approach

The thermodynamics of the Bose-Fermi-Hubbard model with direct interaction between neighbour bosonic particles is considered in this work at finite temperature. The hard-core boson case is considered and the pseudospin formalism is used. Charge susceptibility of the system is calculated and the possibilities of the transitions to the charge density wave order, superfluid and supersolid phases are analysed. We derive an analytic formula for the grand canonical potential and analyse the thermodynamically stable states.     

High-pressure theoretical and experimental study of HgWO4

HgWO₄ at ambient pressure is characterized using a combination of ab initio calculations, X-ray diffraction and Raman scattering measurements. The effect of low pressure and temperature on the structural stability is analysed. Extending our ab initio study to the range of higher pressures, a sequence of stable phases up to 30 GPa is proposed.     

Determination of the density of Pb-Sn melts and segregation phenomena on their crystallization

The density of liquid lead-tin alloys (17.64, 25.34, 25.61, 26.07, and 33.88 at % Pb) has been determined using an γ-ray attenuation technique over the temperature range from the liquidus line to 750–1040 K. The experimental uncertainty of the density measurements is estimated to be within the range of ±0.20–0.25%. It has been shown that the temperature hysteresis of the melt density and thermal expansion, which was observed previously in several studies, disappears after stirring the melts. The distribution of the components through the height of the liquid and solid samples and its variation with time and during melting-crystallization has been investigated. The study shows that the melts do not segregate on long standing (about 4 hours) near the liquidus temperature. However, crystallization and melting of homogeneous Pb-Sn alloys is accompanied by liquation phenomenon, which is enhanced as the sample composition deviates from the eutectic one. The text was submitted by the authors in English.     

Molecular structure and physical parameters of some ferroelectric liquid crystal systems

We investigate thermodynamic properties and spontaneous polarization of ferroelectric liquid crystal (SmC^*LC) systems: smectic C matrix + chiral adding (ChA). The fact of existence of an optimum concentration of ChA, for which the studied SmC^*LC systems have the maximum value of spontaneous polarization, and a certain concentration, at which the phase transition of the chiral tilted smectic C phase to the smectic A phase occurs, is established. Temperature dependence curves for the free energy, configuration energy, specific heat, and the orientational order parameter are calculated in the temperature range of existence of liquid crystal mesophases. The curves are in good agreement with the experimentally observed trends of the SmC^*LC properties.