Pressure dependence of crystal structure and molecular packing in anthracene

Anthracene molecular crystal has been investigated up to a pressure of 10.5 GPa at room temperature using variable shape variable size Monte Carlo simulations in an isothermal–isobaric ensemble. We have reported various structural quantities, such as cell parameters and unit cell volume, as a function of pressure and compared them with the experimental results [J. Chem. Phys. 119, 1078 (2003)]. The pressure dependence of angles θ, δ and χ which describe the relative packing of molecules in the crystal has been calculated. We report that anthracene molecular crystal does not exhibit any first order phase transition up to a pressure of 10.5 GPa which is consistent with the experimental observations by Oehzelt et al. [Phys. Rev. B 66, 174104 (2002)]. The calculated equation of state (EOS) has been fitted to a Murnaghan-type EOS with good agreement. The calculated bulk modulus and the pressure derivative of bulk modulus are 8.2 GPa and 8.9 respectively which are in agreement with the experimentally calculated values.     

High pressure since P. W. Bridgman - A personal view

Abstract

In this paper an attempt is made to review high pressure research over the past three or four decades from a personal viewpoint. Focus is first on two technical developments; the introduction of shock wave techniques in solids, and the extension of the static pressure range by a factor of 20–30.

The major emphasis is on the introduction of high pressure into the mainstream of modern science through the concept that pressure is a powerful, indeed an essential tool in understanding molecular and electronic properties of matter. These properties are classified under the headings (q) structure (x-rays) (b) molecular dynamics (c) pressure tuning spectroscopy (d) chemical and electronic rate phenomena (e) chemical synthesis.     

Potential applications of high pressures in pharmaceutical science and medicine

Abstract

High pressure (HP) technology appears to be of paramount interest for valuable applications in pharmacy and medicine, owing to academic studies and industrial developments of this technology for food processing. The main potential application of HP between 100 MPa and 1200 MPa (1 kbar to 12kbar) is the inactivation of biological agents (bacteria, moulds, yeasts, virus and even prions) which allows the sterilization (i.e, “pascalization”) of thermo-, chemo-, or radio-sensitive materials and chemicals. The immunogenicity of some pressure-killed bacteria or virus suggests also the potentiality of making new vaccines. Another interest of the HP technology is the possibility of storage at subzero temperatures without freezing of cells and animal tissues to be used for cells and organs for transplant. HP engineering may be used for enzyme-catalyzed synthesis of fine chemicals, production of modified proteins of medical or pharmaceutical interest, and organic synthesis of pharmaceuticals.     

Temperature and pressure dependencies of the crystal structure of the organic superconductor (TMTSF) 2 ClO 4

The crystal structure of (TMTSF)₂ClO₄ has been determined at (7 K, 1 bar) and at (7 K, 5 kbar) with a high accuracy. For the latter, low temperature and pressure were applied simultaneously using a X-ray diffraction instrumentation designed in our laboratory, these results are the first for molecular compounds. The effects of lowering the temperature are not the same as those produced by increasing the pressure. At (7 K, 1 bar) the anion ordering which occurs in this compound, and which is characterised by the appearance of b ^* /2 superlattice reflections, is well observed. This anion ordering leads to the presence of two independent stacks of TMTSF cations which is the only case found in the Bechgaard salts family. The comparison of the low temperature crystal structures under atmospheric pressure and at 5 kbar shows that the centres of mass are nearly the same, independent of the pressure: the interchain interactions do not depend on the doubling of the unit cell. Under pressure, the ordering (0, 1/2, 0) does not occur at any temperature. These structural data are confirmed by the quantum chemical calculations which show that the difference in the site energy of the two independent cations is 100 meV. Received 10 April 2000 and Received in final form 27 September 2000     

High pressure electrical resistivity studies on Ga-Te glasses

Abstract

Bulk, semiconducting Ga_xTe_100-x (17 ≤ x ≤ 25) glasses are prepared by the meltingh quenching method and electrical resistivity measurements are carried out at high pressures up to 8 GPa and low temperatures down to 77 K in a Bridgman anvil system. It is found that all the Ga_xTe_100-x, glasses exhibit metallization under pressure, with a continuous decrease in electrical resistivity and activation energy for conduction. Further, the high pressure metallic phases of Ga_xTe_100-x samples are found to be crystalline.     

Pressure calibration in high pressure EXAFS experiments

Abstract

To avoid glitches due to Bragg reflection we use boron carbide anvils for our high pressure EXAFS experiments. Since boron carbide precludes the ruby fluorescence pressure calibration we use the EXAFS of selected materials to measure the reduction with pressure of the nearest neighbor distance and infer the pressure from an isothermal equation of state. The requirements for a good pressure calibrant and the difficulties in EXAFS data analysis are discussed. Cu, NaBr and RbCl have successfully been tested as pressure calibrants.     

Pressure induced resistivity changes in diluted magnetic semiconductors of (Hg,Fe)Se and (Hg,Mn)Te

Abstract

Electrical resistance measurements have been performed on samples of Hg_1-xFe_xSe and Hg_1-xMn_xTe for × ≤ 0.30 as a function of pressure to 7.0 GPa. Both materials crystallize in the B3 structure at atmospheric pressure and room temperature and belong to the class of materials known as diluted magnetic semiconductors. At elevated pressures, HgTe undergoes a phase change from the B3 to B9 phase which is accompanied by an increase in the electrical resistivity of several orders of magnitude. The results reported here are that increasing × in both materials serves to increase the B3-B9 transition pressure. The implication being that partial replacement of the Hg-bonds by Fe-Se and Mn-Te, respectively, increases the stability range of the B3 phase     

Demixing in binary systems at high pressure

Abstract

It is a well-recognized fact that the application of pressure has a large influence on the properties of pure substances. It will be shown here that this is even more true in the case of mixtures. Under high pressure, several new and interesting phenomena are observed. Some recent results will be presented and discussed.     

The kiel/berlin pressure cells for neutron powder diffraction

Abstract

Two piston/cylinder type pressure cells for neutron powder diffraction are presented. They provide a large sample volume and hence allow rapid data collection at moderate flux neutron sources. Structure refinements from the diffraction data are possible. The maximum attainable pressure is above 2 GPa for Ti/Zr zero-scattering pressure cylinders. Both cells may be equipped with a micro-furnace. This allows measurement up to 700 K, simultaneously to the application of pressure. The low temperature setup for the cell-I is presented which will allow experiments down to 1·5K.     

Solidification of selenium melt under pressure up to 6.5 GPa

Abstract

High purity selenium samples were melted under high pressure (≤6.4 GPa) and quenched at various rates ranging from 2 K s^?1 to 500 K s^?1 and the recovered material was examined by X-ray diffraction and electron microscopy. In the entire range of pressure and cooling rate, the melt was found to solidify into a polycrystalline aggregate of the trigonal phase of selenium. The samples obtained by slow cooling of the melt at 6.4 GPa contain, in addition to crystalline phase, regions which appear to be amorphous.     

A tof neutron diffraction system for high pressure and low temperature

Abstract

Time-of-flight method of neutron diffraction is applied for materials under high pressure and low temperature. Extra-scattering from the pressure cell is reduced by geometrical design and by shielding with boron-plastics. Temperature is controled by adjusting the supply of liquid nitrogen: Successive transformations with pressure are observed in heavy ice.     

Structural study of ND4Br at high pressure

Abstract

A structure of ND₄Br has been studied at pressures up to 9 GPa by means of time-of-flight neutron diffraction. A phase transition to the high pressure phase V was observed at P=8·2(5)GPa. It was found that the phase V has a tetragonal structure with an antiparallel ordering of ammonium ions, space group P4/nmm which is in strong resemblance with low temperature modification ND₄Br(III). Deuterium positional parameter as a function of pressure was obtained.     

Developments of nano-polycrystalline diamond anvil cells for neutron diffraction experiments

ABSTRACT

A new high pressure cell for neutron diffraction experiments using nano-polycrystalline anvils is presented. The cell design, off-line pressure generation tests and a gas-loading procedure for this cell are described. The performance is illustrated by powder neutron diffraction patterns of ice VII to ～82?GPa. We also demonstrate the feasibility of single crystal neutron diffraction experiments of Fe₃O₄ at ambient conditions using this cell and discuss the current limitation and future developments.     

High pressure studies on single crystals of zirconium sulphoselenides

Abstract

Electrical resistance measurements have been performed on ZrS₂, ZrSe₂, and ZrSSe single crystals grown by Iodine vapour transport technique in the pressure range 10 kilobar to 80 kilobar (1 GPa-8GPa). It is observed that all these crystals show a decrease in resistance with increase in pressure. The results have been discussed on the basis of energy band.     

New packaging solutions for high pressure treatments of food

Abstract

High pressure is a promising technology for developing new processes in food treatment. In most cases, pre-packaged foods are used in high pressure treatments (HP). Consequently, the behavior of the package under treatment conditions is an important factor. The following work was devoted to the evaluation of different packages under high pressure conditions in the presence of different substances used as simulants. Three main characterizations were carried out after HP treatment: mechanical resistance, barrier properties and integrity.

The experimental results using multilayer plastic films (PA/PE) have led to the selection of several solutions which may be used as packages for high pressure treatments.     

Pressure induced superconductivity in Bismuth

Abstract

In this paper, we report the high pressure band structure and superconductivity calculated for Bi at 9 GPa. The band structure calculation was performed using the LMTO method. The theoretically calculated value of superconducting transition temperature is found to be in good agreement with the experimentally observed value. It is concluded that the same mechanism of s → electron transfer under pressure, which has been used by us for explaining the pressure induced superconductivity in many solids, is found to be responsible for promoting T, in Bi also.     

Long time relaxation effect of liquid castor oil under high pressure conditions

Abstract

The relaxation times of Liquid castor oil have been investigated by observing the scattered light of a He-Ne gas laser during the slow or rapid application of pressure up to 0.7 GPa at 293 K. For the application of pressure above 0.36 GPa the strong increase of the scattered light intensity approximately by a factor of 10² has been observed 12 hours after an application of pressure. This pressure effect is also observed for subsequent pressure cycles, i.e., the pressure effect is reversible. Experimental results are interpreted according to a model of a liquid of long rod molecules.     

High pressure study of the 2D polymeric phase of C60 by means of raman spectroscopy

Abstract

The effect of high hydrostatic pressure, up to 12GPa, on the intramolecular phonon frequencies and the material stability of the two-dimensional tetragonal Cm polymer has been studied by means of Raman spectroscopy in the spectral range of the radial intramolecular modes (200-800cm^?1). A number of new Raman modes appear in the spectrum for pressures ～ 1.4 and ～ 5.0 GPa. The pressure coefficients for the majority of the phonon modes exhibit changes to lower values at P=4.0 GPa, which may be related to a structural modification of the 2D polymer to a more isotropic phase. The peculiarities observed in the Raman spectra are reversible and the material is stable in the pressure region investigated.