A large volume pressure cell for high temperatures

Abstract

Studies of matter under very high pressure at synchrotron radiation sources are mostly done using pressure cells with single-crystal diamond anvils. In some cases the available volume (≤ 10^?3mm³)in such cells causes problems especially at high temperature and for crystal synthesis. To ensure sufficient homogeneity of pressure and temperature, the use of cells with large sample volumes (≥ 1 mm³) is necessary.

Existing devices for such measurements are compared with a novel setup which consists of a toroidal anvil arrangement and a lightweight (50 kg) press with 250 tonnes (2.5 MN) capacity. Preliminary tests of this instrument with synchrotron radiation are reported.

Presented at the IUCr Workshop on ‘Synchrotron Radiation Instrumentation for HighPressure Crystallography’. Daresbury Laboratory 20-21 July 1991     

Integral equation formulation for buoyancy-driven convection problems

An integral equation formulation for buoyancy-driven convection problems is developed and illustrated. Buoyancy-driven convection in a bounded cylindrical geometry with a free surface is studied for a range of aspect ratios and Nusselt numbers. The critical Rayleigh number, the nature of the cellular motion, and the heat transfer enhancement are computed using linear theory. Green's functions are used to convert the linear problem into linear Fredholm integral equations. Theorems are proved which establish the properties of the eigenvalues and eigenfunctions of the linear integral operator which appears in these equations.     

The ammonia hydrates—model mixed-hydrogen-bonded systems

The three ammonia hydrates (ammonia dihydrate, ADH; ammonia monohydrate, AMH and ammonia hemihydrate, AHH) are amongst the simplest systems to contain mixed nitrogen oxygen–hydrogen bonds. In addition, ADH and AMH are believed to be common in the outer solar system, and their properties are important for modelling bodies such as Uranus, Neptune and Titan. Neutron diffraction studies have provided the first full information on structures at ambient pressure with detailed information on the H-bond geometry and state of hydrogen ordering. High-pressure studies of AMH reveal a rich and complex high-pressure phase diagram. This work reveals that ammonia hydrates are likely to provide a wide variety of geometries for the study of mixed H-bonds. It also provides new experimental information on which to base modelling of Titan.     

Heavy metal complexes of macrocyclic trithioethers

We report here two crystal structures of heavy metal complexes containing macrocyclic trithioethers. The trithioether, 10S3 (1,4,7-trithiacyclodecane) complexes Pb(II) to form a compound of the formula [Pb(10S3)](ClO₄)₂(H₂O) (Compound 1). This behavior contrasts the related trithioether 9S3 (1,4,7-trithianonane), which readily forms a bis Pb(II) complex. Crystals of Compound 1 consist of [Pb(10S3)(H₂O)]²⁺ units and perchlorate anions which are weakly bound to the Pb²⁺ center. The two perchlorate anions serve as bidentate ligands to a single lead(II) center. However, a third oxygen in one of the perchlorates also serves as a bridging group between two lead(II) centers, linking the [Pb(10S3)(H₂O)]²⁺ units into zigzag chains. Thus, a highly irregular nine-coordinate (S₃O₆) environment around the Pb²⁺ion is formed by the three sulfur donors from the 10S3 ligand, one coordinated water, and five oxygens from three different perchlorate anions. Also, the lone pair electrons on the Pb center are stereochemically active. Compound 2, [Cd(9S3)₂](ClO₄)₂ 2 CH₃NO₂, forms a distorted octahedral structure with unusual hexakis(thioether) coordination to the Cd(II) center. Bond distances and angles are very comparable to those found in the structure of the tetrafluoroborate salt of the complex cation. (1): P2₁/c, a = 9.1720(6) Å, b = 15.7687(11) Å, c = 11.8427(8) Å, = 93.0180(10) , V = 1710.4 (2) ų, Z = 4; (2): P2₁/c , a = 10.4305(10) Å, b = 15.2059(6) Å, c = 9.4919(7) Å, = 99.324(9), V = 1485.6(2) ų, Z = 2.     

High-pressure transitions in methane hydrate

Three recent studies of the high-pressure transformations of methane hydrate [Chem. Phys. Lett. 325 (2000) 490; Proc. Natl. Acad. Sci. 97 (2000) 13484; Nature 410 (2001) 661] have reported apparently different behaviours. Detailed comparison of our X-ray and neutron diffraction data with those obtained in earlier work shows that there is in fact consistent behaviour on isothermal compression at room temperature with a transition from the clathrate I structure hydrate to a hexagonal hydrate with an unknown structure at 0.9 GPa.