Structural phase transition in NH₄F under extreme pressure conditions.

Ammonium fluoride (NH₄F) exhibits a variety of crystalline phases depending on temperature and pressure. By employing Raman spectroscopy and synchrotron X-ray diffraction beyond megabar pressures (up to 140 GPa), we have here observed a novel dense solid phase of NH₄F, characterised by the tetragonal P4/nmm structure also observed in other ammonium halides under less extreme pressure conditions, typically a few GPa. Using detailed ab-initio calculations and reevaluating earlier theoretical models pertaining to other ammonium halides, we examine the microscopic mechanisms underlying the transition from the low-pressure cubic phase (P-43m) to the newly identified high-pressure tetragonal phase (P4/nmm).

Krypton and the Fundamental Flaw of the Lennard-Jones Potential.

We have performed a series of neutron scattering experiments on supercritical krypton. Our data and analysis allow us to characterize the Frenkel line crossover in this model monatomic fluid. The data from our measurements was analyzed using Empirical Potential Structure Refinement to determine the short- and medium-range structure of the fluids.

How to determine solubility in binary mixtures from neutron scattering data: The case of methane and water.

It has recently been discovered that, when subjected to moderate amounts of pressure, methane dissolves in water to form binary mixtures of up to 40% molar methane. No significant solubility of water in methane is known. In these mixtures, the water hydrogen-bond network is largely complete and surrounds the methane molecules.

Structural Markers of the Frenkel Line in the Proximity of Widom Lines.

We have performed a neutron scattering experiment on supercritical fluid nitrogen at 160 K (1.27 ) over a wide pressure range (7.8 MPa/0.

Ammonium fluoride's analogy to ice: Possibilities and limitations.

Ammonium fluoride, NHF, is often seen as an analog to ice, with several of its solid phases closely resembling known ice phases. While its ionic and hydrogen-ordered nature puts topological constraints on the ice-like network structures it can form, it is not clear what consequences these constraints have for NHF compound formation and evolution. Here, we explore computationally the reach and eventual limits of the ice analogy for ammonium fluoride.

Structure and nature of ice XIX.

Ice is a material of fundamental importance for a wide range of scientific disciplines including physics, chemistry, and biology, as well as space and materials science. A well-known feature of its phase diagram is that high-temperature phases of ice with orientational disorder of the hydrogen-bonded water molecules undergo phase transitions to their ordered counterparts upon cooling. Here, we present an example where this trend is broken.

Squeezing Oil into Water under Pressure: Inverting the Hydrophobic Effect.

The molecular structure of dense homogeneous fluid water-methane mixtures has been determined for the first time using high-pressure neutron-scattering techniques at 1.7 and 2.2 GPa.

Transition from Gas-like to Liquid-like Behavior in Supercritical N.

We have studied in detail the transition from gas-like to rigid liquid-like behavior in supercritical N at 300 K (2.4 ). Our study combines neutron diffraction and Raman spectroscopy with ab initio molecular dynamics simulations.

On a new nitrogen sX hydrate from ice XVII.

Recently, a new gas hydrate structure has been discovered. This structure, sX, is unique in a sense that it is so far the only gas hydrate with chiral channels. It is formed by hydrogen-water or carbon dioxide-water mixtures at pressures above 0.

Pressure-Induced Miscibility Increase of CH in HO: A Computational Study Using Classical Potentials.

Methane and water demix under normal (ambient) pressure and temperature conditions because of the polar nature of water and the apolar nature of methane. Recent experimental work has shown, though, that increasing the pressure to values between 1 and 2 GPa (10-20 kbar) leads to a marked increase of methane solubility in water, for temperatures which are well below the critical temperature for water. Here, we perform molecular dynamics simulations based on classical force fields-which are well-used and have been validated at ambient conditions-for different values of pressure and temperature.

Ostwald's rule of stages and metastable transitions in the hydrogen-water system at high pressure.

Although the hydrogenous analogue of the D2-D2O system has been well explored in the regimes above 1 GPa, and below 0.2 GPa, there have been very few studies in the region between these pressures. The recent discovery in the range 0.

Magnetic and structural changes in LaCoMnO at high pressure.

The structural and magnetic properties of LaCoMnO have been studied as a function of pressure by neutron powder diffraction and DC magnetometry. The material is confirmed to exhibit rhombohedral R [Formula: see text] c symmetry between ambient pressure and 6 GPa. We have determined the bulk modulus B of the sample using a second-order Birch-Murnaghan equation of state which yielded: B   =  140(9) GPa and V [Formula: see text].

Topologically frustrated ionisation in a water-ammonia ice mixture.

Water and ammonia are considered major components of the interiors of the giant icy planets and their satellites, which has motivated their exploration under high P-T conditions. Exotic forms of these pure ices have been revealed at extreme (~megabar) pressures, notably symmetric, ionic, and superionic phases. Here we report on an extensive experimental and computational study of the high-pressure properties of the ammonia monohydrate compound forming from an equimolar mixture of water and ammonia.

When immiscible becomes miscible-Methane in water at high pressures.

At low pressures, the solubility of gases in liquids is governed by Henry's law, which states that the saturated solubility of a gas in a liquid is proportional to the partial pressure of the gas. As the pressure increases, most gases depart from this ideal behavior in a sublinear fashion, leveling off at pressures in the 1- to 5-kbar (0.1 to 0.

A Chiral Gas-Hydrate Structure Common to the Carbon Dioxide-Water and Hydrogen-Water Systems.

We present full in situ structural solutions of carbon dioxide hydrate-II and hydrogen hydrate C at elevated pressures using neutron and X-ray diffraction. We find both hydrates adopt a common water network structure. The structure exhibits several features not previously found in hydrates; most notably it is chiral and has large open spiral channels along which the guest molecules are free to move.

Survey of Emergency and Essential Surgical, Obstetric and Anaesthetic Services Available in Bangladeshi Government Health Facilities.

Objective: Evaluate the capacity of government-run hospitals in Bangladesh to provide emergency and essential surgical, obstetric and anaesthetic services.

Methods: Cross-sectional survey of 240 Bangladeshi Government healthcare facilities using the World Health Organisation Situational Analysis Tool to Assess Emergency and Essential Surgical Care (SAT). This tool evaluates the ability of a healthcare facility to provide basic surgical, obstetric and anaesthetic care based on 108 queries that detail the infrastructure and population demographics, human resources, surgical interventions and reason for referral, and available surgical equipment and supplies.

Ablation index, a novel marker of ablation lesion quality: prediction of pulmonary vein reconnection at repeat electrophysiology study and regional differences in target values.

Aims: Force-Time Integral (FTI) is commonly used as a marker of ablation lesion quality during pulmonary vein isolation (PVI), but does not incorporate power. Ablation Index (AI) is a novel lesion quality marker that utilizes contact force, time, and power in a weighted formula. Furthermore, only a single FTI target value has been suggested despite regional variation in left atrial wall thickness.

Pressure-induced localisation of the hydrogen-bond network in KOH-VI.

Using a combination of ab initio crystal structure prediction and neutron diffraction techniques, we have solved the full structure of KOH-VI at 7 GPa. Rather than being orthorhombic and proton-ordered as had previously be proposed, we find that this high-pressure phase of potassium hydroxide is tetragonal (space group I4/mmm) and proton disordered. It has an unusual hydrogen bond topology, where the hydroxyl groups form isolated hydrogen-bonded square planar (OH)4 units.

Urea and deuterium mixtures at high pressures.

Urea, like many network forming compounds, has long been known to form inclusion (guest-host) compounds. Unlike other network formers like water, urea is not known to form such inclusion compounds with simple molecules like hydrogen. Such compounds if they existed would be of interest both for the fundamental insight they provide into molecular bonding and as potential gas storage systems.

On the stability of the disordered molecular alloy phase of ammonia hemihydrate.

The disordered-molecular-alloy phase (DMA) of ammonia hydrates [J. S. Loveday and R.

The crystal structure of methane B at 8 GPa--an α-Mn arrangement of molecules.

From a combination of powder and single-crystal synchrotron x-ray diffraction data we have determined the carbon substructure of phase B of methane at a pressure of ∼8 GPa. We find this substructure to be cubic with space group I4¯3m and 58 molecules in the unit cell. The unit cell has a lattice parameter a = 11.

An improved method for calibrating time-of-flight Laue single-crystal neutron diffractometers.

A robust and comprehensive method for determining the orientation matrix of a single-crystal sample using the neutron Laue time-of-flight (TOF) technique is described. The new method enables the measurement of the unit-cell parameters with an uncertainty in the range 0.015-0.

Strength analysis and optimisation of double-toroidal anvils for high-pressure research.

We used the finite element method for stress and deformation analysis of the large sample volume double-toroidal anvil and gasket assembly used with the Paris-Edinburgh press for neutron scattering, in order to investigate the failure of this assembly observed repeatedly in experiments at a load of approximately 240 tonnes. The analysis is based on a new approach to modelling an opposed anvil device working under extreme stress conditions. The method relies on use of experimental data to validate the simulation in the absence of the material property data available for high pressure conditions.

Pressure-induced dehydration and the structure of ammonia hemihydrate-II.

The structure of the crystalline ammonia-bearing phase formed when ammonia monohydrate liquid is compressed to 3.5(1) GPa at ambient temperature has been solved from a combination of synchrotron x-ray single-crystal and neutron powder-diffraction studies. The solution reveals that rather than having the ammonia monohydrate (AMH) composition as had been previously thought, the structure has an ammonia hemihydrate composition.

Unflued gas heaters and respiratory symptoms in older people with asthma.

Background: Unflued gas heaters (UFGHs) and cookers are a major contributor to air pollution in homes. Gas appliances have been associated with adverse respiratory outcomes in children and, less consistently, adults. There have been very few studies on the effects of gas appliances on the respiratory health of older people.