The high-pressure alpha/beta phase transition in lead sulphide (PbS): X-ray powder diffraction and quantum mechanical calculations

The high-pressure behaviour of PbS was investigated by angular dispersive X-ray powder diffraction up to pressures of 6.8 GPa. Experiments were accompanied by first principles calculations at the density functional theory level. By combining both methods reliable data for the elastic properties of rock-salt type alpha- and high-pressure beta-PbS could be obtained. beta-PbS could be determined to crystallise in the CrB-type (B33), with space group Cmcm. The reversible ferro-elastic alpha/beta transition is of first order. It is accompanied by a large volume discontinuity of about 5% and a coexistence region of the two phases. A gliding mechanism of {001} bilayers along one of the cubic -directions governs the phase transition which can be described in terms of group/subgroup relationships via a common subgroup, despite its reconstructive character. The quadrupling of the primitive unit cell indicates a wave vector (0,0,pi/a) on the Delta-line of the Brillouin zone.Comment: 7 pages, 5 figures, 3 tables submitted to EP

A computational investigation into the suitability of purely siliceous zeolites as reverse osmosis membranes

Desalination by reverse osmosis is an increasingly important source of potable water in many countries. The interest in developing new, more effective membranes is, therefore, great. One set of materials that have been suggested as a possible new type of desalination membrane are nanoporous materials. In this work computational methods are used to investigate the behavior of water within five different zeolitic systems. Quantum mechanical calculations are used to construct a set of force-field parameters for two atomistic models. Molecular dynamics simulations of the zeolites show that water will diffuse through zeolites at a rate faster than that obtained with the composite membranes currently used in commercial desalination. In addition, the thermodynamics of salt rejection have been investigated using the free energy perturbation method. The results of these calculations show that the chloride ion finds the zeolitic environment strongly unfavorable compared to the bulk solution. In the case of the sodium ion, the energetic difference between the zeolite environment and solution is less significant, but charge separation prevents sodium from permeating the membrane