Lithium oxide: a quantum-corrected and classical Monte Carlo study.

Extensive Monte Carlo simulations of lithium oxide, Li2O, an important material for fusion applications over a wide range of temperatures have been performed. In the low temperature range 1-500 K, quantum path-integral corrections to the enthalpy and unit cell size were determined. We show that classical Monte Carlo underestimates both these quantities and the difference between unit cell parameters with and without quantum corrections is large enough that such corrections should be included in any comparison between theory and experiment. Over the range 300-1000 K, the formation energies of Schottky and Frenkel defects are calculated and compared with those from direct free energy minimisation in the quasiharmonic approximation, which also includes quantum corrections; the Monte Carlo results highlight the onset of failure of the quasiharmonic approximation even at modest temperatures and suggest only a small variation of the defect enthalpies with temperature. Several possible diffusion mechanisms are identified. While an interstitialcy mechanism activates at around 900-1000 K, lithium vacancy migration dominates from 500 K. The estimated migration energy of the Li-vacancy jump (0.28 eV) agrees very well with the most recent NMR study. At temperatures above 1000 K, the superionic phase transition and subsequent melting are simulated and there is good agreement with available experimental data. Our simulations predict a rapid rise in the heat capacity and the thermal expansion coefficient which continues up to the melting point which leaves two interesting questions for future experimental studies: (i) whether above the superionic transition the heat capacity and the thermal expansion coefficient in antifluorite Li2O rise up to the melting point, as in our simulations, or fall, as observed in several fluorites, and (ii) the subsequent change in the heat capacity during melting.

Three-dimensional kinetic Monte Carlo simulations of diamond chemical vapor deposition.

A three-dimensional kinetic Monte Carlo model has been developed to simulate the chemical vapor deposition of a diamond (100) surface under conditions used to grow single-crystal diamond (SCD), microcrystalline diamond (MCD), nanocrystalline diamond (NCD), and ultrananocrystalline diamond (UNCD) films. The model includes adsorption of CHx (x = 0, 3) species, insertion of CHy (y = 0-2) into surface dimer bonds, etching/desorption of both transient adsorbed species and lattice sidewalls, lattice incorporation, and surface migration but not defect formation or renucleation processes. A value of ∼200 kJ mol(-1) for the activation Gibbs energy, ΔG(‡) etch, for etching an adsorbed CHx species reproduces the experimental growth rate accurately. SCD and MCD growths are dominated by migration and step-edge growth, whereas in NCD and UNCD growths, migration is less and species nucleate where they land. Etching of species from the lattice sidewalls has been modelled as a function of geometry and the number of bonded neighbors of each species. Choice of appropriate parameters for the relative decrease in etch rate as a function of number of neighbors allows flat-bottomed etch pits and/or sharp-pointed etch pits to be simulated, which resemble those seen when etching diamond in H2 or O2 atmospheres. Simulation of surface defects using unetchable, immobile species reproduces other observed growth phenomena, such as needles and hillocks. The critical nucleus for new layer growth is 2 adjacent surface carbons, irrespective of the growth regime. We conclude that twinning and formation of multiple grains rather than pristine single-crystals may be a result of misoriented growth islands merging, with each island forming a grain, rather than renucleation caused by an adsorbing defect species.

Post-transplantation primary central nervous system lymphoma in a patient with systemic lupus erythematosus and prolonged use of immunosuppressant.

Post-transplantation primary central nervous system lymphoma is an uncommon and fatal post-transplant lymphoproliferative disorder. Such lymphomas have been described in only a few case series in the literature. The incidence of this condition is rising with improved survival after organ transplantation. A case of post-transplantation primary central nervous system lymphoma in a young Chinese woman with systemic lupus erythematosus is described here. She presented with right-sided weakness and memory loss after tooth extraction 2 weeks before admission. Contrast computed tomography of the brain demonstrated a contrast rim-enhancing lesion over the left frontal lobe. With a history of recent dental procedure, long-term immunosuppressive therapy and computed tomography findings, cerebral abscess was highly suspected. Emergency operation was performed. Histopathology showed post-transplantation primary central nervous system lymphoma, with cells positive for B-cell marker CD20. Immunosuppressant was stopped and she was treated with radiotherapy and rituximab (anti-CD20 monoclonal antibody). She remained disease-free at 16 months. Post-transplantation primary central nervous system lymphoma is rare with variable presentation and radiological features. We believe rituximab may have a role in the treatment of such lymphomas.

The embedded many-body expansion for energetics of molecular crystals.

Reliable prediction of molecular crystal energetics is a vital goal for computational chemistry. Here we show that accurate results can be obtained from a monomer-based many-body expansion truncated at the two-body level, with the monomer and dimer calculations suitably embedded in a model of the crystalline environment. By including the two dominant effects--electrostatics and exchange-repulsion--we are able to capture the important nonadditive terms in the energy, and approach very closely results from full periodic second-order Møller-Plesset calculations. The advantage of the current scheme is that extension to coupled-cluster and explicitly correlated F12 methods is completely straightforward. We demonstrate the approach through calculations on carbon dioxide, hydrogen fluoride, and ice XIh and XIc. In accord with previous studies, we find these two ice polymorphs to be very close in energy, with our periodic coupled-cluster single double triple-F12 calculation giving the hexagonal structure more stable by around 0.3 kJ mol(-1).

Comparison of the incremental and hierarchical methods for crystalline neon.

We present a critical comparison of the incremental and hierarchical methods for the evaluation of the static cohesive energy of crystalline neon. Both of these schemes make it possible to apply the methods of molecular electronic structure theory to crystalline solids, offering a systematically improvable alternative to density functional theory. Results from both methods are compared with previous theoretical and experimental studies of solid neon and potential sources of error are discussed. We explore the similarities of the two methods and demonstrate how they may be used in tandem to study crystalline solids.

Surface diffusion and surface growth in nanofilms of mixed rocksalt oxides.

We have modelled the surface diffusion and growth of BaO and SrO both in the homoepitaxial and heteroepitaxial (BaO on SrO and SrO on BaO) cases. The diffusion proceeds most favourably by an exchange mechanism involving the surface layer. When impurities are adsorbed on the surface this can lead to intermixing between the layers. This strongly suggests that ionic materials may not be grown on a substrate with a similar structure without significant intermixing. Island growth begins with the formation of individual clusters which grow and merge together.

Phosphorus carbides: theory and experiment.

The recent finding that radio frequency plasma activation of CH(4)/PH(3) gas mixtures can yield films with P : C ratios < or = 3 has served to trigger further research into new 'phosphorus carbide' materials. Theoretical and experimental results relating to periodic and amorphous materials, respectively, are presented here: (i) The electronic structure and stability of different crystalline phosphorus carbide P(x)C(y) phases have been studied using first-principles density-functional theory. Calculations have been carried out for P(4)C(3+8 n) (n= 0-4), PC, and PC(3) and the most likely periodic structures examined in detail. Particular attention is paid to the composition PC(3), for which there are several possibilities of similar energy. (ii) Recent experimental efforts have involved use of pulsed laser ablation methods to produce hydrogen-free phosphorus carbide thin films. Mechanically hard, electrically conducting diamond like carbon films containing 0- approximately 26 at.% P have been deposited on both Si and quartz substrates by 193 nm PLA of graphite/phosphorus targets (containing varying percentages of phosphorus), at a range of substrate temperatures (T(sub)= 298-700 K), in vacuum, and analysed via laser Raman and X-ray photoelectron spectroscopy.

The 'zero charge' partitioning behaviour of noble gases during mantle melting.

Noble-gas geochemistry is an important tool for understanding planetary processes from accretion to mantle dynamics and atmospheric formation. Central to much of the modelling of such processes is the crystal-melt partitioning of noble gases during mantle melting, magma ascent and near-surface degassing. Geochemists have traditionally considered the 'inert' noble gases to be extremely incompatible elements, with almost 100 per cent extraction efficiency from the solid phase during melting processes. Previously published experimental data on partitioning between crystalline silicates and melts has, however, suggested that noble gases approach compatible behaviour, and a significant proportion should therefore remain in the mantle during melt extraction. Here we present experimental data to show that noble gases are more incompatible than previously demonstrated, but not necessarily to the extent assumed or required by geochemical models. Independent atomistic computer simulations indicate that noble gases can be considered as species of 'zero charge' incorporated at crystal lattice sites. Together with the lattice strain model, this provides a theoretical framework with which to model noble-gas geochemistry as a function of residual mantle mineralogy.

Dopant incorporation into garnet solid solutions--a breakdown of Goldschmidt's first rule.

Atomistic simulations suggest trace elements are more soluble in a 50:50 pyrope (Mg3Al2Si3O12)-grossular (Ca3Al2Si3O12) garnet mixture than in either end-member; consistent with partitioning experiments, and, contrary to Goldschmidt's first rule, large trace element cations may substitute for Mg2+, small trace elements for Ca2+.

Applications of momentum-space similarity.

Momentum-space similarity indices were used in studies linking chemical structure to observed activity. These included (a) the biological activity of various molecules that are of interest due to their capacity for HIV inhibition; and (b) the hyperpolarisabilities of series of conjugated molecules. Study (a) included comparisons of the total valence densities of different molecules or the densities associated with particular molecular fragments. Study (b) involved, for each molecule, a comparison of the momentum-space densities of the highest occupied (HOMO) and lowest unoccupied (LUMO) molecular orbitals. The momentum-space approach, which is most sensitive to features of the long-range valence electron density, turned out to be particularly useful for cases such as these, in which the physical property or biological activity has no obvious dependence on the bonding topology of the molecules.

A novel approach to molecular similarity.

We review briefly the general problem of assessing the similarity between one molecule and another. We propose a novel approach to the quantitative estimation of the similarity of two electron distributions. The procedure is based on momentum space concepts, and avoids many of the difficulties associated with the usual position space definitions. Results are presented for the model systems CH3CH2CH3, CH3OCH3, CH3SCH3, H2O and H2S.