Dan J. Wilson

Crystal growth and elastic properties of orthorhombic Bi2Ga4O9

The combination of favourable oxygen conductivity at high temperatures with mechanical strength make Bi-containing compounds with mullite-type crystal structures strong candidates for use as electrolytes of solid fuel cells. Large single crystals of orthorhombic Bi2Ga4O9 with dimensions up to 20 × 20 × 10 mm3 were grown by the top-seeded solution growth technique. Their elastic constants at room temperature were determined for the first time using resonant ultrasound spectroscopy. The values given in GPa are c11 = 143.4(2), c22 = 161.7(2), c33 = 224.2(3), c44 = 69.6(1), c55 = 49.2(1), c66 = 76.5(2), c12 = 73.7(2), c13 = 62.2(3) and c23 = 70.3(3). Further, the crystal structure and the elastic properties of Bi2Ga4O9 were studied at 0 K by parameter-free ab initio calculations based on density-functional theory. On average the computed elastic constants differ from the experimental values by about 10%, indicating the reliability of the theoretical approach. Like in other mullite-type compounds the anisotropy of the longitudinal elastic stiffness is clearly controlled by the structurally dominant octahedral chains running parallel to [001]. The deviations from Cauchy relations show a significant anisotropy of the type g22>g11≈g33 which is related to the covalent character of the bonding interactions within the infinite –Bi–O–Bi–O– bond chains parallel to [010]. The mean elastic stiffness of Bi2Ga4O9 is about 40% smaller than for 2/1-mullite and sillimanite. This discrepancy can be attributed to the mechanically very soft behaviour of the Bi 6s2 lone electron pair. Its stereochemical activity is clearly evident from both the asymmetry of the bismuth coordination polyhedron and the calculated electron density maps.

Framework flexibility and the negative thermal expansion mechanism of copper(I) oxideCu2O

The negative thermal expansion (NTE) mechanism in Cu2O has been characterised via mapping of different Cu2O structural flexibility models onto phonons obtained using ab-initio lattice dynamics. Low frequency acoustic modes that are responsible for the NTE in this material correspond to vibrations of rigid O-Cu-O rods. There is also some small contribution from higher frequency optic modes that correspond to rotations of rigid and near-rigid OCu4 tetrahedra as well as of near-rigid O-Cu-O rods. The primary NTE mode also drives a ferroelastic phase transition at high pressure; our calculations predict this to be to an orthorhombic structure with space group Pnnn.

New tools to support collaboration and virtual organizations

We describe the use of new eScience tools to support collaboration, including the use of XML data representations to support shared viewing of the information content of data, metadata tools for documenting data and Web 2.0 social networking tools for documenting ideas and the collaboration process. This latter work has led to the development of the http://SciSpace.net Web resource.

High temperature elastic properties of Mg-cordierite: experimental studies and atomistic simulations

Abstract The temperature dependence of the elastic stiffness coefficients of natural orthorhombic Mg-cordierite was studied between 295 K and 1573 K using resonant ultrasound spectroscopy. The measurements revealed a continuous decrease of all the elastic constants with increasing temperature. The bulk modulus softens from about 129(2) GPa at 295 K to 110(2) GPa at 1473 K. Irreversible anomalies in the temperature evolution of the resonance frequencies of certain eigenmodes were observed above 920 K due to the escape of volatiles and the occurrence of microcracks. However, the dehydrated samples still showed integrity on the macroscopic scale. Therefore, despite the occurence of the micro-cracks, a reasonable quantitative analysis of the high-temperature RUS data of cordierite samples was still feasible. The thermal expansion was studied between 100 K and 1570 K using dilatometry. The new data are consistent with earlier experimental results and confirm the expansion of the a and b unit cell parameters and the contraction of the c parameter with increasing temperature. Posäsible contributions of the Al/Si disorder to the elastic properties of Mg-cordierite were estimated on the basis of force-field and quantum mechanical calculations. The behaviour of individual elastic stiffness coefficients was followed across the order/disorder transition by Monte Carlo simulations. The simulations predicted a decrease in the bulk modulus with increasing Al/Si disorder. However, this effect is much smaller than that observed experimentally. The measured decrease in the elastic stiffness coefficients is mainly due to phonon softening effects.

Information Delivery in Computational Mineral Science: The eMinerals Data Handling System

Escience technologies are designed to address problems arising in the practice of computational science. The outcome of computational science is the generation of data, and a significant portion of these problems concern Information Delivery: the process of disseminating data across virtual organizations, and of contextualizing that data so that relevant quantities may be extracted and examined with the minimum of difficulty. This paper describes some of these issues encountered in the particular field of computational mineral science, and the solutions that the eMinerals consortium has developed to address them.

An e-Science data infrastructure for simulations within Grid computing environment: methods, approaches and practice

Grid‐based simulation usually involves large quantities of data at each stage of the simulation process. These data include simulation input and output files, intermediate results files, log and error files, associated metadata, and information capturing the processes that generate the data. The question of how to effectively store and manage data files within a Grid computing environment is increasingly becoming an important issue. This paper illustrates how we built a lightweight e‐Science infrastructure for data management within a Grid computing environment, including the integration of data curation activities into the entire Grid‐based simulation process. Rather than focusing on specific implementation details, we aim to identify the key issues and research challenges, describing how various existing technologies and tools can be best integrated to address these requirements and challenges. Although the case of quantum mechanical simulation of materials properties is used in the paper, much of the discussion is as generic as possible so that approaches, methods and practice (e.g. integrated approach, workflow taxonomy and development approach, simple but useful semantic annotation approach) can be applied to wider domains and disciplines to facilitat the digital research. A comparison between our approach and Cloud computing, and lessons learned in data management within the Grid computing environment, are also presented. Copyright

A Service-Oriented Framework for Running Quantum Mechanical Simulations of Material Properties in a Grid Environment

In this paper, a service-oriented framework for running quantum mechanical simulation of material properties over Grids is proposed, and a prototype framework has been developed. The framework consists of portal and workflow systems, a set of class libraries and application programming interfaces, defined service specifications, schemas, and configuration files. The framework can be instantiated to submit specific quantum mechanical simulation (e.g., CASTEP) jobs to a Grid from a Web browser with the required tasks managed and coordinated by the workflow without human interaction. The paper details analysis, design, and implementation of a prototype framework. In the test case, the prototype framework is instantiated to submit a CASTEP simulation job to available Grid resources in order to calculate the equation of state of a material.

Atomistic model of diopside-K-jadeite (CaMgSi2O6-KAlSi2O6) solid solution

Atomistic model was proposed to describe the thermodynamics of mixing in the diopside-K-jadeite solid solution (CaMgSi2O6-KAlSi2O6). The simulations were based on minimization of the lattice energies of 800 structures within a 2 × 2 × 4 supercell of C2/c diopside with the compositions between CaMgSi2O6 and KAlSi2O6 and with variable degrees of order/disorder in the arrangement of Ca/K cations in M2 site and Mg/Al in Ml site. The energy minimization was performed with the help of a force-field model. The results of the calculations were used to define a generalized Ising model, which included 37 pair interaction parameters. Isotherms of the enthalpy of mixing within the range of 273–2023 K were calculated with a Monte Carlo algorithm, while the Gibbs free energies of mixing were obtained by thermodynamic integration of the enthalpies of mixing. The calculated T-X diagram for the system CaMgSi2O6-KAlSi2O6 at temperatures below 1000 K shows several miscibility gaps, which are separated by intervals of stability of intermediate ordered compounds. At temperatures above 1000 K a homogeneous solid solution is formed. The standard thermodynamic properties of K-jadeite (KAlSi2O6) evaluated from quantum mechanical calculations were used to determine location of several mineral reactions with the participation of the diopside-K-jadeite solid solution. The results of the simulations suggest that the low content of KalSi2O6 in natural clinopyroxenes is not related to crystal chemical factors preventing isomorphism, but is determined by relatively high standard enthalpy of this end member.