The first principle pseudopotential calculations based on the Perdew-BurkeErnzerhof (PBE) form of generalized gradient approximation (GGA) within density functional theory (DFT) has been utilized to investigate the stabilities of insoluble discharge products of oxygen and sulphur in the Na-O and Na-S batteries. Their structural, mechanical and electronic properties were determined. The lattice parameters were well reproduced and agree with the available experimental data. The heats of formation predict that all structures are generally stable and Na2S has the lowest value. The elastic constants suggest that all the structures are mechanically stable which in good agreement with the calculated phonon dispersions.

Li-ion batteries have transformed portable electronics and will play a key role in the electrification of transport. However, the highest energy storage possible for Li-ion batteries is insufficient for the long-term needs of society. Here we consider a study on rechargeable lithium−sulfur (Li−S) batteries which hold great potential for high-performance energy storage systems because they have a high theoretical specific energy, low cost, and are eco-friendly. This work employs computational modelling methods to explore stability, structural and electronic properties of discharge products formed in the Li-S/Se battery, especially Li2S/Se, which has potential to offer higher theoretical specific energy and remedies the challenges that Li-S battery encounters. First principle methods were used to calculate thermodynamic properties of Li2S and Li2Se, which agreed with available experimental results. A cluster expansion technique generated new stable phases of Li2S/Se system and Monte Carlo simulations determined concentration and temperature ranges in which the systems mix. Interatomic Born Meyer potential models for Li2S and Li2Se were derived and validated and used to explore high temperature structural and transport properties of Li2S/Se.

We present the results of atomistic simulations using derived interatomic potentials for the pyrite-structured metal chalcogenides FeS(2), PtSb(2) and PtAs(2). Structural and elastic constants were calculated and compared with experimental measurements. Surface energies of low-index surfaces were calculated and closely reflected the measured stabilities of these compounds. Equivalent surfaces on the pyrite and marcasite structures of FeS(2) explained the experimentally observed intergrowths of the two phases.

Surface Brillouin scattering has been used to study the variation of the velocities of the Rayleigh surface acoustic wave (RSAW) and the high frequency pseudo-SAW (HFPSAW) with azimuthal angle on the (I 10) surface of iron pyrite (FeS2) over the temperature range 293 to 573 K. The elastic constants of this cubic compound, namely C-11, C-12 and C-44 were extracted by the simultaneous fitting of the velocity data to the results of calculations using surface elastodynamic Green's functions and decrease gradually with increasing temperature. An interatomic potential model has been developed for iron pyrite and has been used to calculate the values of the elastic constants over the same range of temperature as the experimental results. There is good agreement between experimental and the computational results. . 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Interatomic potential parameters have been derived at simulated temperatures of 0 K and 300 K to model pyrite FeS 2 . The predicted pyrite structures are within 1% of those determined experimentally, while the calculated bulk modulus is within 7%. The model is also able to simulate the properties of marcasite, even though no data for this phase were included in the fitting procedure. There is almost no difference in results obtained for pyrite using the two potential sets; however, when used to model FeS 2 marcasite, the potential fitted at 0 K performs better. The potentials have also been used to study the high-pressure behaviour of pyrite up to 44 GPa. The calculated equation of state gives good agreement with experiment and shows that the Fe–S bonds shorten more rapidly that the S–S dimer bonds. The behaviour of marcasite at high pressure is found to be similar to that of pyrite.

Atomistic simulation techniques are used to investigate the surface structure, stability and reactivity of pyrite. We introduce a potential model for FeS2 which reproduces experimental structural parameters, elastic constants and hydration energies of pyrite. We modeled the {100}, {110}, and {111} surfaces of pyrite and calculated the {100} surface to be the most stable and to show little surface relaxation, in agreement with experiment. The surfaces were hydrated by associative adsorption of water molecules which stabilized all three surfaces, especially the unstable {111} surface. The calculated adsorption energy of -47 kT mol(-1) for water on the {100} surface agrees well with an adsorption energy of -42 kJ mol(-1), determined for the stoichiometric (100) surface by temperature-programmed desorption.(1) Adsorption of water molecules at surface sites of lower coordination (four- or three- coordinated) showed increased reactivity of these sites. We calculated an increase in adsorption energy of 50-60 kJ mol(-1) per loss of bond. We next created stepped {100} planes in to model a more realistic {100} surface with one-dimensional defects. Four different steps were investigated in two orthogonal directions. Because of the asymmetry of the sulfur dimers, the geometry of the dimers on the edge showed the dimers either leaning forward (F-steps) or backward (B-steps) with respect to the {100} terrace. We used water molecules as a probe of the reactivity of the different surface sites. Corresponding adsorption sites (terrace, edge or below the step) on the F- and B-steps were found to have different reactivities toward water due to the different adsorption modes of the probe molecule. On the B-steps the increased reactivity of the low-coordinated edge iron atom toward water (approximately -70 kT mol(-1)) was outweighed by the network of interactions of the water molecule to atoms on terrace and step wall in the position below the step (-91 kJ mol(-1)). On the F-steps the four-coordinated edge site was calculated to be the most reactive adsorption site.

Full relaxation of volume and internal parameters of iron pyrite and marcasite FeS2, has been studied using a plane-wave pseudopotential method within the Local Density Approximation to Density Functional Theory (LDA-DFT). We find that the internal parameter u of pyrite decreases with hydrostatic compression. The P-V equation of state falls slightly closer to the experimental curve than a previous unrelaxed Tight-Binding Linear Muffin-Tin Orbital (TB-LMTO) calculation. The optimized parameters are used in a TB-LMTO calculation to predict the electronic structure from which we find a larger band gap on marcasite compared to pyrite. Ab initio calculations of elastic constants for pyrite were performed using the Full Potential (FP) LMTO method and agree to within 7% with experiment.

A revisited electronic structure study of iron pyrite, FeS2, has been performed using a new Tight-Binding Linear Muffin-Tin Orbital (TB-LMTO) technique in which the radii of overlapping MT spheres are determined from a full potential construction. The interstitial spheres were chosen to provide an efficient packing of space while ensuring that the overlap between the spheres remain small. We have found that this treatment of interstitial spheres results in a dramatic improvement in the description of the electronic structure and the binding energy curves for FeS2 in comparison with a previous LMTO calculation. In particular, the energy band gap, the equilibrium lattice constant and the bulk modulus are all in much better agreement with experimental observations. Moreover, the calculated equation of state is in excellent accord with recent measured P-V data up to pressures of 15GPa with overall deviations of less than 10%. The predicted reflectivity spectrum of FeS2 as a function of pressure gives the observed behaviour of the optical edge. The bonding behaviour the orthorhombic marcasite phase of FeS2 is also discussed within this new TB-LMTO formalism.