Sulfur is a very promising cathode material for rechargeable energy storage devices. However, sulfur cathodes undergo a noticeable volume variation upon cycling, which induces mechanical stress. In spite of intensive investigation of the electrochemical behavior of the lithiated sulfur compounds, their mechanical properties are not very well understood. In order to fill this gap, we developed a ReaxFF interatomic potential to describe Li-S interactions and performed molecular dynamics (MD) simulations to study the structural, mechanical, and kinetic behavior of the amorphous lithiated sulfur (a-LixS) compounds. We examined the effect of lithiation on material properties such as ultimate strength, yield strength, and Young's modulus. Our results suggest that with increasing lithium content, the strength of lithiated sulfur compounds improves, although this increment is not linear with lithiation. The diffusion coefficients of both lithium and sulfur were computed for the a-LixS system at various stages of Li-loading. A grand canonical Monte Carlo (GCMC) scheme was used to calculate the open circuit voltage profile during cell discharge. The Li-S binary phase diagram was constructed using genetic algorithm based tools. Overall, these simulation results provide insight into the behavior of sulfur based cathode materials that are needed for developing lithium-sulfur batteries.

We present an evolutionary algorithm which predicts stable atomic structures and phase diagrams by searching the energy landscape of empirical and ab initio Hamiltonians. Composition and geometrical degrees of freedom may be varied simultaneously. We show that this method utilizes information from favorable local structure at one composition to predict that at others, achieving far greater efficiency of phase diagram prediction than a method which relies on sampling compositions individually. We detail this and a number of other efficiency-improving techniques implemented in the genetic algorithm for structure prediction code that is now publicly available. We test the efficiency of the software by searching the ternary Zr-Cu-Al system using an empirical embedded-atom model potential. In addition to testing the algorithm, we also evaluate the accuracy of the potential itself. We find that the potential stabilizes several correct ternary phases, while a few of the predicted ground states are unphysical. Our results suggest that genetic algorithm searches can be used to improve the methodology of empirical potential design.

Density-functional theory energies, forces, and elastic constants determine the parameterization of an empirical, modified embedded-atom method potential for molybdenum. The accuracy and transferability of the potential are verified by comparison to experimental and density-functional data for point defects, phonons, thermal expansion, surface and stacking fault energies, and ideal shear strength. Searching the energy landscape predicted by the potential using a genetic algorithm verifies that it reproduces not only the correct bcc ground state of molybdenum but also all low energy metastable phases. The potential is also applicable to the study of plastic deformation and used to compute energies, core structures, and Peierls stresses of screw and edge dislocations.

Single-layer materials represent a new materials class with properties that are potentially transformative for applications in nanoelectronics and solar energy harvesting. With the goal of discovering novel two-dimensional (2D) materials with unusual compositions and structures, we have developed a grand canonical evolutionary algorithm that searches the structure and composition space while constraining the thickness of the structures. Coupling the algorithm to first principles total energy methods, we show that this approach can successfully identify known 2D materials and find novel low-energy ones. We present the details of the algorithm, including suitable objective functions, and illustrate its potential with a study of the Sn-S and C-Si binary materials systems. The algorithm identifies several new 2D structures of InP, recovers known 2D structures in the binary Sn-S and C-Si systems, and finds two new 1D Si defects in graphene with formation energies below that of isolated substitutional Si atoms.

Synchrotron x-ray diffraction experiments have been carried out on Eu metal at ambient temperature to pressures as high as 92 GPa (0.92 Mbar). Following the well-known bcc-to-hcp transition at 12 GPa, a mixed phase region is observed from 18 to 66 GPa until finally a single orthorhombic (Pnma) phase persists from 66 to 92 GPa. These results are compared to predictions from density functional theory calculations. Under pressure the relatively large molar volume V mol of divalent Eu is rapidly diminished, equaling or falling below V mol (P) for neighboring trivalent lanthanides above 15 GPa. The present results suggest that above 15 GPa Eu is neither divalent nor fully trivalent to pressures as high as 92 GPa. I. IntroductionIn contrast to the other lanthanide metals, which are trivalent, Eu and Yb retain the divalency of their free-atom state. As a result, their atomic volumes are significantly larger and their structures do not fit into the normal structure sequence across the trivalent lanthanide series (hcp → Sm-type → double hcp → fcc → distorted fcc) either with increasing pressure or decreasing atomic number [1][2][3]. At pressures near 1 Mbar, Yb takes on the hexagonal hP3 structure exhibited by the light actinides Sm and Nd under pressure, providing evidence that for pressures of 1 Mbar and above Yb has joined the regular lanthanide series and become fully trivalent [4]. The equation of state (EOS) of Yb is consistent with this conclusion [4].Structure studies on Eu metal at ambient temperatures, on the other hand, have only been carried out to 43 GPa [5,6], revealing a bcc-to-hcp transition at 12 GPa accompanied by a 4% volume collapse, with a new close-packed structure appearing near 17 GPa. From the fact that the EOS of Eu approaches that of trivalent Gd near 20 GPa, it was concluded that at this pressure a significant increase in Eu's valence must have occurred [5]. L III absorption [7] and Mössbauer effect [8,9] studies reportedly indicate that at 10 GPa Eu's valence has already increased to approximately 2.5, with a further increase to 2.64 at pressures of 34 GPa. Theoretical predictions of the pressure necessary for the full divalent-to-trivalent transition in Eu vary from 35 GPa [2, 10] to 71 GPa [11].Should sufficiently high pressure be applied to bring Eu to full trivalency Eu 3+ (4f 6 where J