Silicon is essential for today's electronics because of its ability to show various electronic behaviors that are relevant to numerous fields of cutting-edge applications. It is not a pollutant and, therefore, an ideal candidate to replace the actual materials in photovoltaics, such as compounds based on the arsenic and heavy metals. However, conventional diamond-like Si is an indirect gap semiconductor and cannot absorb solar photons directly. This justifies intensive theoretical and experimental research for the direct-bandgap forms of silicon. Our recent high-pressure studies of the chemical interaction and phase transformations in the Na-Si system, revealed a number of interesting routes to new and known silicon compounds and allotropes. The pressure-temperature range of their formation is suitable for large-volume synthesis and future industrial scaling. The variety of properties observed (e.g. quasi-direct bandgap of open-framework allotrope Si-24) allows us to suggest future applications. (C) 2016 The Authors. Published by Elsevier Ltd.

Recent satellite inferences of hydrous sulfates as recurrent minerals on the surface of icy planetary bodies link with the potential mineral composition of their interior. Blodite, a mixed Mg-Na sulfate, is here taken as representative mineral of icy satellites surface to investigate its crystal structure and stability at conditions of the interior of icy bodies. To this aim we performed in situ synchrotron angle-dispersive X-ray powder diffraction experiments on natural blodite at pressures up to similar to 10.4 GPa and temperatures from similar to 118.8 K to similar to 490.0 K using diamond anvil cell technique to investigate the compression behavior and establish a low-to-high temperature equation of state that can be used as reference when modeling the interior of sulfate-rich icy satellites such as Ganymede.

The lithium-carbon binary system possesses a broad range of chemical compounds, which exhibit fascinating chemical bonding characteristics, which give rise to diverse and technologically important properties. While lithium carbides with various compositions have been studied or suggested previously, the crystal structures of these compounds are far from well understood. In this work, we present the first comprehensive survey of all ground state (GS) structures of lithium carbides over a broad range of thermodynamic conditions, using ab initio density functional theory (DFT) crystal structure searching methods. Thorough searches were performed for 29 stoichiometries ranging from Li12C to LiC12 at 0 and 40 GPa. Based on formation enthalpies from optimized van der Waals density functional calculations, three thermodynamically stable phases (Li4C3, Li2C2, and LiC12) were identified at 0 GPa, and seven thermodynamically stable phases (Li8C, Li6C, Li4C, Li8C3, Li2C, Li3C4, and Li2C3) were predicted at 40 GPa. A rich diversity of carbon bonding, including monomers, dimers, trimers, nanoribbons, sheets, and frameworks, was found within these structures, and the dimensionality of carbon connectivity existing within each phase increases with increasing carbon concentration. We find that the well-known composition LiC6 is actually a metastable one. We also find a unique coexistence of carbon monomers and dimers within the predicted thermodynamically stable phase Li8C3, and different widths of carbon nanoribbons coexist in a metastable phase of Li2C2 (Imm2). Interesting mixed sp(2)-sp(3) carbon frameworks are predicted in metastable phases with composition LiC6.

Periodic mesoporous hexagonal boron nitride was investigated as a precursor for nanoscale forms of cubic boron nitride. Cubic boron nitride (c-BN) nanocrystals were obtained at a pressure of 14 GPa and a temperature of 1300 degrees C. The synthesized nanocrystals have diameters of similar to 50 nm and are solutionprocessible. Mesoporous c-BN formed at a pressure of 10 GPa and 1000 degrees C. The mesoporous c-BN has a pore size distribution centered around 3 nm and a surface area of 122 m(2)/g as determined by electron tomography. At pressures of 8 and 6 GPa near monodisperse h-BN nanodiscs with diameters of 0.5-1 mm and 0.2-0.5 mm were formed, respectively. The discs formed colloidal solutions in acetone. The hydrodynamic radius of the discs matched the radii determined by dynamic light scattering indicating absence of nanodisc aggregation in solution.

The Group 14 element silicon possesses a complex free-energy landscape with many (local) minima, allowing for the formation of a variety of unusual structures, some of which may be stabilized at ambient conditions. Such exotic silicon allotropes represent a significant opportunity to address the ever-increasing demand for novel materials with tailored functionality since these exotic forms are expected to exhibit superlative properties including optimized band gaps for solar power conversion. The application of pressure is a well-recognized and uniquely powerful method to access exotic states of silicon since it promotes large changes to atomic bonding. Conventional high-pressure syntheses, however, lack the capability to access many of these local minima and only four forms of exotic silicon allotropes have been recovered over the last 50 years. However, more recently, significant advances in high pressure methodologies and the use of novel precursor materials have yielded at least three more recoverable exotic Si structures. This review aims to give an overview of these innovative methods of high-pressure application and precursor selection and the recent discoveries of new Si allotropes. The background context of the conventional pressure methods and multitude of predicted new phases are also provided. This review also offers a perspective for possible access to many further exotic functional allotropes not only of silicon but also of other materials, in a technologically feasible manner. Published by AIP Publishing.

We studied the low-pressure (0-10 GPa) phase diagram of crystalline benzene using quantum Monte Carlo and density functional theory (DFT) methods. We performed diffusion quantum Monte Carlo (DMC) calculations to obtain accurate static phase diagrams as benchmarks for modern van der Waals density functionals. Using density functional perturbation theory, we computed the phonon contributions to the free energies. Our DFT enthalpy-pressure phase diagrams indicate that the Pbca and P2(1)/c structures are the most stable phases within the studied pressure range. The DMC Gibbs free-energy calculations predict that the room temperature Pbca to P2(1)/c phase transition occurs at 2.1(1) GPa. This prediction is consistent with available experimental results at room temperature. Our DMC calculations give 50.6 +/- 0.5 kJ/mol for crystalline benzene lattice energy. Published by AIP Publishing.

The potassium (K) and water (H2O) cycles in subduction zones are predominately controlled by the stability of K-and H2O-bearing minerals, such as K-mica, lawsonite, and dense hydrous magnesium silicates (DHMS). K-micas (muscovite or phlogopite) are the principal H2O and K hosts in subduction zones and Earth's upper mantle and play a significant role in the deep H2O and K cycles. The Mg-10 angstrom phase, normally appearing in hydrated peridotite in high-pressure experiments, has been considered as an important water-carrier in subducted hydrated peridotite. In this study, we found a K-bearing Al-10 angstrom phase in the MORB+H2O system (hydrated basalt) at high pressures according to X-ray diffraction and stoichiometry. We experimentally constrained its stability field at high pressure. By considering newly and previously documented compositions of the 10 angstrom phase and micas, we confirmed a continuous solid solution or mixed layering between the 10 angstrom phase and K-mica at the interlayer site, suggesting that the K cycle and the H2O cycle in subduction zones are coupled. From the discussion of the effect of f(H2O) on stability of the Al-10 angstrom phase, we conclude that a cold subduction zone can host and carry more bulk H2O and K into Earth's deep mantle than a hot one. This work expands the stability regions of the 10 angstrom phase from the ultramafic system (Mg-10 angstrom phase) to the mafic system (Al-10 angstrom phase), and emphasizes the significance of the 10 angstrom phase for the deep H2O and K cycle in subduction zone.

The power conversion efficiency for solar cells fabricated using organometal halide perovskites (OMHPs) has risen to more than 20% in a short span of time, making OMHPs promising solar materials for harnessing energy from sunlight. The hybrid perovskite architecture that consists of organic molecular cations and an inorganic lattice could also potentially serve as a robust platform for materials design to realize functionalities beyond photovoltaic applications. Taking methyl ammonium lead iodide (MAPbI(3)) as an example, we explore the response of organometal halide perovskites to various stimuli, using all-atom molecular dynamics simulations with a first-principles-based interatomic potential. We find that a large electric field is necessary to introduce a sizable molecular ordering at room temperature in unstrained MAPbI(3). Molecular dipoles in epitaxially strained MAPbI(3) are more susceptible to an electric field. We also report various caloric effects in MAPbI(3). The adiabatic thermal change is estimated directly by introducing different driving fields in the simulations. We find that MAPbI(3) exhibits both electrocaloric and mechanocaloric effects at room temperature. Local structural analysis reveals that the rearrangement of molecular cations in response to electric and stress fields is responsible for the caloric effects. The enhancement of caloric response could be realized through strain engineering and chemical doping.

Through use of in situ Raman spectroscopy and single-crystal/powder X-ray diffraction, we resolve the "C-0" phase structure discovered recently in the H-2 + H2O system. This phase forms at similar to 400 MPa and 280 K with the nominal composition (H2O)(2)H-2 and three formula units per unit cell. The hexagonal structure is chiral, consisting of interpenetrating spiral chains of hydrogen-bonded water molecules and rotationally disordered H-2 molecules, and shows topological similarities with the mineral quartz. Like other clathrate hydrates and forms of ice, the protons of H2O molecules within C-0 are disordered. The large zeolite-like channels accommodate significant amounts of hydrogen (5.3% by weight) in a unique hydrogen-bonded lattice, which might be applicable to the thermodynamic conditions found on icy planetary bodies.

Two of the three natural quasiperiodic crystals found in the Khatyrka meteorite show a composition within the Al-Cu-Fe system. Icosahedrite, with formula Al63Cu24Fe13, coexists with the new Al62Cu31Fe2 quasicrystal plus additional Al-metallic minerals such as stolperite (AlCu), kryachkoite [(AlCu)(6)(Fe, Cu)], hollisterite (AlFe3), khatyrkite (Al2Cu) and cupalite (AlCu), associated to high-pressure phases like ringwoodite/ahrensite, coesite, and stishovite. These high-pressure minerals represent the evidence that most of the Khatyrka meteoritic fragments formed at least at 5 GPa and 1200 degrees C, if not at more extreme conditions. On the other hand, experimental studies on phase equilibria within the representative Al-Cu Fe system appear mostly limited to ambient pressure conditions, yet. This makes the interpretation of the coexisting mineral phases in the meteoritic sample quite difficult.