Using density functional theory, we show that the long-believed transition-metal tetraborides (TB4) of tungsten and molybdenum are in fact triborides (TB3). This finding is supported by thermodynamic, mechanical, and phonon instabilities of TB4, and it challenges the previously proposed origin of superhardness of these compounds and the predictability of the generally used hardness model. Theoretical calculations for the newly identified stable TB3 structure correctly reproduce their structural and mechanical properties, as well as the experimental x-ray diffraction pattern. However, the relatively low shear moduli and strengths suggest that TB3 cannot be intrinsically stronger than c-BN. The origin of the lattice instability of TB3 under large shear strain that occurs at the atomic level during plastic deformation can be attributed to valence charge depletion between boron and metal atoms, which enables easy sliding of boron layers between the metal ones.

Among transition metal nitrides, tungsten nitrides possess unique and/or superior chemical, mechanical, and thermal properties. Preparation of these nitrides, however, is challenging because the incorporation of nitrogen into tungsten lattice is thermodynamically unfavorable at atmospheric pressure. To date, most materials in the W-N system are in the form of thin films produced by nonequilibrium processes and are often poorly crystallized, which severely limits their use in diverse technological applications. Here we report synthesis of tungsten nitrides through new approaches involving solid-state ion exchange and nitrogen degassing under pressure. We unveil a number of novel nitrides including hexagonal and rhombohedral W2N3. The final products are phase-pure and well-crystallized in bulk forms. For hexagonal W2N3, hexagonal WN, and cubic W3N4, they exhibit elastic properties rivaling or even exceeding cubic-BN. All four nitrides are prepared at a moderate pressure of 5 GPa, the lowest among high-pressure synthesis of transition metal nitrides, making it practically feasible for massive and industrial-scale production.

Hydrostatic pressure, as an alternative of chemical pressure to tune the crystal structure and physical properties, is a significant technique for novel function material design and fundamental research. In this article, we report the phase stability and visible light response of the organolead bromide perovskite, CH3NH3PbBr3 (MAPbBr(3)), under hydrostatic pressure up to 34 GPa at room temperature: Two phase transformations below 2 GPa (from Pm (3) over barm to Im (3) over bar, then to Pnma) and a reversible amorphization starting from about 2 GPa were observed, which could be attributed to the tilting of PbBr6 octahedra and destroying of long-range ordering of MA cations, respectively. The visible light response of MAPbBr3 to pressure was studied by in situ photoluminescence, electric resistance, photocurrent measurements and first-principle simulations. The anomalous band gap evolution during compression with red-shift followed by blue-shift is explained by the competition between compression effect and pressure-induced amorphization. Along with the amorphization process accomplished around 25 GPa, the resistance increased by 5 orders of magnitude while the system still maintains its semiconductor characteristics and considerable response to the visible light irradiation. Our results not only show that hydrostatic pressure may provide an applicable tool for the organohalide perovskites based photovoltaic device functioning as switcher or controller, but also shed light on the exploration of more amorphous organometal composites as potential light absorber.

The suggestion by Zaritsky & Lin (ZL) that a vertical extension of the red clump feature (the VRC) in color-magnitude diagrams (CMDs) of the Large Magellanic Cloud is consistent with a significant population of foreground stars to the LMC that could account for the observed microlensing optical depth has been challenged by various investigators. We respond by (1) examining each of the challenges presented, to determine whether any or all of those arguments invalidate the claims made by ZL, and (2) presenting new photometric and spectroscopic data obtained in an attempt to resolve this issue. We systematically discuss why the objections raised so far do not unequivocally refute ZL's claim. We conclude that although the CMD data do not mandate the existence of a foreground population, they are entirely consistent with a foreground population associated with the LMC that contributes significantly (similar to 50%) to the observed microlensing optical depth. From our new data, we conclude that less than or similar to 40% of the VRC stars are young, massive red clump stars, because (1) synthetic CMDs created using the star formation history derived independently from Hubble Space Telescope data suggest that fewer than 50% of the VRC stars are young, massive red clump stars, (2) the angular distribution of the VRC stars is more uniform than that of the young (<1 Gyr) main-sequence stars, and (3) the velocity dispersion of the VRC stars in the region of the LMC examined by ZL, 18.4 +/- 2.8 km s(-1) (95% confidence limits), is inconsistent with the expectation for a young disk population. Each of these arguments is predicated on assumptions, and the conclusions are uncertain. Therefore, an exact determination of the contribution to the microlensing optical depth by the various hypothesized foreground populations, and the subsequent conclusions regarding the existence of halo MACHOs, requires a detailed knowledge of many complex astrophysical issues, such as the initial mass function, star formation history, and post-main-sequence stellar evolution.

The Transiting Exoplanet Survey Satellite (TESS) will search for planets transiting bright and nearby stars. TESS has been selected by NASA for launch in 2017 as an Astrophysics Explorer mission. The spacecraft will be placed into a highly elliptical 13.7-day orbit around the Earth. During its two-year mission, TESS will employ four wide-field optical CCD cameras to monitor at least 200,000 main-sequence dwarf stars with I-C less than or similar to 13 for temporary drops in brightness caused by planetary transits. Each star will be observed for an interval ranging from one month to one year, depending mainly on the star's ecliptic latitude. The longest observing intervals will be for stars near the ecliptic poles, which are the optimal locations for follow-up observations with the James Webb Space Telescope. Brightness measurements of preselected target stars will be recorded every 2 min, and full frame images will be recorded every 30 min. TESS stars will be 10-100 times brighter than those surveyed by the pioneering Kepler mission. This will make TESS planets easier to characterize with follow-up observations. TESS is expected to find more than a thousand planets smaller than Neptune, including dozens that are comparable in size to the Earth. Public data releases will occur every four months, inviting immediate community-wide efforts to study the new planets. The TESS legacy will be a catalog of the nearest and brightest stars hosting transiting planets, which will endure as highly favorable targets for detailed investigations.

A scientific forum on The Future Science of Exoplanets and Their Systems, sponsored by Europlanet(*) and the International Space Science Institute (ISSI)(dagger) and co-organized by the Center for Space and Habitability (CSH)(double dagger) of the University of Bern, was held during December 5 and 6, 2012, in Bern, Switzerland. It gathered 24 well-known specialists in exoplanetary, Solar System, and stellar science to discuss the future of the fast-expanding field of exoplanetary research, which now has nearly 1000 objects to analyze and compare and will develop even more quickly over the coming years. The forum discussions included a review of current observational knowledge, efforts for exoplanetary atmosphere characterization and their formation, water formation, atmospheric evolution, habitability aspects, and our understanding of how exoplanets interact with their stellar and galactic environment throughout their history. Several important and timely research areas of focus for further research efforts in the field were identified by the forum participants. These scientific topics are related to the origin and formation of water and its delivery to planetary bodies and the role of the disk in relation to planet formation, including constraints from observations as well as star-planet interaction processes and their consequences for atmosphere-magnetosphere environments, evolution, and habitability. The relevance of these research areas is outlined in this report, and possible themes for future ISSI workshops are identified that may be proposed by the international research community over the coming 2-3 years.

Understanding the patterns of marine microbial diversity (Bacteria + Archaea) is essential, as variations in their alpha- and beta-diversities can affect ecological processes. Investigations of microbial diversity from global oceanographic expeditions and basin-wide transects show positive correlations between microbial diversity and either temperature or productivity, but these studies rarely captured seasonality, especially in polar regions. Here, using multiannual alpha-diversity data from eight time series in the northern and southern hemispheres, we show that marine microbial community richness and evenness generally correlate more strongly with daylength than with temperature or chlorophyll a (a proxy for photosynthetic biomass). This pattern is observable across time series found in the northern and southern hemispheres regardless of collection method, DNA extraction protocols, targeted 16S rRNA hypervariable region, sequencing technology, or bioinformatics pipeline.

The geochemistry of asteroidal magmas provides fundamental clues to the processes involved in the origin and early evolution of planetary bodies. Although sulfides are important reservoirs for a diverse suite of major and trace elements, it is currently unclear whether the interiors of asteroid Vesta and the Angrite Parent Body were sulfide liquid saturated during petrogenesis of non-cumulate eucrites and volcanic angrites. To assess the potential of sulfide liquid saturation in the interiors of these bodies, high pressure (P) - temperature (T) experiments were used to quantify the sulfur concentrations at sulfide saturation (SCSS) for volcanic angrites and non-cumulate eucrites. The sulfide-silicate partitioning behavior of various trace elements was simultaneously quantified to study their geochemical behavior at sulfide liquid saturation.