X-ray synchrotron diffraction (XRD) measurements of single-crystal and powder molybdenum disulfide MoS2 are performed at pressures (P) up to 78 GPa and temperatures (T) of 20 to 298 K in diamond-anvil cells. The results on single crystals demonstrate a sharp pressure induced isosymmetric phase transition of 2H(c) to 2H(a) modification at 23 GPa at 40 and 300 K. The structure of the high-pressure 2H(a) phase previously inferred theoretically and from powder XRD data is confirmed by our single-crystal XRD data solution, which also definitively determines the atomic potions as a function of pressure. No additional periodicity (commensurate or incommensurate) or distortion is found in the whole P-T range of this study. These results suggest that a previously proposed hypothetic charge-density-wave phase does not host pressure induced superconductivity experimentally found above 90 GPa.

We have obtained Gemini Planet Imager (GPI)J-,H-,K1-, andK2-Spec observations of the iconic debris ring around the young, main-sequence star HR 4796A. We applied several point-spread function (PSF) subtraction techniques to the observations (Mask-and-Interpolate, RDI-NMF, RDI-KLIP, and ADI-KLIP) to measure the geometric parameters and the scattering phase function for the disk. To understand the systematic errors associated with PSF subtraction, we also forward-modeled the observations using a Markov Chain Monte Carlo framework and a simple model for the disk. We found that measurements of the disk geometric parameters were robust, with all of our analyses yielding consistent results; however, measurements of the scattering phase function were challenging to reconstruct from PSF-subtracted images, despite extensive testing. As a result, we estimated the scattering phase function using disk modeling. We searched for a dependence of the scattering phase function with respect to the GPI filters but found none. We compared theH-band scattering phase function with that measured by Hubble Space Telescope STIS at visual wavelengths and discovered a blue color at small scattering angles and a red color at large scattering angles, consistent with predictions and laboratory measurements of large grains. Finally, we successfully modeled the SPHEREH2HR 4796A scattered phase function using a distribution of hollow spheres composed of silicates, carbon, and metallic iron.

We report a new hydrogen clathrate hydrate synthesized at 1.2 GPa and 298 K documented by single-crystal x-ray diffraction, Raman spectroscopy, and first-principles calculations. The oxygen sublattice of the new clathrate hydrate matches that of ice II, while hydrogen molecules are in the ring cavities, which results in the trigonal R3c or R (3) over barc space group (proton ordered or disordered, respectively) and the composition of (H2O)(6)H-2. Raman spectroscopy and theoretical calculations reveal a hydrogen disordered nature of the new phase C-1', distinct from the well-known ordered C-1 clathrate, to which this new structure transforms upon compression and/or cooling. This new clathrate phase can be viewed as a realization of a disordered ice II, unobserved before, in contrast to all other ordered ice structures.

Optical spectroscopic observations of white dwarf stars selected from catalogs based on the Gaia DR2 database reveal nine new gaseous debris disks that orbit single white dwarf stars, about a factor of 2 increase over the previously known sample. For each source we present gas emission lines identified and basic stellar parameters, including abundances for lines seen with low-resolution spectroscopy. Principle discoveries include (1) the coolest white dwarf (T-eff 12,720 K) with a gas disk; this star, WD0145+234, has been reported to have undergone a recent infrared outburst; (2) co-location in velocity space of gaseous emission from multiple elements, suggesting that different elements are well mixed; (3) highly asymmetric emission structures toward SDSS J0006+2858, and possibly asymmetric structures for two other systems; (4) an overall sample composed of approximately 25% DB and 75% DA white dwarfs, consistent with the overall distribution of primary atmospheric types found in the field population; and (5) never-before-seen emission lines from Na in the spectra of Gaia J0611-6931, semi-forbidden Mg, Ca, and Fe lines toward WD 0842+572, and Si in both stars. The currently known sample of gaseous debris disk systems is significantly skewed toward northern hemisphere stars, suggesting a dozen or so emission line stars are waiting to be found in the southern hemisphere.

The thermal expansion at constant pressure of solid CD4 III is calculated for the low-temperature region where only the rotational tunneling modes are essential and the effect of phonons and librons can be neglected. It is found that in mK region there is a giant peak of the negative thermal expansion. The height of this peak is comparable or even exceeds the thermal expansion of solid N-2, CO, O-2, or CH4 in their triple points. It is shown that like in the case of light methane, the effect of pressure is quite unusual: as evidenced from the pressure dependence of the thermodynamic Gruneisen parameter (which is negative and large in the absolute value), solid CD4 becomes increasingly quantum with rising pressure.

The young (50-400 Myr) A3V star beta Leo is a primary target to study the formation history and evolution of extrasolar planetary systems as one of the few stars with known hot (similar to 1600 K), warm (similar to 600 K), and cold (similar to 120 K) dust belt components. In this paper, we present deep mid-infrared measurements of the warm dust brightness obtained with the Large Binocular Telescope Interferometer (LBTI) as part of its exozodiacal dust survey (HOSTS). The measured excess is 0.47% 0.050% within the central 1.5 au, rising to 0.81% 0.026% within 4.5 au, outside the habitable zone of beta Leo. This dust level is 50 10 times greater than in the solar system's zodiacal cloud. Poynting-Robertson drag on the cold dust detected by Spitzer, and Herschel underpredicts the dust present in the habitable zone of beta Leo, suggesting an additional delivery mechanism (e.g., comets) or an additional belt at similar to 5.5 au. A model of these dust components is provided that implies the absence of planets more than a few Saturn masses between similar to 5 au and the outer belt at similar to 40 au. We also observationally constrain giant planets with the LBTI imaging channel at 3.8 mu m wavelength. Assuming an age of 50 Myr, any planet in the system between approximately 5-50 au must be less than a few Jupiter masses, consistent with our dust model. Taken together, these observations showcase the deep contrasts and detection capabilities attainable by the LBTI for both warm exozodiacal dust and giant exoplanets in or near the habitable zone of nearby stars.

With the exception of lithium, alkali metals do not react with elemental nitrogen either at ambient conditions or at elevated temperatures, requiring the search for alternative synthetic routes to their nitrogen-containing compounds. Here using a controlled decomposition of sodium azide (NaN3) at high pressure conditions, we synthesize two novel compounds, Na-3(N-2)(4) and NaN2, both containing dinitrogen anions. NaN2 synthesized at 4 GPa might be the common intermediate in high-pressure solid-state metathesis reactions, where NaN3 is used as a source of nitrogen, while Na-3(N-2)(4) opens a new class of compounds, where [N-2] units accommodate a noninteger formal charge of 0.75-. This finding can dramatically extend the expected compositions in other group 1 and 2 metal-nitrogen systems. Electronic structure calculations show the metallic character for both compounds.

We present the discovery of an extreme flaring event from Proxima Cen by the Australian Square Kilometre Array Pathfinder (ASKAP), Atacama Large Millimeter/submillimeter Array (ALMA), Hubble Space Telescope (HST), Transiting Exoplanet Survey Satellite (TESS), and the du Pont Telescope that occurred on 2019 May 1. In the millimeter and FUV, this flare is the brightest ever detected, brightening by a factor of >1000 and >14,000 as seen by ALMA and HST, respectively. The millimeter and FUV continuum emission trace each other closely during the flare, suggesting that millimeter emission could serve as a proxy for FUV emission from stellar flares and become a powerful new tool to constrain the high-energy radiation environment of exoplanets. Surprisingly, optical emission associated with the event peaks at a much lower level with a time delay. The initial burst has an extremely short duration, lasting for <10 s. Taken together with the growing sample of millimeter M dwarf flares, this event suggests that millimeter emission is actually common during stellar flares and often originates from short burst-like events.

Alkaline earth metal peroxides are typical examples of ionic compounds containing polyanions. We herein report a stable BaO2 phase at high pressure up to 130 GPa found via a first-principles computational structure search and high-pressure experimental investigations. The identified monoclinic structure (space group C2/m) can be derived by sublattice distortions of Ba atoms and peroxide groups associated with the phonon mode softening of the lower-pressure Cmmm structure. Contrary to the previous expectation of polymerization of the peroxide group at elevated pressure, this phase retains the peroxide group and, interestingly, exhibits an insulating behavior demonstrating an increase of the band gap under compression. Our synchrotron x-ray diffraction (XRD) measurements could not distinguish between Cmmm and C2/m BaO2 definitively because the difference in XRD patterns is very subtle. However, our data do not show any sign of polymerization transition up to 120 GPa. Raman spectra of the O-O peroxide vibration show a small anomaly in frequency at 110 GPa, which is qualitatively like that predicted theoretically due to the Cmmm to C2/m phase transition, thus supporting the predicted transformation.

The ultrafast synthesis of epsilon-Fe3N1+x in a diamond-anvil cell (DAC) from Fe and N-2 under pressure was observed using serial exposures of an X-ray free electron laser (XFEL). When the sample at 5 GPa was irradiated by a pulse train separated by 443 ns, the estimated sample temperature at the delay time was above 1400 K, confirmed by in situ transformation of alpha- to gamma-iron. Ultimately, the Fe and N-2 reacted uniformly throughout the beam path to form Fe3N1.33, as deduced from its established equation of state (EOS). We thus demonstrate that the activation energy provided by intense X-ray exposures in an XFEL can be coupled with the source time structure to enable exploration of the time-dependence of reactions under high-pressure conditions.