Nitrogen and water are very abundant in nature; however, the way they chemically react at extreme pressure-temperature conditions is unknown. Below 6 GPa, they have been reported to form clathrate compounds. Here, we present Raman spectroscopy and x-ray diffraction studies in the H2O-N-2 system at high pressures up to 140 GPa. We find that clathrates, which form locally in our diamond cell experiments above 0.3 GPa, transform into a fine grained state above 6 GPa, while there is no sign of formation of mixed compounds. We point out size effects in fine grained crystallites, which result in peculiar Raman spectra in the molecular regime, but x-ray diffraction shows no additional phase or deviation from the bulk behavior of familiar solid phases. Moreover, we find no sign of ice doping by nitrogen, even in the regimes of stability of nonmolecular nitrogen.

Companions embedded in the cavities of transitional circumstellar disks have been observed to exhibit excess luminosity at H alpha, an indication that they are actively accreting. We report 5 yr (2013-2018) of monitoring of the position and H alpha excess luminosity of the embedded, accreting low-mass stellar companion HD 142527 B from the MagAO/VisAO instrument. We use pyklip, a Python implementation of the Karhunen-Loeve Image Processing algorithm, to detect the companion. Using pyklip forward modeling, we constrain the relative astrometry to 1-2 mas precision and achieve sufficient photometric precision (+/- 0.2 mag, 3% error) to detect changes in the H alpha contrast of the companion over time. In order to accurately determine the relative astrometry of the companion, we conduct an astrometric calibration of the MagAO/VisAO camera against 20 yr of Keck/NIRC2 images of the Trapezium cluster. We demonstrate agreement of our VisAO astrometry with other published positions for HD 142527 B, and use orbitize! to generate a posterior distribution of orbits fit to the relative astrometry of HD 142527 B. Our data suggest that the companion is close to periastron passage, on an orbit significantly misaligned with respect to both the wide circumbinary disk and the recently observed inner disk encircling HD 142527 A. We translate observed H alpha contrasts for HD 142527 B into mass accretion rate estimates on the order of 4-9 x 10(-10) M (circle dot) yr(-1). Photometric variation in the H alpha excess of the companion suggests that the accretion rate onto the companion is variable. This work represents a significant step toward observing accretion-driven variability onto protoplanets, such as PDS 70 b&c.

Synthesis and characterization of nitrogen-rich materials is important for the design of novel high energy density materials due to extremely energetic low-order nitrogen-nitrogen bonds. The balance between the energy output and stability may be achieved if polynitrogen units are stabilized by resonance as in cyclo-N-5(-) pentazolate salts. Here we demonstrate the synthesis of three oxygen-free pentazolate salts Na2N5, NaN5 and NaN5 center dot N-2 from sodium azide NaN3 and molecular nitrogen N-2 at similar to 50 GPa. NaN5 center dot N-2 is a metal-pentazolate framework (MPF) obtained via a self-templated synthesis method with nitrogen molecules being incorporated into the nanochannels of the MPF. Such self-assembled MPFs may be common in a variety of ionic pentazolate compounds. The formation of Na2N5 demonstrates that the cyclo-N-5 group can accommodate more than one electron and indicates the great accessible compositional diversity of pentazolate salts.

We explore whether assumptions about dust grain shape affect the resulting estimates of the composition and grain size distribution of the AU Microscopii (AU Mic) debris disk from scattered-light data collected by Lomax et al. The near edge-on orientation of the AU Mic debris disk makes it ideal for studying the effect of the scattering phase function on the measured flux ratios as a function of wavelength and projected distance. Previous efforts to model the AU Mic debris disk have invoked a variety of dust grain compositions and explored the effect of porosity, but did not undertake a systematic effort to explore a full range of size distributions and compositions to understand possible degeneracies in fitting the data. The degree to which modeling dust grains with more realistic shapes compounds these degeneracies has also not previously been explored. We find differences in the grain properties retrieved depending on the grain shape model used. We also present here our calculations of porous grains of size parameters x = 0.1 to 48 and complex refractive indices (m = n + i kappa) ranging from n = 1.1 to 2.43 and k = 0 to 1.0, covering multiple compositions at visible and near-infrared wavelengths such as ice, silicates, amorphous carbon, and tholins.

Inspired by the rich physical properties of IV-VI compounds, we choose polycrystalline Pb0.99Cr0.01Se to investigate its structural, vibrational, and electrical transport properties under pressure up to 50 GPa. The structural transitions from the B1 to Pnma phase and then to the B2 phase in this sample are verified by the x-ray diffraction and Raman scattering measurements. The formation of the intermediate phase is suggested to be mediated by Peierls distortion, and the broad hump in the temperature-dependent resistivity in the intermediate phase gives further evidence of this phenomenon. When the material evolves into the B2 phase, superconductivity is observed to emerge, accompanied by suppressing the broad hump of resistivity at intermediate temperatures. Meanwhile, Hall coefficient measurements indicate that the carrier type changes during the structural transitions. These results suggest that the superconductivity in the B2 phase for this material is originated by "melting" the Peierls lattice distortion. By extending the present findings to other similar IV-VI semiconductors, we propose that all group IV-VI compounds could exhibit superconductivity in their B2 phase due to the lattice melting at high pressures.

Improving direct detection capability close to the star through improved star subtraction and post-processing techniques is vital for discovering new low-mass companions and characterizing known ones at longer wavelengths. We present results of 17 binary star systems observed with the Magellan adaptive optics system (MagAO) and the Clio infrared camera on the Magellan Clay Telescope using binary differential imaging (BDI). BDI is an application of reference differential imaging (RDI) and angular differential imaging (ADI) applied to wide binary star systems (2 arcsec < Delta rho < 10 arcsec) within the isoplanatic patch in the infrared. Each star serves as the point spread function (PSF) reference for the other, and we performed PSI' estimation and subtraction using principal component analysis. We report contrast and mass limits for the 35 stars in our initial survey using BDI with MagAO/Clio in L' and 3.95 mu m bands. Our achieved contrasts varied between systems, and spanned a range of contrasts from 3.0 to 7.5 magnitudes and a range of separations from 0.2 to 2 arcsec. Stars in our survey span a range of masses, and our achieved contrasts correspond to late-type M-dwarf masses down to similar to 10 M-Jup. We also report detection of a candidate companion signal at 0.2 arcsec (18 au) around HIP 67506 A (SpT G5V, mass similar to 1.2 M-circle dot), which we estimate to be similar to 60 - 90 M-Jup. We found that the effectiveness of BDI is highest for approximately equal brightness binaries in high-Strehl conditions.

Most of the studied two-dimensional (2D) materials are based on highly symmetric hexagonal structural motifs. In contrast, lower-symmetry structures may have exciting anisotropic properties leading to various applications in nano-electronics. In this work we report the synthesis of nickel diazenide NiN2 which possesses atomic-thick layers comprised of Ni2N3 pentagons forming Cairo-type tessellation. The layers of NiN2 are weakly bonded with the calculated exfoliation energy of 0.72 J/m(2), which is just slightly larger than that of graphene. The compound crystallizes in the space group of the ideal Cairo tiling (P4/mbm) and possesses significant anisotropy of elastic properties. The single-layer NiN2 is a direct-band-gap semiconductor, while the bulk material is metallic. This indicates the promise of NiN2 to be a precursor of a pentagonal 2D material with a tunable direct band gap.

The high-precision X-ray diffraction setup for work with diamond anvil cells (DAC5) in interaction chamber 2 (IC2) of the High Energy Density instrument of the European X-ray Free-Electron Laser is described. This includes beamline optics, sample positioning and detector systems located in the multipurpose vacuum chamber. Concepts for pump-probe X-ray diffraction experiments in the DAC are described and their implementation demonstrated during the First User Community Assisted Commissioning experiment. X-ray heating and diffraction of Bi under pressure, obtained using 20 fs X-ray pulses at 17.8 keV and 2.2 MHz repetition, is illustrated through splitting of diffraction peaks, and interpreted employing finite element modeling of the sample chamber in the DAC.

We have performed a combined experimental and theoretical study of ethane and methane at high pressures of up to 120 GPa at 300 K using x-ray diffraction and Raman spectroscopies and the USPEX ab initio evolutionary structural search algorithm, respectively. For ethane, we have determined the crystallization point, for room temperature, at 2.7 GPa and also the low pressure crystal structure (phase A). This crystal structure is orientationally disordered (plastic phase) and deviates from the known crystal structures for ethane at low temperatures. Moreover, a pressure induced phase transition has been identified, for the first time, at 13.6 GPa to a monoclinic phase B, the structure of which is solved based on good agreement with the experimental results and theoretical predictions. For methane, our x-ray diffraction measurements are in agreement with the previously reported high-pressure structures and equation of state (EOS). We have determined the EOSs of ethane and methane, which provides a solid basis for the discussion of their relative stability at high pressures.

In the phase diagram of water, superionic ices with highly mobile protons within the stable oxygen sublattice have been predicted at high pressures. However, the existence of superionic ices and the location of the melting line have been challenging to determine from both theory and experiments, yielding contradictory results depending on the employed techniques and the interpretation of the data. Here we report high-pressure and high-temperature synchrotron X-ray diffraction and optical spectroscopy measurements of water in a laser-heated diamond anvil cell and reveal first-order phase transitions to ices with body-centred and face-centred cubic oxygen lattices. Based on the distinct density, increased optical conductivity and the greatly decreased fusion enthalpies, we assign these observed structures to the theoretically predicted superionic ice phases. Our measurements determine the pressure-temperature stability fields of superionic ice phases and the melting line, suggesting the presence of face-centred cubic superionic ice in water-rich giant planets, such as Neptune and Uranus. The melting line determined here is at higher temperatures than previously determined in static compression experiments, but it is in agreement with theoretical calculations and data from shock-wave experiments.