64 research outputs found
Ternary inorganic compounds containing carbon, nitrogen, and oxygen at high pressures
Ternary C_{x}N_{y}O_{z} compounds are actively researched as novel high
energy density and ultrahard materials. Although some synthesis work has been
performed at ambient conditions, very little is known about the high pressure
chemistry of of C_{x}N_{y}O_{z} compounds. In this work, first principles
variable-composition evolutionary structure prediction calculations are
performed with the goal of discovering novel mixed C_{x}N_{y}O_{z} materials at
ambient and high pressure conditions. By systematically searching ternary
variable composition crystalline materials, the full ternary phase diagram is
constructed in the range of pressures from 0 to 100 GPa. The search finds the
C_{2}N_{2}O crystal containing extended covalent network of C, N, and O atoms,
having space group symmetry Cmc2_{1}, and stable above just 10 GPa. Several
other novel metastable (CO)_{x}-(N)_{y} crystalline compounds discovered during
the search, including two polymorphs of C_{2}NO_{2} and two polymorphs of
C_{3}N_{2}O_{3} crystals are found to be energetically favorable compared to
polymeric carbon monoxide (CO) and nitrogen. Predicted new compounds are
characterized by their Raman spectra and equations of state
Novel Potassium Polynitrides at High Pressures
Polynitrogen compounds have attracted great interest due to their potential
applications as high energy density materials. Most recently, a rich variety of
alkali polynitrogens (R_{x}N_{y}; R=Li, Na, and Cs) have been predicted to be
stable at high pressures and one of them, CsN_{5} has been recently
synthesized. In this work, various potassium polynitrides are investigated
using first-principles crystal structure search methods. Several novel
molecular crystals consisting of N_{4} chains, N_{5} rings, and N_{6} rings
stable at high pressures are discovered. In addition, an unusual nitrogen-rich
metallic crystal with stoichiometry K_{2}N_{16} consisting of a planar
two-dimensional extended network of nitrogen atoms arranged in fused eighteen
atom rings is found to be stable above 70 GPa. An appreciable electron transfer
from K to N atoms is responsible for the appearance of unexpected chemical
bonding in these crystals. The thermodynamic stability and high pressure phase
diagram is constructed. The electronic and vibrational properties of the
layered polynitrogen K_{2}N_{16} compound are investigated, and the
pressure-dependent IR-spectrum is obtained to assist in experimental discovery
of this new high-nitrogen content material
Pentazole and Ammonium Pentazolate: Crystalline Hydro-Nitrogens at High Pressure
Two new crystalline compounds, pentazole (N_{5}H) and ammonium pentazolate
(NH_{4})(N_{5}), both featuring cyclo-{\rm N_{5}^{-}} are discovered using
first principles evolutionary search of the nitrogen-rich portion of the
hydro-nitrogen binary phase diagram (N_{x}H_{y}, x\geqy) at high pressures.
Both crystals consist of the pentazolate N_{5}^{-} anion and ammonium
NH_{4}^{+} or hydrogen H^{+} cations. These two crystals are predicted to be
thermodynamically stable at pressures above 30 GPa for (NH_{4})(N_{5}) and 50
GPa for pentazole N_{5}H. The chemical transformation of ammonium azide
(NH_{4})(N_{3}) mixed with di-nitrogen (N_{2}) to ammonium pentazolate
(NH_{4})(N_{5}) is predicted to become energetically favorable above 12.5 GPa.
To assist in identification of newly synthesized compounds in future
experiments, the Raman spectra of both crystals are calculated and mode
assignments are made as a function of pressure up to 75 GPa
Novel Rubidium Poly-Nitrogen Materials at High Pressure
First-principles crystal structure search is performed to predict novel
rubidium poly-nitrogen materials at high pressure by varying the stoichiometry,
i. e. relative quantities of the constituent rubidium and nitrogen atoms. Three
compounds of high nitrogen content, RbN_{5}, RbN_{2}, and Rb_{4}N_{6}, are
discovered. Rubidium pentazolate (RbN5) becomes thermodynamically stable at
pressures above \unit[30]{GPa}. The charge transfer from Rb to N atoms enables
aromaticity in cyclo-N_{^{_{5}}}^{-} while increasing the ionic bonding in the
crystal. Rubidium pentazolate can be synthesized by compressing rubidium azide
(RbN3) and nitrogen (N2) precursors above \unit[9.42]{GPa}, and its
experimental discovery is aided by calculating the Raman spectrum and
identifying the features attributed to N_{^{_{5}}}^{-} modes. The two other
interesting compounds, RbN2 containing infinitely-long single-bonded nitrogen
chains, and Rb_{4}N_{6} consisting of single-bonded N_{6} hexazine rings,
become thermodynamically stable at pressures exceeding \unit[60]{GPa}. In
addition to the compounds with high nitrogen content, Rb_{3}N_{3}, a new
compound with 1:1 RbN stoichiometry containing bent N_{3} azides is found to
exist at high pressures
Tin-selenium compounds at ambient and high pressures
SnxSey crystalline compounds consisting of Sn and Se atoms of varying
composition are systematically investigated at pressures from 0 to 100 GPa
using the first-principles evolutionary crystal structure search method based
on density functional theory (DFT). All known experimental phases of SnSe and
SnSe2 are found without any prior input. A second order polymorphic phase
transition from SnSe-Pnma phase to SnSe-Cmcm phase is predicted at 2.5 GPa.
Initially being semiconducting, this phase becomes metallic at 7.3 GPa. Upon
further increase of pressure up to 36.6 GPa, SnSe-Cmcm phase is transformed to
CsCl-type SnSe-Pm3m phase, which remains stable at even higher pressures. A
metallic compound with different stoichiometry, Sn3Se4-I43d, is found to be
thermodynamically stable from 18 GPa to 70 GPa. Known semiconductor tin
diselenide SnSe2-P3m1 phase is found to be thermodynamically stable from
ambient pressure up to 18 GPa. Initially being semiconducting, it experiences
metalization at pressures above 8 GPa
High-Pressure Synthesis of a Pentazolate Salt
The pentazolates, the last all-nitrogen members of the azole series, have
been notoriously elusive for the last hundred years despite enormous efforts to
make these compounds in either gas or condensed phases. Here we report a
successful synthesis of a solid state compound consisting of isolated
pentazolate anions N5-, which is achieved by compressing and laser heating
cesium azide (CsN3) mixed with N2 cryogenic liquid in a diamond anvil cell. The
experiment was guided by theory, which predicted the transformation of the
mixture at high pressures to a new compound, cesium pentazolate salt (CsN5).
Electron transfer from Cs atoms to N5 rings enables both aromaticity in the
pentazolates as well as ionic bonding in the CsN5 crystal. This work provides a
critical insight into the role of extreme conditions in exploring unusual
bonding routes that ultimately lead to the formation of novel high nitrogen
content species
Rapid West Nile Virus Antigen Detection
We compared the VecTest WNV antigen assay with standard methods of West Nile virus (WNV) detection in swabs from American Crows (Corvus brachyrhynchos) and House Sparrows (Passer domesticus). The VecTest detected WNV more frequently than the plaque assay and was comparable to a TaqMan reverse transcription–polymerase chain reaction
The First Post-Kepler Brightness Dips of KIC 8462852
We present a photometric detection of the first brightness dips of the unique variable star KIC 8462852 since the end of the Kepler space mission in 2013 May. Our regular photometric surveillance started in October 2015, and a sequence of dipping began in 2017 May continuing on through the end of 2017, when the star was no longer visible from Earth. We distinguish four main 1-2.5% dips, named "Elsie," "Celeste," "Skara Brae," and "Angkor", which persist on timescales from several days to weeks. Our main results so far are: (i) there are no apparent changes of the stellar spectrum or polarization during the dips; (ii) the multiband photometry of the dips shows differential reddening favoring non-grey extinction. Therefore, our data are inconsistent with dip models that invoke optically thick material, but rather they are in-line with predictions for an occulter consisting primarily of ordinary dust, where much of the material must be optically thin with a size scale <<1um, and may also be consistent with models invoking variations intrinsic to the stellar photosphere. Notably, our data do not place constraints on the color of the longer-term "secular" dimming, which may be caused by independent processes, or probe different regimes of a single process
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