500 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
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 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
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
Individual health students and negative factors affecting it
The health of modern students is formed by integration physical, sanitary and hygienic culture. Nonobservance of the simple principles of a healthy lifestyle conducts to numerous diseases and health deteriorationЗдоровье современной студенческой молодежи формируется путем слияния физической и санитарно-гигиенической культуры. Несоблюдение простых принципов здорового образа жизни ведет к многочисленным болезням и ухудшению здоровь
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
Spatially localized solutions of the Hammerstein equation with sigmoid type of nonlinearity
We study the existence of fixed points to a parameterized Hammerstein operator Hβ, β∈(0,∞], with sigmoid type of nonlinearity. The parameter β<∞ indicates the steepness of the slope of a nonlinear smooth sigmoid function and the limit case β=∞ corresponds to a discontinuous unit step function. We prove that spatially localized solutions to the fixed point problem for large β exist and can be approximated by the fixed points of H∞. These results are of a high importance in biological applications where one often approximates the smooth sigmoid by discontinuous unit step function. Moreover, in order to achieve even better approximation than a solution of the limit problem, we employ the iterative method that has several advantages compared to other existing methods. For example, this method can be used to construct non-isolated homoclinic orbit of a Hamiltonian system of equations. We illustrate the results and advantages of the numerical method for stationary versions of the FitzHugh–Nagumo reaction–diffusion equation and a neural field model
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