We describe our modelling of the radiatively cooling shocks and their thin shells with various numerical tools in different physical and calculational setups. We inspect structure of the dense shell, ...its formation and evolution, pointing out physical and numerical factors that sustain its shape and also may lead to instabilities. We have found that under certain physical conditions, the circular shaped shells show a strong bending instability and successive fragmentation on Cartesian grids soon after their formation, while remain almost unperturbed when simulated on polar meshes. We explain this by physical Rayleigh–Taylor-like instabilities triggered by corrugation of the dense shell surfaces by numerical noise. Conditions for these instabilities follow from both the shell structure itself and from episodes of transient acceleration during re-establishing of dynamical pressure balance after sudden radiative cooling onset. They are also easily excited by physical perturbations of the ambient medium. The widely mentioned non-linear thin shell instability, in contrast, in tests with physical perturbations is shown to have only limited chances to develop in real radiative shocks, as it seems to require a special spatial arrangement of fluctuations to be excited efficiently. The described phenomena also set new requirements on further simulations of the radiatively cooling shocks in order to be physically correct and free of numerical artefacts.
The structural and magnetic properties of Cr2O3 have been studied by means of X-ray and neutron powder diffraction at high pressures up to 35 GPa. The lattice compression of the rhombohedral crystal ...structure of R3¯c symmetry is slightly anisotropic with anomaly in the pressure behavior of the c/a parameters ratio at P ≈ 20 GPa of presumably magnetic nature. The oxygen octahedra around chromium ions become more symmetric with close values of shared and unshared bonds under high pressure. The antiferromagnetic structure of Cr2O3 remains stable in the studied pressure range up to 35 GPa. The pressure coefficient of the Néel temperature, (1/TN)(dTN/dP) = + 0.0091 GPa−1, is significantly less in comparison with perovskite-like compounds containing Cr3+ and Mn4+ ions of similar electronic configuration.
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•The magnetoelectrically active AFM phase is stable at pressures up to 35 GPa.•The pressure coefficient of TN is unusually small compared to perovskite compounds.•The pressure behavior of interatomic distances is determined by neutron diffraction.•The relationship between the structural and magnetic properties is analyzed.•The dominant role of direct exchange magnetic interactions in Cr2O3 is evidenced.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK, ZRSKP
We draw attention to recent high-explosive (HE) experiments which provide compression of macroscopic amount of matter to high, even record, values of pressure in comparison with other HE experiments. ...The observed bounce after the compression corresponds to processes in core-collapse supernova explosions after neutrino trapping. Conditions provided in the experiments resemble those in core-collapse supernovae, permitting their use for laboratory astrophysics. A unique feature of the experiments is compression at low entropy. The values of specific entropy are close to those obtained in numerical simulations during the process of collapse in supernova explosions, and much lower than those obtained at laser ignition facilities, another type of high-compression experiment. Both in supernovae and HE experiments the bounce occurs at low entropy, so the HE experiments provide a new platform to realize some supernova collapse effects in laboratory, especially to study hydrodynamics of collapsing flows and the bounce. Due to the good resolution of diagnostics in the compression of macroscopic amounts of material with essential effects of nonideal plasma in EOS, and observed development of 3D instabilities, these experiments may serve as a useful benchmark for astrophysical hydrodynamic codes.
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The crystal structure of elemental Sn was investigated by synchrotron X-ray diffraction at ultra high pressures up to ∼230 GPa creating in diamond anvil cells. Above 70 GPa, a pure
bcc
structure of ...Sn was observed, which is stable up to 160GPa, until an occurrence of the
hcp
phase was revealed. At the onset of the
bcc
-
hcp
transition at pressure of about 160GPa, the drop of the unit cell volume is about 1%. A mixture of the
bcc
-
hcp
states was observed at least up to 230GPa, and it seems that this state could exist even up to higher pressures. The fractions of the
bcc
and
hcp
phases were evaluated in the pressure range of the phase coexistence 160–230 GPa. The difference between static and dynamic compression and its effect on the
V
–
P
phase diagram of Sn are discussed.
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DOBA, EMUNI, FIS, FZAB, GEOZS, GIS, IJS, IMTLJ, IZUM, KILJ, KISLJ, MFDPS, NLZOH, NUK, OBVAL, OILJ, PILJ, PNG, SAZU, SBCE, SBJE, SBMB, SBNM, SIK, UILJ, UKNU, UL, UM, UPUK, VKSCE, ZAGLJ
The crystal, magnetic structure and vibrational spectra of multiferroic GaFeO3 have been studied by means of neutron, X-ray powder diffraction and Raman spectroscopy at pressures up to 6.2 and ...42 GPa, respectively. A presence of Fe/Ga antisite disorder leads to a formation of the ferrimagnetic ground state with the Néel temperature TN = 292 K at ambient pressure. Upon compression, the magnetic ground state symmetry remains the same and the Néel temperature increases with a pressure coefficient (1/TN)(dTN/dP) = 0.011(1) GPa−1. Application of high pressure above 21 GPa leads to a gradual structural phase transition from the polar orthorhombic Pc21n phase to nonpolar orthorhombic Pbnm phase. It is accompanied by anomalies in the pressure behaviour of several Raman modes. Pressure dependencies of lattice parameters and Raman modes frequencies in the observed structural phases were obtained.
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•Magnetic structure of GaFeO3 is changed under pressure.•The Néel temperature of GaFeO3 is close to room temperature.•The Néel temperature of GaFeO3 raises under pressure.•The spontaneous magnetization exhibits a tendency towards increase under pressure in GaFeO3.•Application of high pressure leads to changes in Raman spectra of GaFeO3 at structural phase transition.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK, ZRSKP
The structural and magnetic properties of multiferroic CuO have been studied by means of neutron and x-ray powder diffraction at pressures up to 11 and 38 GPa, respectively, and by first-principles ...theoretical calculations. Anomalous lattice compression is observed, with enlargement of the lattice parameter a, reaching a maximum at P=13GPa, followed by its reduction at higher pressures. The lattice distortion of the monoclinic structure at high pressures is accompanied by a progressive change of the oxygen coordination around Cu atoms from the square fourfold towards the octahedral sixfold coordination. The pressure-induced evolution of the structural properties and electronic structure of CuO was successfully elucidated in the framework of full-electronic density functional theory calculations with range-separated HSE06, and meta–generalized gradient approximation hybrid M06 functionals. The antiferromagnetic (AFM) ground state with a propagation vector q=(0.5,0,−0.5) remains stable in the studied pressure range. From the obtained structural parameters, the pressure dependencies of the principal superexchange magnetic interactions were analyzed, and the pressure behavior of the Néel temperature as well as the magnetic transition temperature from the intermediate incommensurate AFM multiferroic state to the commensurate AFM ground state were evaluated. The estimated upper limit of the Néel temperature at P=38GPa is about 260 K, not supporting the previously predicted existence of the multiferroic phase at room temperature and high pressure.
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Poly-nitrogen compounds have been considered as potential high energy density materials for a long time due to the large number of energetic N-N or N=N bonds. In most cases high nitrogen content and ...stability at ambient conditions are mutually exclusive, thereby making the synthesis of such materials challenging. One way to stabilize such compounds is the application of high pressure. Here, through a direct reaction between Fe and N
in a laser-heated diamond anvil cell, we synthesize three ironnitrogen compounds Fe
N
, FeN
and FeN
. Their crystal structures are revealed by single-crystal synchrotron X-ray diffraction. Fe
N
, synthesized at 50 GPa, is isostructural to chromium carbide Cr
C
. FeN
has a marcasite structure type and features covalently bonded dinitrogen units in its crystal structure. FeN
, synthesized at 106 GPa, features polymeric nitrogen chains of N
units. Based on results of structural studies and theoretical analysis, N
units in this compound reveal catena-polytetraz-1-ene-1,4-diyl anions.
Experimental studies of electrons produced in a laser wakefield accelerator indicate trapping initiated by ionization of target gas atoms. Targets composed of helium and controlled amounts of various ...gases were found to increase the beam charge by as much as an order of magnitude compared to pure helium at the same electron density and decrease the beam divergence from 5.1+/-1.0 to 2.9+/-0.8 mrad. The measurements are supported by particle-in-cell modeling including ionization. This mechanism should allow generation of electron beams with lower emittance and higher charge than in preionized gas.
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The structural and magnetic properties of the Ca3Co2O6 spin-chain compound have been studied by means of neutron and x-ray powder diffraction at pressures up to 6.8 and 32 GPa, respectively. A ...suppression of the initial spin-density wave state (TN=25K) and stabilization of the collinear commensurate antiferromagnetic (AFM) state at high pressures (TNC=26K at P=2.1 GPa) were observed. The pressure behavior of the competing intra- and interchain magnetic interactions was analyzed on the basis of obtained structural data and their role in the formation of the magnetic phase diagram is discussed. The pressure behavior of the Néel temperature of the commensurate AFM phase was evaluated within the mean field theory approach and a good agreement with the experimental value dTNC/dP=0.65K/GPa was obtained.
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