arXiv subjects

Alexander F. Goncharov

Publications and source records attributed to Alexander F. Goncharov.

68 records · Page 4Linked to original sources

arXiv2014

Synthesis of Sodium Polyhydrides at High Pressures

The only known compound of sodium and hydrogen is archetypal ionic NaH. Application of high pressure is known to promote states with higher atomic coordination, but extensive searches for polyhydrides with unusual stoichiometry remain unsuccessful in spite of several theoretical predictions. Here we report the first observation of formation of polyhydrides of Na (NaH3 and NaH7) above 40 GPa and 2000 K. We combined synchrotron x-ray diffraction and Raman spectroscopy in a laser heated diamond anvil cell and theoretical Ab-Initio Random Structure search, which both agree in stable structures and compositions. Our results support the formation of multicenter bonding in a material with unusual stoichiometry. These results are applicable to the design of new energetic solids and high-temperature superconductors based on hydrogenrich materials.

cond-mat.mtrl-sci↗

arXiv2014

Backbone NxH Compounds at High Pressures

Optical and synchrotron x-ray diffraction diamond anvil cell experiments have been combined with first principles theoretical structure predictions to investigate mixed N2 and H2 up to 55 GPa. We found the formation of oligomeric NxH (x>1) compounds using mechano- and photochemistry at pressures above 47 and 10 GPa, respectively, and room temperature. These compounds can be recovered to ambient pressure at T<130 K, whereas at room temperature, they can be metastable down to 3.5 GPa. Our results suggest new pathways for synthesis of environmentally benign high energy-density materials and alternative planetary ice.

cond-mat.mtrl-sci↗

arXiv2014

Crystal field and related phenomena in hexagonal close-packed H2 and D2 under pressure

The lattice distortion parameter (the deviation of the c/a ratio from the ideal value 1.633), orientational order parameter, and crystal-field parameter in hexagonal close-packed (hcp) lattice of p-H2, o-D2 and n-H2 are calculated using the semi-empirical lattice-dynamic approach. It is shown that the lattice distortion in the J-even species is two order of magnitude smaller compared with that found in n-H2, and n-D2. The difference is due to the splitting of the J-odd rotational levels in the J-even - J-odd mixtures.

cond-mat.mtrl-sci↗

arXiv2013

Unexpected stable stoichiometries of sodium chlorides

Sodium chloride (NaCl), or rocksalt, is well characterized at ambient pressure. Due to the large electronegativity difference between Na and Cl atoms, it has highly ionic chemical bonding, with stoichiometry 1:1 dictated by charge balance, and B1-type crystal structure. Here, by combining theoretical predictions and diamond anvil cell experiments we show that new materials with different stoichiometries emerge at pressure as low as 20 GPa. Compounds such us Na3Cl, Na2Cl, Na3Cl2, NaCl3 and NaCl7 are theoretically stable and have unusual bonding and electronic properties. To test this prediction, at 55-80 GPa we synthesized cubic and orthorhombic NaCl3 at 55-70 GPa and 2D-metallic tetragonal Na3Cl. This proves that novel compounds, violating chemical intuition, can be thermodynamically stable even in simplest systems at non-ambient conditions.

cond-mat.mtrl-sci↗

arXiv2012

Bonding, structures, and band gap closure of hydrogen at high pressures

We have studied dense hydrogen and deuterium experimentally up to 320 GPa and using ab initio molecular dynamic (MD) simulations up to 370 GPa between 250 and 300 K. Raman and optical absorption spectra show significant anharmonic and quantum effects in mixed atomic and molecular dense phase IV of hydrogen. In agreement with these observations, ab initio MD simulations near 300 K show extremely large atomic motions, which include molecular rotations, hopping and even pair fluctuations suggesting that phase IV may not have a well-defined crystalline structure. The structurally diverse layers (molecular and graphene-like) are strongly coupled thus opening an indirect band gap; moreover, at 300 GPa we find fast synchronized intralayer structural fluctuations. At 370 GPa the mixed structure collapses to form a metallic molecular Cmca-4 phase, which exhibit a new interstitial valence charge bonding scheme.

cond-mat.mtrl-sci↗

arXiv2012

Unexpected stable stoichiometries of sodium chlorides

At ambient pressure, sodium, chlorine, and their only known compound NaCl, have well-understood crystal structures and chemical bonding. Sodium is a nearly-free-electron metal with the bcc structure. Chlorine is a molecular crystal, consisting of Cl2 molecules. Sodium chloride, due to the large electronegativity difference between Na and Cl atoms, has highly ionic chemical bonding, with stoichiometry 1:1 dictated by charge balance, and rocksalt (B1-type) crystal structure in accordance with Pauling's rules. Up to now, Na-Cl was thought to be an ultimately simple textbook system. Here, we show that under pressure the stability of compounds in the Na-Cl system changes and new materials with different stoichiometries emerge at pressure as low as 25 GPa. In addition to NaCl, our theoretical calculations predict the stability of Na3Cl, Na2Cl, Na3Cl2, NaCl3 and NaCl7 compounds with unusual bonding and electronic properties. The bandgap is closed for the majority of these materials. Guided by these predictions, we have synthesized cubic NaCl3 at 55-60 GPa in the laser-heated diamond anvil cell at temperatures above 2000 K.

cond-mat.mtrl-sci↗

arXiv2012

To the Editor: comment on Eremets and Troyan, Nature Mater. 10, 927-931 (2011)

Eremets and Troyan (Nature Mater. 10, 927-931 (2011)) claim that they produced the conducting liquid hydrogen state at 270 GPa and 295 K. Their evidence consists of disappearance of Raman signals, visual observations, and measurements of electrical conductivity in diamond anvil cells (DAC). However, there is no proof that the reported observations are due to transformations in hydrogen.

cond-mat.other↗

arXiv2012

Equation of state and Raman-active $E_{2g}$ lattice phonon in phases I, II, and III of solid hydrogen and deuterium

We present results of lattice dynamics calculations of the $P-V$ equation of state and the pressure dependence of the Raman-active $E_{2g}$ lattice phonon for $p-$H$_2$ and $o-$D$_2$ in a wide pressure range up to $\sim$2 Mbar using our recently developed semi-empirical many-body potential, and density-functional theory. Comparison with existing body of experimental and theoretical results showed that the employed many-body potential is a reliable basis for high-precision calculations for phases I, II, and III of solid hydrogens.

cond-mat.mtrl-sci↗

arXiv2010

Pressure-induced phase transition in the electronic structure of palladium nitride

We present a combined theoretical and experimental study of the electronic structure and equation of state (EOS) of crystalline PdN2. The compound forms above 58 GPa in the pyrite structure and is metastable down to 11 GPa. We show that the EOS cannot be accurately described within either the local density or generalized gradient approximations. The Heyd-Scuseria-Ernzerhof exchange-correlation functional (HSE06), however, provides very good agreement with experimental data. We explain the strong pressure dependence of the Raman intensities in terms of a similar dependence of the calculated band gap, which closes just below 11 GPa. At this pressure, the HSE06 functional predicts a first-order isostructural transition accompanied by a pronounced elastic instability of the longitudinal-acoustic branches that provides the mechanism for the experimentally observed decomposition. Using an extensive Wannier function analysis, we show that the structural transformation is driven by a phase transition of the electronic structure, which is manifested by a discontinuous change in the hybridization between Pd-d and N-p electrons as well as a conversion from single to triple bonded nitrogen dimers. We argue for the possible existence of a critical point for the isostructural transition, at which massive fluctuations in both the electronic as well as the structural degrees of freedom are expected.

cond-mat.mtrl-sci↗

arXiv2010

Thermodynamic ground states of platinum metal nitrides

The thermodynamic stabilities of various phases of the nitrides of the platinum metal elements are systematically studied using density functional theory. It is shown that for the nitrides of Rh, Pd, Ir and Pt two new crystal structures, in which the metal ions occupy simple tetragonal lattice sites, have lower formation enthalpies at ambient conditions than any previously proposed structures. The region of stability with respect to those structures extends to 17 GPa for PtN2. Calculations show that the PtN2 simple tetragonal structures at this pressure are thermodynamically stable also with respect to phase separation. The fact that the local density and generalized gradient approximations predict different values of the absolute formation enthalpies as well different relative stabilities between simple tetragonal and the pyrite or marcasite structures are further discussed.

cond-mat.mtrl-sci↗

arXiv2001

Raman Scattering of Metals to Very High Pressures

We review recent development in Raman techniques and diamond anvil cell technology, that allow measurements of Raman of metals in the megabar pressure range. The Raman technique includes holographic transmission optics and single-stage spectrographs with CCD array detectors. Use of synthetic high-purity diamonds and anglular excitation geometry reduces spurious background scattering substantially, which makes observable very weak Raman peaks in metals. We give several examples of studies including the hexagonal close packed (hcp) $ε$-Fe, Fe$_{(1-x)}$Ni${_x}$, and Re and the recently discovered high-temperature superconductor MgB$_2$. The shift of phonon modes with pressure is used for determination of the mode-Grüneisen parameters, estimation of shear elastic moduli and provide information on structural and electronic transitions.

cond-mat.mtrl-sci↗

arXiv2001

Spectroscopic Studies of the Vibrational and Electronic Properties of Solid Hydrogen to 285 GPa

We report Raman scattering and visible to near-infrared absorption spectra of solid hydrogen under static pressure up to 285 GPa at 85-140 K. We obtain pressure dependences of vibron and phonon modes in agreement with previously determined to lower pressures. The results indicate the stability of the ordered molecular phase III to the highest pressure reached and provide constraints on the insulator-to-metal transition pressure.

cond-mat.mtrl-sci↗

arXiv2001

High-Pressure Amorphous Nitrogen

The phase diagram and stability limits of diatomic solid nitrogen have been explored in a wide pressure--temperature range by several optical spectroscopic techniques. A newly characterized narrow-gap semiconducting phase $η$ has been found to exist in a range of 80--270 GPa and 10--510 K. The vibrational and optical properties of the $η$ phase produced under these conditions indicate that it is largely amorphous and back transforms to a new molecular phase. The band gap of the $η$ phase is found to decrease with pressure indicating possible metallization by band overlap above 280 GPa.

cond-mat.mtrl-sci↗

arXiv2001

Raman spectrum and lattice parameters of MgB2 as a function of pressure

We report Raman spectra and synchrotron x-ray diffraction measurements of lattice parameters of polycrystalline MgB2 under hydrostatic pressure conditions up to 15 GPa. An anomalously broadened Raman band at 620 cm-1 is observed that exhibits a large linear pressure shift of its frequency. The large mode damping and Gruneisen parameter indicate a highly anharmonic nature of the mode, broadly consistent with theoretical predictions for the E2g in-plane boron stretching mode. The results obtained may provide additional constraints on the electron-phonon coupling in the system.

cond-mat.supr-con↗