61 research outputs found
Stable reconstruction of the (110) surface and its role in pseudocapacitance of rutile-like RuO2
Surfaces of rutile-like RuO2, especially the most stable (110) surface, are
important for catalysis, sensing and charge storage applications. Structure,
chemical composition, and properties of the surface depend on external
conditions. Using the evolutionary prediction method USPEX, we found stable
reconstructions of the (11) surface. Two stable reconstructions, RuO4-(2x1) and
RuO2-(1x1), were found, and the surface phase diagram was determined. The new
RuO4-(2x1) reconstruction is stable in a wide range of environmental
conditions, its simulated STM image perfectly matches experimental data, it is
more thermodynamically stable than previously proposed reconstructions, and
explains well pseudocapacitance of RuO2 cathodes.Comment: 10 pages, 5 figure
Actinium hydrides , , as high-temperature conventional superconductors
Stability of numerous unexpected actinium hydrides was predicted via
evolutionary algorithm USPEX. Electron-phonon interaction was investigated for
the hydrogen-richest and most symmetric phases: Rm-,
I4/mmm- and Pm2-. Predicted structures of
actinium hydrides are consistent with all previously studied Ac-H phases and
demonstrate phonon-mediated high-temperature superconductivity with Tc in the
range 204-251 K for Rm- at 200 GPa and 199-241 K for
Pm2- at 150 GPa which was estimated by directly solving
of Eliashberg equation. Actinium belongs to the series of d1-elements
(Sc-Y-La-Ac) that form high-Tc superconducting (HTSC) hydrides. Combining this
observation with p0-HTSC hydrides ( and ), we propose that
p0- and d1-atoms with low-lying empty orbitals tend to form phonon-mediated
HTSC metal polyhydrides
Antiferromagnetic Stabilization in Ti8O12
Using the evolutionary algorithm USPEX and DFT+U calculations, we predicted a
high-symmetry geometric structure of bare Ti8O12 cluster composed of 8 Ti atoms
forming a cube, which O atoms are at midpoints of all of its edges, in
excellent agreement with experimental results. Using Natural Bond Orbital
analysis, Adaptive Natural Density Partitioning algorithm, electron
localization function and partial charge plots, we find the origin of the
particular stability of bare Ti8O12 cluster: unique chemical bonding where
eight electrons of Ti atoms interacting with each other in antiferromagnetic
fashion to lower the total energy of the system. The bare Ti8O12 is thus an
unusual molecule stabilized by d-orbital antiferromagnetic coupling.Comment: 8 pages, 5 figure
On Distribution of Superconductivity in Metal Hydrides
Using the data on the superconducting critical temperature () for a
number of metal hydrides, we found a rule that makes it possible to predict the
maximum based only on the information about the electronic structure of
metal atoms. Using this guiding principle, we explored the hydride systems for
which no reliable information existed, predicted new higher hydrides in the
K-H, Zr-H, Hf-H, Ti-H, Mg-H, Sr-H, Ba-H, Cs-H, and Rb-H systems at high
pressures, and calculated their . Results of the study of actinides and
lanthanides show that they form highly symmetric superhydrides .
However, actinide hydrides do not exhibit high-temperature superconductivity
(except Th-H system) and might not be considered as promising materials for
experimental studies, as well as all -elements with m > 4, including metal
hydrides of the noble elements. Designed neural network allowing the prediction
of of various hydrides shows good accuracy and was used to estimate
upper limit for of the materials with absence of the data. The
developed rule, based on regular behavior of the maximum achievable critical
temperature as a function of number of electrons, enables targeted
predictions about the existence of new high- superconductors
Stable magnesium peroxide at high pressure
Rocky planets are thought to comprise compounds of Mg and O as these are
among the most abundant elements, but knowledge of their stable phases may be
incomplete. MgO is known to be remarkably stable to very high pressure and
chemically inert under reduced condition of the Earth's lower mantle. However,
in 'icy' gas giants as well as in exoplanets oxygen may be a more abundant
constituent (Ref. 1,2). Here, using synchrotron x-ray diffraction in
laser-heated diamond anvil cells, we show that MgO and oxygen react at
pressures above 94 GPa and T = 2150 K with the formation of the theoretically
predicted I4/mcm MgO2 (Ref.3). Raman spectroscopy detects the presence of a
peroxide ion (O22-) in the synthesized material as well as in the recovered
specimen. Likewise, energy-dispersive x-ray spectroscopy confirms that the
recovered sample has higher oxygen content than pure MgO. Our finding suggests
that MgO2 may substitute MgO in rocky mantles and rocky planetary cores under
highly oxidizing conditions
Iron superhydrides FeH5 and FeH6: stability, electronic properties and superconductivity
Recently a big number of works devoted to search for new hydrides with record
high-temperature superconductivity and at the same time the successful
synthesis of potential high-TC superconducting FeH5 was reported. We present a
systematic search for stable compounds in the Fe-H system using
variable-composition version of the evolutionary algorithm USPEX. All known
(FeH, FeH3, FeH5) and several new Fe3H5, Fe3H13 and FeH6 iron hydrides were
found to be stable, resulting in a very complex phase diagram with rich
structural relationships between phases. We calculate electronic properties of
two potentially high-TC FeH5 and FeH6 phases in the pressure range from 150 to
300 GPa. Indeed, hydrogen-rich FeH5 and FeH6 phases were found to be
superconducting within Bardeen-Cooper-Schrieffer theory, with TC values of up
to 46 K.Comment: 10 pages, 3 figures, 1 table + Supporting information (7 pages, 10
figures
Formation of As-As Interlayer Bonding in the collapsed tetragonal phase of NaFeAs under pressure
NaFeAs is investigated experimentally using powder x-ray diffraction
and Raman spectroscopy at pressures up to 23 GPa at room temperature and using
ab-initio calculations. The results reveal a pressure-induced structural
modification at 4 GPa from the starting tetragonal to a collapsed tetragonal
phase. We determined the changes in interatomic distances under pressure that
allowed us to connect the structural changes and superconductivity. The
transition is related to the formation of interlayer As-As bonds at the expense
of weakening of Fe-As bonds in agreement with recent theoretical predictions.Comment: 14 Pages,8 figure
Raman spectroscopy and X-ray diffraction of sp3-CaCO3 at lower mantle pressures
The exceptional ability of carbon to form sp2 and sp3 bonding states leads to
a great structural and chemical diversity of carbon-bearing phases at
non-ambient conditions. Here we use laser-heated diamond anvil cells combined
with synchrotron x-ray diffraction, Raman spectroscopy, and first-principles
calculations to explore phase transitions in CaCO3 at P > 40 GPa. We find that
post-aragonite CaCO3 transforms to the previously predicted P21/c-CaCO3 with
sp3-hybridized carbon at 105 GPa (~30 GPa higher than the theoretically
predicted crossover pressure). The lowest enthalpy transition path to
P21/c-CaCO3 includes reoccurring sp2- and sp3-CaCO3 intermediate phases and
transition states, as reveled by our variable-cell nudged elastic band
simulation. Raman spectra of P21/c-CaCO3 show an intense band at 1025 cm-1,
which we assign to the symmetric C-O stretching vibration based on empirical
and first principles calculations. This Raman band has a frequency that is ~20
% lower than the symmetric C-O stretching in sp2-CaCO3, due to the C-O bond
length increase across the sp2-sp3 transition, and can be used as a fingerprint
of tetrahedrally-coordinated carbon in other carbonates
Superconductivity of and polyhydrides
We explore high-pressure phase stability and superconductivity of lanthanum
hydrides (m=4-11,16). We predict stability of a hitherto unreported
polyhydride - at pressures above 150 GPa; at 200 GPa its
predicted superconducting is 156 K, critical field (0) ~ 35 T
and superconducting gap is up to 35 meV. We revisit superconductivity of the
recently discovered and find its to be up to 259 K (170 GPa)
from solving the Eliashberg equation and 271 K from solving the gap equation in
SCDFT which also allowed us to compute the Coulomb pseudopotential for
and . Presence of several polymorph modifications of LaH10
may explain the variety in the experimentally measured values for
[1,2]Comment: 11 pages including 4 figures, 2 tables, Supporting Information (23
pages
Synthesis of ultra-incompressible sp3-hybridized carbon nitride
Search of materials with C-N composition hold a great promise in creating
materials which would rival diamond in hardness due to the very strong and
relatively low-ionic C-N bond. Early experimental and theoretical works on C-N
compounds were based on structural similarity with binary A3B4 structural-
types; however, the synthesis of C3N4 remains elusive. Here we explored an
unbiased synthesis from the elemental materials at high pressures and
temperatures. Using in situ synchrotron X-ray diffraction and Raman
spectroscopy we demonstrate synthesis of highly incompressible Pnnm CN compound
with sp3 hybridized carbon is synthesized above 55 GPa and 7000 K. This result
is supported by first principles evolutionary search, which finds that Pnnm CN
is the most stable compound above 10.9 GPa. On pressure release below 6 GPa the
synthesized CN compound amorphizes reattaining its 1:1 stoichiometry as
confirmed by Energy-Dispersive X-ray Spectroscopy. This work underscores the
importance of understanding of novel high-pressure chemistry rules and it opens
a new route for synthesis of superhard materials.Comment: 24 pages, 12 figure
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