1,484 research outputs found
Evolutionary Metadynamics: a Novel Method to Predict Crystal Structures
A novel method for crystal structure prediction, based on metadynamics and
evolutionary algorithms, is presented here. This technique can be used to
produce efficiently both the ground state and metastable states easily
reachable from a reasonable initial structure. We use the cell shape as
collective variable and evolutionary variation operators developed in the
context of the USPEX method [Oganov, Glass, \textit{J. Chem. Phys.}, 2006,
\textbf{124}, 244704; Lyakhov \textit{et al., Comp. Phys. Comm.}, 2010,
\textbf{181}, 1623; Oganov \textit{et al., Acc. Chem. Res.}, 2011, \textbf{44},
227] to equilibrate the system as a function of the collective variables. We
illustrate how this approach helps one to find stable and metastable states for
AlSiO, SiO, MgSiO, and carbon. Apart from predicting crystal
structures, the new method can also provide insight into mechanisms of phase
transitions.Comment: 7 pages, 7 figures; CrystEngComm 2012, The Royal Society of Chemistr
Dynamics and certain mechanisms in the changes of the skeletal-muscular system of man under bedrest conditions
Bed rest conditions evaluated varied in the longitudinal axis of the body, perpendicular to the vector gravitational forces, and the cranial portion of the body inclined from the horizontal. The duration of bed rest fluctuated in various experiments from 30 to 182 days. The state of muscle and neuromuscular system was judged on the basis of the recording of various functional indices, as well as by certain results of morphological and biochemical studies and data from the study of motor functions
Evolutionary search for novel superhard materials: Methodology and applications to forms of carbon and TiO2
We have developed a method for prediction of the hardest crystal structures
in a given chemical system. It is based on the evolutionary algorithm USPEX
(Universal Structure Prediction: Evolutionary Xtallography) and
electronegativity-based hardness model that we have augmented with bond-valence
model and graph theory. These extensions enable correct description of the
hardness of layered, molecular, and low-symmetry crystal structures. Applying
this method to C and TiO2, we have (i) obtained a number of low-energy carbon
structures with hardness slightly lower than diamond and (ii) proved that TiO2
in any of its possible polymorphs cannot be the hardest oxide, its hardness
being below 17 GPa.Comment: Submitted in November 2010; revised in March 2011; resubmitted 24
June 2011; published 12 September 2011. 8 pages, 2 tables, 3 figure
Towards the theory of hardness of materials
Recent studies showed that hardness, a complex property, can be calculated
using very simple approaches or even analytical formulae. These form the basis
for evaluating controversial experimental results (as we illustrate for
TiO2-cotunnite) and enable a systematic search for novel hard materials, for
instance, using global optimization algorithms (as we show on the example of
SiO2 polymorphs)
Boron: a Hunt for Superhard Polymorphs
Boron is a unique element, being the only element, all known polymorphs of
which are superhard, and all of its crystal structures are distinct from any
other element. The electron-deficient bonding in boron explains its remarkable
sensitivity to even small concentrations of impurity atoms and allows boron to
form peculiar chemical compounds with very different elements. These
complications made the study of boron a great challenge, creating also a unique
and instructive chapter in the history of science. Strange though it may sound,
the discovery of boron in 1808 was ambiguous, with pure boron polymorphs
established only starting from the 1950s-1970s, and only in 2007 was the stable
phase at ambient conditions determined. The history of boron research from its
discovery to the latest discoveries pertaining to the phase diagram of this
element, the structure and stability of beta-boron, and establishment of a new
high-pressure polymorph, gamma-boron, is reviewed
Experiment K-6-10. Effects of zero gravity on myofibril protein content and isomyosin distribution in rodent skeletal muscle
The purpose of this experiment was to investigate the effects of 12 days of zero gravity (0G) exposure (Cosmos 1887 Biosputnik) on the enzymatic properties, protein content, and isomyosin distribution of the myofibril fraction of the slow-twitch vastus intermedius (VI) and the fast-twitch vastus lateralis (VL) muscles of adult male rats. Measurements were obtained on three experimental groups (n=5 each group) designated as flight-group (FG), vivarium-control (VC), and synchronous-control (SC). Body weight of the FG was significantly lower than the two control groups (p less than 0.05). Compared to the two control groups, VI weight was lower by 23 percent (p less than 0.10); whereas no such reduction was observed for the VL muscle. Myofibril yields (mg protein/g of muscle) in the VI were 35 percent lower in the FG compared to the controls (p less than 0.05); whereas, no such pattern was apparent for the VL muscle. When myofibril yields were expressed on a muscle basis (mg/g x muscle weight), the loss of myofibril protein was more exaggerated and suggests that myofibril protein degradation is an early event in the muscle atrophy response to 0G. Analysis of myosin isoforms indicated that slow-myosin was the primary isoform lost in the calculated degradation of total myosin. No evidence of loss of the fast isomyosins was apparent for either muscle following space flight. Myofibril ATPase activity of the VI was increased in the FG compared to controls, which is consistent with the observation of preferential slow-myosin degradation. These data suggest that muscles containing a high percent of slow-twitch fibers undergo greater degrees of myofibril protein degradation than do muscles containing predominantly fast-twitch fibers in response to a relatively short period of 0G exposure, and the primary target appears to be the slow-myosin molecule
Fe-C and Fe-H systems at pressures of the Earth's inner core
The solid inner core of the Earth is predominantly composed of iron alloyed
with several percent Ni and some lighter elements, Si, S, O, H, and C being the
prime candidates. There have been a growing number of papers investigating C
and H as possible light elements in the core, but the results are
contradictory. Here, using ab initio simulations, we study the Fe-C and Fe-H
systems at inner core pressures (330-364 GPa). Using the evolutionary structure
prediction algorithm USPEX, we have determined the lowest-enthalpy structures
of possible carbides (FeC, Fe2C, Fe3C, Fe4C, FeC2, FeC3, FeC4 and Fe7C3) and
hydrides (Fe4H, Fe3H, Fe2H, FeH, FeH2, FeH3, FeH4) and have found that Fe2C
(Pnma) is the most stable iron carbide at pressures of the inner core, while
FeH, FeH3 and FeH4 are stable iron hydrides at these conditions. For Fe3C, the
cementite structure (Pnma) and the Cmcm structure recently found by random
sampling are less stable than the I-4 and C2/m structures found here. We found
that FeH3 and FeH4 adopt chemically interesting thermodynamically stable
structures, in both compounds containing trivalent iron. The density of the
inner core can be matched with a reasonable concentration of carbon, 11-15
mol.percent (2.6-3.7 wt.percent) at relevant pressures and temperatures. This
concentration matches that in CI carbonaceous chondrites and corresponds to the
average atomic mass in the range 49.3-51.0, in close agreement with inferences
from the Birch's law for the inner core. Similarly made estimates for the
maximum hydrogen content are unrealistically high, 17-22 mol.percent (0.4-0.5
wt.percent), which corresponds to the average atomic mass in the range
43.8-46.5. We conclude that carbon is a better candidate light alloying element
than hydrogen.Comment: Published in Physics-Uspekhi: full text will soon appear at
http://ufn.ru/en/articles/2012/5/c/ (currently, only abstract is available
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