Arkady S. Mikhaylushkin

Body-centered cubic iron-nickel alloy in Earth's core.

Cosmochemical, geochemical, and geophysical studies provide evidence that Earths core contains iron with substantial (5 to 15%) amounts of nickel. The iron-nickel alloy Fe0.9Ni0.1 has been studied in situ by means of angle-dispersive x-ray diffraction in internally heated diamond anvil cells (DACs), and its resistance has been measured as a function of pressure and temperature. At pressures above 225 gigapascals and temperatures over 3400 kelvin, Fe0.9Ni0.1 adopts a body-centered cubic structure. Our experimental and theoretical results not only support the interpretation of shockwave data on pure iron as showing a solid-solid phase transition above about 200 gigapascals, but also suggest that iron alloys with geochemically reasonable compositions (that is, with substantial nickel, sulfur, or silicon content) adopt the bcc structure in Earths inner core.

Quenching of bcc-Fe from high to room temperature at high-pressure conditions : a molecular dynamics simulation

The new high-temperature (T), high-pressure (P), body-centered cubic (bcc) phase of iron has probably already been synthesized in recent diamond anvil cell (DAC) experiments (Mikhaylushkin et al 2007 Phys. Rev. Lett. 99 165505). These DAC experiments on iron revealed that the high-PT phase on quenching transforms into a mixture of close-packed phases. Our molecular dynamics simulation and structural analysis allow us to provide a probable interpretation of the experiments. We show that quenching of the high-PT bcc phase simulated with the embedded-atom model also leads to the formation of the mixture of close-packed phases. Therefore, the assumption of the stability of the high-PT bcc iron phase is consistent with experimental observation.

Structure and Bonding of γ-B28: Is the High Pressure Form of Elemental Boron Ionic?

The recently characterized crystal structure of metastable γ-B(28) is analyzed from a crystal chemical point of view, and the electron requirement of its building units and that of their linkage is determined. The structure consists of unique B(2) dumbbells and B(12) icosahedra, which are connected through two-center and three-center, two-electron bonds. The different bonding motifs are ascertained by theoretical calculations of difference charge distributions. Chemical bonding in high pressure γ-B(28) bears great resemblance to α-B(12) which is the simplest boron modification. The previously made description of γ-B(28) as ionic in terms of (B(2))(δ+) and (B(12))(δ-) is not supported.

Structural and magnetic properties of FeHx (x=0.25; 0.50; 0.75)

The structural and magnetic properties of the FeHx (x=0.25; 0.50; 0.75) compounds have been studied using the projector augmented wave (PAW) method within the generalized gradient approximation (GGA). We compare the hcp, dhcp and fcc structures and find that for the considered concentrations of hydrogen the hcp structure is most stable in a wide pressure range. The magnetic behavior of iron is crucially influenced by hydrogen. In particular, the local moment on a Fe atom depends on the number of hydrogen atoms in the atom surroundings. Iron atoms, which are crystallographically equivalent in their original structures (hcp, fcc) but have different number of hydrogen neighbors, are shown to have different local magnetic moments. This finding suggests that the experimental observations of two magnetic moments in iron hydride can be explained by nonstoichiometry of the hydride and might not be a direct evidence for the presence of the dhcp phase.

Transport properties of Al-Si solid solutions: theory

Abstract Al–Si solid solutions synthesized under high pressure demonstrate striking physical properties, among which are enhanced superconductivity and peculiarities of low-temperature transport properties. To find the reason behind the latter we have performed a first-principles study of the electronic spectra and Fermi surfaces of Al–Si solid solutions. Two electronic topological transitions (ETTs), taking place in the system with increasing concentration of Si and pressure, have been revealed. Based on these data and the theory of ETTs for substitutional solid solutions we have calculated concentration dependencies of the resistivity, thermoelectric power and Hall constant. We show the results to be in quantitative agreement with experiment and to reproduce nicely the experimentally observed peculiarities.

Effects of O and N impurities on the nanostructural evolution during growth of Cr/Sc multilayers

Transition metal multilayers are prime candidates for high reflectivity soft x-ray multilayer mirrors. In particular, Cr/Sc multilayers in the amorphous state have proven to give the highest reflec ...

The influence of hydrogen contamination on the structural stability of CoSn under compression.

The binary CoSn compound has a unique ground state large-void crystal structure, whose stability under pressure has recently been examined. Whereas theoretical results predicted a series of phase transformations, the room temperature experiments did not observe any structural change. We suggest that the large void of a CoSn-type structure could contain natural impurities such as hydrogen, which can influence the thermodynamic stability of a CoSn system and explain the unusual disagreement between the theoretical and experimental results. Based on first-principles calculations we reveal that the contamination of CoSn by hydrogen only results in a subtle change of structural parameters and the equation of state of CoSn, but drastically increases the stability of the CoSn-type phase in comparison with the high-pressure phases predicted earlier. We argue that the hardly detectable natural impurities of light elements in porous compounds like CoSn are able to change the phase equilibria.