Biomedical subjects

Viktor V Struzhkin

Publications and source records attributed to Viktor V Struzhkin.

12 recordsLinked to original sources

PubMed2006

Reduced radiative conductivity of low-spin (Mg,Fe)O in the lower mantle.

Optical absorption spectra have been measured at pressures up to 80 gigapascals (GPa) for the lower-mantle oxide magnesiowüstite (Mg,Fe)O. Upon reaching the high-spin to low-spin transition of Fe2+ at about 60 GPa, we observed enhanced absorption in the mid- and near-infrared spectral range, whereas absorption in the visible-ultraviolet was reduced. The observed changes in absorption are in contrast to prediction and are attributed to d-d orbital charge transfer in the Fe2+ ion. The results indicate that low-spin (Mg,Fe)O will exhibit lower radiative thermal conductivity than high-spin (Mg,Fe)O, which needs to be considered in future geodynamic models of convection and plume stabilization in the lower mantle.

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PubMed2006

Valence band x-ray emission spectra of compressed germanium.

We report measurements of the valence band width in compressed Ge determined from x-ray emission spectra below the Ge K edge. The width of the valence band does not show any pressure dependence in the semiconducting diamond-type structure of Ge below 10 GPa. On the other hand, in the metallic beta-Sn phase above 10 GPa the valence band width increases under compression. Density-functional calculations show an increasing valence band width under compression both in the semiconducting phase (contrary to experiment) and in the metallic beta-Sn phase of Ge (in agreement with observed pressure-induced broadening). The pressure-independent valence band width in the semiconducting phase of Ge appears to require theoretical advances beyond the density-functional theory or the GW approximation.

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PubMed2005

X-ray emission spectroscopy with a laser-heated diamond anvil cell: a new experimental probe of the spin state of iron in the Earth's interior.

Synchrotron-based X-ray emission spectroscopy (XES) is well suited to probing the local electronic structure of 3d transition metals such as Fe and Mn in their host phases. The laser-heated diamond anvil cell technique is uniquely capable of generating ultra-high static pressures and temperatures in excess of 100 GPa and 3000 K. Here X-ray emission spectroscopy and X-ray diffraction have been interfaced with the laser-heated diamond cell for studying the electronic spin states of iron in magnesiowüstite-(Mg0.75,Fe0.25)O and its crystal structure under lower-mantle conditions. X-ray emission spectra of the ferrous iron in a single crystal of magnesiowüstite-(Mg0.75,Fe0.25)O indicate that a high-spin to low-spin transition of ferrous iron occurs at 54 to 67 GPa and 300 K and the ferrous iron remains in the high-spin state up to 47 GPa and 1300 K. This pilot study points to the unique capability of the synchrotron-based XES and X-ray diffraction techniques for addressing the issue of electronic spin transition or crossover in 3d transition metals and compounds under extreme high-P-T conditions.

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PubMed2005

Spin transition of iron in magnesiowüstite in the Earth's lower mantle.

Iron is the most abundant transition-metal element in the mantle and therefore plays an important role in the geochemistry and geodynamics of the Earth's interior. Pressure-induced electronic spin transitions of iron occur in magnesiowüstite, silicate perovskite and post-perovskite. Here we have studied the spin states of iron in magnesiowüstite and the isolated effects of the electronic transitions on the elasticity of magnesiowüstite with in situ X-ray emission spectroscopy and X-ray diffraction to pressures of the lowermost mantle. An observed high-spin to low-spin transition of iron in magnesiowüstite results in an abnormal compressional behaviour between the high-spin and the low-spin states. The high-pressure, low-spin state exhibits a much higher bulk modulus and bulk sound velocity than the low-pressure, high-spin state; the bulk modulus jumps by approximately 35 percent and bulk sound velocity increases by approximately 15 percent across the transition in (Mg0.83,Fe0.17)O. Although no significant density change is observed across the electronic transition, the jump in the sound velocities and the bulk modulus across the transition provides an additional explanation for the seismic wave heterogeneity in the lowermost mantle. The transition also affects current interpretations of the geophysical and geochemical models using extrapolated or calculated thermal equation-of-state data without considering the effects of the electronic transition.

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PubMed2005

Hard superconducting nitrides.

Detailed study of the equation of state, elasticity, and hardness of selected superconducting transition-metal nitrides reveals interesting correlations among their physical properties. Both the bulk modulus and Vickers hardness are found to decrease with increasing zero-pressure volume in NbN, HfN, and ZrN. The computed elastic constants from first principles satisfy c11 > c12 > c44 for NbN, but c11 > c44 > c12 for HfN and ZrN, which are in good agreement with the neutron scattering data. The cubic delta-NbN superconducting phase possesses a bulk modulus of 348 GPa, comparable to that of cubic boron nitride, and a Vickers hardness of 20 GPa, which is close to sapphire. Theoretical calculations for NbN show that all elastic moduli increase monotonically with increasing pressure. These results suggest technological applications of such materials in extreme environments.

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PubMed2005

Synchrotron radiation and high pressure: new light on materials under extreme conditions.

With the steady development of static high-pressure techniques in recent years, it is now possible to probe in increasing detail the novel behavior of materials subjected to extreme conditions of multimegabar pressures (>300 GPa) and temperatures from cryogenic states to thousands of degrees. By and large, the growth in this area has been made possible by accelerating developments in diamond-anvil cell methods coupled with new synchrotron radiation techniques. Significant advances have occurred in high-pressure powder and single-crystal diffraction, spectroscopy, inelastic scattering, radiography, and infrared spectroscopy. A brief overview of selected highlights in each of these classes of experiments is presented that illustrate both the state-of-the-art as well as current technical and scientific challenges. The experiments have been made possible by the development of a spectrum of new techniques at both third- and second-generation high-energy sources together with key advances in high-pressure technology. The results have implications for a variety of problems in physics, chemistry, materials science, geoscience, planetary science, and biology.

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PubMed2004

High pressure-temperature Raman measurements of H2O melting to 22 GPa and 900 K.

The melting curve of H(2)O has been measured by in situ Raman spectroscopy in an externally heated diamond anvil cell up to 22 GPa and 900 K. The Raman-active OH-stretching bands and the translational modes of H(2)O as well as optical observations are used to directly and reliably detect melting in ice VII. The observed melting temperatures are higher than previously reported x-ray measurements and significantly lower than recent laser-heating determinations. However, our results are in accord with earlier optical determinations. The frequencies and intensities of the OH-stretching peaks change significantly across the melting line while the translational mode disappears altogether in the liquid phase. The observed OH-stretching bands of liquid water at high pressure are very similar to those obtained in shock-wave Raman measurements.

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PubMed2004

Electronic spin state of iron in lower mantle perovskite.

The electronic spin state of iron in lower mantle perovskite is one of the fundamental parameters that governs the physics and chemistry of the most voluminous and massive shell in the Earth. We present experimental evidence for spin-pairing transition in aluminum-bearing silicate perovskite (Mg,Fe)(Si,Al)O(3) under the lower mantle pressures. Our results demonstrate that as pressure increases, iron in perovskite transforms gradually from the initial high-spin state toward the final low-spin state. At 100 GPa, both aluminum-free and aluminum-bearing samples exhibit a mixed spin state. The residual magnetic moment in the aluminum-bearing perovskite is significantly higher than that in its aluminum-free counterpart. The observed spin evolution with pressure can be explained by the presence of multiple iron species and the occurrence of partial spin-paring transitions in the perovskite. Pressure-induced spin-pairing transitions in the perovskite would have important bearing on the magnetic, thermoelastic, and transport properties of the lower mantle, and on the distribution of iron in the Earth's interior.

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PubMed2003

Iron partitioning in Earth's mantle: toward a deep lower mantle discontinuity.

We measured the spin state of iron in ferropericlase (Mg0.83Fe0.17)O at high pressure and found a high-spin to low-spin transition occurring in the 60- to 70-gigapascal pressure range, corresponding to depths of 2000 kilometers in Earth's lower mantle. This transition implies that the partition coefficient of iron between ferropericlase and magnesium silicate perovskite, the two main constituents of the lower mantle, may increase by several orders of magnitude, depleting the perovskite phase of its iron. The lower mantle may then be composed of two different layers. The upper layer would consist of a phase mixture with about equal partitioning of iron between magnesium silicate perovskite and ferropericlase, whereas the lower layer would consist of almost iron-free perovskite and iron-rich ferropericlase. This stratification is likely to have profound implications for the transport properties of Earth's lowermost mantle.

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PubMed2002

Nature of the high-pressure transition in Fe2O3 hematite.

We present a new method to separate the crystallographic and electronic phase transitions in hematite using x-ray emission spectroscopy and x-ray diffraction. Our observations, based on the behavior of a metastable high-pressure phase in the stability domain of the low-pressure phase, show that the electronic transition is preempted by the crystallographic transition. The former occurs only afterwards in the high-pressure phase, possibly as a result of a Mott transition. The idea that the electronic transition drives the transition in hematite is therefore invalidated. Such methods should help elucidate the mechanics and the driving forces behind a number of first-order high-pressure phase transitions.

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PubMed2002

Superconductivity in dense lithium.

Superconductivity in compressed lithium is observed by magnetic susceptibility and electrical resistivity measurements. A superconducting critical temperature (Tc) is found ranging from 9 to 16 kelvin at 23 to 80 gigapascals. The pressure dependence of Tc suggests multiple phase transitions, consistent with theoretical predictions and reported x-ray diffraction results. The observed values for Tc are much lower than those theoretically predicted, indicating that more sophisticated theoretical treatments similar to those proposed for metallic hydrogen may be required to understand superconductivity in dense phases of lithium.

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PubMed2002

Hydrogen clusters in clathrate hydrate.

High-pressure Raman, infrared, x-ray, and neutron studies show that H2 and H2O mixtures crystallize into the sII clathrate structure with an approximate H2/H2O molar ratio of 1:2. The clathrate cages are multiply occupied, with a cluster of two H2 molecules in the small cage and four in the large cage. Substantial softening and splitting of hydrogen vibrons indicate increased intermolecular interactions. The quenched clathrate is stable up to 145 kelvin at ambient pressure. Retention of hydrogen at such high temperatures could help its condensation in planetary nebulae and may play a key role in the evolution of icy bodies.

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