arXiv subjects

Vitali Prakapenka

Publications and source records attributed to Vitali Prakapenka.

23 records · Page 2Linked to original sources

arXiv2018

Observation of a metastable intermediate during solid-solid phase transformation in response to rapid compression

In order to probe the mechanism of solid-solid phase transformations, we have applied ultrafast shock wave compression (120 picosecond duration) and ultrashort (130 femtosecond) x-ray diffraction at the Linac Coherent Light Source (LCLS) to probe the compression-induced phase transition pathway in zirconium. Surprisingly, rather than transform from alpha-Zr to the more disordered hex-3 equilibrium omega-Zr phase, in its place we find the formation of a non-equilibrium body-centered cubic (bcc) metastable intermediate. Theoretically hypothesized for several decades, this bcc intermediate state has now been found to be dynamically stabilized under uniaxial loading at sub-nanosecond timescales. Molecular dynamics simulations of shock-wave propagation in zirconium predict this transformation via the dynamical intermediate state. In contrast with longer timescale experiments where the phase diagram alone is an adequate predictor of the crystalline structure of a material, our recent study highlights the importance of metastability and time-dependence in the kinetics of phase transformation at extreme conditions.

cond-mat.mtrl-sci↗

arXiv2018

Uranium polyhydrides at moderate pressures: prediction, synthesis, and expected superconductivity

Hydrogen-rich hydrides attract great attention due to recent theoretical (1) and then experimental discovery of record high-temperature superconductivity in H3S (Tc = 203 K at 155 GPa (2)). Here we search for stable uranium hydrides at pressures up to 500 GPa using ab initio evolutionary crystal structure prediction. Chemistry of the U-H system turned out to be extremely rich, with 14 new compounds, including hydrogen-rich UH5, UH6, U2H13, UH7, UH8, U2H17, and UH9. Their crystal structures are based on either common f.c.c. or h.c.p. uranium sublattice and unusual H8 cubic clusters. Our high-pressure experiments at 1-103 GPa confirm the predicted UH7, UH8, and three different phases of UH5, raising confidence about predictions of the other phases. Many of the newly predicted phases are expected to be high-temperature superconductors. The highest-Tc superconductor is UH7 predicted to be thermodynamically stable at pressures above 22 GPa (with Tc = 44-54 K) and this phase remains dynamically stable upon decompression to zero pressure (with Tc = 57-66 K).

cond-mat.mtrl-sci↗

arXiv2017

First non-icosahedral boron allotrope synthesized at high pressure and high temperature

Theoretical predictions of pressure-induced phase transformations often become long-standing enigmas because of limitations of contemporary available experimental possibilities. Hitherto the existence of a non-icosahedral boron allotrope has been one of them. Here we report on the first non-icosahedral boron allotrope, which we denoted as ζ-B, with the orthorhombic α-Ga-type structure (space group Cmce) synthesized in a diamond anvil cell at extreme high-pressure high-temperature conditions (115 GPa and 2100 K). The structure of ζ-B was solved using single-crystal synchrotron X-ray diffraction and its compressional behavior was studied in the range of very high pressures (115 GPa to 135 GPa). Experimental validation of theoretical predictions reveals the degree of our up-to-date comprehension of condensed matter and promotes further development of the solid state physics and chemistry.

cond-mat.mtrl-sci↗

arXiv2016

Size-Dependent Structural Phase Transitions in SrTiO3 Nanoparticles

Understanding the structural phase diagram of nano scale SrTiO3 has important implications on the basic physics and applications of the general class of transition metal oxide perovskites. Pressure dependent structural measurements on monodispersed nanoscale SrTiO3 samples with average diameters of 10 to ~80 nm were conducted. A robust pressure independent polar structure was detected in the 10 nm sample for pressures of up to 13 GPa while a size dependent cubic to tetragonal transition occurs (at P = Pc) for larger particle sizes. The results suggest that the growth of ~10 nm STO particles on substrates with large lattice mismatch will not alter the polar state of the system for a large range of strain values, possibly enabling device use.

cond-mat.mtrl-sci↗

arXiv2016

Hydrogen sulfide at high pressure: change in stoichiometry

Hydrogen-sulfide (H2S) was studied by x-ray synchrotron diffraction (XRD) and Raman spectroscopy up to 150 GPa at 180-295 K and by quantum-mechanical variable-composition evolutionary simulations. The experiments show that H2S becomes unstable with respect to formation of new compounds with different structure and composition, including Cccm and a body-centered-cubic (bcc) like (R3m or Im-3m) H3S, the latter one predicted previously to show a record-high superconducting transition temperature, Tc of 203 K. These experiments provide experimental ground for understanding of this record high Tc. The experimental results are supported by theoretical structure searches that suggest the stability of new H3S, H4S3, H5S8, H3S5, and HS2 compounds at elevated pressures.

cond-mat.mtrl-sci↗