Sergey N. Tkachev

Vibrational, elastic, and structural properties of cubic silicon carbide under pressure up to 75 GPa: Implication for a primary pressure scale

We present results of concomitant measurements of synchrotron x-ray diffraction (XRD), Brillouin, and Raman spectroscopy on the single crystal samples of cubic silicon carbide (3C-SiC) under quasi-hydrostatic pressures up to 65 GPa, as well as x-ray diffraction and Raman spectroscopy up to 75 GPa. We determined the equation of state of 3C-SiC and pressure dependencies of the zone-center phonon, elastic tensor, and mode Gruneisen parameters. Cubic SiC lattice was found to be stable up to 75 GPa, but there is a tendency for destabilization above 40 GPa, based on softening of a transverse sound velocity. By applying the concomitant density and elasticity measurements, we determined the pressure on the SiC sample without referring to any other pressure scale thus establishing a new primary pressure scale with a 2%–4% precision up to 65 GPa. We proposed corrections to the existing ruby and neon pressure scales, and also calibrated cubic SiC as a pressure marker for the x-ray diffraction and Raman experiments.

Evolution of Self-Assembled ZnTe Magic-Sized Nanoclusters

Three families of ZnTe magic-sized nanoclusters (MSNCs) were obtained exclusively using polytellurides as a tellurium precursor in a one-pot reaction by simply varying the reaction temperature and time only. Different ZnTe MSNCs exhibit different self-assembling or aggregation behavior, owing to their different structure, cluster size, and dipole-dipole interactions. The smallest family of ZnTe MSNCs (F323) does not reveal a crystalline structure and as a result assembles into lamellar triangle plates. Continuous heating of as synthesized ZnTe F323 assemblies resulted in the formation of ZnTe F398 MSNCs with wurzite structure and concomitant transformation into lamellar rectangle assemblies with the organization of nanoclusters along the ⟨002⟩ direction. Further annealing of ZnTe F398 assembled lamellar rectangles leads to full organization of MSNCs in all directions and formation of larger ZnTe F444 NCs that spontaneously form ultrathin nanowires following an oriented attachment mechanism. The key step in control over the size distribution of ZnTe ultrathin nanowires is, in fact, the growth mechanism of ZnTe F398 MSNCs; namely, the step growth mechanism enables formation of more uniform nanowires compared to those obtained by continuous growth mechanism. High yield of ZnTe nanowires is achieved as a result of the wurzite structure of F398 precursor. Transient absorption (TA) measurements show that all three families possess ultrafast dynamics of photogenerated electrons, despite their different crystalline structures.

The Sm:YAG primary fluorescence pressure scale

Primary pressure determinations involve the measurement of pressure without recourse to secondary standard materials. These measurements are essential for ensuring the accuracy of pressures measured in gasketed high-pressure devices. In this study, the wavelength of optical fluorescence bands and the density of single crystal Sm-doped yttrium aluminum garnet Y3Al5O12 (Sm:YAG) have been calibrated as a primary pressure scale up to 58 GPa. Absolute pressures were obtained by integrating the bulk modulus determined via Brillouin spectroscopy with respect to volumes measured simultaneously by X-ray diffraction. A third-order Birch-Murnaghan equation of state of Sm:YAG yields V0 = 1735.15(26) A3, KT0 = 185(1.5) GPa, and K` = 4.18(5). The accompanied pressure-induced shifts of the fluorescence lines Y1 and Y2 of Sm:YAG were calibrated to the primary pressure, thus creating a highly accurate fluorescence pressure scale. These shifts are described as P = (A/B) * {[1 + (Δλ/λ0)]B − 1} with A = 2089.91(23.04), B = −4.43(1.07) for Y1, and A = 2578.22(48.70), B = −15.38(1.62) for Y2 bands, where ∆λ = λ − λ0, λ and λ0 are wavelengths in nanometer at pressure and ambient conditions. The sensitivity in the pressure determination of the Sm:YAG fluorescence shift is 0.32 nm/GPa, which is identical to that of the ruby scale. Sm:YAG can be considered elastically isotropic up to 58 GPa, implying insensitivity of the determined pressure to the crystallographic orientation under nonhydrostatic or quasi-hydrostatic conditions. The Sm:YAG fluorescence shift is apparently also independent of crystallographic orientation, in contrast to that of ruby. Since the Y fluorescence band of Sm:YAG is insensitive to temperature changes, this material is highly suitable for the measurement of pressure at elevated temperatures.

The Frenkel Line: a direct experimental evidence for the new thermodynamic boundary

Supercritical fluids play a significant role in elucidating fundamental aspects of liquid matter under extreme conditions. They have been extensively studied at pressures and temperatures relevant to various industrial applications. However, much less is known about the structural behaviour of supercritical fluids and no structural crossovers have been observed in static compression experiments in any temperature and pressure ranges beyond the critical point. The structure of supercritical state is currently perceived to be uniform everywhere on the pressure-temperature phase diagram, and to change only in a monotonic way even moving around the critical point, not only along isotherms or isobars. Conversely, we observe structural crossovers for the first time in a deeply supercritical sample through diffraction measurements in a diamond anvil cell and discover a new thermodynamic boundary on the pressure-temperature diagram. We explain the existence of these crossovers in the framework of the phonon theory of liquids using molecular dynamics simulations. The obtained results are of prime importance since they imply a global reconsideration of the mere essence of the supercritical phase. Furthermore, this discovery may pave the way to new unexpected applications and to the exploration of exotic behaviour of confined fluids relevant to geo- and planetary sciences.

Elasticity of single‐crystal superhydrous phase B at simultaneous high pressure‐temperature conditions

We investigated the combined effect of pressure and temperature on the elasticity of single-crystal superhydrous phase B (Shy-B) using Brillouin scattering and X-ray diffraction up to 12 GPa and 700 K. Using the obtained elasticity, we modeled the anisotropy of Shy-B along slab geotherms, showing that Shy-B has a low anisotropy and cannot be the major cause of the observed anisotropy in the region. Modeled velocities of Shy-B show that Shy-B will be shown as positive velocity anomalies at the bottom transition zone. Once Shy-B is transported to the topmost lower mantle, it will exhibit a seismic signature of lower velocities than topmost lower mantle. We speculate that an accumulation of hydrous phases, such as Shy-B, at the topmost lower mantle with a weight percentage of ~17–26% in the peridotite layer as subduction progresses could help explain the observed 2–3% low shear velocity anomalies in the region.

Quasicrystals at extreme conditions: The role of pressure in stabilizing icosahedral Al63Cu24Fe13 at high temperature

Abstract Icosahedrite, the first natural quasicrystal with composition Al63Cu24Fe13, was discovered in several grains of the Khatyrka meteorite, a CV3 carbonaceous chondrite. The presence of icosahedrite associated with high-pressure phases like ahrensite and stishovite indicates formation at high pressures and temperatures due to an impact-induced shock. Previous experimental studies on the stability of synthetic icosahedral AlCuFe have either been limited to ambient pressure, for which they indicate incongruent melting at ~1123 K, or limited to room-temperature, for which they indicate structural stability up to about 35 GPa. These data are insufficient to experimentally constrain the formation and stability of icosahedrite under the conditions of high pressure and temperature that formed the Khatyrka meteorite. Here we present the results of room-temperature, high-pressure diamond-anvil cells measurements of the compressional behavior of synthetic icosahedrite up to ~50 GPa. High P-T experiments were also carried out using both laser-heated diamond-anvil cells combined with in situ synchrotron X‑ray diffraction (at ~42 GPa) and multi-anvil apparatus (at 21 GPa) to investigate the structural evolution and crystallization of possible coexisting phases. The results demonstrate that the quasiperiodic order of icosahedrite is retained over the P-T range explored. We find that pressure acts to stabilize the icosahedral symmetry at temperatures much higher than previously reported. Direct solidification of AlCuFe quasicrystals from an unusual Al-Cu-rich melt is possible but it is limited to a narrow temperature range. Alternatively, quasicrystals may form after crystallization through solidsolid reactions of Al-rich phases. In either case, our results show that quasicrystals can preserve their structure even after hypervelocity impacts spanning a broad range of pressures and temperatures.

Phase Transition and Equation of State of Dense Hydrous Silica up to 63 GPa

Although it has previously been considered to be essentially anhydrous, Al-free stishovite can contain up to ∼1.3 wt% of H2O, perhaps through the direct substitution (Si4+ → 4H+), according to recent studies. Yet, the stability of such substitution and its impact on the properties of silica and rutile-structured hydrous phases (such as δ-AlOOH and phase H) are unknown at the conditions of the deeper mantle. We have synthesized hydrous and anhydrous Al-free stishovite samples at 723K and 9GPa, and 1473K and 10GPa, respectively. Synchrotron X-ray diffraction patterns show that the unit-cell volume of hydrous stishovite is 1.3% greater than that of anhydrous stishovite at 1bar, suggesting significant incorporation of OH in the crystal structure (3.2 ± 0.5 wt% H2O). At 300K, we found a lower and broader transition pressure from rutile type to CaCl2 type (28–42GPa) in hydrous dense silica. We also found that hydrous silica polymorphs are more compressible than their anhydrous counterparts. After the phase transition, the unit-cell volume of hydrous silica becomes the same as that of anhydrous silica, showing that the proton incorporation through a direct substitution can be further stabilized at high pressure. The lower pressure transition and the pressure stabilization of the proton incorporation in silica would provide ways to transport and store water in the lower mantle in silica-rich heterogeneities, such as subducted oceanic crust.

Elastic and mechanical softening in boron-doped diamond

Alternative approaches to evaluating the hardness and elastic properties of materials exhibiting physical properties comparable to pure diamond have recently become necessary. The classic linear relationship between shear modulus (G) and Vickers hardness (HV), along with more recent non-linear formulations based on Pugh’s modulus extending into the superhard region (HV > 40 GPa) have guided synthesis and identification of novel superabrasives. These schemes rely on accurately quantifying HV of diamond-like materials approaching or potentially exceeding the hardness of the diamond indenter, leading to debate about methodology and the very definition of hardness. Elasticity measurements on such materials are equally challenging. Here we used a high-precision, GHz-ultrasonic interferometer in conjunction with a newly developed optical contact micrometer and 3D optical microscopy of indentations to evaluate elasticity-hardness relations in the ultrahard range (HV > 80 GPa) by examining single-crystal boron-doped diamond (BDD) with boron contents ranging from 50–3000 ppm. We observe a drastic elastic-mechanical softening in highly doped BDD relative to the trends observed for superhard materials, providing insight into elasticity-hardness relations for ultrahard materials.

Pressure-induced phase transitions of β-type pyrochlore CsTaWO6

The β-type pyrochlore CsTaWO6 was studied by synchrotron X-ray diffraction (XRD) and Raman scattering methods up to pressures of 43 GPa using a diamond anvil cell (DAC). With increasing pressure, the cubic pyrochlore in space group of Fd-m transforms to an orthorhombic structure (space group: Pnma) at 5.9 GPa and then to a monoclinic structure (space group: P21/c) at ∼18 GPa. The structural evolution in CsTaWO6 is a continuous process and experimental results suggest that the initial cubic phase has a tetragonal distortion at ambient conditions. Both XRD and Raman measurements indicate that the pressure-induced phase transitions in CsTaWO6 are reversible. These results may provide a structural explanation of previous experimental resistivity measurement results for the isostructural superconductor K(Cs)Os2O6 at high pressure conditions.

Acoustic Microscopy and Surface Brillouin Scattering of Amorphous Carbon Pressure-Synthesized from C60

Abstract : Here, we report successful measurements by surface Brillouin scattering (SBS) and scanning acoustic microscopy (SAM) of the elastic properties of small specimens of amorphous carbon obtained from C(60) under high pressure and temperature. The super hard phases of amorphous carbon were synthesized from C(60) at pressure 13 - 13.5 GPa and temperature 800-900 degrees C. Two types of acoustic waves have been detected by 5155 in super hard samples: surface Rayleigh wave and bulk longitudinal wave. The longitudinal velocity (v(L)) in the hardest sample is slightly lower than longitudinal wave velocity in diamond in 11O direction. Simultaneous measurements of the Rayleigh and longitudinal wave velocities make it possible to determine shear and bulk elastic model of the specimens. Obtained elastic properties for amorphous carbon synthesized under pressure 13.5 GPa and temperature 900 degrees C are close to those for diamond, indicating that bonds among amorphous carbon network are diamond bonding dominated.