Compact high-temperature cell for Brillouin scattering measurements

A compact ceramic high-temperature cell for Brillouin spectroscopy was designed and tested. The cell can be mounted onto a three- or four-circle goniometer and allows collection of the full set of elastic constants of minerals to temperatures in excess of 1500 K from samples with dimensions of 100×100×20 µm or smaller. As a test of the instrument the single-crystal elastic constants of MgO were measured to 1510(10) K, and are found to be in excellent agreement with earlier independent results. The high-temperature cell should be useful for other types of spectroscopic measurements, and is especially useful in situations where spectral properties vary with the scattering geometry

Brillouin scattering study on the single-crystal elastic properties of natrolite and analcime zeolites

Copyright © 2005 American Institute of Physics. This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Institute of Physics. The following article appeared in Journal of Applied Physics 98 (2005) and may be found at http://link.aip.org/link/?jap/98/053508The Brillouin light-scattering technique was used to investigate the single-crystal elastic properties of two aluminosilicate zeolites, natrolite (NAT) and analcime (ANA), at ambient conditions. An inversion of the acoustic velocity data results in the full set of elastic stiffness moduli (Cij's) for both materials. From the single-crystal moduli the aggregate adiabatic bulk moduli (Ks), shear moduli (G), and Poisson's ratios (v) were found to be Ks=48.5(1.0) GPa, G=31.6(1.0) GPa, and v =0.232(5) for NAT, and Ks=59.8(1.2) GPa, G=32.1(1.0) GPa, and v=0.272(5) for ANA (Voigt-Reuss-Hill averages). The bulk and shear moduli of both zeolites are relatively low compared with those of densely packed aluminosilicates, reflecting an open framework structure of (Al,SiO4) tetrahedra which is easily deformed by bending the Si–O–Al angles. As expected for a less dense crystal, NAT is softer and more compressible than ANA. An evaluation of the directional Young's moduli shows that the compressibility of NAT is nearly uniform along the [100] and [010] axes, while [001] is stiffer, in agreement with previous compression studies. We do not find experimental evidence of negative Poisson's ratios for NAT zeolites as predicted by recent theoretical calculations

Ahrensite, γ-Fe_2SiO_4, a new shock-metamorphic mineral from the Tissint meteorite: Implications for the Tissint shock event on Mars

Ahrensite (IMA 2013-028), γ-Fe_2SiO_4, is the natural Fe-analog of the silicate-spinel ringwoodite (γ-Mg_2SiO_4). It occurs in the Tissint Martian meteorite, where it forms through the transformation of the fayalite-rich rims of olivine megacrysts or Fe-rich microphenocrysts in contact with shock melt pockets. The typical sequence of phase assemblages traversing across a Tissint melt pocket into olivine is: quenched melt or fayalite-pigeonite intergrowth ⇒ bridgmanite + wüstite ⇒ ahrensite and/or ringwoodite ⇒ highly-deformed olivine + nanocrystalline ringwoodite ⇒ deformed olivine. We report the first comprehensive set of crystallographic, spectroscopic, and quantitative chemical analysis of type ahrensite, and show that concentrations of ferric iron and inversion in the type material of this newly approved mineral are negligible. We also report the occurrence of nanocrystalline ringwoodite in strained olivine and establish correlations between grain size and distance from melt pockets. The ahrensite and ringwoodite crystals show no preferred orientation, consistent with random nucleation and incoherent growth within a highly strained matrix of olivine. Grain sizes of ahrensite immediately adjacent to melt pockets are consistent with growth during a shock of moderate duration (1–10 ms)

Controlled formation of metastable germanium polymorphs

The nucleation of metastable germanium polymorphs on decompression is studied using in situ synchrotron x-ray diffraction. We show that the transition pathway is critically dependent on the hydrostaticity. Quasihydrostatic conditions result in the nucleation of the rhombohedral r8 phase, followed by the cubic bc8 and hexagonal diamond phases. In contrast, the presence of shear yields the tetragonal st12 phase. Thus, targeted nucleation of a metastable polymorph is now possible. This observation has implications for the technological exploitation of Ge, but also for other tetrahedral systems

Long-Range Ordered Carbon Clusters: A Crystalline Material with Amorphous Building Blocks

Solid-state materials can be categorized by their structures into crystalline (having periodic translation symmetry), amorphous (no periodic and orientational symmetry), and quasi-crystalline (having orientational but not periodic translation symmetry) phases. Hybridization of crystalline and amorphous structures at the atomic level has not been experimentally observed. We report the discovery of a long-range ordered material constructed from units of amorphous carbon clusters that was synthesized by compressing solvated fullerenes. Using x-ray diffraction, Raman spectroscopy, and quantum molecular dynamics simulation, we observed that, although carbon-60 cages were crushed and became amorphous, the solvent molecules remained intact, playing a crucial role in maintaining the long-range periodicity. Once formed, the high-pressure phase is quenchable back to ambient conditions and is ultra-incompressible, with the ability to indent diamond

Phase transition kinetics revealed in laser-heated dynamic diamond anvil cells

We report on a novel approach to dynamic compression of materials that bridges the gap between previous static- and dynamic- compression techniques, allowing to explore a wide range of pathways in the pressure-temperature space. By combining a dynamic-diamond anvil cell setup with double-sided laser-heating and in situ X-ray diffraction, we are able to perform dynamic compression at high temperature and characterize structural transitions with unprecedented time resolution. Using this method, we investigate the $\gamma-\epsilon$ phase transition of iron under dynamic compression for the first time, reaching compression rates of hundreds of GPa/s and temperatures of 2000 K. Our results demonstrate a distinct response of the $\gamma-\epsilon$ and $\alpha-\epsilon$ transitions to the high compression rates achieved. These findings open up new avenues to study tailored dynamic compression pathways in the pressure-temperature space and highlight the potential of this platform to capture kinetic effects in a diamond anvil cell.Comment: Reworked the text and figures to be more in line with the format of PR

Single-crystal elasticity and sound velocities of (Mg_(0.94)Fe_(0.06))O ferropericlase to 20 GPa

The single-crystal elastic properties of high-spin (Mg_(0.94)Fe_(0.06))O ferropericlase were measured by Brillouin spectroscopy on a sample compressed to 20 GPa with diamond anvil cells using methanol-ethanol-water as a pressure-transmitting medium. At room pressure, the adiabatic bulk (K_0S) and shear (μ_0S) moduli are K_0S = 163 ± 3 GPa and μ_0S = 121 ± 2 GPa, in excellent agreement with ultrasonic results from the same bulk sample (Jacobsen et al., 2002). A fit to all our high-pressure Brillouin data using a third-order finite-strain equation of state yields the following pressure derivatives of the adiabatic bulk and shear moduli: K′_0S = 3.9 ± 0.2 and μ′_0S = 2.1 ± 0.1. Within the uncertainties, we find that K_0S and K′_0S of (Mg_0.94)Fe_(0.06))O are unchanged from MgO. However, μ_0S and μ′_0S of (Mg_(0.94)Fe_(0.06))O are reduced by 8% and 11%, respectively. The aggregate compressional (VP) and shear (VS) wave velocities are reduced by 4% and 6%, respectively, as compared to MgO. The pressure dependence of the single-crystal elastic moduli and aggregate sound velocities is linear within the investigated pressure range. The elastic anisotropy of (Mg_(0.94)Fe_(0.06))O is about 10% greater than that of MgO at ambient conditions. At the highest pressure obtained here, the elastic anisotropy of (Mg_(0.94)Fe_(0.06))O is close to zero. On the basis of our measurements and earlier ultrasonic measurements, we find that the pressure derivatives of shear moduli obtained at room pressure for low iron concentrations (<20 mol% FeO) of high-spin ferropericlase are inconsistent with those inferred from the lower mantle PREM model

Phase transition kinetics revealed by <i>in situ</i> x-ray diffraction in laser-heated dynamic diamond anvil cells

We report successful coupling of dynamic loading in a diamond anvil cell and stable laser heating, which enables compression rates up to 500 GPa/s along high-temperature isotherms. Dynamic loading in a diamond-anvil cell allows exploration of a wider range of pathways in the pressure-temperature space compared to conventional dynamic compression techniques. By in situ x-ray diffraction, we are able to characterize and monitor the structural transitions with the appropriate time resolution i.e., millisecond timescales. Using this method, we investigate the γ−ε phase transition of iron under dynamic compression, reaching compression rates of hundreds of GPa/s and temperatures of 2000 K. Our results demonstrate a distinct response of the γ−ε and α− ε transitions to the high compression rates achieved, possibly due to the different transition mechanisms. These findings open up new avenues to study tailored dynamic compression pathways in the pressure-temperature space and highlight the potential of this platform to capture kinetic effects (over ms time scales) in a diamond anvil cell