Stanislav V. Sinogeikin

Single-crystal elasticity of pyrope and MgO to 20 GPa by Brillouin scattering in the diamond cell

The single-crystal elastic properties of synthetic pyrope (Mg3Al2Si3O12) and periclase (MgO) have been measured by Brillouin scattering in a diamond anvil cell (DAC) up to 20 GPa. A 16:3:1 mixture of methanol–ethanol–water was used as a pressure-transmitting medium. Above the freezing pressure of this medium (∼14 GPa), heat treatment and accompanying stress relaxation produces quasi-hydrostatic conditions. An analysis of geometric errors associated with the DAC indicates that the DAC introduces an additional uncertainty in velocity of ≤0.5% (as compared to measurements in air), if there is no vignetting of the incident and scattered beams. The nonhydrostaticity caused by freezing of the pressure-transmitting medium results in lower velocities and elastic moduli than are obtained under hydrostatic conditions, and this leads to an overestimation of the second pressure derivatives of the elastic moduli. Fitting our hydrostatic data to finite-strain equations of state yields the following adiabatic bulk (KS) and shear (μ) moduli and their pressure derivatives: KS=163.2(10), K′=3.96(10), Kʺ=−0.044(20), μ=130.2(10), μ′=2.35(10), μʺ=−0.040(20) for MgO and KS=171.2(20), K′=4.1(3), μ=93.7 (20), μ′=1.3(2) for pyrope, where primes indicate pressure derivatives of moduli. Our results for MgO are in excellent agreement with previous ultrasonic measurements performed at lower pressures, and in particular the values of K′ agree to within a few percent. Our data are in good agreement with recent compression measurements on pyrope to pressures exceeding 20 GPa, suggesting that Brillouin scattering is an accurate method for high-pressure density and elastic moduli measurements. Pyrope is nearly elastically isotropic at ambient conditions and remains isotropic over the pressure range studied here. In contrast, the elastic anisotropy of MgO is observed to decrease dramatically with increasing pressure, becoming elastically isotropic at ∼21.5 GPa.

Sound velocities and elastic properties of Fe‐bearing wadsleyite and ringwoodite

The sound velocities and single-crystal elastic moduli of β phase (wadsleyite) and γ phase (ringwoodite) of (Mg,Fe)2SiO4 with Fe/(Fe+Mg) ratios of ∼0.075 and ∼0.09, respectively, have been determined at ambient conditions by Brillouin spectroscopy. Both compressional and shear wave aggregate velocities decrease with increasing Fe content in both phases, but the magnitude of this decrease is different for the two phases. The adiabatic bulk modulus, Ks, of Fe;-bearing β-Mg2SiO4 (Ks = 170±2 GPa) is indistinguishable from that of the Mg end-member within experimental uncertainty, whereas Ks of γ-(Mg,Fe)2SiO4 increases rapidly with increasing iron content. The shear moduli of both phases decrease with increasing Fe content. Our measurements indicate that the velocity and impedance contrasts between olivine and β-(Mg,Fe)2SiO4 are independent of Fe content for Mg-rich compositions, but the contrast for the β → γ-(Mg,Fe)2SiO4 transition increases significantly with increasing Fe content. The new data support a previous estimate of 40±10% for the olivine content of the upper mantle and suggest that less than 50% (Mg,Fe)2SiO4 is sufficient to account for the observed impedance contrasts at depths of both 410 km and 520 km. Unless the effect of Fe on elastic properties is accounted for, it is difficult to account for both the 410 and 520 km discontinuities with a single olivine content.

BX90: A new diamond anvil cell design for X-ray diffraction and optical measurements

We present a new design of a universal diamond anvil cell, suitable for different kinds of experimental studies under high pressures. Main features of the cell are an ultimate 90-degrees symmetrical axial opening and high stability, making the presented cell design suitable for a whole range of techniques from optical absorption to single-crystal X-ray diffraction studies, also in combination with external resistive or double-side laser heating. Three examples of the cell applications are provided: a Brillouin scattering of neon, single-crystal X-ray diffraction of α-Cr(2)O(3), and resistivity measurements on the (Mg(0.60)Fe(0.40))(Si(0.63)Al(0.37))O(3) silicate perovskite.

Long-range ordered carbon clusters: A crystalline material with amorphous building blocks

Amorphous Crystals One usually thinks of a crystal as containing atoms, molecules, or other ordered units arranged in a periodic fashion to make a larger structure. L. Wang et al. (p. 825; see the Perspective by D. Wang and Fernandez-Martinez) compressed a solvate of xylene in C60 (fullerene) to very high pressures at room temperature, which caused the fullerene molecules to collapse and form amorphous clusters but with each cluster retaining its position in the original crystal lattice because of the interaction with the intercalated xylene molecules. Thus, a crystal was formed from amorphous constituents. On heating at atmospheric pressure, the xylene disappeared and the sample became completely amorphous. Amorphous carbon clusters can be constructed into a crystalline material. 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.

Sound velocities and elastic properties of γ-Mg2SiO4 to 873 K by Brillouin spectroscopy

Abstract The sound velocities and single-crystal elastic moduli of spinel-structured g-Mg2SiO4 were measured to 873 K by Brillouin spectroscopy using a new high-temperature cell designed for singlecrystal measurements. These are the first reported acoustic measurements of g-Mg2SiO4 elasticity at high temperatures. A linear decrease of elastic moduli and sound velocities with temperature adequately describes the data. The adiabatic bulk modulus, KS, shear modulus, m, and respective temperature derivatives for γ-Mg2SiO4 are: KS = 185(3) GPa, μ = 120.4(2.0) GPa, (∂KS/∂T)P = -0.024(3) GPa/K and (∂m/∂T)P = -0.015(2) GPa/K. Extrapolation of our data to transition zone pressures and temperatures indicates that the shear and compressional impedance contrasts associated with β- → ∂-(Mg,Fe)2SiO4 transition are sufficient to produce an observable discontinuity at 520 km depth, even with a moderate (30-50%) amount of olivine

The post-stishovite phase transition in hydrous alumina-bearing SiO2 in the lower mantle of the earth

Silica is the most abundant oxide component in the Earth mantle by weight, and stishovite, the rutile-structured (P42/mnm) high-pressure phase with silica in six coordination by oxygen, is one of the main constituents of the basaltic layer of subducting slabs. It may also be present as a free phase in the lower mantle and at the core–mantle boundary. Pure stishovite undergoes a displacive phase transition to the CaCl2 structure (Pnnm) at ≈55 GPa. Theory suggests that this transition is associated with softening of the shear modulus that could provide a significant seismic signature, but none has ever been observed in the Earth. However, stishovite in natural rocks is expected to contain up to 5 wt % Al2O3 and possibly water. Here we report the acoustic velocities, densities, and Raman frequencies of aluminum- and hydrogen-bearing stishovite with a composition close to that expected in the Earth mantle at pressures up to 43.8(3) GPa [where (3) indicates an uncertainty of 0.3 GPa]. The post-stishovite phase transition occurs at 24.3(5) GPa (at 298 K), far lower than for pure silica at 50–60 GPa. Our results suggest that the rutile–CaCl2 transition in natural stishovite (with 5 wt % Al2O3) should occur at ≈30 GPa or ≈1,000-km depth at mantle temperatures. The major changes in elastic properties across this transition could make it visible in seismic profiles and may be responsible for seismic reflectors observed at 1,000- to 1,400-km depth.

Elasticity of single-crystal calcite and rhodochrosite by Brillouin spectroscopy

Abstract The single-crystal elastic moduli of natural samples of both calcite (CaCO3) and rhodochrosite (MnCO3) have been measured by Brillouin spectroscopy under ambient condition. Based on the trigonal unit cell, the elastic constants C11, C33, C44, C12, C13, and C14 are 149.4(7), 85.2(18), 34.1(5), 57.9(11), 53.5(9), -20.0(2), and 223.9(15), 132.6(41), 44.5(9), 93.4(21), 76.0(23), -17.3(6) GPa for CaCO3 and MnCO3, respectively. Our data for calcite are in good agreement with earlier data obtained by ultrasonic experiments. The off-diagonal elastic constants (C12, C13, and C14) for rhodochrosite have systematically larger values than the trend defined by other isostructural carbonates, in all of which the divalent cations are alkaline-earth metals. This is a distinctive signature of transition- metal-bearing oxides, which is present in silicates and simple oxides as well.

Elastic properties of hydrous ringwoodite

Abstract Sound velocities and single-crystal elastic moduli of hydrous γ-Mg2SiO4 (ringwoodite) containing 2.3 wt% H2O have been measured by Brillouin spectroscopy at ambient conditions. The aggregate elastic moduli (VRH averages) are KS = 165.8(5) GPa and m = 107.4(3) GPa for the adiabatic bulk modulus and shear modulus, respectively. Although the aggregate elastic moduli and acoustic velocities of hydrous ringwoodite are smaller than those of anhydrous ringwoodite by about 10% and 3.6% respectively, our results indicate that water has a smaller effect on the elastic moduli and velocities of ringwoodite than implied by previous studies. Our new results are consistent with systematic relationships between changes in the elastic properties and density that accompany the hydration of mineral phases.

Elasticity of pyrope and majorite–pyrope solid solutions to high temperatures

Abstract Majorite–garnet solid solutions are major mineral phases in the Earth’s upper mantle and transition zone. Here we present the first Brillouin scattering measurements of the elasticity of majorite (Mj, Mg 4 Si 4 O 12 )–pyrope (Py, Mg 3 Al 2 Si 3 O 12 ) solid solutions (Mj 50 Py 50 and Mj 80 Py 20 ) and single-crystal elasticity of pure synthetic pyrope at temperatures up to 800°C. The temperature derivatives of the adiabatic bulk ( K S ) and shear ( μ ) moduli for all compositions along the Mj–Py join are the same within the experimental uncertainties (−∂ K S /∂ T =14.0–14.5(20) MPa/K, −∂ μ /∂ T =8.3–9.2(10) MPa/K). The temperature dependence of the acoustic velocities for Mj–Py solid solutions is about half that of other major transition zone minerals. This implies that temperature variations in the transition zone, inferred from lateral velocity heterogeneity, can be significantly underestimated if the properties of majoritic garnet are not taken into account.

High-pressure induced phase transitions of Y2O3 and Y2O3:Eu3+

We investigated high-pressure induced phase transitions in Y2O3 and Eu-doped Y2O3 (Y2O:Eu3+) using angular dispersive synchrotron x-ray diffraction, Raman spectroscopy, and photoluminescence (PL). With increasing pressure, we observed a series of phase transformations in Y2O3:Eu3+, which followed a structure sequence of cubic→monoclinic→hexagonal, while Y2O3 followed a sequence of cubic→hexagonal. During decompression, both hexagonal structured Y2O3 and Y2O3:Eu3+ transformed into monoclinic phases which were quenchable back to ambient pressure. Raman and PL measurements shed additional light on the different phase transition behavior in these two samples.