Crystal structure of hydrous wadsleyite with 2.8% H 2 O and compressibility to 60 GPa

ABSTRACT Hydrous wadsleyite (β-Mg 2 SiO 4 ) with 2.8 wt% water content has been synthesized at 15 GPa and 1250 °C in a multi-anvil press. The unit-cell parameters are: a = 5.6686(8), b = 11.569(1), c = 8.2449(9) Å, β = 90.14(1)°, and V = 540.7(1) Å 3 , and the space group is I2/m. The structure was refined in space groups Imma and I2/m. The room-pressure structure differs from that of anhydrous wadsleyite principally in the increased cation distances around O1, the non-silicate oxygen. The compression of a single crystal of this wadsleyite was measured up to 61.3(7) GPa at room temperature in a diamond anvil cell with neon as pressure medium by X-ray diffraction at Sector 13 at the Advanced Photon Source, Argonne National Laboratory. The experimental pressure range was far beyond the wadsleyite-ringwoodite phase-transition pressure at 525 km depth (17

The effect of compressive strain on the Raman modes of the dry and hydrated BaCe0.8Y0.2O3 proton conductor

The BaCe0.8Y0.2O3-{\delta} proton conductor under hydration and under compressive strain has been analyzed with high pressure Raman spectroscopy and high pressure x-ray diffraction. The pressure dependent variation of the Ag and B2g bending modes from the O-Ce-O unit is suppressed when the proton conductor is hydrated, affecting directly the proton transfer by locally changing the electron density of the oxygen ions. Compressive strain causes a hardening of the Ce-O stretching bond. The activation barrier for proton conductivity is raised, in line with recent findings using high pressure and high temperature impedance spectroscopy. The increasing Raman frequency of the B1g and B3g modes thus implies that the phonons become hardened and increase the vibration energy in the a-c crystal plane upon compressive strain, whereas phonons are relaxed in the b-axis, and thus reveal softening of the Ag and B2g modes. Lattice toughening in the a-c crystal plane raises therefore a higher activation barrier for proton transfer and thus anisotropic conductivity. The experimental findings of the interaction of protons with the ceramic host lattice under external strain may provide a general guideline for yet to develop epitaxial strained proton conducting thin film systems with high proton mobility and low activation energy

Lattice stability of nickel titanate under high pressure up to 30.3 GPa

Studying the lattice stability of ilmenite-type compounds under extreme conditions such as high temperature and high pressure is of great significance both for understanding the intrinsic mechanism of structural transformations between various forms of ABO3 compounds and for guiding the design of functional materials. Herein, lattice transformations of ilmenite-type compounds represented by nickel titanate (NiTiO3) have been studied by using in-situ high-pressure Raman spectroscopy up to 30.3 GPa for the first time. No phase transitions have been observed in the studied pressure range. However, our data clearly show a structural distortion in the local cationic octahedron-NiO6 of NiTiO3 starting at 15 GPa, which has resulted from the pressure-induced Jahn–Teller effect. Our data also indicate the ilmenite structure NiTiO3 to become more symmetrical under high pressure, and we did not find any amorphization up to 30.3 GPa. This research provides basic information on the ilmenite NiTiO3 structure that it is more stable than other analog ilmenite structures previously studied by other researchers

Lattice dynamics of NiTiO3 under high pressure: Raman evidence under two pressure-transmitting mediums

Ilmenite-type NiTiO3 has been studied by Raman spectroscopy under hydrostatic and non-hydrostatic pressure. The rhombohedral structure does not undergo phase transitions up to 35 GPa but evidence of it is found under non-hydrostatic pressure at 45 GPa. The pressure evolution of different modes will be discussed in detail. Our findings not only provide direct experimental evidence of difference in the lattice of NiTiO3 between hydrostatic and non-hydrostatic conditions, but also offer a new insight to understand the phase stability region of ilmenite minerals in the deep earth

Raman, Brillouin, and nuclear magnetic resonance spectroscopic studies on shocked borosilicate glass

Using Brillouin and Raman scattering and NMR techniques, we have investigated the elastic and structural properties of four post-shocked specimens of borosilicate glass, recovered from peak pressures of 19.8, 31.3, 40.3, and 49.1 GPa. The Raman spectra of shock-wave compressed borosilicate glass for peak pressures of 19.8 and 31.3 GPa show two new peaks at 606 cm^ and near 1600 cm^, while a peak at ～923 cm^. disappears in these glasses following shock-loading. The mode at 606 cm^ is correlated with four-membered rings, composed of one BO_4 and three SiO_4 tetrahedra (a reedmergneritelike configuration). Modes near ～1600 cm^ are of uncertain origin, while that at 923 cm^ may associated with silica tetrahedra with two nonbridging oxygens, although standard models of this type of glass suggest that total nonbridging oxygen contents should be low. The Raman spectra for the shocked samples at 40.3 and 49.1 GPa are similar to that of the unshocked sample, suggesting that much of the irreversible density and structural changes are recoverable following adiabatic decompression and thermal relaxation. This reversibility for the highest pressure shocked samples is in accord with the Brillouin results, which show an increase in the product of sound velocity and index of refraction at pressures up to 20 GPa. The Raman band initially at 450 cm^, which corresponds to the bending vibration mode of the Si-O-Si, Si-O-B (with primarily six-membered rings in the network) reaches a maximum frequency of 470 cm^ and narrowing at a peak shock pressure of 31.3 GPa, and then also decreases to its initial values for samples shocked at 40.3 and 49.1 GPa. This shift toward higher frequency under shock-wave compression indicates the average Si-O-Si, Si-O-B angles decrease with pressure. The narrowing of this band suggests a narrower distribution of Si-O-Si angles in the shocked samples for peak pressures of 19.8 and 31.3 GPa. ^B NMR spectra for all four shocked glasses are similar, and indicate ratios of BO_3 to BO_4 that are not greatly changed from the starting material. However, changes in peak shapes suggest significant changes in the connectivity of the B and Si components of the network, with more silicon neighbors surrounding BO_4 tetrahedra in the shocked glasses, and a modest increase in the number of nonring related BO_3 groups following shock-loading. Thus, the irreversible effects of shock-loading appear to be to generate smaller rings of tetrahedra (hence decreasing the average T-O-T bond angle), and to increase the average number of neighbors of Si around boron tetrahedra