Heat capacity, Raman, and Brillouin scattering studies of M2O–MgO–WO3–P2O5 glasses (M=K,Rb)

The authors report the results of temperature-dependent Brillouin scattering from both transverse and longitudinal acoustic waves, heat capacity studies as well as room temperature Raman scattering studies on M2O–MgO–WO3–P2O5 glasses (M=K,Rb). These results were used to obtain information about structure and various properties of the studied glasses such as fragility, elastic moduli, ratio of photoelastic constants, and elastic anharmonicity. They have found that both glasses have similar properties but replacement of K+ ions by Rb+ ions in the glass network leads to decrease of elastic parameters and P44 photoelastic constant due to increase of fragility. Based on Brillouin spectroscopy they show that a linear correlation between longitudinal and shear elastic moduli holds over a large temperature range. This result supports the literature data that the Cauchy-type relation represents a general rule for amorphous solids. An analysis of the Boson peak revealed that the form of the frequency distribution of the excess density of states is in agreement with the Euclidean random matrix theory. The reason of the observed shift of the maximum frequency of the Boson peak when K+ ions are substituted for Rb+ ions is also briefly discussed

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

High-pressure phase of brucite stable at Earth's mantle transition zone and lower mantle conditions

We investigate the high-pressure phase diagram of the hydrous mineral brucite, Mg(OH)(2), using structure search algorithms and ab initio simulations. We predict a high-pressure phase stable at pressure and temperature conditions found in cold subducting slabs in Earth’s mantle transition zone and lower mantle. This prediction implies that brucite can play a much more important role in water transport and storage in Earth’s interior than hitherto thought. The predicted high-pressure phase, stable in calculations between 20 and 35 GPa and up to 800 K, features MgO(6) octahedral units arranged in the anatase–TiO(2) structure. Our findings suggest that brucite will transform from a layered to a compact 3D network structure before eventual decomposition into periclase and ice. We show that the high-pressure phase has unique spectroscopic fingerprints that should allow for straightforward detection in experiments. The phase also has distinct elastic properties that might make its direct detection in the deep Earth possible with geophysical methods

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