Comparative Compressibilities of Silicate Spinels: Anomalous Behavior of (Mg,Fe)2SiO4

Quantitative analysis of hydrogen sites and occupancy in deep mantle hydrous wadsleyite using single crystal neutron diffraction

Evidence from seismological and mineralogical studies increasingly indicates that water from the oceans has been transported to the deep earth to form water-bearing dense mantle minerals. Wadsleyite [(Mg, Fe²⁺)₂SiO₄] has been identified as one of the most important host minerals incorporating this type of water, which is capable of storing the entire mass of the oceans as a hidden reservoir. To understand the effects of such water on the physical properties and chemical evolution of Earth's interior, it is essential to determine where in the crystal structure the hydration occurs and which chemical bonds are altered and weakened after hydration. Here, we conduct a neutron time-of-flight single-crystal Laue diffraction study on hydrous wadsleyite. Single crystals were grown under pressure to a size suitable for the experiment and with physical qualities representative of wet, deep mantle conditions. The results of this neutron single crystal diffraction study unambiguously demonstrate the method of hydrogen incorporation into the wadsleyite, which is qualitatively different from that of its denser polymorph, ringwoodite, in the wet mantle. The difference is a vital clue towards understanding why these dense mantle minerals show distinctly different softening behaviours after hydration.

Comparative ab initio study of lattice dynamics and thermodynamics of Fe2SiO4- and Mg2SiO4-spinels

Lattice dynamics and thermodynamic properties of antiferromagnetic Fe(2)SiO(4)-spinel have been studied using density functional theory. Phonon dispersions are obtained for several hydrostatic pressures up to 20 GPa. They are used to calculate thermodynamic properties within the quasiharmonic approximation. Comparison with ab initio results obtained for Mg(2)SiO(4)-spinel is made in order to study the effect of the cation exchange on the dynamic and thermodynamic properties of (Mg, Fe)(2)SiO(4)-spinel. The obtained results have been compared with the available experimental data.

Mineral phases of the Earth´s mantle

Our knowledge of the structure of the Earth´s interior has been obtained by analysing seismic waves that travel in the Earth, and the reference Earth global models used by geophysicists are essentially seismological. Depth profiles of the seismic waves velocities reveal that the deep Earth is divided in several shells, separated by velocity and density discontinuities. The main discontinuity located at a depth of 2900 km corresponds to the transition between the mantle and the core. The Earth´s mantle can be further divided into the upper mantle and the lower mantle, with a transition zone characterised by two prominent increases in velocities observed at 410- and 660-km depths. This article will be focused on the mineral phases of the Earth´s mantle. The interpretation of seismological models in terms of chemical composition and temperature relies on the knowledge of the nature, structure and elastic properties of the candidate materials. We will describe to what extent recent advances in experimental mineral physics and X-ray diffraction have yielded essential knowledge on the structure and high-pressure high-temperature behaviour of pertinent materials, and major improvements in our understanding of the chemical and mineralogical composition of the Earth´s mantle.

Strength weakening by nanocrystals in ceramic materials

A key question in nanomechanics concerns the grain size effects on materials' strength. Correct solution to this question is critical to design and tailor the properties of materials for particular applications. The full map of grain sizes-hardness/yield stress relationship in metals has been built. However, for ceramic materials, the similar studies and understandings are really lacking. Here we employed a novel technique to comparatively study the mechanical features of titanium dioxide (TiO(2)) with different crystallite sizes. On the basis of peak profile analysis of the X-ray diffraction data, we determined yield strength for nanocrystalline and bulk TiO(2). Our results reveal a remarkable reduction in yield strength as the grain size decreases from 30-40 microm to approximately 10 nm, providing the only evidence of a strength weakening by nanocrystals relative to their bulk counterparts. This finding infers an inverse Hall-Petch effect, the first of its kind for ceramic materials, and a dramatic strength weakening after the breakdown of classic Hall-Petch relation below a characteristic grain size.

HighP-TNano-Mechanics of Polycrystalline Nickel

We have conducted highP-Tsynchrotron X-ray and time-of-flight neutron diffraction experiments as well as indentation measurements to study equation of state, constitutive properties, and hardness of nanocrystalline and bulk nickel. Our lattice volume-pressure data present a clear evidence of elastic softening in nanocrystalline Ni as compared with the bulk nickel. We show that the enhanced overall compressibility of nanocrystalline Ni is a consequence of the higher compressibility of the surface shell of Ni nanocrystals, which supports the results of molecular dynamics simulation and a generalized model of a nanocrystal with expanded surface layer. The analytical methods we developed based on the peak-profile of diffraction data allow us to identify "micro/local" yield due to high stress concentration at the grain-to-grain contacts and "macro/bulk" yield due to deviatoric stress over the entire sample. The graphic approach of our strain/stress analyses can also reveal the corresponding yield strength, grain crushing/growth, work hardening/softening, and thermal relaxation under highP-Tconditions, as well as the intrinsic residual/surface strains in the polycrystalline bulks. From micro-indentation measurements, we found that a low-temperature annealing (T < 0.4 Tm) hardens nanocrystalline Ni, leading to an inverse Hall-Petch relationship. We explain this abnormal Hall-Petch effect in terms of impurity segregation to the grain boundaries of the nanocrystalline Ni.

Redefinition of the mode Grüneisen parameter for polyatomic substances and thermodynamic implications

Although the value of the thermal Grüneisen parameter (gamma(th)) should be obtained by averaging spectroscopic measurements of mode Grüneisen parameters [gamma(i) identical with (K(T)/nu(i)) delta nu(i)/ delta P, where K(T) is isothermal bulk modulus, nu is frequency, and P is pressure], in practice, the average <gamma(i)> is up to 25% lower than gamma(th). This discrepancy limits the accuracy of inferring physical properties from spectroscopic data and their application to geophysics. The problem arises because the above formula is physically meaningful only for monatomic or diatomic solids. We redefine gamma(i) to allow for the presence of functional groups in polyatomic crystal structures, and test the formula against spinel- and olivine-group minerals that have well-constrained spectra at pressure, band assignments, thermodynamic properties, and elastic moduli, and represent two types of functional groups. Our revised formula [gamma(i) identical with (K(X)/nu(i)) delta nu(i)/ delta P] uses polyhedral bulk moduli (K(X)) appropriate to the particular atomic motion associated with each vibrational mode, which results in equal values for <gamma(i)>, gamma(th), and gamma(LA) (the Grüneisen parameter of the longitudinal acoustic mode). Similar revisions lead to the pressure derivatives of these parameters being equal. Accounting for differential compression intrinsic to structures with functional groups improves the accuracy with which spectroscopic models predict thermodynamic properties and link to elastic properties.