Ultimate Mechanical Properties of Forsterite

The ultimate mechanical properties, as characterized here by the ideal strengths of Mg2SiO4 forsterite, have been calculated using first-principles calculations and generalized gradient approximation under tensile and shear loading. The ideal tensile strengths (ITS) and ideal shear strengths (ISS) are computed by applying homogeneous strain increments along high-symmetry directions ([100], [010], and [001]) and low index shear plane ((100), (010), and (001)) of the orthorhombic lattice. We show that the ultimate mechanical properties of forsterite are highly anisotropic, with ITS ranging from 12.1 GPa along [010] to 29.3 GPa along [100], and ISS ranging from 5.6 GPa for simple shear deformation along (100) to 11.5 GPa for shear along (010).

Metallurgical Wastes Employed as Catalysts and Photocatalysts for Water Treatment: A Review

Metallurgical slags are a mass-produced industrial solid waste, often destined to landfills; the volumes disposed represent an environmental burden. Over the last three decades, applications have been found for these wastes, mainly as a low-cost additive in building materials. More recently, their unique chemical properties have attracted attention to produce high-added-value materials for environmental applications, to be used as adsorbents, catalysts, or a source of reactive species in environmental engineering. Such uses can be classified as a function of the added value generated, technological complexity, and environmental impact. This review will focus specifically on the modification and use of slags for catalysis, photocatalysis, and photocatalytic production of hydrogen, which have received relatively little attention in literature. A summary will be presented about the general requirements for using unmodified slags as well as slag processed under alkaline or acidic conditions for advanced oxidation processes. Then, an overview will be given of the use of slags as photocatalysts in water treatment, organized according to the origin of the product (steel, copper, magnesium, ferromanganese), as well as emerging reports on the photocatalytic production of hydrogen, in contrast to the use of highly specific titania-based products developed for the same purpose.

Nb K-edge x-ray absorption investigation of the pressure induced amorphization in A-site deficient double perovskite La1/3NbO3

Nb K-edge x-ray absorption spectroscopy is utilized to investigate the changes in the local structure of the A-site deficient double perovskite La1/3NbO3 which undergoes a pressure induced irreversible amorphization. EXAFS results show that with increasing pressure up to 7.5 GPa, the average Nb-O bond distance decreases in agreement with the expected compression and tilting of the NbO6 octahedra. On the contrary, above 7.5 GPa, the average Nb-O bond distance show a tendency to increase. Significant changes in the Nb K-edge XANES spectrum with evident low energy shift of the pre-peak and the absorption edge is found to happen in La1/3NbO3 above 6.3 GPa. These changes evidence a gradual reduction of the Nb cations from Nb(5+) towards Nb(4+) above 6.3 GPa. Such a valence change accompanied by the elongation of the average Nb-O bond distances in the octahedra, introduces repulsion forces between non-bonding adjacent oxygen anions in the unoccupied A-sites. Above a critical pressure, the Nb reduction mechanism can no longer be sustained by the changing local structure and amorphization occurs, apparently due to the build-up of local strain. EXAFS and XANES results indicate two distinct pressure regimes having different local and electronic response in the La1/3NbO3 system before the occurence of the pressure induced amorphization at  ∼14.5 GPa.

Mechanism of pressure-induced amorphization

Several crystalline substances have been found to be transformed into the amorphous state under compressed condition at kinetically low temperature. Dynamical lattice-instability due to elastic deformation by shear and stress induces the reversible amorphization, some of which produces memory glass. On the other hand the irreversible modes are attributed to the plastic deformation by the nucleation of high-pressure form in the parent lattice but thermal energy is not kinetically high enough to provide the large crystallite size coherent to the X-ray radiation. They can be defined as X-ray amorphous. These reversible and irreversible transformations arise from the hindrance to sufficient atomic mobility. These pressure-induced amorphizations are the precursor phenomena of the phase transformation to high-pressure polymorphs. Successive structure changes of the pressure-induced amorphization are investigated under various pressure and temperature by X-ray diffractometry, EXAFS and Raman spectroscopy. The amorphization has been also simulated by the molecular dynamics.

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.

Electronic Transitions in Perovskite: Possible Nonconvecting Layers in the Lower Mantle

We measured the spin state of iron in magnesium silicate perovskite (Mg(0.9),Fe(0.1))SiO(3) at high pressure and found two electronic transitions occurring at 70 gigapascals and at 120 gigapascals, corresponding to partial and full electron pairing in iron, respectively. The proportion of iron in the low spin state thus grows with depth, increasing the transparency of the mantle in the infrared region, with a maximum at pressures consistent with the D" layer above the core-mantle boundary. The resulting increase in radiative thermal conductivity suggests the existence of nonconvecting layers in the lowermost mantle.

Antiferromagnetism in Pressure-Amorphized Fe 2 SiO 4

Amorphous Fe(2)SiO(4) synthesized at elevated pressures exhibits a Néel transition at a temperature identical to that observed in the crystalline form, T(N) = 65 (+/-2) kelvin at zero pressure. This behavior contrasts sharply with observations on other disordered systems, such as spin glasses, which characteristically exhibit strong "frustration" of the spins and consequent marked suppression of the Néel transition.

Memory Glass: An Amorphous Material Formed from AlPO4

A glass exhibiting structural memory has been produced through the compression of a single crystal of AlPO(4) berlinite to 18 gigapascals at 300 kelvin. The unique and extraordinary characteristic of this glass is that upon decompression below 5 gigapascals it transforms back into a single crystal with the same orientation as the starting crystal. This glass has a "memory" of the previous crystallographic orientation of the crystal from which it forms.

Ultrahigh-Pressure Melting of Lead: A Multidisciplinary Study

Measurements of the melting temperature of lead, carried out to pressures of 1 megabar (10(11) pascal) and temperatures near 4000 kelvin by means of a laser-heated diamond cell, are in excellent agreement with the results of previous shock-wave experiments. The data are analyzed by means of first principles quantum mechanical calculations, and the agreement documents the reliability of current experimental and theoretical techniques for studies of melting at ultrahigh pressures. These studies have potentially wide-ranging applications, from planetary science to condensed matter physics.