Occurrence and characterization of tremolite asbestos from the Mid Atlantic Ridge

Tremolite is one of the most common amphibole species and, in the fibrous form (i.e., characterized by crystals/particles consisting of fibres with length > 5 µm, width < 3 µm and aspect ratio > 3), one of the six asbestos minerals. Until now the attention of crystallographers has focused only on samples from continental environment. Here we report the first chemical and structural data of a tremolite asbestos found along the Mid Atlantic Ridge (MAR) at the eastern intersection of the Romanche Transform Fault (Equatorial MAR). Tremolite is associated with chlorite and lizardite and was formed through the green shale facies lower than zeolite in a predominantly fluid system. MAR tremolite asbestos shows very slight deviations from the ideal crystal structure of tremolite. Differences in cation site partitioning were found with respect to tremolite asbestos from ophiolitic complexes, attributed to the different chemical–physical conditions during the mineral formation. In particular, oceanic tremolite asbestos is enriched in Al and Na, forming a trend clearly distinct from the continental tremolites.

Формирование Ферми-дуг при T<< T-=SUB=-c-=/SUB=- в окрестности d-волновых узлов структурно неоднородных ВТСП YBa-=SUB=-2-=/SUB=-Cu-=SUB=-3-=/SUB=-O-=SUB=-6.92-=/SUB=-

The effect of structural inhomogeneity on the superconducting gap near d-wave for optimally doped high-Tc superconductors YBa2Cu3O6.92 sites is considered. For this purpose, the heat capacity was investigated in the temperature range T = 2-10 K and in magnetic fields H = 0-9 T for a series of fine-crystalline samples with different degrees of controlled structural disordering. The information on the features of superconducting gap near d-wave sites in structurally disordered samples is obtained. It is shown that the d-wave sites inherent in an ideal crystal structure under the influence of structural disorder are modified with the formation of small Fermi arcs in the vicinity of nodal points at T << Tc, not covered by a superconducting gap. In this case, the gap itself remains at T ≤ Tc in other directions of the Brillouin zone and coexists with the Fermi arcs. This transformation is accompanied by an increase in the steepness of the nodal slope, a decrease of the Volovik effect, and the generation of a linear term γ (0) T of the metallic type in the temperature dependence of heat capacity, which should not be present in superconductors with an ideal crystal structure and the nature of which has not yet been established.

Electronic Structure and Ionic Transport in Phosphate Glass with Pyrophosphate Structural Elements

The electronic structure of lithium pyrophosphates (Li4P2O7) and sodium (Na4P2O7) was calculated by the density functional theory method. The calculations were performed for ordered (crystalline) and disordered (corresponding to glassy) structural states of lithium and sodium pyrophosphates. The disordering of the structure is simulated by ab initio molecular dynamics from crystal state with the same atomic basis as used for ideal crystal electronic calculations.

Structure analysis of materials at the order–disorder borderline using three-dimensional electron diffraction

Diffuse scattering, observed as intensity distribution between the Bragg peaks, is associated with deviations from the average crystal structure, generally referred to as disorder. In many cases crystal defects are seen as unwanted perturbations of the periodic structure and therefore they are often ignored. Yet, when it comes to the structure analysis of nano-volumes, what electron crystallography is designed for, the significance of defects increases. Twinning and polytypic sequences are other perturbations from ideal crystal structure that are also commonly observed in nanocrystals. Here we present an overview of defect types and review some of the most prominent studies published on the analysis of defective nanocrystalline structures by means of three-dimensional electron diffraction.

Механизм плавления при быстром нагреве

The behavior of an ideal crystal during fast heating when the settled life time of particles is substantially larger than the characteristic time of changing temperature is considered. A calculation model is a crystal with a simple cubic lattice, in which the interaction between particles is described by the Lenard-Jones potential. Disordering of the crystal on heating is considered to be a result of the formation of vacancies. Vacancies are shown to form during fast heating only in the case if the temperature is higher than a critical temperature T _c. An equation of state describing the behavior of a crystal with vacancies during fast heating has been derived. It is found that there is a temperature at which the crystal lattice losses the mechanical stability. This temperature is identified as the melting temperature on fast heating.

Exotic Spectra and Lattice Vibrations of Ice X Using the DFT Method

A typical vibrational spectrum in the ice phase has four separate bands: Translation, libration, bending, and stretching. Ice X, the final ice phase under high pressure, shows an exotic vibrational spectrum. Based on harmonic approximation, an ideal crystal of ice X has one peak, at 998 cm−1, for Raman scattering and two peaks, at 450 cm−1 and 1507 cm−1, for infrared absorption in this work. These three characteristic peaks are indicators of the phase transition between ice VII and VIII and ice X. Despite many experimental and theoretical works on ice X, only this study has clearly indicated these characteristic peaks in the region of the IR band. The phonon density of states shows quite different features than ice VIII, which could be verified by inelastic neutron scattering in the future. The dynamic processes of 15 vibrational normal modes are discussed and the typical hydrogen bonds are missing.

Exotic spectra and lattice vibrations of ice X using the DFT method

A typical vibrational spectrum in the ice phase has four separate bands: translation, libration, bending and stretching. Ice X, the final ice phase under high pressure, shows an exotic vibrational spectrum. Theoretically, an ideal crystal of ice X only has one peak at 998 cm-1 for Raman scattering and two peaks at 450 cm-1 and 1507 cm-1 for infrared absorption in this work. These three characteristic peaks are indicators of the phase transition between ice VII/VIII and ice X. Despite much experimental and theoretical work on ice X, only this study has clearly indicated these characteristic peaks in the region of the IR band. The phonon density of states shows quite different features than ice VIII, which could be verified by inelastic neutron scattering in the future. The dynamic processes of 15 vibrational normal modes are discussed and the typical hydrogen bonds are missing.