Crystallographic point group | EPFL Graph Search

In crystallography, a crystallographic point group is a set of symmetry operations, corresponding to one of the point groups in three dimensions, such that each operation (perhaps followed by a translation) would leave the structure of a crystal unchanged i.e. the same kinds of atoms would be placed in similar positions as before the transformation. For example, in many crystals in the cubic crystal system, a rotation of the unit cell by 90 degrees around an axis that is perpendicular to one of the faces of the cube is a symmetry operation that moves each atom to the location of another atom of the same kind, leaving the overall structure of the crystal unaffected. In the classification of crystals, each point group defines a so-called (geometric) crystal class. There are infinitely many three-dimensional point groups. However, the crystallographic restriction on the general point groups results in there being only 32 crystallographic point groups. These 32 point groups are one-and-t

Mechanical spectroscopy of interface stress relaxation in magnesium matrix composites

Abu Salek Md. Fahim Chowdhury

Magnesium matrix composites made of pure (99.98%) magnesium reinforced with long carbon (Mg/C) and stainless steel (Mg/steel) fibers were processed by low pressure infiltration method. Mg/C composites were then oriented from the crystallographic point of view by Bridgman technique and finally three composites were obtained with γ = 0°, 45° and 90° (γ is the angle between the matrix-fiber interface and the normal to the basal plane). The results show that the damping level in the composite for γ = 0° is almost 3 and 7 times higher than that of the composites with γ = 45° and γ = 90°, respectively. From the result analysis it was possible to conclude that the damping originates in the hysteretic motion of dislocations in the magnesium matrix. As such a mechanism is not thermally activated, high damping is observed over a broad frequency range. The complete evolution of the mechanical loss as a function of the vibration amplitude has been measured. The obtained result has been found to be in excellent agreement with the Granato-Lücke model for dislocation breakaway. The fit of the experimental points with the theoretical curve and the straight Granato-Lücke plot confirm that the microstructure evolution is controlled by the breakaway of dislocation segments from randomly segregated point defects (hysteretic damping). According to the Granato-Lücke model the critical stress for the breakaway could be estimated. As a consequence of this analysis, the transient damping due to interface thermal stress relaxation has been interpreted by a model based on a solid friction mechanism. It has been shown that the model of Mayencourt and Schaller justifies the whole mechanical loss behavior. The model predicts two fit parameters C1 and C2, which account for the respective evolution of the mobile dislocation density and interfacial strength. An anomalous behavior is observed for the parameter C2 as well upon heating as upon cooling, which is being reflected in the shear modulus evolution. This anomalous behavior is due to a "bonding-debonding" mechanism at the fiber-matrix interface due to the motion of dislocations, which depends on the orientation of the glide plane. Also, at high temperature, the interface is strong, which helps in creep resistance of the composite, and at low temperature, the interface is weak, which is positive for improving toughness by interface crack deflection.

EPFL2009

Induced giant piezoelectricity in centrosymmetric oxides

Dragan Damjanovic, Mahmoud Hadad, Paul Muralt, Daesung Park, Lukas Riemer, Vasiliki Tileli

Piezoelectrics are materials that linearly deform in response to an applied electric field. As a fundamental prerequisite, piezoelectric materials must have a noncentrosymmetric crystal structure. For more than a century, this has remained a major obstacle for finding piezoelectric materials. We circumvented this limitation by breaking the crystallographic symmetry and inducing large and sustainable piezoelectric effects in centrosymmetric materials by the electric field–induced rearrangement of oxygen vacancies. Our results show the generation of extraordinarily large piezoelectric responses [with piezoelectric strain coefficients (d33) of ~200,000 picometers per volt at millihertz frequencies] in cubic fluorite gadolinium-doped CeO2−x films, which are two orders of magnitude larger than the responses observed in the presently best-known lead-based piezoelectric relaxor–ferroelectric oxide at kilohertz frequencies. These findings provide opportunities to design piezoelectric materials from environmentally friendly centrosymmetric ones.

2022

Molecular structure of the lipoamide dehydrogenase domain of a surface antigen from Neisseria meningitidis

The protein p64k from the surface of the Neisseria meningitidis bacteria has been characterized as a two-domain protein. It contains a dihydrolipoamide dehydrogenase domain of 482 residues, involving a FAD prosthetic group as a cofactor, and a smaller lipoic acid binding domain of 86 residues. The two domains are joined by a flexible segment rich in alanine and proline residues. The structure of the dihydrolipoamide dehydrogenase domain was determined by X-ray diffraction. It was solved by a combination of molecular replacement and multiple isomorphous replacement techniques and refined to 2.7 Angstrom resolution. In the crystal, the recombinant p64k mimics the functional homo-dimer by using one of the crystallographic 2-fold axes. The reactive disulphide bridge Cys161-Cys166 is in the oxidised state and the FAD is bound in an extended conformation. This main domain contains the major antigenic determinant of the protein, an extended loop of 32 residues at the surface of the protein. A mis-attribution at residue 553 in the sequence has been detected by inspection of electron density maps and the geometry. However, when compared to the other dihydrolipoamide dehydrogenases, there are some significant differences: (1) an unusual number of cis-proline residues and (2) a new motif built around a 2-fold axis by the sulphur atoms of residues Met558, Cys560 and their symmetry related equivalents. (C) 1997 Academic Press Limited.

1997

Mechanical spectroscopy of interface stress relaxation in magnesium matrix composites

Abu Salek Md. Fahim Chowdhury

EPFL2009