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Dielectric Properties
Published in Daniel D. Pollock, PHYSICAL PROPERTIES of MATERIALS for ENGINEERS 2ND EDITION, 2020
The anisotropic nature of s and d emphasizes the importance of the degree of symmetry of crystals. A crystal has a center of symmetry if two equal and opposite vectors drawn from a central point in a structure cell end at ions of the same species. The ions of crystals with symmetrical structure cells move in a uniform way and preserve their symmetry under the influence of a mechanical stress, either tension or compression. Thus, no changes in polarization take place in these crystals as a result of applied stress. Where the structure cell of a crystal has some degree of asymmetry, the dipole moment may be decreased as a result of mechanical compression or increased by tension. Of the natural 32 crystal types, 21 are unsymmetrical to some degree. However, many materials of these types have piezoelectric properties that are too small to be of significance.
Crystalline Structure of Different Semiconductors
Published in Jyoti Prasad Banerjee, Suranjana Banerjee, Physics of Semiconductors and Nanostructures, 2019
Jyoti Prasad Banerjee, Suranjana Banerjee
If the center of symmetry is at the origin (0, 0, 0), then corresponding to a lattice point r(x, y, z), an identical lattice point −r(x¯,y¯,z¯) exists. The center of symmetry is considered to be a reflection through a point instead of reflection at a plane. The inversion operation is equivalent to a rotation of π followed by reflection in a plane perpendicular to the rotation axis. Thus, considering all symmetry elements, a cube has a total number of 23 elements of symmetry, out of which the rotational axes of symmetry is 13, mirror planes of symmetry is 9, and the inversion symmetry is 1.
Electromagnetic Waves in Anisotropic and Optically Active Media
Published in Vladimir V. Mitin, Dmitry I. Sementsov, An Introduction to Applied Electromagnetics and Optics, 2016
Vladimir V. Mitin, Dmitry I. Sementsov
In some natural solid-state crystals, the two structural—right-handed and left-handed—forms are found. Crystals possess optical activity if they have no center of symmetry. An example of such a crystal is quartz, which in nature can exist in two types—right-handed and left-handed. Let us note that in the optical range, it is possible to create artificial optical activity in a medium. Thus, when an optically inactive substance is placed in an external magnetic field, a rotation of the polarization plane of a wave propagating in the medium is observed.
Synthesis, X-ray crystal structure, Hirshfeld surface analysis, DFT, AIM, ELF, RDG and molecular docking studies of bis[4-(dimethylamino)pyridinium]di-µ-chlorido-bis[dichloridomercurate(II)]
Published in Journal of Coordination Chemistry, 2022
Mouna Harzallah, Mouna Medimagh, Noureddine Issaoui, Thierry Roisnel, Ayed Brahim
The asymmetric unit of the title compound comprises two 4-(dimethylamino)pyridinium cations and two half [Hg2Cl6]2- anions. The structure is a coordination polymer and the two independent [Hg2(1,2)Cl6]2- anions are found to adopt a centrosymmetric arrangement. Each anion appears to be a distorted edge-shared bi-tetrahedron, similar to that reported by Larock et al. [24], with its center of mass coincident with a crystallographic center of symmetry.