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In mineralogy and crystallography, a crystal structure is a unique arrangement of atoms in a crystal. A crystal structure is composed of a motif, a set of atoms arranged in a particular way, and a lattice. Motifs are located upon the points of a lattice, which is an array of points repeating periodically in three dimensions. The points can be thought of as forming identical tiny boxes, called unit cells, that fill the space of the lattice. The lengths of the edges of a unit cell and the angles between them are called the lattice parameters. The symmetry properties of the crystal are embodied in its space group. A crystal's structure and symmetry play a role in determining many of its properties, such as cleavage, electronic band structure, and optical properties.
*******s [hide]
1 Unit cell
2 Classification of crystals by symmetry
2.1 The seven crystal systems
2.2 The 14 Bravais Lattices
2.3 The 32 Point Groups
2.4 The 230 Space Groups
2.5 Summary Table
3 Physical properties
3.1 Defects or impurities in crystals
3.2 Crystal symmetry and physical properties
4 Crystal structure prediction
5 See also
6 References
7 External links
[edit] Unit cell
The crystal structure of a material or the arrangement of atoms in a crystal structure can be described in terms of its unit cell. The unit cell is a tiny box containing one or more motifs, a spatial arrangement of atoms. The unit cells stacked in threedimensional space describe the bulk arrangement of atoms of the crystal. The crystal structure has a three dimensional shape. The unit cell is given by its lattice parameters, the length of the cell edges and the angles between them, while the positions of the atoms inside the unit cell are described by the set of atomic positions (xi,yi,zi) measured from a lattice point.
Although there are an infinite number of ways to specify a unit cell, for each crystal structure there is a conventional unit cell, which is chosen to display the full symmetry of the crystal (see below). However, the conventional unit cell is not always the smallest possible choice. A primitive unit cell of a particular crystal structure is the smallest possible volume one can construct with the arrangement of atoms in the crystal such that, when stacked, completely fills the space. This primitive unit cell will not always display all the symmetries inherent in the crystal. A WignerSeitz cell is a particular kind of primitive cell which has the same symmetry as the lattice. In a unit cell each atom has an identical environment when stacked in 3 dimensional space. In a primitive cell, each atom may not have the same environment.
Unit cell definition using parallelepiped with lengths a, b, c and angles between the sides given by α,β,γ[1]
[edit] Classification of crystals by symmetry
The defining property of a crystal is its inherent symmetry, by which we mean that under certain 'operations' the crystal remains unchanged. For example, rotating the crystal 180 degrees about a certain axis may result in an atomic configuration which is identical to the original configuration. The crystal is then said to have a twofold rotational symmetry about this axis. In addition to rotational symmetries like this, a crystal may have symmetries in the form of mirror planes and translational symmetries, and also the socalled compound symmetries which are a combination of translation and rotation/mirror symmetries. A full classification of a crystal is achieved when all of these inherent symmetries of the crystal are identified. [2]
[edit] The seven crystal systems
These crystal systems are a grouping of crystal structures according to the axial system used to describe their lattice. Each crystal system consists of a set of three axes in a particular geometrical arrangement. There are seven unique crystal systems.
The 7 Crystal systems
(From least to most symmetric) The 14 Bravais Lattices
1. triclinic
(none)
2. monoclinic
(1 diad) simple basecentered
3. orthorhombic
(3 perpendicular diads) simple basecentered bodycentered facecentered
4. rhombohedral
(aka, trigonal)
(1 triad)
5. tetragonal
(1 tetrad) simple bodycentered
6. hexagonal
(1 hexad)
7. cubic
(4 triads) simple (SC) bodycentered (BCC) facecentered (FCC)
The simplest and most symmetric, the cubic (or isometric) system, has the symmetry of a cube, that is, it exhibits four threefold rotational axes oriented at 109.5 degrees (the tetrahedral angle) with respect to each other. These threefold axes lie along the body diagonals of the cube. This definition of a cubic is correct, although many textbooks incorrectly state that a cube is defined by three mutually perpendicular axes of equal length – if this were true there would be far more than 14 Bravais lattices. The other six systems, in order of decreasing symmetry, are hexagonal, tetragonal, rhombohedral (also known as trigonal), orthorhombic, monoclinic and triclinic. Some crystallographers consider the hexagonal crystal system not to be its own crystal system, but instead it is a part of the trigonal crystal system. The crystal system and Bravais lattice of a crystal describe the (purely) translational symmetry of the crystal.
[edit] The 14 Bravais Lattices
When the crystal systems are combined with the various possible lattice centerings, we arrive at the Bravais lattices. They describe the geometric arrangement of the lattice points, and thereby the translational symmetry of the crystal. In three dimensions, there are 14 unique Bravais lattices which are distinct from one another in the translational symmetry they contain. All crystalline materials recognized until now (not including quasicrystals) fit in one of these arrangements. The fourteen threedimensional lattices, classified by crystal system, are shown to the right. The Bravais lattices are sometimes referred to as space lattices.
The crystal structure consists of the same group of atoms, the basis, positioned around each and every lattice point. This group of atoms therefore repeats indefinitely in three dimensions according to the arrangement of one of the 14 Bravais lattices. The characteristic rotation and mirror symmetries of the group of atoms, or unit cell, is described by its crystallographic point group.
[edit] The 32 Point Groups
The crystallographic point group or crystal class is the mathematical group comprising the symmetry operations that leave at least one point unmoved and that leave the appearance of the crystal structure unchanged. These symmetry operations include
reflection, which reflects the structure across a reflection plane
rotation, which rotates the structure a specified portion of a circle about a rotation axis
inversion which changes the sign of the coordinate of each point with respect to a center of symmetry or inversion point
improper rotation, which consists of a rotation about an axis followed by an inversion.
Rotation axes (proper and improper), reflection planes, and centers of symmetry are collectively called symmetry elements. There are 32 possible crystal classes. Each one can be classified into one of the seven crystal systems.
[edit] The 230 Space Groups
The space group of the crystal structure is composed of the translational symmetry operations in addition to the operations of the point group. These include
pure translations which move a point along a vector
screw axes, which rotate a point around an axis while translating parallel to the axis
glide planes, which reflect a point through a plane while translating it parallel to the plane.
There are 230 distinct space groups.
[edit] Summary Table
The following table shows the numerical relations:
Crystal system No. of point groups No. of bravais lattices No. of space groups
1. Triclinic 2 1 2
2. Monoclinic 3 2 13
3. Orthorhombic 3 4 59
4. Rhombohedral (aka, Trigonal) 5 1 25
5. Tetragonal 7 2 68
6. Hexagonal 7 1 27
7. Cubic 5 3 36
Total 32 14 230
[edit] Physical properties
[edit] Defects or impurities in crystals
Real crystals feature defects or irregularities in the ideal arrangements described above and it is these defects that critically determine many of the electrical and mechanical properties of real materials. When one atom substitutes for one of the principal atomic components within the crystal structure, alteration in the electrical and thermal properties of the material may ensue.[3] Impurities may also manifest as spin impurities in certain materials. Research on magnetic impurities demonstrates that substantial alteration of certain properties such as specific heat may be affected by small concentrations of an impurity, as for example impurities in semiconducting ferromagnetic alloys may lead to different properties as first predicted in the late 1960s.[4][5] Dislocations in the crystal lattice allow shear at lower stress than that needed for a perfect crystal structure.[6]
[edit] Crystal symmetry and physical properties
Twenty of the 32 crystal classes are socalled piezoelectric, and crystals belonging to one of these classes (point groups) display piezoelectricity. All 21 piezoelectric classes lack a center of symmetry. Any material develops a dielectric polarization when an electric field is applied, but a substance which has such a natural charge separation even in the absence of a field is called a polar material. Whether or not a material is polar is determined solely by its crystal structure. Only 10 of the 32 point groups are polar. All polar crystals are pyroelectric, so the 10 polar crystal classes are sometimes referred to as the pyroelectric classes.
There are a few crystal structures, notably the perovskite structure, which exhibit ferroelectric behaviour. This is analogous to ferromagnetism, in that, in the absence of an electric field during production, the ferroelectric crystal does not exhibit a polarisation. Upon the application of an electric field of sufficient magnitude, the crystal becomes permanently polarised. This polarisation can be reversed by a sufficiently large countercharge, in the same way that a ferromagnet can be reversed. However, it is important to note that, although they are called ferroelectrics, the effect is due to the crystal structure, not the presence of a ferrous metal. the angle between the normals to the two intersecting faces is called interfacial angle.
Incommensurate crystals have periodvarying translational symmetry. The period between nodes of symmetry is constant in most crystals. The distance between nodes in an incommensurate crystal is dependent on the number of nodes between it and the base node.
[edit] Crystal structure prediction
The difficulty of predicting stable crystal structures based on the knowledge of only the chemical composition (the number of possible structures is often astronomically large) has long frustrated scientists and been a stumbling block on the way to fully computational materials design. Now, with more powerful algorithms and highperformance computing, structures of medium complexity can be predicted using such approaches as evolutionary alorithms, random sampling, or metadynamics.
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