1.
Crystallography
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Crystallography is the experimental science of determining the arrangement of atoms in the crystalline solids. The word crystallography derives from the Greek words crystallon cold drop, frozen drop, with its meaning extending to all solids with some degree of transparency, and graphein to write. In July 2012, the United Nations recognised the importance of the science of crystallography by proclaiming that 2014 would be the International Year of Crystallography, X-ray crystallography is used to determine the structure of large biomolecules such as proteins. Before the development of X-ray diffraction crystallography, the study of crystals was based on measurements of their geometry. This involved measuring the angles of crystal faces relative to other and to theoretical reference axes. This physical measurement is carried out using a goniometer, the position in 3D space of each crystal face is plotted on a stereographic net such as a Wulff net or Lambert net. The pole to face is plotted on the net. Each point is labelled with its Miller index, the final plot allows the symmetry of the crystal to be established. Crystallographic methods now depend on analysis of the patterns of a sample targeted by a beam of some type. X-rays are most commonly used, other beams used include electrons or neutrons and this is facilitated by the wave properties of the particles. Crystallographers often explicitly state the type of beam used, as in the terms X-ray crystallography and these three types of radiation interact with the specimen in different ways. X-rays interact with the distribution of electrons in the sample. Electrons are charged particles and therefore interact with the charge distribution of both the atomic nuclei and the electrons of the sample. Neutrons are scattered by the atomic nuclei through the nuclear forces, but in addition. They are therefore also scattered by magnetic fields, when neutrons are scattered from hydrogen-containing materials, they produce diffraction patterns with high noise levels. However, the material can sometimes be treated to substitute deuterium for hydrogen, because of these different forms of interaction, the three types of radiation are suitable for different crystallographic studies. An image of an object is made using a lens to focus the beam. However, the wavelength of light is three orders of magnitude longer than the length of typical atomic bonds and atoms themselves
2.
Symmetry operation
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In the context of molecular symmetry, a symmetry operation is a permutation of atoms such that the molecule or crystal is transformed into a state indistinguishable from the starting state. Two basic facts follow from this definition, which emphasize its usefulness, physical properties must be invariant with respect to symmetry operations. Symmetry operations can be collected together in groups which are isomorphous to permutation groups, wavefunctions need not be invariant, because the operation can multiply them by a phase or mix states within a degenerate representation, without affecting any physical property. These are denoted by Cnm and are rotations of 360°/n, performed m times, the superscript m is omitted if it is equal to one. C1, rotation by 360°, is called the Identity operation and is denoted by E or I, cnn, n rotations 360°/n is also an Identity operation. These are denoted by Snm and are rotations of 360°/n followed by reflection in a perpendicular to the rotation axis. S1 is usually denoted as σ, an operation about a mirror plane. S2 is usually denoted as i, an operation about an inversion centre. When n is an even number Snn = E, but when n is odd Sn2n = E. Rotation axes, mirror planes and inversion centres are symmetry elements, the rotation axis of highest order is known as the principal rotation axis. It is conventional to set the Cartesian z axis of the molecule to contain the principal rotation axis, there is a C2 rotation axis which passes through the carbon atom and the midpoints between the two hydrogen atoms and the two chlorine atoms. Define the z axis as co-linear with the C2 axis, the xz plane as containing CH2, a C2 rotation operation permutes the two hydrogen atoms and the two chlorine atoms. Reflection in the yz plane permutes the hydrogen atoms while reflection in the xz plane permutes the chlorine atoms, the four symmetry operations E, C2, σand σ form the point group C2v. Note that if any two operations are carried out in succession the result is the same as if an operation of the group had been performed. In addition to the rotations of order 2 and 3 there are three mutually perpendicular S4 axes which pass half-way between the C-H bonds and six mirror planes. In crystals screw rotations and/or glide reflections are additionally possible and these are rotations or reflections together with partial translation. The Bravais lattices may be considered as representing translational symmetry operations, combinations of operations of the crystallographic point groups with the addition symmetry operations produce the 230 crystallographic space groups. Cotton Chemical applications of theory, Wiley,1962,1971
3.
Crystal
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A crystal or crystalline solid is a solid material whose constituents are arranged in a highly ordered microscopic structure, forming a crystal lattice that extends in all directions. In addition, macroscopic single crystals are usually identifiable by their geometrical shape, the scientific study of crystals and crystal formation is known as crystallography. The process of crystal formation via mechanisms of crystal growth is called crystallization or solidification, the word crystal derives from the Ancient Greek word κρύσταλλος, meaning both ice and rock crystal, from κρύος, icy cold, frost. Examples of large crystals include snowflakes, diamonds, and table salt, most inorganic solids are not crystals but polycrystals, i. e. many microscopic crystals fused together into a single solid. Examples of polycrystals include most metals, rocks, ceramics, a third category of solids is amorphous solids, where the atoms have no periodic structure whatsoever. Examples of amorphous solids include glass, wax, and many plastics, Crystals are often used in pseudoscientific practices such as crystal therapy, and, along with gemstones, are sometimes associated with spellwork in Wiccan beliefs and related religious movements. The scientific definition of a crystal is based on the arrangement of atoms inside it. A crystal is a solid where the form a periodic arrangement. For example, when liquid water starts freezing, the change begins with small ice crystals that grow until they fuse. Most macroscopic inorganic solids are polycrystalline, including almost all metals, ceramics, ice, rocks, solids that are neither crystalline nor polycrystalline, such as glass, are called amorphous solids, also called glassy, vitreous, or noncrystalline. These have no periodic order, even microscopically, there are distinct differences between crystalline solids and amorphous solids, most notably, the process of forming a glass does not release the latent heat of fusion, but forming a crystal does. A crystal structure is characterized by its cell, a small imaginary box containing one or more atoms in a specific spatial arrangement. The unit cells are stacked in three-dimensional space to form the crystal, the symmetry of a crystal is constrained by the requirement that the unit cells stack perfectly with no gaps. There are 219 possible crystal symmetries, called space groups. These are grouped into 7 crystal systems, such as cubic crystal system or hexagonal crystal system, Crystals are commonly recognized by their shape, consisting of flat faces with sharp angles. Euhedral crystals are those with obvious, well-formed flat faces, anhedral crystals do not, usually because the crystal is one grain in a polycrystalline solid. The flat faces of a crystal are oriented in a specific way relative to the underlying atomic arrangement of the crystal. This occurs because some surface orientations are more stable than others, as a crystal grows, new atoms attach easily to the rougher and less stable parts of the surface, but less easily to the flat, stable surfaces
4.
Quasicrystal
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A quasiperiodic crystal, or quasicrystal, is a structure that is ordered but not periodic. A quasicrystalline pattern can continuously fill all space, but it lacks translational symmetry. Aperiodic tilings were discovered by mathematicians in the early 1960s, and, some twenty years later, the discovery of these aperiodic forms in nature has produced a paradigm shift in the fields of crystallography. Quasicrystals had been investigated and observed earlier, but, until the 1980s, in 2009, after a dedicated search, a mineralogical finding, icosahedrite, offered evidence for the existence of natural quasicrystals. Roughly, an ordering is non-periodic if it lacks translational symmetry, symmetrical diffraction patterns result from the existence of an indefinitely large number of elements with a regular spacing, a property loosely described as long-range order. Experimentally, the aperiodicity is revealed in the symmetry of the diffraction pattern. In 1982 materials scientist Dan Shechtman observed that certain aluminium-manganese alloys produced the unusual diffractograms which today are seen as revelatory of quasicrystal structures. Due to fear of the communitys reaction, it took him two years to publish the results for which he was awarded the Nobel Prize in Chemistry in 2011. In 1961, Hao Wang asked whether determining if a set of tiles admits a tiling of the plane is an unsolvable problem or not. He conjectured that it is solvable, relying on the hypothesis that every set of tiles that can tile the plane can do it periodically. Nevertheless, two later, his student Robert Berger constructed a set of some 20,000 square tiles that can tile the plane. As further aperiodic sets of tiles were discovered, sets with fewer and fewer shapes were found, in 1976 Roger Penrose discovered a set of just two tiles, now referred to as Penrose tiles, that produced only non-periodic tilings of the plane. These tilings displayed instances of fivefold symmetry, around the same time Robert Ammann created a set of aperiodic tiles that produced eightfold symmetry. Mathematically, quasicrystals have been shown to be derivable from a method that treats them as projections of a higher-dimensional lattice. Icosahedral quasicrystals in three dimensions were projected from a six-dimensional hypercubic lattice by Peter Kramer and Roberto Neri in 1984, the tiling is formed by two tiles with rhombohedral shape. Shechtman first observed ten-fold electron diffraction patterns in 1982, as described in his notebook, the observation was made during a routine investigation, by electron microscopy, of a rapidly cooled alloy of aluminium and manganese prepared at the US National Bureau of Standards. In the summer of the same year Shechtman visited Ilan Blech, Blech responded that such diffractions had been seen before. Around that time, Shechtman also related his finding to John Cahn of NIST who did not offer any explanation, Shechtman quoted Cahn as saying, Danny, this material is telling us something and I challenge you to find out what it is
5.
Translational symmetry
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In geometry, a translation slides a thing by a, Ta = p + a. In physics and mathematics, continuous translational symmetry is the invariance of a system of equations under any translation, discrete translational symmetry is invariant under discrete translation. More precisely it must hold that ∀ δ A f = A, laws of physics are translationally invariant under a spatial translation if they do not distinguish different points in space. According to Noethers theorem, space translational symmetry of a system is equivalent to the momentum conservation law. Translational symmetry of a means that a particular translation does not change the object. Fundamental domains are e. g. H + a for any hyperplane H for which a has an independent direction. This is in 1D a line segment, in 2D an infinite strip, Note that the strip and slab need not be perpendicular to the vector, hence can be narrower or thinner than the length of the vector. In spaces with higher than 1, there may be multiple translational symmetry. For each set of k independent translation vectors the symmetry group is isomorphic with Zk, in particular the multiplicity may be equal to the dimension. This implies that the object is infinite in all directions, in this case the set of all translations forms a lattice. The absolute value of the determinant of the matrix formed by a set of vectors is the hypervolume of the n-dimensional parallelepiped the set subtends. This parallelepiped is a region of the symmetry, any pattern on or in it is possible. E. g. in 2D, instead of a and b we can take a. In general in 2D, we can take pa + qb and ra + sb for integers p, q, r and this ensures that a and b themselves are integer linear combinations of the other two vectors. If not, not all translations are possible with the other pair, each pair a, b defines a parallelogram, all with the same area, the magnitude of the cross product. One parallelogram fully defines the whole object, without further symmetry, this parallelogram is a fundamental domain. The vectors a and b can be represented by complex numbers, for two given lattice points, equivalence of choices of a third point to generate a lattice shape is represented by the modular group, see lattice. With rotational symmetry of order two of the pattern on the tile we have p2, the rectangle is a more convenient unit to consider as fundamental domain than a parallelogram consisting of part of a tile and part of another one
6.
Crystallographic restriction theorem
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The crystallographic restriction theorem in its basic form was based on the observation that the rotational symmetries of a crystal are usually limited to 2-fold, 3-fold, 4-fold, and 6-fold. However, quasicrystals can occur with other diffraction pattern symmetries, such as 5-fold, crystals are modeled as discrete lattices, generated by a list of independent finite translations. Because discreteness requires that the spacings between lattice points have a bound, the group of rotational symmetries of the lattice at any point must be a finite group. The strength of the theorem is not all finite groups are compatible with a discrete lattice, in any dimension. The special cases of 2D and 3D are most heavily used in applications, a rotation symmetry in dimension 2 or 3 must move a lattice point to a succession of other lattice points in the same plane, generating a regular polygon of coplanar lattice points. We now confine our attention to the plane in which the symmetry acts, now consider an 8-fold rotation, and the displacement vectors between adjacent points of the polygon. If a displacement exists between any two points, then that same displacement is repeated everywhere in the lattice. So collect all the edge displacements to begin at a lattice point. The edge vectors become radial vectors, and their 8-fold symmetry implies a regular octagon of lattice points around the collection point, but this is impossible, because the new octagon is about 80% as large as the original. The significance of the shrinking is that it is unlimited, the same construction can be repeated with the new octagon, and again and again until the distance between lattice points is as small as we like, thus no discrete lattice can have 8-fold symmetry. The same argument applies to any rotation, for k greater than 6. A shrinking argument also eliminates 5-fold symmetry, Consider a regular pentagon of lattice points. If it exists, then we can take every other edge displacement and assemble a 5-point star, the vertices of such a star are again vertices of a regular pentagon with 5-fold symmetry, but about 60% smaller than the original. The existence of quasicrystals and Penrose tilings shows that the assumption of a translation is necessary. And without the discrete lattice assumption, the construction not only fails to reach a contradiction. Thus 5-fold rotational symmetry cannot be eliminated by an argument missing either of those assumptions, Consider two lattice points A and B separated by a translation vector r. Consider an angle α such that a rotation of angle α about any point is a symmetry of the lattice. Rotating about point B by α maps point A to a new point A. Similarly, since both rotations mentioned are symmetry operations, A and B must both be lattice points
7.
Point group
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In geometry, a point group is a group of geometric symmetries that keep at least one point fixed. Point groups can exist in a Euclidean space with any dimension, point groups can be realized as sets of orthogonal matrices M that transform point x into point y, y = Mx where the origin is the fixed point. Point-group elements can either be rotations or else reflections, or improper rotations and these are the crystallographic point groups. Point groups can be classified into groups and achiral groups. The chiral groups are subgroups of the orthogonal group SO, they contain only orientation-preserving orthogonal transformations. The achiral groups contain also transformations of determinant −1, in an achiral group, the orientation-preserving transformations form a subgroup of index 2. Finite Coxeter groups or reflection groups are those point groups that are generated purely by a set of reflectional mirrors passing through the same point, a rank n Coxeter group has n mirrors and is represented by a Coxeter-Dynkin diagram. Coxeter notation offers a bracketed notation equivalent to the Coxeter diagram, with symbols for rotational. There are only two one-dimensional point groups, the identity group and the reflection group, point groups in two dimensions, sometimes called rosette groups. The subset of pure reflectional point groups, defined by 1 or 2 mirrors, can also be given by their Coxeter group and related polygons. The symmetry of the groups can be doubled by an isomorphism. Point groups in three dimensions, sometimes called point groups after their wide use in studying the symmetries of small molecules. They come in 7 infinite families of axial or prismatic groups, the reflection point groups, defined by 1 to 3 mirror planes, can also be given by their Coxeter group and related polyhedra. The group can be doubled, written as, mapping the first and last mirrors onto each other, doubling the symmetry to 48, the four-dimensional point groups are listed in Conway and Smith, Section 4, Tables 4. 1-4.3. The following list gives the four-dimensional reflection groups, each group is specified as a Coxeter group, and like the polyhedral groups of 3D, it can be named by its related convex regular 4-polytope. Front-back symmetric groups like and can be doubled, shown as double brackets in Coxeters notation, the following table gives the five-dimensional reflection groups, by listing them as Coxeter groups. The following table gives the six-dimensional reflection groups, by listing them as Coxeter groups, the following table gives the seven-dimensional reflection groups, by listing them as Coxeter groups. The following table gives the eight-dimensional reflection groups, by listing them as Coxeter groups, S. M. Coxeter, Kaleidoscopes, Selected Writings of H. S. M. Coxeter, edited by F. Arthur Sherk, Peter McMullen, Anthony C
8.
Johann F. C. Hessel
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Johann Friedrich Christian Hessel was a German physician and professor of mineralogy at the University of Marburg. The origins of geometric crystallography, for which Hessels work was noteworthy, Hessel also made contributions to classical mineralogy, as well. A crystal form here denotes a set of symmetrically equivalent planes with Miller indices enclosed in braces, for example, a cube-shaped crystal of fluorite has six equivalent faces. The entire set is denoted as, the indices for each of the individual six faces are enclosed by parentheses and these are designated, and. The cube belongs to the isometric or tessular class, as do an octahedron and tetrahedron, the essential symmetry elements of the isometric class is the existence of a set of three 4-fold, four 3-fold, and six 2-fold rotation axes. In the earlier classification schemes by the German mineralogists Christian Samuel Weiss and Friedrich Mohs the isometric class had been designated sphäroedrisch and tessularisch, as of Hessels time, not all of the 32 possible symmetries had actually been observed in real crystals. Hessels work originally appeared in 1830 as an article in Gehler’s Physikalische Wörterbuch and it went unnoticed until it was republished in 1897 as part of a collection of papers on crystallography in Oswald’s Klassiker der Exakten Wissenschaften. Prior to this posthumous re-publication of Hessels investigations, similar findings had been reported by the French scientist Auguste Bravais in Extrait J. Math, pures et Applique ́es and by the Russian crystallographer Alex V. Gadolin in 1867. However, the 32 classes of symmetry are one-and-the-same as the 32 crystallographic point groups. Briefly, a crystal is similar to three-dimensional wallpaper, in that it is a repetition of some motif. The motif is created by point group operations, while the wallpaper, the symmetry of the motif is the true point group symmetry of the crystal and it causes the symmetry of the external forms. Specifically, the crystals external morphological symmetry must conform to the components of the space group symmetry operations. Under favorable circumstances, point groups can be determined solely by examination of the crystal morphology and this is not always possible because, of the many forms normally apparent or expected in a typical crystal specimen, some forms may be absent or show unequal development. The word habit is used to describe the external shape of a crystal specimen. In general, a substance may crystallize in different habits because the rates of the various faces need not be the same. Following the work of the Swiss mathematician Simon Antoine Jean LHuilier, in this case, the sum of the valence and the number of faces does not equal two plus the number of edges. Such exceptions can occur when a polyhedron possesses internal cavities, which, in turn, Hessel found this to be true with lead sulfide crystals inside calcium fluoride crystals. Hessel also found Eulers formula disobeyed with interconnected polyhedra, for example, in the field of classical mineralogy, Hessel showed that the plagioclase feldspars could be considered solid solutions of albite and anorthite
9.
Birefringence
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Birefringence is the optical property of a material having a refractive index that depends on the polarization and propagation direction of light. These optically anisotropic materials are said to be birefringent, the birefringence is often quantified as the maximum difference between refractive indices exhibited by the material. Crystals with non-cubic crystal structures are often birefringent, as are plastics under mechanical stress and this effect was first described by the Danish scientist Rasmus Bartholin in 1669, who observed it in calcite, a crystal having one of the strongest birefringences. A mathematical description of wave propagation in a birefringent medium is presented below, following is a qualitative explanation of the phenomenon. Thus rotating the material around this axis does not change its optical behavior and this special direction is known as the optic axis of the material. Light whose polarization is perpendicular to the axis is governed by a refractive index no. Light whose polarization is in the direction of the optic axis sees an optical index ne, for any ray direction there is a linear polarization direction perpendicular to the optic axis, and this is called an ordinary ray. The magnitude of the difference is quantified by the birefringence, Δ n = n e − n o, the propagation of the ordinary ray is simply described by no as if there were no birefringence involved. However the extraordinary ray, as its name suggests, propagates unlike any wave in an optical material. Its refraction at a surface can be using the effective refractive index. However it is in fact an inhomogeneous wave whose power flow is not exactly in the direction of the wave vector and this causes an additional shift in that beam, even when launched at normal incidence, as is popularly observed using a crystal of calcite as photographed above. Rotating the calcite crystal will cause one of the two images, that of the ray, to rotate slightly around that of the ordinary ray. When the light propagates either along or orthogonal to the optic axis, in the first case, both polarizations see the same effective refractive index, so there is no extraordinary ray. In the second case the extraordinary ray propagates at a different phase velocity but is not an inhomogeneous wave, for instance, a quarter-wave plate is commonly used to create circular polarization from a linearly polarized source. The case of so-called biaxial crystals is substantially more complex and these are characterized by three refractive indices corresponding to three principal axes of the crystal. For most ray directions, both polarizations would be classified as extraordinary rays but with different effective refractive indices, being extraordinary waves, however, the direction of power flow is not identical to the direction of the wave vector in either case. The two refractive indices can be determined using the index ellipsoids for given directions of the polarization, note that for biaxial crystals the index ellipsoid will not be an ellipsoid of revolution but is described by three unequal principle refractive indices nα, nβ and nγ. Thus there is no axis around which a rotation leaves the optical properties invariant, for this reason, birefringent materials with three distinct refractive indices are called biaxial