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Crystal From Wikipedia, the free encyclopedia
"Xtal" redirects here. For other uses, see Xtal (disambiguation).
This article is about crystalline solids. For the type of glass, see Lead glass. For other uses, see Crystal (disambiguation).
external image 220px-Amethystemadagascar2.jpg
A crystal of amethyst quartz
external image 220px-Crystalline_polycrystalline_amorphous2.svg.png
Microscopically, a single crystal has atoms in a near-perfect periodic arrangement; a polycrystal is composed of many microscopic crystals (called "crystallites" or "grains"); and an amorphous solid (such as glass) has no periodic arrangement even microscopically.
A crystal or crystalline solid is a solid material whose constituents, such as atoms, molecules or ions, 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, consisting of flat faces with specific, characteristic orientations.
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 is derived from the Ancient Greek word κρύσταλλος (krustallos), meaning both “ice” and “rock crystal”,[1[[|]]] from κρύος (kruos), "icy cold, frost".[2[[|]]][3[[|]]]
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, and ice. 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.


Crystal structure (microscopic)

Halite (table salt, NaCl): Microscopic and macroscopic
Halite crystal (microscopic)Halite crystal (microscopic)
Microscopic structure of a halite crystal. (Purple is sodium ion, green is chlorine ion.) There is cubic symmetry in the atoms' arrangement.
Halite crystal (Macroscopic )Halite crystal (Macroscopic )
Macroscopic (~16cm) halite crystal. The right-angles between crystal faces are due to the cubic symmetry of the atoms' arrangement.
Main article: Crystal structure
The scientific definition of a "crystal" is based on the microscopic arrangement of atoms inside it, called the crystal structure. A crystal is a solid where the atoms form a periodic arrangement. (Quasicrystals are an exception, see below.)
Not all solids are crystals. For example, when liquid water starts freezing, the phase change begins with small ice crystals that grow until they fuse, forming a polycrystalline structure. In the final block of ice, each of the small crystals (called "crystallites" or "grains") is a true crystal with a periodic arrangement of atoms, but the whole polycrystal does not have a periodic arrangement of atoms, because the periodic pattern is broken at the grain boundaries. Most macroscopic inorganic solids are polycrystalline, including almost all metals, ceramics, ice, rocks, etc. 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 (an arrangement of atoms in a crystal) is characterized by its unit 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 crystallographic space groups. These are grouped into 7 crystal systems, such as cubic crystal system (where the crystals may form cubes or rectangular boxes, such as halite shown at right) or hexagonal crystal system (where the crystals may form hexagons, such as ordinary water ice).

Crystal faces and shapes

external image 350px-Crystal_facet_formation.svg.png
As a halite crystal is growing, new atoms can very easily attach to the parts of the surface with rough atomic-scale structure and many dangling bonds. Therefore, these parts of the crystal grow out very quickly (yellow arrows). Eventually, the whole surface consists of smooth, stable faces, where new atoms cannot as easily attach themselves.
Crystals are commonly recognized by their shape, consisting of flat faces with sharp angles. These shape characteristics are not necessary for a crystal—a crystal is scientifically defined by its microscopic atomic arrangement, not its macroscopic shape—but the characteristic macroscopic shape is often present and easy to see.
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 (also called facets) of a euhedral crystal are oriented in a specific way relative to the underlying atomic arrangement of the crystal: They are planes of relatively low Miller index.[4[[|]]] This occurs because some surface orientations are more stable than others (lower surface energy). 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. Therefore, the flat surfaces tend to grow larger and smoother, until the whole crystal surface consists of these plane surfaces. (See diagram on right.)
One of the oldest techniques in the science of crystallography consists of measuring the three-dimensional orientations of the faces of a crystal, and using them to infer the underlying crystal symmetry.
A crystal's habit is its visible external shape. This is determined by the crystal structure (which restricts the possible facet orientations), the specific crystal chemistry and bonding (which may favor some facet types over others), and the conditions under which the crystal formed.

Occurrence in nature

external image 220px-Ice_crystals.jpg
Ice crystals.
external image 220px-CalciteEchinosphaerites.jpg
Fossil shell with calcite crystals.


By volume and weight, the largest concentrations of crystals in the Earth are part of its solid bedrock. Crystals found in rocks typically range in size from a fraction of a millimetre to several centimetres across, although exceptionally large crystals are occasionally found. As of 1999, the world's largest known naturally occurring crystal is a crystal of beryl from Malakialina, Madagascar, 18 m (59 ft) long and 3.5 m (11 ft) in diameter, and weighing 380,000 kg (840,000 lb).[5[[|]]]
Some crystals have formed by magmatic and metamorphic processes, giving origin to large masses of crystalline rock. The vast majority of igneous rocks are formed from molten magma and the degree of crystallization depends primarily on the conditions under which they solidified. Such rocks as granite, which have cooled very slowly and under great pressures, have completely crystallized; but many kinds of lava were poured out at the surface and cooled very rapidly, and in this latter group a small amount of amorphous or glassy matter is common. Other crystalline rocks, the metamorphic rocks such as marbles, mica-schists and quartzites, are recrystallized. This means that they were at first fragmental rocks like limestone, shale and sandstone and have never been in a molten condition nor entirely in solution, but the high temperature and pressure conditions of metamorphism have acted on them by erasing their original structures and inducing recrystallization in the solid state.[6[[|]]]
Other rock crystals have formed out of precipitation from fluids, commonly water, to form druses or quartz veins. The evaporites such as halite, gypsum and some limestones have been deposited from aqueous solution, mostly owing to evaporation in arid climates.


Water-based ice in the form of snow, sea ice and glaciers is a very common manifestation of crystalline or polycrystalline matter on Earth. A single snowflake is typically a single crystal, while an ice cube is a polycrystal.

Organigenic crystals

Many living organisms are able to produce crystals, for example calcite and aragonite in the case of most molluscs or hydroxylapatite in the case of vertebrates.

Polymorphism and allotropy

Main articles: Polymorphism (materials science) and Allotropy
The same group of atoms can often solidify in many different ways. Polymorphism is the ability of a solid to exist in more than one crystal form. For example, water ice is ordinarily found in the hexagonal form Ice Ih, but can also exist as the cubic Ice Ic, the rhombohedral ice II, and many other forms. The different polymorphs are usually called different phases.
In addition, the same atoms may be able to form noncrystalline phases. For example, water can also form amorphous ice, while SiO2 can form both fused silica (an amorphous glass) and quartz (a crystal). Likewise, if a substance can form crystals, it can also form polycrystals.
For pure chemical elements, polymorphism is known as allotropy. For example, diamond and graphite are two crystalline forms of carbon, while amorphous carbon is a noncrystalline form. Polymorphs, despite having the same atoms, may have wildly different properties. For example, diamond is among the hardest substances known, while graphite is so soft that it is used as a lubricant.
Polyamorphism is a similar phenomenon where the same atoms can exist in more than one amorphous solid form.


Main articles: Crystallization and Crystal growth
external image 170px-1-cooling-crystallizer-schladen.JPG
Vertical cooling crystallizer in a beet sugar factory.
Crystallization is the process of forming a crystalline structure from a fluid or from materials dissolved in a fluid. (More rarely, crystals may be deposited directly from gas; see thin-film deposition and epitaxy.)
Crystallization is a complex and extensively-studied field, because depending on the conditions, a single fluid can solidify into many different possible forms. It can form a single crystal, perhaps with various possible phases, stoichiometries, impurities, defects, and habits. Or, it can form a polycrystal, with various possibilities for the size, arrangement, orientation, and phase of its grains. The final form of the solid is determined by the conditions under which the fluid is being solidified, such as the chemistry of the fluid, the ambient pressure, the temperature, and the speed with which all these parameters are changing.
Specific industrial techniques to produce large single crystals (called boules) include the Czochralski process and the Bridgman technique. Other less exotic methods of crystallization may be used, depending on the physical properties of the substance, including hydrothermal synthesis, sublimation, or simply solvent-based crystallization.
Large single crystals can be created by geological processes. For example, selenite crystals in excess of 10 meters are found in the Cave of the Crystals in Naica, Mexico.[7[[|]]] For more details on geological crystal formation, see above.
Crystals can also be formed by biological processes, see above. Conversely, some organisms have special techniques to prevent crystallization from occurring, such as antifreeze proteins.

Defects, impurities, and twinning

Main articles: Crystallographic defect, Impurity, Crystal twinning and Mosaicity
external image 170px-Vector_de_Burgers.PNG
Two types of crystallographic defects. Top right: edge dislocation. Bottom right: screw dislocation.
An ideal crystal has every atom in a perfect, exactly repeating pattern. However, in reality, most crystalline materials have a variety of crystallographic defects, places where the crystal's pattern is interrupted. The types and structures of these defects may have a profound effect on the properties of the materials.
A few examples of crystallographic defects include vacancy defects (an empty space where an atom should fit), interstitial defects (an extra atom squeezed in where it does not fit), and dislocations (see figure at right). Dislocations are especially important in materials science, because they help determine the mechanical strength of materials.
Another common type of crystallographic defect is an impurity, meaning that the "wrong" type of atom is present in a crystal. For example, a perfect crystal of diamond would only contain carbon atoms, but a real crystal might perhaps contain a few boron atoms as well. These boron impurities change the diamond's color to slightly blue. Likewise, the only difference between ruby and sapphire is the type of impurities present in a corundum crystal.
external image 220px-Pyrite_60608.jpg
Twinned pyrite crystal group.
In semiconductors, a special type of impurity, called a dopant, drastically changes the crystal's electrical properties. Semiconductor devices, such as transistors, are made possible largely by putting different semiconductor dopants into different places, in specific patterns.
Twinning is a phenomenon somewhere between a crystallographic defect and a grain boundary. Like a grain boundary, a twin boundary has different crystal orientations on its two sides. But unlike a grain boundary, the orientations are not random, but related in a specific, mirror-image way.
Mosaicity is a spread of crystal plane orientations. A mosaic crystal is supposed to consist of smaller crystalline units that are somewhat misaligned with respect to each other.

Chemical bonds

Crystalline structures occur in all classes of materials, with all types of chemical bonds. Almost all metal exists in a polycrystalline state; amorphous or single-crystal metals must be produced synthetically, often with great difficulty. Ionically bonded crystals can form upon solidification of salts, either from a molten fluid or upon crystallization from a solution. Covalently bonded crystals are also very common, notable examples being diamond, silica, and graphite. Polymer materials generally will form crystalline regions, but the lengths of the molecules usually prevent complete crystallization. Weak van der Waals forces can also play a role in a crystal structure; for example, this type of bonding loosely holds together the hexagonal-patterned sheets in graphite.


Attractive forces
Melting point
Other properties
Positive and negative ions
Electrostatic attractions
Hard, brittle, good electrical conductor in molten state
Polar molecules
London force and dipole-dipole attraction
Soft, non-conductor or extremely poor conductor of electricity in liquid state
Non-polar molecules
London force
Soft conductor


external image 220px-Ho-Mg-ZnQuasicrystal.jpg
The material HoMgZn forms quasicrystals, which can take on the macroscopic shape of a dodecahedron. (Only a quasicrystal, not a normal crystal, can take this shape.) The edges are 2 mm long.
Main article: Quasicrystal
A quasicrystal consists of arrays of atoms that are ordered but not strictly periodic. They have many attributes in common with ordinary crystals, such as displaying a discrete pattern in x-ray diffraction, and the ability to form shapes with smooth, flat faces.
Quasicrystals are most famous for their ability to show five-fold symmetry, which is impossible for an ordinary periodic crystal (see crystallographic restriction theorem).
The International Union of Crystallography has redefined the term "crystal" to include both ordinary periodic crystals and quasicrystals ("any solid having an essentially discrete diffraction diagram"[8[[|]]]).
Quasicrystals, first discovered in 1982, are quite rare in practice. Only about 100 solids are known to form quasicrystals, compared to about 400,000 periodic crystals measured to date.[9[[|]]] The 2011 Nobel Prize in Chemistry was awarded to Dan Shechtman for the discovery of quasicrystals.[10[[|]]]

Special properties from anisotropy

See also: Crystal optics
Crystals can have certain special electrical, optical, and mechanical properties that glass and polycrystals normally cannot. These properties are related to the anisotropy of the crystal, i.e. the lack of rotational symmetry in its atomic arrangement. One such property is the piezoelectric effect, where a voltage across the crystal can shrink or stretch it. Another is birefringence, where a double image appears when looking through a crystal. Moreover, various properties of a crystal, including electrical conductivity, electrical permittivity, and Young's modulus, may be different in different directions in a crystal. For example, graphite crystals consist of a stack of sheets, and although each individual sheet is mechanically very strong, the sheets are rather loosely bound to each other. Therefore, the mechanical strength of the material is quite different depending on the direction of stress.
Not all crystals have all of these properties. Conversely, these properties are not quite exclusive to crystals. They can appear in glasses or polycrystals that have been made anisotropic by working or stress—for example, stress-induced birefringence.


Main article: Crystallography
Crystallography is the science of measuring the crystal structure (in other words, the atomic arrangement) of a crystal. One widely used crystallography technique is X-ray diffraction. Large numbers of known crystal structures are stored in crystallographic databases.


  • external image 200px-Insulincrystals.jpg

Insulin crystals grown in earth orbit.

  • external image 200px-Hoar_frost_macro2.jpg

Hoar frost: A type of ice crystal (picture taken from a distance of about 5 cm).

  • external image 200px-Gallium_crystals.jpg

Gallium, a metal that easily forms large crystals.

  • external image 198px-Apatite-Rhodochrosite-Fluorite-169799.jpg

An apatite crystal sits front and center on cherry-red rhodochroite rhombs, purple fluorite cubes, quartz and a dusting of brass-yellow pyrite cubes.

  • external image 75px-Monokristalines_Silizium_f%C3%BCr_die_Waferherstellung.jpg

Boules of silicon, like this one, are an important type of industrially-produced single crystal.

  • external image 200px-Bornite-Chalcopyrite-Pyrite-180794.jpg

A specimen consisting of a bornite-coated chalcopyrite crystal nestled in a bed of clear quartz crystals and lustrous pyrite crystals. The bornite-coated crystal is up to 1.5 cm across.

See also


  1. "The Nobel Prize in Chemistry 2011". Retrieved 2011-12-29.

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Kilde: (25.dec.2015 kl. 13:09)


Crystal growth From Wikipedia, the free encyclopedia
Snow crystallization in Akureyri 2005-02-26 19-03-37.jpegSnow crystallization in Akureyri 2005-02-26 19-03-37.jpeg
Crystallization · Crystal growth
Recrystallization · Seed crystal
Protocrystalline · Single crystal
Methods and technology
Bridgman–Stockbarger technique
Czochralski process
Fractional crystallization
Fractional freezing
Hydrothermal synthesis
Laser-heated pedestal growth
Crystal bar process
Nucleation · Crystal
Crystal structure · Solid

external image 300px-Quartz%2C_Tibet.jpg
Quartz is one of the several thermodynamically stable crystalline forms of silica, SiO2
A crystal is a solid material whose constituent atoms, molecules, or ions are arranged in an orderly repeating pattern extending in all three spatial dimensions. Crystal growth is a major stage of a crystallization process, and consists in the addition of new atoms, ions, or polymer strings into the characteristic arrangement of a crystalline Bravais lattice. The growth typically follows an initial stage of either homogeneous or heterogeneous (surface catalyzed) nucleation, unless a "seed" crystal, purposely added to start the growth, was already present.
The action of crystal growth yields a crystalline solid whose atoms or molecules are typically close packed, with fixed positions in space relative to each other. The crystalline state of matter is characterized by a distinct structural rigidity and virtual resistance to deformation (i.e. changes of shape and/or volume). Most crystalline solids have high values both of Young's modulus and of the shear modulus of elasticity. This contrasts with most liquids or fluids, which have a low shear modulus, and typically exhibit the capacity for macroscopic viscous flow.



external image 40px-Edit-clear.svg.png
This article may be too technical for most readers to understand. Please help improve this article to make it understandable to non-experts, without removing the technical details. The talk page may contain suggestions. (December 2015)
Crystalline solids are typically formed by cooling and solidification from the molten (or liquid) state. According to the Ehrenfest classification of first-order phase transitions, there is a discontinuous change in volume (and thus a discontinuity in the slope or first derivative with respect to temperature, dV/dT) at the melting point. Within this context, the crystal and melt are distinct phases with an interfacial discontinuity having a surface of tension with a positive surface energy. Thus, a metastable parent phase is always stable with respect to the nucleation of small embryos or droplets from a daughter phase, provided it has a positive surface of tension. Such first-order transitions must proceed by the advancement of an interfacial region whose structure and properties vary discontinuously from the parent phase.[1[[|]]][2[[|]]][3[[|]]][4[[|]]]
The process of nucleation and growth generally occurs in two different stages. In the first nucleation stage, a small nucleus containing the newly forming crystal is created. Nucleation occurs relatively slowly as the initial crystal components must impinge on each other in the correct orientation and placement for them to adhere and form the crystal. After crystal nucleation, the second stage of growth rapidly ensues. Crystal growth spreads outwards from the nucleating site. In this faster process, the elements which form the motif add to the growing crystal in a prearranged system, the crystal lattice, started in crystal nucleation. As first pointed out by Frank, perfect crystals would only grow exceedingly slowly. Real crystals grow comparatively rapidly because they contain dislocations (and other defects), which provide the necessary growth points, thus providing the necessary catalyst for structural transformation and long-range order formation.[5[[|]]]


The conditions of a homogeneous environment are often approximated to but rarely ever realized. Crystal growth always involves some form of transport of matter or heat (or both). And homogeneous conditions for the transport process can only exist for spherical, cylindrical, or infinite plane surfaces. A polyhedral crystal cannot grow (remaining polyhedral) with uniform levels of supersaturation (or supercooling) over its faces. In general, the supersaturation is greatest at its corners. This refutes the assumption that the growth rate is a function of orientation and local supersaturation.
Thus, the crystal face must grow as a whole. The growth rate of the entire face is determined by the driving force (level of supersaturation) at the point of emergence of the predominant point of growth (e.g. a dislocation, a foreign particle acting as catalyst, or crystal twin). The defect-free habit face can thus resist a finite level of supersaturation without any growth at all.
Josiah Willard Gibbs was the first to point out that in the growth of a perfect crystal, the first derivative of the free energy with respect to mass becomes periodically undefinable — at each time that an additional layer on the crystal face is completed. There is discontinuity in the chemical potential at each such point.
In one sense, the crystal can then be in equilibrium with environments having a range of chemical potentials. In another sense, it is not in equilibrium. There are available states of lower free energy. But any free energy barrier must be passed by a fluctuation, or nucleation process, in order to access it. The fundamental thermodynamic effect of a screw dislocation is to eliminate this discontinuity in the chemical potential, by making it impossible to ever complete a single crystal face.


Main article: Nucleation
external image 300px-Silver_surface_crystal_growth_SEM.png
Silver crystal growing on a ceramic substrate.
Nucleation can be either homogeneous, without the influence of foreign particles, or heterogeneous, with the influence of foreign particles. Generally, heterogeneous nucleation takes place more quickly since the foreign particles act as a scaffold for the crystal to grow on, thus eliminating the necessity of creating a new surface and the incipient surface energy requirements.
Heterogeneous nucleation can take place by several methods. Some of the most typical are small inclusions, or cuts, in the container the crystal is being grown on. This includes scratches on the sides and bottom of glassware. A common practice in crystal growing is to add a foreign substance, such as a string or a rock, to the solution, thereby providing nucleation sites for facilitating crystal growth and reducing the time to fully crystallize.
The number of nucleating sites can also be controlled in this manner. If a brand-new piece of glassware or a plastic container is used, crystals may not form because the container surface is too smooth to allow heterogeneous nucleation. On the other hand, a badly scratched container will result in many lines of small crystals. To achieve a moderate number of medium-sized crystals, a container which has a few scratches works best. Likewise, adding small previously made crystals, or seed crystals, to a crystal growing project will provide nucleating sites to the solution. The addition of only one seed crystal should result in a larger single crystal.
Some important features during growth are the arrangement, the origin of growth, the interface form (important for the driving force), and the final size. When origin of growth is only in one direction for all the crystals, it can result in the material becoming very anisotropic (different properties in different directions). The interface form determines the additional free energy for each volume of crystal growth.
Lattice arrangement in metals often takes the structure of body centered cubic, face centered cubic, or hexagonal close packed. The final size of the crystal is important for mechanical properties of materials. (For example, in metals it is widely acknowledged that large crystals can stretch further due to the longer deformation path and thus lower internal stresses.).

Mechanisms of growth

external image 300px-ImgSalt.jpg
An example of the cubic crystals typical of the rock-salt structure.
File:CitricAcid Crystalisation Timelapse.oggFile:CitricAcid Crystalisation Timelapse.oggPlay media

Time-lapse of growth of a citric acid crystal. The video covers an area of 2.0 by 1.5 mm and was captured over 7.2 min.
The interface between a crystal and its vapor can be molecularly sharp at temperatures well below the melting point. An ideal crystalline surface grows by the spreading of single layers, or equivalently, by the lateral advance of the growth steps bounding the layers. For perceptible growth rates, this mechanism requires a finite driving force (or degree of supercooling) in order to lower the nucleation barrier sufficiently for nucleation to occur by means of thermal fluctuations.[6[[|]]] In the theory of crystal growth from the melt, Burton and Cabrera have distinguished between two major mechanisms:[7[[|]]][8[[|]]][9[[|]]]

  • Non-uniform lateral growth. The surface advances by the lateral motion of steps which are one interplanar spacing in height (or some integral multiple thereof). An element of surface undergoes no change and does not advance normal to itself except during the passage of a step, and then it advances by the step height. It is useful to consider the step as the transition between two adjacent regions of a surface which are parallel to each other and thus identical in configuration — displaced from each other by an integral number of lattice planes. Note here the distinct possibility of a step in a diffuse surface, even though the step height would be much smaller than the thickness of the diffuse surface.
  • Uniform normal growth. The surface advances normal to itself without the necessity of a stepwise growth mechanism. This means that in the presence of a sufficient thermodynamic driving force, every element of surface is capable of a continuous change contributing to the advancement of the interface. For a sharp or discontinuous surface, this continuous change may be more or less uniform over large areas each successive new layer. For a more diffuse surface, a continuous growth mechanism may require change over several successive layers simultaneously.

Non-uniform lateral growth is a geometrical motion of steps — as opposed to motion of the entire surface normal to itself. Alternatively, uniform normal growth is based on the time sequence of an element of surface. In this mode, there is no motion or change except when a step passes via a continual change. The prediction of which mechanism will be operative under any set of given conditions is fundamental to the understanding of crystal growth. Two criteria have been used to make this prediction:

Driving force

Consider next the necessary requirements for the appearance of lateral growth. It is evident that the lateral growth mechanism will be found when any area in the surface can reach a metastable equilibrium in the presence of a driving force. It will then tend to remain in such an equilibrium configuration until the passage of a step. Afterward, the configuration will be identical except that each part of the step but will have advanced by the step height. If the surface cannot reach equilibrium in the presence of a driving force, then it will continue to advance without waiting for the lateral motion of steps.
Thus, Cahn concluded that the distinguishing feature is the ability of the surface to reach an equilibrium state in the presence of the driving force. He also concluded that for every surface or interface in a crystalline medium, there exists a critical driving force, which, if exceeded, will enable the surface or interface to advance normal to itself, and, if not exceeded, will require the lateral growth mechanism.
Thus, for sufficiently large driving forces, the interface can move uniformly without the benefit of either a heterogeneous nucleation or screw dislocation mechanism. What constitutes a sufficiently large driving force depends upon the diffuseness of the interface, so that for extremely diffuse interfaces, this critical driving force will be so small that any measurable driving force will exceed it. Alternatively, for sharp interfaces, the critical driving force will be very large, and most growth will occur by the lateral step mechanism.
Note that in a typical solidification or crystallization process, the thermodynamic driving force is dictated by the degree of supercooling.


external image 300px-SilverSulfideWhiskers1.jpg
Silver sulfide whiskers growing out of surface-mount resistors.
It is generally believed that the mechanical and other properties of the crystal are also pertinent to the subject matter, and that crystal morphology provides the missing link between growth kinetics and physical properties. The necessary thermodynamic apparatus was provided by Josiah Willard Gibbs'study of heterogeneous equilibrium. He provided a clear definition of surface energy, by which the concept of surface tension is made applicable to solids as well as liquids. He also appreciated that an anisotropic surface free energy implied a non-spherical equilibrium shape, which should be thermodynamically defined as the shape which minimizes the total surface free energy.[13[[|]]]
It may be instructional to note that whisker growth provides the link between the mechanical phenomenon of high strength in whiskers and the various growth mechanisms which are responsible for their fibrous morphologies. (Prior to the discovery of carbon nanotubes, single-crystal whiskers had the highest tensile strength of any materials known). Some mechanisms produce defect-free whiskers, while others may have single screw dislocations along the main axis of growth — producing high strength whiskers.
The mechanism behind whisker growth is not well understood, but seems to be encouraged by compressive mechanical stresses including mechanically induced stresses, stresses induced by diffusion of different elements, and thermally induced stresses. Metal whiskers differ from metallic dendrites in several respects. Dendrites are fern-shaped like the branches of a tree, and grow across the surface of the metal. In contrast, whiskers are fibrous and project at a right angle to the surface of growth, or substrate.


external image Dendrite_formation.gif
NASA animation of dendrite formation in microgravity.
external image 350px-Dendrites01.jpg
Manganese dendrites on a limestone bedding plane from Solnhofen, Germany. Scale in mm.
Very commonly when the supersaturation (or degree of supercooling) is high, and sometimes even when it is not high, growth kinetics may be diffusion-controlled. Under such conditions, the polyhedral crystal form will be unstable, it will sprout protrusions at its corners and edges where the degree of supersaturation is at its highest level. The tips of these protrusions will clearly be the points of highest supersaturation. It is generally believed that the protrusion will become longer (and thinner at the tip) until the effect of interfacial free energy in raising the chemical potential slows the tip growth and maintains a constant value for the tip thickness.
In the subsequent tip-thickening process, there should be a corresponding instability of shape. Minor bumps or "bulges" should be exaggerated — and develop into rapidly growing side branches. In such an unstable (or metastable) situation, minor degrees of anisotropy should be sufficient to determine directions of significant branching and growth. The most appealing aspect of this argument, of course, is that it yields the primary morphological features of dendritic growth.

See also


  • Atkins, P.W., Physical Chemistry (W.H. Freeman & Co., New York, 1997) ISBN 0-7167-3465-6
  1. Gibbs, J.W. (1874–1878) On the Equilibrium of Heterogeneous Substances, Collected Works, Longmans, Green & Co., New York. PDF,

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Kilde: (25.dec.2015 kl. 13:11)


Cocrystal From Wikipedia, the free encyclopedia
The definition of a cocrystal has been debated in the crystallography field. This definition can encompass many types of compounds, including hydrates, solvates and clathrates, which represent the basic principle of host-guest chemistry. Hundreds of examples of cocrystallization are reported annually.
Although the exact definition of a cocrystal remains topic of debate, most solid-state chemists agree nowadays that they can be defined as “solids that are crystalline single phase materials composed of two or more different molecular and/or ionic compounds generally in a stoichiometric ratio which are neither solvates nor simple salts.”[1[[|]]] Several subclassifications exist.[2[[|]]][3[[|]]]



The first reported cocrystal, quinhydrone, was studied by Friedrich Wöhler in 1844. Quinhydrone is a cocrystal of quinone and hydroquinone (known archaically as quinol). He found that this material was made up of a 1:1 molar combination of the components. Quinhydrone was analyzed by numerous groups over the next decade and several related cocrystals were made from halogenated quinones.[4[[|]]]
Many cocrystals discovered in the late 1800s and early 1900s were reported in Organische Molekulverbindungen, published by Paul Pfeiffer in 1922.[4[[|]]] This book separated the cocrystals into two categories; those made of inorganic:organic components, and those made only of organic components. The inorganic:organic cocrystals include organic molecules cocrystallized with alkali and alkaline earth salts, mineral acids, and halogens as in the case of the halogenated quinones. A majority of the organic:organic cocrystals contained aromatic compounds, with a significant fraction containing di- or trinitro aromatic compounds. The existence of several cocrystals containing eucalyptol, a compound which has no aromatic groups, was an important finding which taught scientists that pi stacking is not necessary for the formation of cocrystals.[4[[|]]]
Cocrystals continued to be discovered throughout the 1900s. Some were discovered by chance and others by screening techniques. Knowledge of the intermolecular interactions and their effects on crystal packing allowed for the engineering of cocrystals with desired physical and chemical properties. In the last decade there has been an enhanced interest in cocrystal research, primarily due to applications in the pharmaceutical industry.[5[[|]]]
Cocrystals represent about 0.5% of the crystal structures archived in the Cambridge Structural Database (CSD).[5[[|]]] However, the study of cocrystals has a long history spanning more than 160 years. They have found use in a number of industries, including pharmaceutical, textile, paper, chemical processing, photographic, propellant, and electronic.[4[[|]]]


The meaning of the term "cocrystal" is subject of disagreement. One definition states that a cocrystal is a crystalline structure composed of at least two components, where the components may be atoms, ions or molecules.[4[[|]]] This definition is sometimes extended to specify that the components be solid in their pure forms at ambient conditions.[6[[|]]] However, it has been argued that this separation based on ambient phase is arbitrary.[7[[|]]] A more inclusive definition is that cocrystals “consist of two or more components that form a unique crystalline structure having unique properties.”[8[[|]]] Due to variation in the use of the term, structures such as solvates and clathrates may or may not be considered cocrystals in a given situation. It should be noted that the difference between a crystalline salt and a cocrystal lies merely in the transfer of a proton. The transfer of protons from one component to another in a crystal is dependent on the environment. For this reason, crystalline salts and cocrystals may be thought of as two ends of a proton transfer spectrum, where the salt has completed the proton transfer at one end and an absence of proton transfer exists for cocrystals at the other end.[8[[|]]]


external image 300px-Meltingptgraph.png
A schematic for the determination of melting point binary phase diagrams from thermal microscopy.
The components interact via non-covalent interactions such as hydrogen bonding, ionic interactions, van der Waals interactions and Π-interactions. The intermolecular interactions and resulting crystal structures can generate physical and chemical properties that differ from the properties of the individual components.[9[[|]]] Such properties include melting point, solubility, chemical stability, and mechanical properties. Some cocrystals have been observed to exist as polymorphs, which may display different physical properties depending on the form of the crystal.[9[[|]]]
Phase diagrams determined from the "contact method" of thermal microscopy is valuable in the detection of cocrystals.[4[[|]]] The construction of these phase diagrams is made possible due to the change in melting point upon cocrystallization. Two crystalline substances are deposited on either side of a microscope slide and are sequentially melted and resolidified. This process creates thin films of each substance with a contact zone in the middle. A melting point phase diagram may be constructed by slow heating of the slide under a microscope and observation of the melting points of the various portions of the slide. For a simple binary phase diagram, if one eutectic point is observed then the substances do not form a cocrystal. If two eutectic points are observed, then the composition between these two points corresponds to the cocrystal.

Production and characterization


There are a multitude of synthetic strategies that are available to prepare cocrystals. However, it may be difficult to prepare single cocrystals for X-ray diffraction, as it has been known to take up to 6 months to prepare these materials.[8[[|]]]
Cocrystals are typically generated through slow evaporation of solutions of the two components. This approach has been successful with molecules of complimentary hydrogen bonding properties, in which case cocrystallization is likely to be thermodynamically favored.[10[[|]]]
A multitude of other methods exist in order to produce cocrystals. Crystallizing with a molar excess of one cocrystal former may produce a cocrystal by a decrease in solubility of that one component. Another method to synthesize cocrystals is to conduct the crystallization in a slurry. As with any crystallization, solvent considerations are important. Changing the solvent will change the intermolecular interactions and possibly lead to cocrystal formation. Also, by changing the solvent, phase considerations may be utilized. The role of a solvent in nucleation of cocrystals remains poorly understood but critical in order to obtain a cocrystal from solution.[10[[|]]]
Cooling molten mixture of cocrystal formers often affords cocrystals. Seeding can be useful.[9[[|]]] Another approach that exploits phase change is sublimation which often forms hydrates.[11[[|]]]
Grinding, both neat and liquid-assisted, is employed to produce cocrystal, e.g., using a mortar and pestle, using a ball mill, or using a vibratory mill. In liquid-assisted grinding, or kneading, a small or substoichiometric amount of liquid (solvent) is added to the grinding mixture. This method was developed in order to increase the rate of cocrystal formation, but has advantages over neat grinding such as increased yield, ability to control polymorph production, better product crystallinity, and applies to a significantly larger scope of cocrystal formers.[12[[|]]] and nucleation through seeding.[11[[|]]]
Supercritical fluids (SCF's) serve as a media for growing cocrystals. Crystal growth be achieved due to unique properties of SCFs by using different supercritical fluid properties: supercritical CO2 solvent power, anti-solvent effect and its atomization enhancement.
Using intermediate phases to synthesize solid-state compounds are also employed. Through the use of a hydrate or an amorphous phase as an intermediate during synthesis in a solid-state route has proven successful in forming a cocrystal. Also, the use of a metastable polymorphic form of one cocrystal former can be employed. In this method, the metastable form acts as an unstable intermediate on the nucleation pathway to a cocrystal. As always, a clear connection between pairwise components of the cocrystal are needed in addition to the thermodynamic requirements in order to form these compounds.[9[[|]]]
Importantly, the phase that is obtained is independent of the synthetic methodology used. It may seem facile to synthesize these materials, but on the contrary the synthesis is far from routine.[10[[|]]]


Cocrystals may be characterized in a wide variety of ways. Powder X-Ray diffraction proves to be the most commonly used method in order to characterize cocrystals. It is easily seen that a unique compound is formed and if it could possibly be a cocrystal or not owing to each compound having its own distinct powder diffractogram.[6[[|]]] Single-crystal X-ray diffraction may prove difficult on some cocrystals, especially those formed through grinding, as this method more often than not provides powders. However, these forms may be formed often through other methodologies in order to afford single crystals.[12[[|]]]
Aside from common spectroscopic methods such as FT-IR and Raman spectroscopy, solid state NMR spectroscopy allows differentiation of chiral and racemic cocrystals of similar structure.[12[[|]]]
Other physical methods of characterization may be employed. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) are two commonly used methods in order to determine melting points, phase transitions, and enthalpic factors which can be compared to each individual cocrystal former.


Cocrystal engineering is relevant to production of energetic materials, pharmaceuticals, and other compounds. Of these, the most widely studied and used application is in drug development and more specifically, the formation, design, and implementation of active pharmaceutical ingredients, or API’s. Changing the structure and composition of the API can greatly influence the bioavailability of a drug.[10[[|]]] The engineering of cocrystals takes advantage of the specific properties of each component to make the most favorable conditions for solubility that could ultimately enhance the bioavailability of the drug. The principal idea is to develop superior physico-chemical properties of the API while holding the properties of the drug molecule itself constant.[11[[|]]]


Cocrystal engineering has become of such great importance in the field of pharmaceuticals that a particular subdivision of multicomponent cocrystals has been given the term pharmaceutical cocrystals to refer to a solid cocrystal former component and a molecular or ionic API (active pharmaceutical ingredient). However, other classifications also exist when one or more of the components are not in solid form under ambient conditions. For example, if one component is a liquid under ambient conditions, the cocrystal might actually be deemed a cocrystal solvate as discussed previously. The physical states of the individual components under ambient conditions is the only source of division among these classifications. The classification naming scheme of the cocrystals might seem to be of little importance to the cocrystal itself, but in the categorization lies significant information regarding the physical properties, such as solubility and melting point, and the stability of API’s.[10[[|]]]
The objective of pharmaceutical cocrystals is have properties that differ from that expected of the pure API’s without making and/or breaking covalent bonds.[13[[|]]] Among the earliest pharmaceutical cocrystals reported are of sulfonamides.[11[[|]]] The area of pharmaceutical cocrystals has thus increased on the basis of interactions between API’s and cocrystal formers. Most commonly, API’s have hydrogen-bonding capability at their exterior which makes them more susceptible to polymorphism, especially in the case of cocrystal solvates which can be known to have different polymorphic forms. Such a case is in the drug sulfathiazole, a common oral and topical antimicrobial,which has over a hundred different solvates. It is thus important in the field of pharmaceuticals to screen for every polymorphic form of a cocrystal before it is considered as a realistic improvement to the existing API. Pharmaceutical cocrystal formation can also be driven by multiple functional groups on the API, which introduces the possibility of binary, ternary, and higher ordered cocrystal forms.[14[[|]]] Nevertheless, the cocrystal former is used to optimize the properties of the API but can also be used solely in the isolation and/or purification of the API, such as a separating enantiomers from each other, as well and removed preceding the production of the drug.[10[[|]]]
It is with reasoning that the physical properties of pharmaceutical cocrystals could then ultimately change with varying amounts and concentrations of the individual components. One of the most important properties to change with varying the concentrations of the components is solubility.[13[[|]]] It has been shown that if the stability of the components is less than the cocrystal formed between them, then the solubility of the cocrystal will be lower than the pure combination of the individual constituents. If the solubility of the cocrystal is lower, this means that there exists a driving force for the cocrystallization to occur.[6[[|]]] Even more important for pharmaceutical applications is the ability to alter the stability to hydration and bioavailability of the API with cocrystal formation, which has huge implications on drug development. The cocrystal can increase or decrease such properties as melting point and stability to relative humidity compared to the pure API and therefore, must be studied on a case to case basis for their utilization in improving a pharmaceutical on the market.[11[[|]]]
A screening procedure has been developed to help determine the formation of cocrystals from two components and the ability to improve the properties of the pure API. First, the solubilities of the individual compounds are determined. Secondly, the cocrystallization of the two components is evaluated. Finally, phase diagram screening and powder X-ray diffraction (PXRD) are further investigated to optimize conditions for cocrystallization of the components.[6[[|]]] This procedure is still done to discover cocrystals of pharmaceutical interest including simple APIs, such as carbamazepine (CBZ), a common treatment for epilepsy, trigeminal neuralgia, and bipolar disorder. CBZ has only one primary functional group involved in hydrogen bonding, which simplifies the possibilities of cocrystal formation that can greatly improve its low dissolution bioavailability.[10[[|]]]
Another example of an API being studied would be that of Piracetam, or (2-oxo-1-pyrrolidinyl)acetamide, which is used to stimulate the central nervous system and thus, enhance learning and memory. Four polymorphs of Piracetam exist that involve hydrogen bonding of the carbonyl and primary amide. It is these same hydrogen bonding functional groups that interact with and enhance the cocrystallization of Piracetam with gentisic acid, a non-steroidal anti-inflammatory drug (NSAID), and with p-hydroxybenzoic acid, an isomer of the aspirin precursor salicylic acid.[10[[|]]] No matter what the API is that is being researched, it is quite evident of the wide applicability and possibility for constant improvement in the realm of drug development, thus making it clear that the driving force of cocrystallization continues to consist of attempting to improve on the physical properties in which the existing cocrystals are lacking.[6[[|]]][10[[|]]]

Energetic materials

Two explosives HMX and CL-20 cocrystallized in a ratio 1:2 to form a hybrid explosive. This explosive had the same low sensitivity of HMX and nearly the same explosive power of CL-20. Physically mixing explosives creates a mixture that has the same sensitivity as the most sensitive component, which cocrystallisation overcomes.[15[[|]]]

See also


  • Polymorphs, Salts, and Cocrystals: What’s in a Name? S. Aitipamula, R. Banerjee, A. K. Bansal, K. Biradha, M. L. Cheney, A. R. Choudhury, G. R. Desiraju, A. G. Dikundwar, R. Dubey, N. Duggirala, P. P. Ghogale, S. Ghosh, P. K. Goswami, N. R. Goud, R. R. K. R. Jetti, P. Karpinski, P. Kaushik, D. Kumar, V. Kumar, B. Moulton, A. Mukherjee, G. Mukherjee, A. S. Myerson, V. Puri, A. Ramanan, T. Rajamannar, C. M. Reddy, N. Rodriguez-Hornedo, R. D. Rogers, T. N. Guru Row, P. Sanphui, N. Shan, G. Shete, A. Singh ,C. C. Sun, J. A. Swift, R. Thaimattam, T. S. Thakur, R. K. Thaper, S. P. Thomas, S. Tothadi, V. R. Vangala, N. Variankaval, P. Vishweshwar, D. R. Weyna, M. J. Zaworotko, Cryst. Growth Des. 2012. doi:10.1021/cg3002948
  1. "Explosives: A bigger bang". The Economist. Sep 15, 2012.


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  • ” How Cocrystallization Affects Solid-State Tautomerism: Stanozolol Case Study” A. Tilborg, G. Springuel, B. Norberg, J. Wouters, T. Leyssens, Cryst. Growth Des. 2014. doi: 10.1021/cg500358h
  • ” Pharmaceutical salts and cocrystals involving amino acids: a brief structural overview of the state-of-art” A. Tilborg, B. Norberg, J. Wouters, Eur.J Med. Chem. 2014. doi: 10.1016/j.ejmech.2013.11.045
  • Stahly, G. P. (2009). "A Survey of Cocrystals Reported Prior to 2000". Crystal Growth & Design 9 (10): 4212. doi:10.1021/cg900873t.
  • Scott L. Childs (2009). Childs, Scott L; Zaworotko, Michael J, eds. "The Reemergence of Cocrystals: The Crystal Clear Writing is on the Wall Introduction to Virtual Special Issue on Pharmaceutical Cocrystals". Crystal Growth & Design 9 (10): 4208. doi:10.1021/cg901002y.
  • Ter Horst, J. H.; Deij, M. A.; Cains, P. W. (2009). "Discovering New Co-Crystals". Crystal Growth & Design 9 (3): 1531. doi:10.1021/cg801200h.
  • Bond, A. D. (2007). "What is a co-crystal?". CrystEngComm 9 (9): 833–834. doi:10.1039/b708112j.
  • Stahly, G. P. (2007). "Diversity in Single- and Multiple-Component Crystals. The Search for and Prevalence of Polymorphs and Cocrystals". Crystal Growth & Design 7 (6): 1007. doi:10.1021/cg060838j.
  • Braga, D.; Grepioni, F.; Maini, L.; Polito, M. (2009). "Crystal Polymorphism and Multiple Crystal Forms". Molecular Networks. Structure and Bonding 132. p. 25. doi:10.1007/430_2008_7. ISBN 978-3-642-01366-9.
  • Vishweshwar, P.; McMahon, J. A.; Bis, J. A.; Zaworotko, M. J. (2006). "Pharmaceutical co-crystals". Journal of Pharmaceutical Sciences 95 (3): 499–516. doi:10.1002/jps.20578. PMID 16444755.
  • Blagden, N.; Berry, D. J.; Parkin, A.; Javed, H.; Ibrahim, A.; Gavan, P. T.; De Matos, L. L.; Seaton, C. C. (2008). "Current directions in co-crystal growth". New Journal of Chemistry 32 (10): 1659. doi:10.1039/b803866j.
  • Friščić, T.; Jones, W. (2009). "Recent Advances in Understanding the Mechanism of Cocrystal Formation via Grinding". Crystal Growth & Design 9 (3): 1621. doi:10.1021/cg800764n.
  • Adivaraha, J. (2008). //Understanding the Mechanisms, Thermodynamics and Kinetics of Cocrystallization to Control Phase Transformations// (PDF) (dissertation). University of Michigan.
  • Cheney, M. L.; Weyna, D. R.; Shan, N.; Hanna, M.; Wojtas, L.; Zaworotko, M. J. (2010). "Supramolecular Architectures of Meloxicam Carboxylic Acid Cocrystals, a Crystal Engineering Case Study". Crystal Growth & Design 10 (10): 4401. doi:10.1021/cg100514g.

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