Crystal

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A crystal or crystalline solid is a solid material whose constituent atom s, molecule s, or ion s are arranged in an order and disorder (physics)|orderly , Crystal structure|repeating pattern extending in all three spatial dimensions . In addition to their microscopic structure, large crystals are usually identifiable by their macroscopic 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 (), meaning both “ ice ” and “ Quartz#Varieties (according to color)|rock crystal ”,, Henry George Liddell , Robert Scott (philologist)|Robert Scott , A Greek-English Lexicon , on Perseus Digital Library from (), "icy cold, frost"., Henry George Liddell, Robert Scott, A Greek-English Lexicon , on Perseus Digital Library

Common crystals include snowflake s, diamond s, and table salt ; however, most common inorganic solids are polycrystal s. Crystals are often symmetrically intergrown to form crystal twin s.

Crystal structure (microscopic)

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. ( Quasicrystal s are an exception, see #Quasicrystals|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 " crystallite s" 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 boundary|grain boundaries . Most macroscopic inorganic solids are polycrystalline, including almost all metal s, ceramic s, ice , Rock (geology)|rocks , etc. Solids that are neither crystalline nor polycrystalline, such as glass , are called amorphous solid s , also called glass y, 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 honeycomb (geometry)|stacked in three-dimensional space to form the crystal.

The crystal structure|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 group|crystallographic space groups . These are grouped into 7 crystal system s, 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 Ice Ih|ordinary water ice ).

Crystal faces and shapes

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 (petrology)|Anhedral crystals do not, usually because the crystal is one grain in a polycrystalline solid.

The flat faces (also called facet s) of a euhedral crystal are oriented in a specific way relative to the underlying crystal structure|atomic arrangement of the crystal : They are plane (mathematics)|planes of relatively low Miller index . The surface science of metal oxides , by Victor E. Henrich, P. A. Cox, page 28, 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 system|crystal symmetry .

Crystal habit|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

Rocks

By volume and weight, the largest concentrations of crystals in the earth are part of the Earth's solid bedrock.

Some crystals have formed by magmatic and metamorphic processes, giving origin to large masses of crystalline rock (geology)|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 glass y matter is common. Other crystalline rocks, the metamorphic rocks such as marble s, mica-schist s and quartzite s, 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.

Other rock crystals have formed out of precipitation from fluids, commonly water, to form druse (geology)|druse s or quartz veins.
The evaporite s such as rock salt , gypsum and some limestones have been deposited from aqueous solution, mostly owing to evaporation in arid climates.

Ice

Water-based ice in the form of snow , sea ice and glacier s 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 mollusc s or hydroxylapatite in the case of vertebrate s.

Polymorphism and allotropy

The same group of atoms can often solidify in many different ways. Polymorphism (materials science)|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|Ice I_h , but can also exist as the cubic ice Ic|Ice I_c , the rhombohedral ice II , and many other forms. The different polymorphs are usually called different Phase (matter)| phases .

In addition, the same atoms may be able to form noncrystalline Phase (matter)|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.

Crystallization

Crystallization is the process of forming a crystalline structure from a fluid or from materials dissolved in a fluid. (More rarely, crystals may be Deposition (phase transition)|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 phase (matter)|phases , stoichiometry|stoichiometries , impurities, Crystallographic defect|defects , and crystal habit|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 boule (crystal)| 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 (chemistry)|sublimation , or simply Recrystallization (chemistry)|solvent-based crystallization .

Large single crystals can be created by geological processes. For example, Selenite (mineral)|selenite crystals in excess of 10 meter s are found in the Cave of the Crystals in Naica, Mexico. For more details on geological crystal formation, see #Rocks|above .

Crystals can also be formed by biological processes, see #Organigenic crystals|above . Conversely, some organisms have special techniques to prevent crystallization from occurring, such as antifreeze protein s.

Defects, impurities, and twinning

An ideal crystal has every atom in a perfect, exactly repeating pattern. However, in reality, most crystalline materials have a variety of crystallographic defect s, 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 defect s (an empty space where an atom should fit), interstitial defect s (an extra atom squeezed in where it does not fit), and dislocation s (see figure at right). Dislocations are especially important in materials science , because they help determine the Strength of materials|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 color|diamond's color to slightly blue. Likewise, the only difference between ruby and sapphire is the type of impurities present in a corundum crystal.

In semiconductor s, a special type of impurity, called a dopant , drastically changes the crystal's electrical properties. Semiconductor device s, such as transistor s, are made possible largely by putting different semiconductor dopants into different places, in specific patterns.

Crystal twinning|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.

Chemical bonds

Crystalline structures occur in all classes of materials, with all types of chemical bond s. Almost all metallic bond|metal exists in a polycrystalline state; amorphous or single-crystal metals must be produced synthetically, often with great difficulty. ionic bond|Ionically bonded crystals can form upon solidification of salt s, either from a molten fluid or upon crystallization from a solution. Covalent ly 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 force s can also play a role in a crystal structure; for example, this type of bonding loosely holds together the hexagonal -patterned sheets in graphite .

Properties

Crystal	Particles	Attractive forces	Melting point	Other properties
Ionic	Positive and negative ion s	Electrostatic attraction s	High	Hard, brittle, good electrical conductor in molten state
Molecular	Polar molecules	London force and dipole-dipole attraction	Low	Soft, non-conductor or extremely poor conductor of electricity in liquid state
Molecular	Non-polar molecules	London force	Low	Soft conductor

Quasicrystals

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").

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. The 2011 Nobel Prize in Chemistry was awarded to Dan Shechtman for the discovery of quasicrystals.

Special properties from anisotropy

Crystals can have certain special electrical, optical, and mechanical properties that glass and polycrystal s 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 piezoelectricity|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 glass es or polycrystal s that have been made anisotropic by Work hardening|working or stress (mechanics)|stress --for example, photoelasticity|stress-induced birefringence .

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 database s.

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