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In
materials science The interdisciplinary field of materials science, also commonly termed materials science and engineering, covers the design and discovery of new materials, particularly solids. The intellectual origins of materials science stem from the Enlighten ...
, polymorphism describes the existence of a solid material in more than one form or
crystal structure In crystallography, crystal structure is a description of the ordered arrangement of atoms, ions or molecules in a crystalline material. Ordered structures occur from the intrinsic nature of the constituent particles to form symmetric patterns that ...
. Polymorphism is a form of isomerism. Any crystalline material can exhibit the phenomenon.
Allotropy Allotropy or allotropism () is the property of some chemical elements to exist in two or more different forms, in the same physical state, known as allotropes of the elements. Allotropes are different structural modifications of an element; the ...
refers to polymorphism for
chemical element 400px|The periodic table of the chemical elements In chemistry, an element is a pure substance consisting only of atoms that all have the same numbers of protons in their atomic nuclei. Unlike chemical compounds, chemical elements cannot be br ...
s. Polymorphism is of practical relevance to
pharmaceutical A medication (also referred to as medicine, pharmaceutical drug, medicinal drug or simply drug) is a drug used to diagnose, cure, treat, or prevent disease. Drug therapy (pharmacotherapy) is an important part of the medical field and relies on ...
s,
agrochemical An agrochemical or agrichemical, a contraction of ''agricultural chemical'', is a chemical product used in agriculture. In most cases, ''agrichemical'' refers to pesticides including insecticides, herbicides, fungicides and nematicides. It may als ...
s,
pigment A pigment is a colored material that is completely or nearly insoluble in water. In contrast, dyes are typically soluble, at least at some stage in their use. Generally dyes are often organic compounds whereas pigments are often inorganic compou ...
s,
dyestuff 300px|Yarn drying after being dyed in the early American tradition, at Conner Prairie living_history_museum">Conner_Prairie_ living_history_museum. A_dye_is_a_[[wiktionary:colored.html" style="text-decoration: none;"class="mw-redirect" title="liv ...
s, [[foods, and [[explosives. According to [[IUPAC, a polymorphic transition is "A reversible transition of a solid crystalline phase at a certain temperature and pressure (the inversion point) to another phase of the same chemical composition with a different crystal structure." Materials with two polymorphs are called dimorphic, with three polymorphs, trimorphic, etc.


Examples

Many compounds exhibit polymorphism. It has been claimed that "every compound has different polymorphic forms, and that, in general, the number of forms known for a given compound is proportional to the time and money spent in research on that compound."


Organic compounds

;Benzamide The phenomenon was discovered in 1832 by
Friedrich Wöhler Friedrich Wöhler () FRS(For) HFRSE (31 July 180023 September 1882) was a German chemist, known for his work in inorganic chemistry, being the first to isolate the chemical elements beryllium and yttrium in pure metallic form. He was the first to ...
and
Justus von Liebig Justus Freiherr von Liebig (12 May 1803 – 18 April 1873) was a German scientist who made major contributions to agricultural and biological chemistry, and is considered one of the principal founders of organic chemistry. As a professor at the ...

Justus von Liebig
. They observed that the silky needles of freshly crystallized
benzamide Benzamide is a white solid with the chemical formula of C6H5C(O)NH2. It is the simplest amide derivative of benzoic acid. It is slightly soluble in water, and soluble in many organic solvents. A number of substituted benzamides exist. See also * ...
slowly converted to rhombic crystals. Present-day analysis identifies three polymorphs for benzamide: the least stable one, formed by flash cooling is the
orthorhombicIn crystallography, the orthorhombic crystal system is one of the 7 crystal systems. Orthorhombic lattices result from stretching a cubic lattice along two of its orthogonal pairs by two different factors, resulting in a rectangular prism with a rect ...

orthorhombic
form II. This type is followed by the
monoclinic In crystallography, the monoclinic crystal system is one of the seven crystal systems. A crystal system is described by three vectors. In the monoclinic system, the crystal is described by vectors of unequal lengths, as in the orthorhombic syste ...

monoclinic
form III (observed by Wöhler/Liebig). The most stable form is
monoclinic In crystallography, the monoclinic crystal system is one of the seven crystal systems. A crystal system is described by three vectors. In the monoclinic system, the crystal is described by vectors of unequal lengths, as in the orthorhombic syste ...

monoclinic
form I. The hydrogen bonding mechanisms are the same for all three phases; however, they differ strongly in their pi-pi interactions. ;Maleic acid In 2006 a new polymorph of
maleic acid Maleic acid or ''cis''-butenedioic acid is an organic compound that is a dicarboxylic acid, a molecule with two carboxyl groups. Its chemical formula is HO2CCH=CHCO2H. Maleic acid is the ''cis''-isomer of butenedioic acid, whereas fumaric acid is ...
was discovered, fully 124 years after the first crystal form was studied. Maleic acid is manufactured on an industrial scale in the chemical industry. It forms salt found in medicine. The new crystal type is produced when a
co-crystalCocrystals are "solids that are crystalline single phase materials composed of two or more different molecular or ionic compounds generally in a stoichiometric ratio which are neither solvates nor simple salts." A broader definition is that cocrystal ...
of
caffeine Caffeine is a central nervous system (CNS) stimulant of the methylxanthine class. It is the world's most widely consumed psychoactive drug. Unlike many other psychoactive substances, it is legal and unregulated in nearly all parts of the world ...

caffeine
and maleic acid (2:1) is dissolved in
chloroform Chloroform, or trichloromethane, is an organic compound with formula CHCl3. It is a colorless, strong-smelling, dense liquid that is produced on a large scale as a precursor to PTFE. It is also a precursor to various refrigerants. It is one of t ...
and when the solvent is allowed to evaporate slowly. Whereas form I has
monoclinic In crystallography, the monoclinic crystal system is one of the seven crystal systems. A crystal system is described by three vectors. In the monoclinic system, the crystal is described by vectors of unequal lengths, as in the orthorhombic syste ...

monoclinic
space group In mathematics, physics and chemistry, a space group is the symmetry group of a configuration in space, usually in three dimensions. In three dimensions, there are 219 distinct types, or 230 if chiral copies are considered distinct. Space groups a ...
''P''21/''c'', the new form has space group ''Pc''. Both polymorphs consist of sheets of molecules connected through
hydrogen bond#REDIRECT Hydrogen bond#REDIRECT Hydrogen bond {{R from other capitalisation ...
{{R from other capitalisation ...
ing of the
carboxylic acid A carboxylic acid is an organic acid that contains a carboxyl group (C(=O)OH) attached to an R-group. The general formula of a carboxylic acid is R−COOH or R−CO2H, with R referring to the alkyl, alkenyl, aryl, or other group. Carboxylic a ...
groups; but, in form I, the sheets alternate with respect of the net dipole moment, whereas, in form II, the sheets are oriented in the same direction. ;1,3,5-Trinitrobenzene After 125 years of study,
1,3,5-trinitrobenzene#REDIRECT 1,3,5-Trinitrobenzene {{R from other capitalisation ...
yielded a second polymorph. The usual form has the space group ''Pbca'', but in 2004, a second polymorph was obtained in the space group ''Pca''21 when the compound was crystallised in the presence of an additive, trisindane. This experiment shows that additives can induce the appearance of polymorphic forms. ;Other organic compounds
Acridine Acridine is an organic compound and a nitrogen heterocycle with the formula C13H9N. Acridines are substituted derivatives of the parent ring. It is a planar molecule that is structurally related to anthracene with one of the central CH groups repl ...

Acridine
has been obtained as seven polymorphs.
Glycine Glycine (symbol Gly or G; ) is an amino acid that has a single hydrogen atom as its side chain. It is the simplest amino acid (since carbamic acid is unstable), with the chemical formula NH2‐CH2‐COOH. Glycine is one of the proteinogenic amino ...
crystallizes as both
monoclinic In crystallography, the monoclinic crystal system is one of the seven crystal systems. A crystal system is described by three vectors. In the monoclinic system, the crystal is described by vectors of unequal lengths, as in the orthorhombic syste ...
and hexagonal crystals. Polymorphism in organic compounds is often the result of conformational polymorphism.


Inorganic compounds

;Binary metal oxides Polymorphism in binary metal oxides has attracted much attention because these materials are of significant economic value. One set of famous examples have the composition SiO2, which form many polymorphs. Important ones include: α-quartz, β-quartz,
tridymite Tridymite is a high-temperature polymorph of silica and usually occurs as minute tabular white or colorless pseudo-hexagonal crystals, or scales, in cavities in felsic volcanic rocks. Its chemical formula is SiO2. Tridymite was first described in ...
,
cristobalite Cristobalite is a mineral polymorph of silica that is formed at very high-temperatures. It is used in dentistry as a component of alginate impression materials as well as for making models of teeth It has the same chemical formula as quartz, SiO ...
,
moganite Moganite is an oxide mineral with the chemical formula SiO2 (silicon dioxide) that was discovered in 1984. It crystallises in the monoclinic crystal system. Moganite is considered a polymorph of quartz: it has the same chemical composition as quar ...

moganite
,
coesite Coesite is a form (polymorph) of silicon dioxide SiO2 that is formed when very high pressure (2–3 gigapascals), and moderately high temperature (), are applied to quartz. Coesite was first synthesized by Loring Coes Jr., a chemist at the Norton ...

coesite
, and
stishovite Stishovite is an extremely hard, dense tetragonal form (polymorph) of silicon dioxide. It is very rare on the Earth's surface, however, it may be a predominant form of silicon dioxide in the Earth, especially in the lower mantle. Stishovite was n ...

stishovite
. "Polymorphism in nanocrystalline binary metal oxides", S. Sood, P.Gouma, Nanomaterials and Energy, 2(NME2), 1-15(2013). ;Other inorganic materials Classical examples of polymorphism are the pair of minerals
calcite 235px|Crystal structure of calcite Calcite is a carbonate mineral and the most stable polymorph of calcium carbonate (CaCO3). The Mohs scale of mineral hardness, based on scratch hardness comparison, defines value 3 as "calcite". Other polymorphs ...

calcite
and
aragonite Aragonite is a carbonate mineral, one of the three most common naturally occurring crystal forms of calcium carbonate, (the other forms being the minerals calcite and vaterite). It is formed by biological and physical processes, including precipi ...

aragonite
, both forms of
calcium carbonate Calcium carbonate is a chemical compound with the formula CaCO3. It is a common substance found in rocks as the minerals calcite and aragonite (most notably as limestone, which is a type of sedimentary rock consisting mainly of calcite) and is t ...

calcium carbonate
. While diamonds are traditionally cubic, hexagonal diamonds occur also.


Factors affecting polymorphism

According to
Ostwald's ruleIn materials science, Ostwald's rule or Ostwald's step rule, conceived by Wilhelm Ostwald, describes the formation of polymorphs. The rule states that usually less stable polymorph crystallizes first. Unstable polymorphs more closely resemble the sta ...
, usually less stable polymorphs crystallize before the stable form. The concept hinges on the idea that unstable polymorphs more closely resemble the state in solution, and thus are kinetically advantaged. The founding case of fibrous vs rhombic benzamide illustrates the case. Another example is provided by two polymorphs of
titanium dioxide Titanium dioxide, also known as titanium(IV) oxide or titania , is the naturally occurring oxide of titanium, chemical formula . When used as a pigment, it is called titanium white, Pigment White 6 (PW6), or CI 77891. Generally, it is sourced fro ...
.Anatase to Rutile Transformation
ART) summarized in the Journal of Materials Science 2011
Polymorphs have disparate stabilities. Some convert rapidly at room (or any) temperature. Most polymorphs of organic molecules only differ by a few kJ/mol in lattice energy. Approximately 50% of known polymorph pairs differ by less than 2 kJ/mol and stability differences of more than 10 kJ/mol are rare. Polymorphism is affected by the details of
crystallisation Crystallization or crystallisation is the process by which a solid forms, where the atoms or molecules are highly organized into a structure known as a crystal. Some of the ways by which crystals form are precipitating from a solution, freezing, ...
. The solvent in all respects affects the nature of the polymorph, including concentration, other components of the solvent, i.e., species that inhibiting or promote certain growth patterns. A decisive factor is often the temperature of the solvent from which
crystallisation Crystallization or crystallisation is the process by which a solid forms, where the atoms or molecules are highly organized into a structure known as a crystal. Some of the ways by which crystals form are precipitating from a solution, freezing, ...
is carried out. Metastable polymorphs are not always reproducibly obtained, leading to cases of "disappearing polymorphs".Crystal Engineering: The Design and Application of Functional Solids, Volume 539, Kenneth Richard Seddon, Michael Zaworotk 1999


In pharmaceuticals

Drugs Uncoated tablets, consisting of about 90% acetylsalicylic acid, along with a minor amount of inert fillers and binders. Aspirin is a _receive_[[regulation.html" "title="pharmaceutical drug">Tablet (pharmacy)">tablets, consisting of about 90% acetylsalicylic acid, along with a mi ...
receive [[regulation">regulatory approval Regulation is the management of complex systems according to a set of rules and trends. In systems theory, these types of rules exist in various fields of biology and society, but the term has slightly different meanings according to context. For ...
for only a single polymorph. In a classic [[patent dispute, the [[GlaxoSmithKline defended its patent for the polymorph type II of the active ingredient in [[Zantac against competitors while that of the polymorph type I had already expired. Polymorphism in drugs can also have direct medical implications since
dissolution Dissolution may refer to: Arts and entertainment Books * ''Dissolution'' (''Forgotten Realms'' novel), a 2002 fantasy novel by Richard Lee Byers * ''Dissolution'' (Sansom novel), a 2003 historical novel by C. J. Sansom Music * Dissolution, in music ...
rates depend on the polymorph. Polymorphic purity of drug samples can be checked using techniques such as powder X-ray diffraction, IR/Raman spectroscopy, and utilizing the differences in their optical properties in some cases.


Case studies

;Ritonavir The antiviral drug
ritonavir Ritonavir (RTV), sold under the brand name Norvir, is an antiretroviral medication used along with other medications to treat HIV/AIDS. This combination treatment is known as highly active antiretroviral therapy (HAART). Often a low dose is used w ...
exists as two polymorphs, which differ greatly in efficacy. Such issues were solved by reformulating the medicine into gelcaps and tablets, rather than the original capsules. ;Acetylsalicylic acid A second polymorph of
acetylsalicylic acid Aspirin, also known as acetylsalicylic acid (ASA), is a medication used to reduce pain, fever, or inflammation. Specific inflammatory conditions which aspirin is used to treat include Kawasaki disease, pericarditis, and rheumatic fever. Aspirin ...
was reported only in 2005. A new crystal type was found after attempted co-crystallization of aspirin and
levetiracetam Levetiracetam, sold under the brand name Keppra among others, is a medication used to treat epilepsy. It is used for partial-onset, myoclonic, or tonic–clonic seizures and is taken either by mouth as an immediate or extended release formulatio ...

levetiracetam
from hot
acetonitrile Acetonitrile, often abbreviated MeCN (methyl cyanide), is the chemical compound with the formula . This colourless liquid is the simplest organic nitrile (hydrogen cyanide is a simpler nitrile, but the cyanide anion is not classed as organic). It i ...
. In form I, pairs of aspirin molecules form centrosymmetric dimers through the
acetyl In organic chemistry, acetyl is a moiety, the acyl with chemical formula CH3CO. It is sometimes represented by the symbol Ac (not to be confused with the element actinium). The acetyl group contains a methyl group single-bonded to a carbonyl. ...
groups with the (acidic)
methyl A methyl group is an alkyl derived from methane, containing one carbon atom bonded to three hydrogen atoms — CH3. In formulas, the group is often abbreviated Me. Such hydrocarbon groups occur in many organic compounds. It is a ver ...
proton to
carbonyl In organic chemistry, a carbonyl group is a functional group composed of a carbon atom double-bonded to an oxygen atom: C=O. It is common to several classes of organic compounds, as part of many larger functional groups. A compound containing a ca ...
hydrogen bond#REDIRECT Hydrogen bond#REDIRECT Hydrogen bond {{R from other capitalisation ...
{{R from other capitalisation ...
s. In form II, each aspirin molecule forms the same hydrogen bonds, but with two neighbouring molecules instead of one. With respect to the hydrogen bonds formed by the
carboxylic acid A carboxylic acid is an organic acid that contains a carboxyl group (C(=O)OH) attached to an R-group. The general formula of a carboxylic acid is R−COOH or R−CO2H, with R referring to the alkyl, alkenyl, aryl, or other group. Carboxylic a ...
groups, both polymorphs form identical dimer structures. The aspirin polymorphs contain identical 2-dimensional sections and are therefore more precisely described as polytypes. ;Paracetamol [[Paracetamol powder has poor compression properties, which poses difficulty in making tablets. A second polymorph was found with more suitable compressive propertiies. ;Cortisone acetate [[Cortisone acetate exists in at least five different polymorphs, four of which are unstable in water and change to a stable form. ;Carbamazepine [[Carbamazepine, [[estrogen, [[paroxetine, and [[chloramphenicol also show polymorphism.


Polytypism

Polytypes are a special case of polymorphs, where multiple [[close-packed crystal structures differ in one dimension only. Polytypes have identical close-packed planes, but differ in the [[Stacking fault|stacking sequence in the third dimension perpendicular to these planes. [[Polymorphs of silicon carbide|Silicon carbide (SiC) has more than 170 known polytypes, although most are rare. All the polytypes of SiC have virtually the same [[density and [[Gibbs free energy. The most common SiC polytypes are shown in Table 1. Table 1: Some polytypes of SiC. A second group of materials with different polytypes are the [[transition metal dichalcogenides, layered materials such as molybdenum disulfide (MoS2). For these materials the polytypes have more distinct effects on material properties, e.g. for MoS2, the 1T polytype is metallic in character, while the 2H form is more semiconducting. Another example is [[Tantalum disulfide, where the common 1T as well as 2H polytypes occur, but also more complex 'mixed coordination' types such as 4Hb and 6R, where the trigonal prismatic and the octahedral geometry layers are mixed. Here, the 1T polytype exhibits a [[charge density wave, with distinct influence on the conductivity as a function of temperature, while the 2H polytype exhibits [[superconductivity. ZnS and CdI2 are also polytypical. It has been suggested that this type of polymorphism is due to kinetics where [[screw dislocations rapidly reproduce partly disordered sequences in a periodic fashion.


Theory

In terms of [[thermodynamics, two types of polymorphic behaviour are recognized. For a monotropic system, plots of the free energies of the various polymorphs against temperature do not cross before all polymorphs melt—in other words, any transition from one polymorph to another below melting point will be irreversible. For an [[wikt:enantiotropism|enantiotropic system, a plot of the free energy against temperature shows a crossing point threshold before the various melting points. It may also be possible to revert interchangeably between the two polymorphs by heating or cooling, or through physical contact with a lower energy polymorph. ]


See also

* [[Isomorphism (crystallography) * [[wikt:dimorphism|Dimorphism (Wiktionary)


References


External links


"Small Molecule Crystallization"
([[PDF) at [[Illinois Institute of Technology website
"SiC and Polytpism"
{{Authority control [[Category:Polymorphism (materials science)| [[Category:Mineralogy [[Category:Gemology [[Category:Crystallography