Organic superconductor
Encyclopedia
In physical chemistry
and condensed matter physics
, an organic superconductor is an organic compound
which exhibits superconductivity
at low temperatures. As of 2007 the highest achieved critical temperature
for an organic superconductor at ambient pressure is 33 kelvin
, observed in the alkali-doped fullerene RbCs2C60.
There are a large number of materials that can be described as organic superconductors. These include the Bechgaard salt
s and Fabre salts which are both quasi one-dimensional, and quasi two-dimensional materials such as k
-BEDT-TTF2X charge transfer salts, λ
-BETS2X compounds, graphite intercalation compound
s and three dimensional
materials such as the alkali
-doped fullerene
s.
In 1979 Klaus Bechgaard
synthesized the first organic superconductor (TMTSF)2PF6 (the corresponding material class was named after him later) with a transition temperature of TC = 1.1 K (at an external pressure of 6.5 kbar).
Organic superconductors are of special interest not only for scientists, looking for room-temperature superconductivity
and for model systems explaining the origin of superconductivity but also for daily life issues as organic compounds are mainly built of carbon
and hydrogen
which belong to the most common elements on earth
in contrast to copper
or silicon
.
of tetramethyltetraselenafulvalene (TMTSF). This two organic molecules are similar expect for the sulfur
-atoms of TMTTF being replaced by selenium
-atoms in TMTSF. The molecules are stacked in columns (with a tendency to dimerization) which are separated by anions. Typical anions are for example octahedral PF6, AsF6 or tetrahedral ClO4 or ReO4.
Both material classes are quasi one-dimensional at room-temperature only conducting along the molecule stacks and share a very rich phase diagram
containing antiferromagnetic ordering
, charge order
, spin-density wave state, dimensional crossover and of course superconductivity
.
Only one Bechgaard-Salt was found to be superconducting at ambient pressure which is (TMTTF)2ClO4 with a transition temperature of TC = 1.4 K. Several other salts become superconducting only under external pressure. The external pressure one would have to apply to drive most Fabre-salts to superconductivity is so high, that under lab conditions superconductivity was observed only in one compound. A selection of the transition temperature and corresponding external pressure of several one-dimensional organic superconductors is shown in the table below.
The amount of possible anions separating two sheets of ET-molecules is nearly infinite. There are simple anions such as I3, polymeric ones such as the very famous Cu[N(CN)2]Br and anions containing solvents for example Ag(CF3)4·112DCBE. The electronic properties of the ET-based crystals are determined by its growing phase, its anion and by the external pressure applied. The external pressure needed to drive an ET-salt with insulating groundstate to a superconducting one is much smaller than those needed for Bechgaard-Salts
. For example κ-(ET)2Cu[N(CN)2]Cl needs only a pressure of about 300 bar to become superconducting, which can be achieved by placing a crystal in grease
which is freezing below 0 °C and then providing sufficient stress to induce the superconducting transition. The crystals are very sensitive (never user tweezers
on them) which can be observed impressively in α-(ET)2I3 lying several hours in the sun
(or, more controlled in an oven at 40 °C). After this treatment one gets αTempered-(ET)2I3 which is superconducting.
In contrast to the Fabre-/Bechgaard-Salts universal phase diagrams for all the ET-based salts have only been proposed yet. For sure such a phase diagram wouldn’t only depend on temperature and pressure (i.e. bandwidth) but also on electronic correlations
. In addition to the superconducting groundstate these materials show charge-order
, antiferromagnetism
or remain metallic down to lowest temperatures. One compound is even predicted to be a spin liquid
.
The highest transition temperatures at ambient pressure and with external pressure are both found in κ-phases with very similar anions. κ-(ET)2Cu[N(CN)2]Br becomes superconducting at TC = 11.8 K at ambient pressure, and a pressure of 300 bar drives deuterated κ-(ET)2Cu[N(CN)2]Cl from an antiferromagnetic to a superconducting groundstate with a transition temperature of TC = 13.1 K. The following table restricts to only a few exemplary superconductors of this class. For more superconductors see ref 1.
Even more superconductors can be found by changing the ET-molecules slightly either by replacing the sulfur atoms by selenium (BEDT-TSF, BETS) or by oxygen (BEDO-TTF, BEDO).
Some two-dimensional organic superconductors of the κ-(ET)2X and λ(BETS)2X families are candidates for the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) phase
when superconductivity is suppressed by an external magnetic field.
molecules. In addition these molecules are no longer flat but bulky which gives rise to a three dimensional, isotropic superconductor. The pure C60 grows in a fcc-lattice
and is an insulator. By placing alkali
atoms in the interstitials the crystal becomes metallic and eventually superconducting at low temperatures.
Unfortunately C60 crystals are not stable at ambient atmosphere. They are grown and investigated in closed capsules, limiting the measurement techniques possible. The highest transition temperature measured so far was TC = 33 K for Cs2RbC60.The highest measured transition temperature of an organic superconductor was found in 1995 in Cs3C60 pressurized with 15 kbar to be TC = 40 K. Under pressure this compound shows a unique behaviour. Usually the highest TC is achieved with the lowest pressure necessary to drive the transition. Further increase of the pressure reduces the transition temperature usually. Different in Cs3C60: Superconductivity sets in at very low pressures of several 100 bar and the transition temperature keeps increasing with increasing pressure. This indicates a completely different mechanism then just broadening of the bandwidth.
). Using TTP (tetrathiapentalene) as basic molecules one receives a variety of new organic molecules serving as cations in organic crystals. And some of them are superconducting. This class of superconductors was only reported recently and investigations are still under process.
and phenanthrene
. Doping the crystal Picene and Phenanthrene with some alkali metals such as potassium
or rubidium
and annealing for several days leads to superconductivity with transition temperatures up to 18 K. For the AxPhenanthrene, the superconductivity is possible unconventional. Bpth phenanthrene and picene are called phenanthrene-edge-type polycyclic aromatic hydrocarbon
. The increasing number of benzene rings results in higher Tc.
Graphite intercalation
Putting foreign molecules or atoms between hexagon graphite
sheets leads to ordered structures and to superconductivity even if neither the foreign molecule or atom nor the graphite layers are metallic. Several stoichiometries
have been synthesized using mainly alkali atoms as anions.
Physical chemistry
Physical chemistry is the study of macroscopic, atomic, subatomic, and particulate phenomena in chemical systems in terms of physical laws and concepts...
and condensed matter physics
Condensed matter physics
Condensed matter physics deals with the physical properties of condensed phases of matter. These properties appear when a number of atoms at the supramolecular and macromolecular scale interact strongly and adhere to each other or are otherwise highly concentrated in a system. The most familiar...
, an organic superconductor is an organic compound
Organic compound
An organic compound is any member of a large class of gaseous, liquid, or solid chemical compounds whose molecules contain carbon. For historical reasons discussed below, a few types of carbon-containing compounds such as carbides, carbonates, simple oxides of carbon, and cyanides, as well as the...
which exhibits superconductivity
Superconductivity
Superconductivity is a phenomenon of exactly zero electrical resistance occurring in certain materials below a characteristic temperature. It was discovered by Heike Kamerlingh Onnes on April 8, 1911 in Leiden. Like ferromagnetism and atomic spectral lines, superconductivity is a quantum...
at low temperatures. As of 2007 the highest achieved critical temperature
Temperature
Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...
for an organic superconductor at ambient pressure is 33 kelvin
Kelvin
The kelvin is a unit of measurement for temperature. It is one of the seven base units in the International System of Units and is assigned the unit symbol K. The Kelvin scale is an absolute, thermodynamic temperature scale using as its null point absolute zero, the temperature at which all...
, observed in the alkali-doped fullerene RbCs2C60.
There are a large number of materials that can be described as organic superconductors. These include the Bechgaard salt
Bechgaard salt
A Bechgaard salt is any one of a number of organic charge-transfer complexes that exhibit superconductivity at low temperatures . They are named for chemist Klaus Bechgaard, who was one of the first scientists to synthesize them and demonstrate their superconductivity with the help of physicist...
s and Fabre salts which are both quasi one-dimensional, and quasi two-dimensional materials such as k
Kappa
Kappa is the 10th letter of the Greek alphabet, used to represent the voiceless velar stop, or "k", sound in Ancient and Modern Greek. In the system of Greek numerals it has a value of 20. It was derived from the Phoenician letter Kaph...
-BEDT-TTF2X charge transfer salts, λ
Lambda
Lambda is the 11th letter of the Greek alphabet. In the system of Greek numerals lambda has a value of 30. Lambda is related to the Phoenician letter Lamed . Letters in other alphabets that stemmed from lambda include the Roman L and the Cyrillic letter El...
-BETS2X compounds, graphite intercalation compound
Graphite intercalation compound
Graphite intercalation compounds are complex materials having formula XCy where element or molecule X is inserted between the graphite layers. In this type of compound, the graphite layers remain largely intact and the guest molecules or atoms are located in between...
s and three dimensional
Three-dimensional space
Three-dimensional space is a geometric 3-parameters model of the physical universe in which we live. These three dimensions are commonly called length, width, and depth , although any three directions can be chosen, provided that they do not lie in the same plane.In physics and mathematics, a...
materials such as the alkali
Alkali
In chemistry, an alkali is a basic, ionic salt of an alkali metal or alkaline earth metal element. Some authors also define an alkali as a base that dissolves in water. A solution of a soluble base has a pH greater than 7. The adjective alkaline is commonly used in English as a synonym for base,...
-doped fullerene
Fullerene
A fullerene is any molecule composed entirely of carbon, in the form of a hollow sphere, ellipsoid, or tube. Spherical fullerenes are also called buckyballs, and they resemble the balls used in association football. Cylindrical ones are called carbon nanotubes or buckytubes...
s.
In 1979 Klaus Bechgaard
Klaus Bechgaard
Klaus Bechgaard is a Danish scientist and chemist, noted for being one of the first scientists in the world to synthesize a number of organic charge transfer complexes and demonstrate their superconductivity, threreof the name Bechgaard salt...
synthesized the first organic superconductor (TMTSF)2PF6 (the corresponding material class was named after him later) with a transition temperature of TC = 1.1 K (at an external pressure of 6.5 kbar).
Organic superconductors are of special interest not only for scientists, looking for room-temperature superconductivity
Superconductivity
Superconductivity is a phenomenon of exactly zero electrical resistance occurring in certain materials below a characteristic temperature. It was discovered by Heike Kamerlingh Onnes on April 8, 1911 in Leiden. Like ferromagnetism and atomic spectral lines, superconductivity is a quantum...
and for model systems explaining the origin of superconductivity but also for daily life issues as organic compounds are mainly built of carbon
Carbon
Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...
and hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
which belong to the most common elements on earth
Abundance of the chemical elements
The abundance of a chemical element measures how relatively common the element is, or how much of the element is present in a given environment by comparison to all other elements...
in contrast to copper
Copper
Copper is a chemical element with the symbol Cu and atomic number 29. It is a ductile metal with very high thermal and electrical conductivity. Pure copper is soft and malleable; an exposed surface has a reddish-orange tarnish...
or silicon
Silicon
Silicon is a chemical element with the symbol Si and atomic number 14. A tetravalent metalloid, it is less reactive than its chemical analog carbon, the nonmetal directly above it in the periodic table, but more reactive than germanium, the metalloid directly below it in the table...
.
One dimensional Fabre- & Bechgaard-Salts
Fabre-Salts are composed of tetramethyltetrathiafulvalene (TMTTF) and Bechgaard-saltsBechgaard salt
A Bechgaard salt is any one of a number of organic charge-transfer complexes that exhibit superconductivity at low temperatures . They are named for chemist Klaus Bechgaard, who was one of the first scientists to synthesize them and demonstrate their superconductivity with the help of physicist...
of tetramethyltetraselenafulvalene (TMTSF). This two organic molecules are similar expect for the sulfur
Sulfur
Sulfur or sulphur is the chemical element with atomic number 16. In the periodic table it is represented by the symbol S. It is an abundant, multivalent non-metal. Under normal conditions, sulfur atoms form cyclic octatomic molecules with chemical formula S8. Elemental sulfur is a bright yellow...
-atoms of TMTTF being replaced by selenium
Selenium
Selenium is a chemical element with atomic number 34, chemical symbol Se, and an atomic mass of 78.96. It is a nonmetal, whose properties are intermediate between those of adjacent chalcogen elements sulfur and tellurium...
-atoms in TMTSF. The molecules are stacked in columns (with a tendency to dimerization) which are separated by anions. Typical anions are for example octahedral PF6, AsF6 or tetrahedral ClO4 or ReO4.
Both material classes are quasi one-dimensional at room-temperature only conducting along the molecule stacks and share a very rich phase diagram
Phase diagram
A phase diagram in physical chemistry, engineering, mineralogy, and materials science is a type of chart used to show conditions at which thermodynamically distinct phases can occur at equilibrium...
containing antiferromagnetic ordering
Antiferromagnetism
In materials that exhibit antiferromagnetism, the magnetic moments of atoms or molecules, usuallyrelated to the spins of electrons, align in a regular pattern with neighboring spins pointing in opposite directions. This is, like ferromagnetism and ferrimagnetism, a manifestation of ordered magnetism...
, charge order
Charge ordering
Charge ordering is a phase transition occurring mostly in strongly correlated materials such as transition metal oxides or organic conductors. Due to the strong interaction, the charge is localized on different sites leading to a disproportionation and an ordered superlattice...
, spin-density wave state, dimensional crossover and of course superconductivity
Superconductivity
Superconductivity is a phenomenon of exactly zero electrical resistance occurring in certain materials below a characteristic temperature. It was discovered by Heike Kamerlingh Onnes on April 8, 1911 in Leiden. Like ferromagnetism and atomic spectral lines, superconductivity is a quantum...
.
Only one Bechgaard-Salt was found to be superconducting at ambient pressure which is (TMTTF)2ClO4 with a transition temperature of TC = 1.4 K. Several other salts become superconducting only under external pressure. The external pressure one would have to apply to drive most Fabre-salts to superconductivity is so high, that under lab conditions superconductivity was observed only in one compound. A selection of the transition temperature and corresponding external pressure of several one-dimensional organic superconductors is shown in the table below.
| Material | TC (K) | pext (kbar) |
|---|---|---|
| (TMTSF)2SbF6 | 0.36 | 10.5 |
| (TMTSF)2PF4 | 1.1 | 6.5 |
| (TMTSF)2AsF6 | 1.1 | 9.5 |
| (TMTSF)2ReO4 | 1.2 | 9.5 |
| (TMTSF)2TaF6 | 1.35 | 11 |
| (TMTTF)2Br | 0.8 | 26 |
Two-dimensional (BEDT-TTF)2X
BEDT-TTF is the short form of bisethylenedithio-tetrathiafulvalene commonly abbreviated with ET. These molecules form planes which are separated by anions. The pattern of the molecules in the planes is not unique but there are several different phases growing, depending on the anion and the growth conditions. Important phases concerning superconductivity are the α- and θ- phase with the molecules ordering in a fishbone structure and the β- and especially κ-phase which order in a checkerboard structure with molecules being dimerized in the κ-phase. This dimerization makes the κ-phases special as they are not quarter- but half-filled systems, driving them into superconductivity at higher temperatures compared to the other phases.The amount of possible anions separating two sheets of ET-molecules is nearly infinite. There are simple anions such as I3, polymeric ones such as the very famous Cu[N(CN)2]Br and anions containing solvents for example Ag(CF3)4·112DCBE. The electronic properties of the ET-based crystals are determined by its growing phase, its anion and by the external pressure applied. The external pressure needed to drive an ET-salt with insulating groundstate to a superconducting one is much smaller than those needed for Bechgaard-Salts
Bechgaard salt
A Bechgaard salt is any one of a number of organic charge-transfer complexes that exhibit superconductivity at low temperatures . They are named for chemist Klaus Bechgaard, who was one of the first scientists to synthesize them and demonstrate their superconductivity with the help of physicist...
. For example κ-(ET)2Cu[N(CN)2]Cl needs only a pressure of about 300 bar to become superconducting, which can be achieved by placing a crystal in grease
Grease (lubricant)
The term grease is used to describe semisolid lubricants. Although the word grease is also used to describe rendered fat of animals, in the context of lubrication, grease typically applies to a material consisting of a soap emulsified with mineral or vegetable oil...
which is freezing below 0 °C and then providing sufficient stress to induce the superconducting transition. The crystals are very sensitive (never user tweezers
Tweezers
Tweezers are tools used for picking up and manipulating objects too small to be easily handled with the human hands. They are probably derived from tongs, pincers, or scissors-like pliers used to grab or hold hot objects since the dawn of recorded history...
on them) which can be observed impressively in α-(ET)2I3 lying several hours in the sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...
(or, more controlled in an oven at 40 °C). After this treatment one gets αTempered-(ET)2I3 which is superconducting.
In contrast to the Fabre-/Bechgaard-Salts universal phase diagrams for all the ET-based salts have only been proposed yet. For sure such a phase diagram wouldn’t only depend on temperature and pressure (i.e. bandwidth) but also on electronic correlations
Electronic correlation
Electronic correlation is the interaction between electrons in the electronic structure of a quantum system.- Atomic and molecular systems :...
. In addition to the superconducting groundstate these materials show charge-order
Charge ordering
Charge ordering is a phase transition occurring mostly in strongly correlated materials such as transition metal oxides or organic conductors. Due to the strong interaction, the charge is localized on different sites leading to a disproportionation and an ordered superlattice...
, antiferromagnetism
Antiferromagnetism
In materials that exhibit antiferromagnetism, the magnetic moments of atoms or molecules, usuallyrelated to the spins of electrons, align in a regular pattern with neighboring spins pointing in opposite directions. This is, like ferromagnetism and ferrimagnetism, a manifestation of ordered magnetism...
or remain metallic down to lowest temperatures. One compound is even predicted to be a spin liquid
Spin liquid
In solid-state physics, spin liquid denotes a state of matter, where local permanent magnetic moments are present in the material, but do not show any sign of ordering down to the lowest temperatures despite comparable strong antiferromagnetic interactions.Even though many theories study spin...
.
The highest transition temperatures at ambient pressure and with external pressure are both found in κ-phases with very similar anions. κ-(ET)2Cu[N(CN)2]Br becomes superconducting at TC = 11.8 K at ambient pressure, and a pressure of 300 bar drives deuterated κ-(ET)2Cu[N(CN)2]Cl from an antiferromagnetic to a superconducting groundstate with a transition temperature of TC = 13.1 K. The following table restricts to only a few exemplary superconductors of this class. For more superconductors see ref 1.
| Material | TC (K) | pext (kbar) |
|---|---|---|
| βH-(ET)2I3 | 1.5 | 0 |
| θ-(ET)2I3 | 3.6 | 0 |
| k-(ET)2I3 | 3.6 | 0 |
| α-(ET)2KHg(SCN)4 | 0.3 | 0 |
| α-(ET)2KHg(SCN)4 | 1.2 | 1.2 |
| β’’-(ET)2SF5CH2CF2SO3 | 5.3 | 0 |
| κ-(ET)2Cu[N(CN)2]Cl | 12.8 | 0.3 |
| κ-(ET)2Cu[N(CN)2]Cl deuterated | 13.1 | 0.3 |
| κ-(ET)2Cu[N(CN)2]Br deuterated | 11.2 | 0 |
| κ-(ET)2Cu(NCS)2 | 10.4 | 0 |
| κ-(ET)4Hg2.89Cl8 | 1.8 | 12 |
| κH-(ET)2Cu(CF3)4·TCE | 9.2 | 0 |
| κH-(ET)2Ag(CF3)4·TCE | 11.1 | 0 |
Even more superconductors can be found by changing the ET-molecules slightly either by replacing the sulfur atoms by selenium (BEDT-TSF, BETS) or by oxygen (BEDO-TTF, BEDO).
Some two-dimensional organic superconductors of the κ-(ET)2X and λ(BETS)2X families are candidates for the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) phase
Fulde-Ferrell-Larkin-Ovchinnikov phase
The Fulde-Ferrell-Larkin-Ovchinnikov phase can arise in a superconductor in large magnetic field. Among its characteristics are Cooper pairs with nonzero total momentum and a spatially non-uniform order parameter.- History :...
when superconductivity is suppressed by an external magnetic field.
Doped Buckminster fullerenes
Superconducting fullerenes (based on the Buckminster fullerene C60) are fairly different from other organic superconductors. The building molecules are no longer manipulated hydrocarbons but pure carbonCarbon
Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...
molecules. In addition these molecules are no longer flat but bulky which gives rise to a three dimensional, isotropic superconductor. The pure C60 grows in a fcc-lattice
Cubic crystal system
In crystallography, the cubic crystal system is a crystal system where the unit cell is in the shape of a cube. This is one of the most common and simplest shapes found in crystals and minerals....
and is an insulator. By placing alkali
Alkali
In chemistry, an alkali is a basic, ionic salt of an alkali metal or alkaline earth metal element. Some authors also define an alkali as a base that dissolves in water. A solution of a soluble base has a pH greater than 7. The adjective alkaline is commonly used in English as a synonym for base,...
atoms in the interstitials the crystal becomes metallic and eventually superconducting at low temperatures.
Unfortunately C60 crystals are not stable at ambient atmosphere. They are grown and investigated in closed capsules, limiting the measurement techniques possible. The highest transition temperature measured so far was TC = 33 K for Cs2RbC60.The highest measured transition temperature of an organic superconductor was found in 1995 in Cs3C60 pressurized with 15 kbar to be TC = 40 K. Under pressure this compound shows a unique behaviour. Usually the highest TC is achieved with the lowest pressure necessary to drive the transition. Further increase of the pressure reduces the transition temperature usually. Different in Cs3C60: Superconductivity sets in at very low pressures of several 100 bar and the transition temperature keeps increasing with increasing pressure. This indicates a completely different mechanism then just broadening of the bandwidth.
| Material | TC (K) | pext (mbar) |
|---|---|---|
| K3C60 | 18 | 0 |
| Rb3C60 | 30.7 | 0 |
| K2CsC60 | 24 | 0 |
| K2RbC60 | 21.5 | 0 |
| K5C60 | 8.4 | 0 |
| Sr6C60 | 6.8 | 0 |
| (NH3)4Na2CsC60 | 29.6 | 0 |
| (NH3)K3C60 | 28 | 14.8 |
More organic superconductors
Next to the three major classes of organic superconductors (SCs) there are more organic systems becoming superconducting at low temperatures or under pressure. A few examples shall be presented here.TTP-based SCs
TMTTF as well as BEDT-TTF are based on the molecule TTF (tetrathiafulvaleneTetrathiafulvalene
Tetrathiafulvalene is a organosulfur compound with the formula 2. Studies on this heterocyclic compound contributed to the development of molecular electronics. TTF is related to the hydrocarbon fulvalene, 2, by replacement of four CH groups with sulfur atoms...
). Using TTP (tetrathiapentalene) as basic molecules one receives a variety of new organic molecules serving as cations in organic crystals. And some of them are superconducting. This class of superconductors was only reported recently and investigations are still under process.
Phenanthrene-type SCs
Instead of using sulfated molecules or the fairly big Buckminster fullerenes recently it became possible to synthesize crystals from the hydrocarbon picenePicene
Picene is a hydrocarbon found in the pitchy residue obtained in the distillation of peat tar and of petroleum. This is distilled to dryness and the distillate repeatedly recrystallized from cymene. It may be synthetically prepared by the action of anhydrous aluminium chloride on a mixture of...
and phenanthrene
Phenanthrene
Phenanthrene is a polycyclic aromatic hydrocarbon composed of three fused benzene rings. The name phenanthrene is a composite of phenyl and anthracene. In its pure form, it is found in cigarette smoke and is a known irritant, photosensitizing skin to light...
. Doping the crystal Picene and Phenanthrene with some alkali metals such as potassium
Potassium
Potassium is the chemical element with the symbol K and atomic number 19. Elemental potassium is a soft silvery-white alkali metal that oxidizes rapidly in air and is very reactive with water, generating sufficient heat to ignite the hydrogen emitted in the reaction.Potassium and sodium are...
or rubidium
Rubidium
Rubidium is a chemical element with the symbol Rb and atomic number 37. Rubidium is a soft, silvery-white metallic element of the alkali metal group. Its atomic mass is 85.4678. Elemental rubidium is highly reactive, with properties similar to those of other elements in group 1, such as very rapid...
and annealing for several days leads to superconductivity with transition temperatures up to 18 K. For the AxPhenanthrene, the superconductivity is possible unconventional. Bpth phenanthrene and picene are called phenanthrene-edge-type polycyclic aromatic hydrocarbon
Polycyclic aromatic hydrocarbon
Polycyclic aromatic hydrocarbons , also known as poly-aromatic hydrocarbons or polynuclear aromatic hydrocarbons, are potent atmospheric pollutants that consist of fused aromatic rings and do not contain heteroatoms or carry substituents. Naphthalene is the simplest example of a PAH...
. The increasing number of benzene rings results in higher Tc.
Graphite intercalationGraphite intercalation compoundGraphite intercalation compounds are complex materials having formula XCy where element or molecule X is inserted between the graphite layers. In this type of compound, the graphite layers remain largely intact and the guest molecules or atoms are located in between...
SCs
Putting foreign molecules or atoms between hexagon graphiteGraphite
The mineral graphite is one of the allotropes of carbon. It was named by Abraham Gottlob Werner in 1789 from the Ancient Greek γράφω , "to draw/write", for its use in pencils, where it is commonly called lead . Unlike diamond , graphite is an electrical conductor, a semimetal...
sheets leads to ordered structures and to superconductivity even if neither the foreign molecule or atom nor the graphite layers are metallic. Several stoichiometries
Stoichiometry
Stoichiometry is a branch of chemistry that deals with the relative quantities of reactants and products in chemical reactions. In a balanced chemical reaction, the relations among quantities of reactants and products typically form a ratio of whole numbers...
have been synthesized using mainly alkali atoms as anions.
Several TCs for unusual SCs
| Material | TC (K) |
|---|---|
| (BDA-TTP)2AsF6 | 5.8 |
| (DTEDT)3Au(CN)2 | 4 |
| K3.3Picene | 18 |
| Rb3.1Picene | 6.9 |
| K3Phenanthrene | 4.95 |
| Rb3Phenanthrene | 4.75 |
| CaC5 | 11.5 |
| NaC2 | 5 |
| KC8 | 0.14 |