Fulleride

{{short description|Chemical compound}} 300px|thumb|Cs₃C₆₀ crystal structure '''Fullerides''' are chemical compounds containing fullerene anions. Common fullerides are derivatives of the most common fullerenes, i.e. C₆₀ and C₇₀. The scope of the area is large because multiple charges are possible, i.e., [C₆₀]^''n''− (''n'' = 1, 2...6), and all fullerenes can be converted to fullerides. The suffix "-ide" implies their negatively charged nature.

Fullerides can be isolated as derivatives with a wide range of cations. Most heavily studied derivatives are those with alkali metals, but fullerides have been prepared with organic cations. Fullerides are typically dark colored solids that generally dissolve in polar organic solvents.

==Structure and bonding== According to electronic structure calculations, the LUMO of C₆₀ is a triply degenerate orbital of t_1u symmetry. Using the technique cyclic voltammetry, C₆₀ can be shown to undergo six reversible reductions starting at −1&nbsp;V referenced to the Fc⁺/Fc couple. Reduction causes only subtle changes in the structure and many derivatives exhibit disorder, which obscures these effects. Many fullerides are subject to Jahn–Teller distortion. In certain cases, e.g. <nowiki>[</nowiki>PPN<nowiki>]</nowiki>₂C₆₀, the structures are highly ordered and slight (10&nbsp;pm) elongation of some C−C bonds is observed.<ref name=CAR/>

==Preparation== Fullerides have been prepared in various ways: *treating with alkali metals to give the alkali metal fullerides: :C₆₀ + 2&nbsp;K → K₂C₆₀ *treating with suitable organic and organometallic reducing agents, such as cobaltocene and tetrakisdimethylaminoethylene. *alkali metal fullerides can be subjected to cation metathesis. In this way the (bis(triphenylphosphine)iminium (PPN⁺) salts have been prepared, e.g. [PPN]₂C₆₀:<ref name=CAR>{{cite journal|title=Discrete Fulleride Anions and Fullerenium Cations|author1=Reed, Christopher A. |author2=Bolskar, Robert D. |journal=Chemical Reviews|year=2000|volume=100|issue=3|pages=1075–1120|doi=10.1021/cr980017o|url=https://escholarship.org/content/qt60b5m71z/qt60b5m71z.pdf?t=o9psl5}}</ref> :K₂C₆₀ + 2 [PPN]Cl → [PPN]₂C₆₀ + 2&nbsp;KCl

The fulleride salt ([K(crypt-222)]⁺)₂[C₆₀]²⁻ salt is synthesized by treating C₆₀ with metallic potassium in the presence of [[2.2.2-Cryptand|[2.2.2]cryptand]].

==Alkali metal derivatives== {| class="wikitable" ! colspan=2|Critical temperatures (''T''_c) of the fulleride salts |- | M₃C₆₀ ||''T''_c (K) |- | Na₃C₆₀ || (non-superconducting) |- | K₃C₆₀ || 18 |- | Rb₃C₆₀ || 28 |- | Cs₃C₆₀ || 40 |}

Particular attention has been paid to alkali metal (Na⁺, K⁺, Rb⁺, Cs⁺) derivatives of C₆₀³⁻ because these compounds exhibit physical properties resulting from intercluster interactions such as metallic behavior. In contrast, in C₆₀, the individual molecules interact only weakly, i.e. with essentially nonoverlapping bands. These alkali metal derivatives are sometimes viewed as arising by intercalation of the metal into C₆₀ lattice. Alternatively, these materials are viewed as n-doped fullerenes.<ref name="O. Gunnarsson 1997 575"> {{cite journal |last = Gunnarsson |first = O. |year = 1997 |title = Superconductivity in fullerides |journal = Reviews of Modern Physics |volume = 69|pages = 575–606 |doi = 10.1103/RevModPhys.69.575 | bibcode = 1997RvMP...69..575G | arxiv = cond-mat/9611150 |issue = 2 }}</ref>

Alkali metal salts of this trianion are superconducting. In M₃C₆₀ (M = Na, K, Rb), the M⁺ ions occupy the interstitial holes in a lattice composed of ccp lattice composed of nearly spherical C₆₀ anions. In Cs₃C₆₀, the cages are arranged in a bcc lattice.

In 1991, it was revealed that potassium-doped C₆₀ becomes superconducting at {{cvt|18|K|C|0}}.<ref> {{cite journal |year = 1991 |title = Superconductivity at 18 K in potassium-doped C₆₀ |journal = Nature |volume = 350 |pages = 600–601 |doi = 10.1038/350600a0 |issue=6319 |bibcode=1991Natur.350..600H |last1 = Hebard |first1 = A. F. |last2 = Rosseinsky |first2 = M. J. |last3 = Haddon |first3 = R. C. |last4 = Murphy |first4 = D. W. |last5 = Glarum |first5 = S. H. |last6 = Palstra |first6 = T. T. M. |last7 = Ramirez |first7 = A. P. |last8 = Kortan |first8 = A. R. |url = https://pure.rug.nl/ws/files/14557724/1991NatureHebard.pdf |hdl = 11370/3709b8a7-6fc1-4b32-8842-ce9b5355b5e4 |hdl-access = free }}</ref> This was the highest transition temperature for a molecular superconductor. Since then, superconductivity has been reported in fullerene doped with various other alkali metals.<ref> {{cite journal |year = 1991 |title = Superconductivity at 28&nbsp;K in Rb_xC₆₀ |journal = Physical Review Letters |volume = 66 |issue = 21 |pages = 2830–2832 |doi = 10.1103/PhysRevLett.66.2830 | bibcode = 1991PhRvL..66.2830R | last1 = Rosseinsky |first1 = M. |last2 = Ramirez |first2 = A. |last3 = Glarum |first3 = S. |last4 = Murphy |first4 = D. |last5 = Haddon |first5 = R. |last6 = Hebard |first6 = A. |last7 = Palstra |first7 = T. |last8 = Kortan |first8 = A. |last9 = Zahurak |first9 = S. |last10 = Makhija |first10 = A. |pmid=10043627 |url = https://pure.rug.nl/ws/files/14557802/1991PhysRevLettRosseinsky.pdf }}</ref><ref> {{cite journal |year = 1991 |title = (Rb_''x''K_1−''x'')₃C₆₀ Superconductors: Formation of a Continuous Series of Solid Solutions |journal = Science |volume = 253|issue = 5022|pages = 886–8 |doi = 10.1126/science.253.5022.886 |pmid = 17751824 | bibcode = 1991Sci...253..886C |last1 = Chen |first1 = C.-C. |last2 = Kelty |first2 = S. P. |last3 = Lieber |first3 = C. M. }}</ref> It has been shown that the superconducting transition temperature in alkaline-metal-doped fullerene increases with the unit-cell volume V.<ref> {{cite journal |year = 1992 |title = Compressibility of M₃C₆₀ Fullerene Superconductors: Relation Between Tc and Lattice Parameter |journal = Science |volume = 255 |issue = 5046 |pages = 833–5 |doi = 10.1126/science.255.5046.833 |pmid = 17756430 | bibcode = 1992Sci...255..833Z |last1 = Zhou |first1 = O. |last2 = Zhu |first2 = Q. |last3 = Fischer |first3 = J. E. |last4 = Coustel |first4 = N. |last5 = Vaughan |first5 = G. B. M. |last6 = Heiney |first6 = P. A. |last7 = McCauley |first7 = J. P. |last8 = Smith |first8 = A. B. }}</ref><ref> {{cite journal |year = 1999 |title = Pressure dependence of superconductivity in the Na₂Rb_0.5Cs_0.5C₆₀ fulleride |journal = Physical Review B |volume = 59|issue = 6 |pages = 4439–4444 |doi = 10.1103/PhysRevB.59.4439 | bibcode = 1999PhRvB..59.4439B |last1 = Brown |first1 = Craig |last2 = Takenobu |first2 = Taishi |last3 = Kordatos |first3 = Konstantinos |last4 = Prassides |first4 = Kosmas |last5 = Iwasa |first5 = Yoshihiro |last6 = Tanigaki |first6 = Katsumi }}</ref> As Cs⁺ is the largest alkali ion, caesium-doped fullerene is an important material in this family. Superconductivity at {{cvt|38|K|C|0}} has been reported in bulk Cs₃C₆₀,<ref name="A. Y. Ganin, et al. 2008 367"> {{cite journal |year = 2008 |title = Bulk superconductivity at 38&nbsp;K in a molecular system |journal = Nature Materials |volume = 7|issue = 5|doi = 10.1038/nmat2179 |pmid = 18425134 |bibcode=2008NatMa...7..367G |pages = 367–71 |last1 = Ganin |first1 = Alexey Y. |last2 = Takabayashi |first2 = Yasuhiro |last3 = Khimyak |first3 = Yaroslav Z. |last4 = Margadonna |first4 = Serena |last5 = Tamai |first5 = Anna |last6 = Rosseinsky |first6 = Matthew J. |last7 = Prassides |first7 = Kosmas }}</ref> but only under applied pressure. The highest superconducting transition temperature of {{cvt|33|K|C|0}} at ambient pressure is reported for Cs₂RbC₆₀.<ref> {{cite journal |year = 1991 |title = Superconductivity at 33&nbsp;K in Cs_xRb_yC₆₀ |journal = Nature |volume = 352|pages = 222–223 |doi = 10.1038/352222a0 |issue=6332 | bibcode = 1991Natur.352..222T |last1 = Tanigaki |first1 = K. |last2 = Ebbesen |first2 = T. W. |last3 = Saito |first3 = S. |last4 = Mizuki |first4 = J. |last5 = Tsai |first5 = J. S. |last6 = Kubo |first6 = Y. |last7 = Kuroshima |first7 = S. }}</ref>

The increase of transition temperature with the unit-cell volume had been believed to be evidence for the BCS mechanism of C₆₀ solid superconductivity, because inter C₆₀ separation can be related to an increase in the density of states on the Fermi level, ''N''(''ε''_F). Therefore, efforts have been made to increase the interfullerene separation, in particular, intercalating neutral molecules into the A₃C₆₀ lattice to increase the interfullerene spacing while the valence of C₆₀ is kept unchanged. However, this ammoniation technique has revealed a new aspect of fullerene intercalation compounds: the Mott transition and the correlation between the orientation/orbital order of C₆₀ molecules and the magnetic structure.<ref name="Y. Iwasa et al. 2003 R495"> {{cite journal |year = 2003 |title = Superconductivity, Mott Hubbard states, and molecular orbital order in intercalated fullerides |journal = Journal of Physics: Condensed Matter |volume = 15|pages = R495 |doi = 10.1088/0953-8984/15/13/202 | bibcode = 2003JPCM...15R.495I |issue = 13 |last1 = Iwasa |first1 = Y |last2 = Takenobu |first2 = T }}</ref>

Fourfold-reduced materials, i.e., those with the stoichiometry A₄C₆₀, are insulating, even though the t_1u band is only partially filled.<ref> {{cite journal |year = 1993 |title = Electronic structure of superconducting Ba₆C₆₀ |journal = Physical Review B |volume = 47|issue = 21 |pages = 14657–14660 |doi = 10.1103/PhysRevB.47.14657 | arxiv = cond-mat/9301006 |bibcode = 1993PhRvB..4714657E |last1 = Erwin |first1 = Steven |last2 = Pederson |first2 = Mark }}</ref> This apparent anomaly may be explained by the Jahn–Teller effect, where spontaneous deformations of high-symmetry molecules induce the splitting of degenerate levels to gain the electronic energy. The Jahn–Teller type electron-phonon interaction is strong enough in C₆₀ solids to destroy the band picture for particular valence states.<ref name="Y. Iwasa et al. 2003 R495"/>

A narrow band or strongly correlated electronic system and degenerated ground states are relevant to explaining superconductivity in fulleride solids. When the interelectron repulsion ''U'' is greater than the bandwidth, an insulating localized electron ground state is produced in the simple Mott–Hubbard model. This explains the absence of superconductivity at ambient pressure in caesium-doped C₆₀ solids.<ref name="A. Y. Ganin, et al. 2008 367"/> Electron-correlation-driven localization of the t_1u electrons exceeds the critical value, leading to the Mott insulator. The application of high pressure decreases the interfullerene spacing, therefore caesium-doped C₆₀ solids turn to metallic and superconducting.

A fully developed theory of C₆₀ solids superconductivity is lacking, but it has been widely accepted that strong electronic correlations and the Jahn–Teller electron–phonon coupling<ref> {{cite journal |year = 2003 |title = Strong Superconductivity with Local Jahn–Teller Phonons in C₆₀ Solids |journal = Physical Review Letters |volume = 90|issue = 16 |article-number = 167006 |doi = 10.1103/PhysRevLett.90.167006 | pmid = 12731998 | bibcode = 2003PhRvL..90p7006H | last1 = Han |first1 = J. |last2 = Gunnarsson |first2 = O. |last3 = Crespi |first3 = V. |s2cid = 8235933 |url = http://pdfs.semanticscholar.org/a82c/8357bbbf7ba4836c523b45c7a25007c44417.pdf |archive-url = https://web.archive.org/web/20191228154949/http://pdfs.semanticscholar.org/a82c/8357bbbf7ba4836c523b45c7a25007c44417.pdf |archive-date = 2019-12-28 }}</ref> produce local electron pairings that show a high transition temperature close to the insulator–metal transition.<ref> {{cite journal |year = 2002 |title = Strongly Correlated Superconductivity |journal = Science |volume = 296|issue = 5577 |pmid = 12089436 |doi = 10.1126/science.1071122|arxiv = cond-mat/0207058 |bibcode = 2002Sci...296.2364C |pages = 2364–6 |last1 = Capone |first1 = M. |last2 = Fabrizio |first2 = M |last3 = Castellani |first3 = C |last4 = Tosatti |first4 = E }} </ref>

==References== <references />

==Further reading== *{{cite journal |year = 1991 |title = Electronic structure of crystalline K₆C₆₀ |journal = Physical Review Letters |volume = 67|issue = 12 |pages = 1610–1613 |doi = 10.1103/PhysRevLett.67.1610 |pmid = 10044199 | bibcode = 1991PhRvL..67.1610E | last1 = Erwin |first1 = Steven |last2 = Pederson |first2 = Mark }} *{{cite journal |year = 1991 |title = Conducting films of C₆₀ and C₇₀ by alkali-metal doping |journal = Nature |volume = 350|pages = 320–322 |doi = 10.1038/350320a0 |issue=6316 | bibcode = 1991Natur.350..320H |last1 = Haddon |first1 = R. C. |last2 = Hebard |first2 = A. F. |last3 = Rosseinsky |first3 = M. J. |last4 = Murphy |first4 = D. W. |last5 = Duclos |first5 = S. J. |last6 = Lyons |first6 = K. B. |last7 = Miller |first7 = B. |last8 = Rosamilia |first8 = J. M. |last9 = Fleming |first9 = R. M. |last10 = Kortan |first10 = A. R. |last11 = Glarum |first11 = S. H. |last12 = Makhija |first12 = A. V. |last13 = Muller |first13 = A. J. |last14 = Eick |first14 = R. H. |last15 = Zahurak |first15 = S. M. |last16 = Tycko |first16 = R. |last17 = Dabbagh |first17 = G. |last18 = Thiel |first18 = F. A. }}

Category:Fullerenes