Potassium fulleride

The simple charge transfer from K to C60 observed in the fee K3C60 is not always the case in potassium fullerides. Similar comparison between the LDA band structure of the bcc K6C60 fulleride and that of the hypothetical bcc pristine solid... 

Powder samples of were prepared as described previously 3, 5, 8). Briefly, the appropriate amounts of Cgo powder and K were sealed in pyrex tubes in vacuo. The tubes were heated at 225°C for 3 to 5 days to form the potassium fulleride compounds. The samples were then resealed in s20 torr HC(g) and annealed at temperatures between 250° and 300°C for periods ranging from 10 to 25 days. NMR measurements are reported for four samples, with nominal stoichiometries Kj sC o (sample I), Kj 0 60 (sample II), Kj.iQo (sample III), and K60C60 (sample IV). Superconducting fractions, as determined by magnetic flux expulsion measurements, were 16, 33, 32, and 0% for samples I to IV, respectively. In all cases, a single superconductivity transition at 19.0 0.5 K was observed. NMR spectra were obtained at a fiequency of 100.5 MHz (9.39-T field). Frequencies are reported in parts per million with respect to tetrameth-ylsilane. The total mass of carbon that contributes to the NMR spectrum was determined for each sample by comparing... 

Whereas Cjq is insoluble and inert in liquid ammonia without any cosolvent, the fulleride anions Cjq"" n = 1 ), generated electrochemically with KI as supporting electrolyte, dissolve completely in this polar medium [15]. Further reductions lead to the ammonia-insoluble potassium salts of the penta- and hexaanions. 

Alkali metal doping of Cjq is also possible by solution-phase techniques [1,118-121]. K CgQ and Rb CgQ containing small fractions of the superconducting M3C50 phases were prepared by allowing toluene solutions of Cjq to react with the alkali metal [118, 119]. During the reaction, the alkali metal fullerides form a black precipitate. In another example, sonication of a solution of Cjq and excess potassium in TMEDA yields K3C5q(THF)j4 with a defined stoichiometry [104],... 

In 1991, scientists at AT T Bell Laboratories discovered a new class of high-temperature superconductors based on fullerene, the allotrope of carbon that contains Cgo molecules (Sections 10.10 and 19.6). Called "buckyballs," after the architect R. Buckminster Fuller, these soccer ball-shaped Cgo molecules react with potassium to give K3C6o- This stable crystalline solid contains a face-centered cubic array of buckyballs, with K+ ions in the cavities between the Cgo molecules (Figure 21.16). At room temperature, K3Q,o is a metallic conductor, but it becomes a superconductor at 18 K. The rubidium fulleride, Rb C o, and a rubidium— thallium-Cfio compound of unknown stoichiometry have higher Tc values of 30 K and 45M8 K, respectively. 

Figure 4.8-29 Near infrared reflectivity of a thin ful-lerite film and fulleride films doped with potassium to x=3 and 6. The dashed line is as calculated from Eq. 4.8-1 to 4.8-3 including multiple reflection, according to Pichler et al., 1994. The four modes in the undoped film (a-0) are marked with arrows.

To make the reactive fulleride compound KeCeo in the Fe-Ceo synthesis, fullerenes and a slight excess of potassium were sealed in a glass tube under vacuum and heated for approximately four days at 250 °C. Both solid-state NMR and Raman spectroscopy were employed to determine that the KeCeo compound was in fact synthesized. The KeCeo product was then reacted in an inert atmosphere with cyclopentadienyl-iron-dicarbonyl-iodide (CpFe(CO)2l) in tetrahydrofuran (THF) to form the complex. The recovered product was dried in an inert atmosphere. Manipulations of air-sensitive materials were carried out in a glove box or using standard Schlenk techniques. THF was distilled just prior to use from sodium benzophenone ketyl. Ceo was obtained from Aldrich, and CpFeCCOjol was obtained from Strew. 

E3.7 Within the fee lattice of fullerides shown in Figure 3.16, there are the equivalent of four close-packed molecules (1/8(8) comers A(6) faces). This lattice contains the equivalent of four octahedral holes and eight tetrahedral holes as shown in Figure 3.18. If the potassium cations occupy all of these holes then we have 12 K ions for every four Cw anions and the formula is (simplified from (Ki2(C i)4). See Figure 24.69 for further visualization of the structure. Review Example 3.9 and Self-Test 3,9 if you had problems with this exercise. 

The alkali metal fullerides, M3C60 (M = Na, K, Rb, and Cs), are a little different. The absence of the sign indicates that these are not endohedral fullerenes. Take K3C6O a representative example. It is prepared from stoichiometric amounts of solid C o potassium vapor. The three potassium atoms transfer their valence electrons to the fuUerene so that K3C60 more accurately written as (K )3(C o ) This special compound becomes a superconductor below the critical temperature, 7, of 19.3 K. (Below the 7, a superconductor is perfectly conducting—that is, it has zero resistance.) It has a face-centered cubic structure of C o anions with potassium cations in both the octahedral and tetrahedral holes as shown in Figure 15.9. (See p. 173 for more details on the positions and numbers of these holes.)... 

When molecular ions have spherical or near spherical stereochemistry they often pack in a similar way to classical ionic solids. For example, in the fulleride KgCgo the spherical Cgo" ions pack in a standard body-centered cubic structure with potassium clusters located in the interstitial sites. ... 

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