Superconductivity intercalation compounds

Titanium disulfide, like the disulfides of Zr, Hf, V, Nb, and Ta, has a layer structure two adjacent close-packed layers of S atoms have Ti atoms in octahedral interstices. These sandwiches are then stacked so that there are adjacent layers of S atoms. Lewis bases such as aliphatic amines can be intercalated between these adjacent sulfur layers similar intercalation compounds can be made with M and MSe2 compounds for M = Ti, Zr, Hf, V, Nb, and Ta. Many of these have potentially useful electrical properties, including use as cathode material for lithium batteries, and superconductivity, and may be compared with the intercalation compounds of... 

The molecule Ceo was first observed in the mass spectrum of laser evaporated carbon by Kroto et al., who proposed the now familiar soccer ball stmcture. Subsequently, macroscopic quantities were synthesized. In the solid state, Ceo adopts a face-centered cubic (fee) stmcture, which can be considered to be based on the close packing of spheres with a radius of 5 A. Two tetrahedral sites and one octahedral site with radii 1.12 and 2.06 A are present in the stmcture. The availability of these empty sites and the electronegativity of Ceo make it a suitable host lattice for reductive intercalation by electropositive cations such as the alkah metals. The octahedral site is larger than any of the alkah cations, but the tetrahedral site is about the same size as Na+. The discovery of Superconductivity in K Ceo (Tc = 18K) has focused considerable interest on these materials. Intercalation compounds can be prepared by direct reaction of the alkali metal with Ceo to form compositions Aj,C6o (x = 2, 3, 4, or 6) depending on the specific A cation. Direct reaction of Ceo with the alkali metals is the most conveiuent route to the AeCeo phases other compositions can be prepared by use of the appropriate stoichiometry or by reaction of AeCeo with Ceo. Other alkali metal reagents such as NasHg2 and NaH have also been used. ... 

The stractures of the alkali metal fullerides may be considered to be intercalation compounds of the fee lattice of the Ceo host formed by filhng the interstitial sites. Superconductivity has been investigated in detail for the alkali metal and mixed alkah metal phases of composition AgCeo where the conduction band is half filled. Critical temperatures as high as 31.3 K (in Rb2CsCeo) have been observed. A clear correlation between unit cell size and Tc is observed, and it has been suggested that the conduction bandwidth controlled by the separation between the molecules is the dominant factor in determining Tc. 

Reductive intercalation compounds of /3-ZrNCl with cobaltocenes have been reported where the metallocene molecules are believed to be oriented with the molecular axes parallel to the layers. The metallocene intercalation compounds also show superconductivity at 14K. 

The sulfides have been studied much less than the oxides, but it is clear that many similarities exist. Like the oxides, a number of stable phases exist and nonstoichiometry is prevalent. The most important sulfide is TiS2, which has the Cdl2 layer structure, in which the Ti atoms occupy octahedral sites between alternate layers of close-packed sulfur atoms. The adjacent sulfur layers are able to intercalate Lewis bases such as aliphatic amines. Similar intercalation compounds can be made with MS2 and MSe2 compounds (M = Ti, Zr, Hf, V, Nb, Ta). Many of these compounds exhibit useful electrical properties including superconductivity see Superconductivity). 

The physics and chemistry of molybdenum disulphide intercalation compounds have been reviewed by Woollam and Somoano . Perhaps the most interesting of these properties is superconductivity below 6.9°K, ° obtained with either organic bases or alkali metals. Some of the intercalation compounds show high alkali ion diffusivity, and this has led to them being considered for use in electrodes for high energy-density batteries . 

There are a vast number of different sorts of superconducting compound. Examples of these are summarized in Table 7.2. They include organic polymers and intercalation compounds (such as RbjCgjj), as well as ceramic oxides and sulfides. In all these compounds the critical temperature is still below the boiling point of liquid nitrogen. Hence, these materials would be very expensive to use in everyday life (as a litre of liquid helium costs 20 times as much as liquid nitrogen). 

Studies have continued on the intercalation of the alkaline-earth metals into lattices of the transition-metal sulphides. Calcium and strontium are intercalated into M0S2 from liquid ammonia solution. X-Ray data reveal a lowering of the crystal symmetry of the sulphide and an increase in the complexity of the structure on intercalation. The intercalation compounds begin to superconduct at ca 4 K (for Ca) and 5.6 K (for Sr), and they show considerable anisotropy with respect to the critical magnetic field. Calcium in liquid ammonia also intercalates with TiS2- Two Ca,TiS2 phases have been identified. The limits of the first are 0.03 0.50, for which a relationship between x and cell parameters a and c... 

ZrNCl Co(Cp)2 o,io has been shown to be a Type II superconductor with a superconducting transition at 14 K. This is the highest for any metallocene intercalation compound. The superconducting transition temperatures for all three metallocene intercalation compounds are the same as those in the alkali metals intercalates with the same doping level reported by Yamanaka et The transition temperature appears not to be dependent on the interlayer separation and on the doping level which suggests that the superconductivity is largely confined to the thin two-dimensional ZrN layers of the host lattice. 

Another property of transition metal dichalcogenides is an intercalation effect. Most of the atoms in the periodic table and organic molecules which are Lewis bases can be intercalated in the van der Waals gap sites, forming intercalation compounds(intercalates). One of the most striking features is a transport phenomenon. Alkali metal intercalates of IV] - and VIb-MX2, for example, NaxMoS2 or LixZrSe2 become metallic and show a superconductivity at low temperatures(10,11). A charge transfer occurs between the alkali atom and the mother crystal. 

Superconductivity. Some Intercalation compounds are superconductors, such as CeoKa, with an onset of superconductivity at 17 K eind zero resistance at 5 Other compounds have even higher critical... 

---