Transition metal compounds superconductivity

The electron density in transition metal complexes is of unusual interest. The chemistry of transition metal compounds is of relevance for catalysis, for solid-state properties, and for a large number of key biological processes. The importance of transition-metal-based materials needs no further mention after the discovery of the high-Tc superconducting cuprates, the properties of which depend critically on the electronic structure in the CuOz planes. 

AnRu2 compounds were prepared for An=Th, Np, Pu and Am. The latter compound has the hexagonal C14 structure. All of them possess a non-magnetic ground state. Moreover, ThRu2 becomes superconducting below 3.56 K (Matthias et al. 1961). The temperature dependence of the electrical resistivity displays a tendency to saturate at temperatures above 80 K (Lawson et al. 1976, Lawson 1985), which is common for many transition metal compounds. 

Because they exhibit interplay of magnetic and superconducting properties, the formation and crystal chemistry of MRgMy4B4 compounds have been examined. Ternary rare-earth and actinide (Th, U, Pu)-transition metal borides of the approxi-... 

Among various superconductors, compounds with the A15 (Cr3Si) crystal structure have the highest critical temperatures. This crystal structure has a simple relationship with the Ll2 structure (Ito and Fujiwara, 1994) as illustrated in Figure 8.9. When the unit cells are aggregated, the face-centered pairs of atoms form uniform chains of transition metal atoms along three orthogonal directions. This feature may be related to the relatively stable superconductivity in compounds with this structure. 

At this point, it may be informative to present a chronological listing of the different discoveries in oxide superconductors reported prior to 1975. In this listing, Table 5, we present the year that the oxide compound was first reported, then the year in which superconductivity was first observed in the system and the group credited for the discovery. Of particular interest is the compound Ba(Pb1 xBix)Os discovered by Sleight at du Pont in 1975. This oxide material adopts the perovskite-type structure and contains no transition metals. 

In 1988, Cava and co-workers also prepared (88a) a quaternary oxide, Ba/K/Bi/O, and observed superconductivity at -28 K. This compound was the first "non-transition metal" oxide with a Tc above the legendary "alloy record" of 23 K. Further studies indicated (88a) that the optimum composition for "high temperature" superconductivity in this system was Ba0 6K0 4BiO3 x, having a Tc of 30.5 K (Figure 17). The samples were multiphase, and the superconducting fraction varied from 3 to 25%. Superconductivity for the rubidium-substituted compound was observed at -28.6 K. 

The investigation of the whole family of these multilayer compounds would help to understand the mechanisms for superconductivity and magnetism in the quaternary rare-earth transition-metal borocarbides (Michor et al. 2001 Baggio-Saitovitch et al. 2001). However this report will be confined to magnetism and superconductivity in singlelayer / Ni2B2C compounds i.e. compounds with (/ C/N) Ni2B2 with n > 1 will not be considered further. 

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