Superconductors, lanthanide oxides

Current availability of individual lanthanides (plus Y and La) in a state of high purity and relatively low cost has stimulated research into potential new applications. These are mainly in the field of solid state chemistry and include solid oxide fuel cells, new phosphors and perhaps most significantly high temperature superconductors... 

Various substitution studies (171-173) were conducted in the early stages of research on these new oxide superconductors. One most dramatic result was the facile substitution of other (magnetic) lanthanide ions for yttrium in the VUI-coordinated site of the structure. The incorporation of these magnetic ions had no effect on the superconductivity nor the Tc of the material— quite astounding, since the presence of magnetic ions in superconductors was previously believed to destroy the phenomenon entirely Table 13 presents several examples of such substituted compounds. 

In view of the oxides of metals such as the alkaline earth, the lanthanides (e.g., Y and La), and copper being constituents of superconductors (366), a more intensive research was initiated for the synthesis of soluble and comparatively (with respect to monodentate) less hydrolyzable alkoxides of such metals as well. 

Shortly after acceptance of my manuscripts on the Chevrel phases (15) and the lanthanide rhodium borides (16) the initial scientific reports on the high copper oxide superconductors... 

Chapter 200, by Peter M. Allenspach and M. Brian Maple, reviews some aspects of the low-temperature heat capacity of the ceramic oxide superconductors. These measurements yield valuable information about the electronic, lattice, magnetic, crystalline electric field and hyperfine nature of the various rare-earth cuprate materials, and in that respect compliment other physical property studies, such as neutron diffraction, inelastic neutron scattering, and various spectroscopic measurements. The authors review the heat-capacity properties of the stoichiometric RBaCu307 compoimds and oxygen-deficient materials, and show that there are significant differences. The heat capacities of other lanthanide cuprates, such as RBa2Cu40g and R2B4CU7O14+X, are also discussed. 

Another significant development was the superconducting system of a mixed oxide of yttrium reported by Wu, Chu and co-workers. The compound has a formula YBa CujO, where x is between 6 and 7 and a T, 93 K which means that the expensive liquid helium can be replaced by cheap liquid nitrogen (77 K). For simplicity, the above system is called the 1-2-3 system based on the ratio of the metals present. This also has a perovskite structure. Similar compounds substituting yttrium by other lanthanides like europium, samarium etc., are known. These are called warm superconductors. A lot of research to obtain high T are going on and superconductors with Tj, as high as 164 K have been reported. 

The situation was rapidly changed after the appearance of two copper oxide superconducting families, La2 cM Cu04, (M = Ca, Sr and Ba) with a Tg of about 40 K and LnBa2Cu7 (Ln = Y and or other lanthanides) with a Tg of about 90 K [2]. In another approach to find new conductors and superconductors, boron (B) system compounds have also been surveyed... 

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