Metallic oxides, superconducting

One of the most exciting and perhaps unexpected discoveries in science within the last decade has been the observation of superconductivity (the complete absence of resistivity to electric current) in metal oxides at temperature < 90 K. This tempera-... 

Another example of the use of neutron diffraction to understand the role of atomic vacancies in producing a superconducting metal oxide phase is work that has been performed on Bao Kq 4fii03. This work demonstrates that at the synthesis temperature (700° C), under the proper conditions, oxygen vacancies are created to allow the formation of the parent phase with bismuth largely in the +3 oxidation state. The presence of the vacancies allows the incorporation of potassium in the... 

Nonstoichiometry is relatively common among mixed metal oxides, in which more than one metal is present. In 1986 it was discovered that certain compounds of this type showed the phenomenon of superconductivity on cooling to about 100 K, their electrical resistance drops to zero (Figure 20.9). A typical formula here is YBa2Cu30 where x varies from 6.5 to 7.2, depending on the method of preparation of the solid. 

One of the most exciting properties of some materials is superconductivity. Some complex metal oxides have the ability to conduct electricity free of any resistance, and thus free of power loss. Many materials are superconducting at very low temperatures (close to absolute zero), but recent work has moved the so-called transition temperature (where superconducting properties appear) to higher and higher values. There are still no superconductors that can operate at room temperature, but this goal is actively pursued. As more current is passed through... 

The normal spinel Li[Ti2]04 is a metallic oxide with a superconducting transition temperature of 13.7 K. The nominal formula is Li+[Ti3+Ti4+]04, in which the Li+ ions occupy the tetrahedral sites while the octahedral sites contain titanium with an average charge Ti3 5+, although as the material is metallic at room temperature the electrons are delocalized in a partly filled 3d band. 

Lai.85Ba0.i5CuO4) lost its resistance at a Tc = 30-35 K. The observation of superconductivity in a metal oxide at temperatures higher than those of metals themselves triggered a change in emphasis in superconductivity research towards this type of material. In 1987 two research groups, at the University of Houston and the University of Alabama, reported that the oxide of formula YBa2Cu307 x (commonly known as T-2-3 superconductor , from the stoichiometric ratios Y Ba Cu of 1 2 3) displayed a Tc of 92 K. 

Gloom for Oxide Superconductors Dismayed at the progress through the years, even with the most promising room-temperature metallic, binary oxides, many scientists abandoned the search for new high temperature oxide superconductors. Also, it should be mentioned that a deep-rooted prejudice had developed which claimed that the BCS theory had imposed a maximum transition temperature limit of 25 K for all superconducting materials, and that this temperature had already been achieved in certain alloys of niobium. Some scientists, however, were steadfast in their determination to break this barrier, optimistic in their outlook, and they continued their search for this unusual phenomenon in other metallic oxide systems. 

It should be mentioned that, of the other first-row transition metal oxides crystallizing with the NaCl structure, none has been found to superconduct down to 2.5 K. Some of these oxides undergo magnetic ordering at low temperature and most behave as semiconductors at all temperatures. These would include MnO, FeO, CoO, and NiO. Studies performed on CuO, which has a different crystalline structure, showed only semiconducting behavior to very low temperatures (1.9 K). 

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. 

A determined search for superconductivity in metallic oxides was initiated in mid-summer of 1983 at the IBM, Zurich Research Laboratories in Riischliken, Switzerland. This research effort was an extension of previous work (145) on oxides, namely, Sr1.xCaxTiOs, which exhibited some unusual structural and ferro-electric transitions (see Section 2.2a). During the summer of 1983, the superconductivity research was focussed on copper-oxide compounds. Muller had projected the need for mixed Cu2+/Cu3+ valence states, Jahn-Teller interactions (associated with Cu2+ ions), and the presence of room temperature metallic conductivity to generate good superconductor candidates. These researchers then became aware of the publication by Michel, Er-Rakho, and Raveau (146) entitled ... 

It is clear that no organic compound with a polymer chain of conjugation has been found to be superconducting. It is equally clear that all known superconductors are metal, compound alloy, or metal-oxide of some kind (including ceramics ). This fact suggests that the quasi-free electrons do play an important role in superconductivity. Thus, the key to the superconductivity mechanism should lie in a combination of Covalon-conduction and quasi-free electrons. 

The discovery in 1986 of high-temperature superconductivity in ceramic cuprates of perovskite structure started a period of very intensive research of transition metal oxides. Soon afterwards, in 1993, the colossal magnetoresistance effect was discovered in manganite perovskites, again leading to an increasing research activity in the field of magnetic oxides. It is... 

Since the discovery of high temperature superconductivity in cuprates, there has been intense interest in transition metal oxides with strongly layered, (quasi) two-dimensional (2D) crystal structures and electronic properties. For several years now alkali-metal intercalated layered cobaltates, particularly Na CoCL (NxCO)withx 0.50 — 0.75, have been pursued for their thermoelectric properties [1] IAX C0O2 is of course of great interest and importance due to its battery applications. The recent discovery[2] and confirmation[3-5] of superconductivity in this system, for x 0.3 when intercalated with H20, has heightened interest in the NxCO system. 

Use of the combustion method in an atmosphere of air or oxygen to prepare complex metal oxides seems obvious. In the last three to four years, a variety of oxides have been prepared using nitrate mixtures with a fuel such as glycine or urea. It seems that almost any ternary or quaternary oxide can be prepared by this method. All the superconducting cuprates have been prepared by this method, although the resulting products in fine particulate form have to be heated to an appropriate high temperature in a desired atmosphere to obtain the final cuprate [18], Table 2 lists typical materials prepared by the combustion method. 

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