Superconductivity lanthanum

The discovery of high-temperature superconductivity in mixed oxides, such as the lanthanum-barium-copper oxide complexes, has created a great deal of interest in these materials. Superconductivity, that is, the absence of any resistance to the flow of electric current, is now possible at temperatures above the temperature of liquid nitrogen (77K). Many problems remain in the development of practical processes for these materials and commercialization is not likely to occur until these problems are solved. Among the several processing techniques now used, CVD appears one of the most successful. 

One of the most exciting developments in materials science in recent years involves mixed oxides containing rare earth metals. Some of these compounds are superconductors, as described in our Chemistry and Technology Box. Below a certain temperature, a superconductor can carry an immense electrical current without losses from resistance. Before 1986, it was thought that this property was limited to a few metals at temperatures below 25 K. Then it was found that a mixed oxide of lanthanum, barium, and copper showed superconductivity at around 30 K, and since then the temperature threshold for superconductivity has been advanced to 135 K. 

The superconducting oxides include both perovskites and Ruddlesden-Popper compounds which have an orthorhombic arrangement of cubic cells, alternatively of the perovskite and sodium chloride structures. The common feature of all of these is the presence of copper as a major component. The first ceramic superconductor was a lanthanum-strontium substituted cuprate (Lai Sr Cu04 z), which is a perovskite, but subsequently the inter-oxide compound Y203 2BaO 3CuO, commonly referred to as a 123 compound, was shown to have superior performance. The speculation concerning the conduction mechanism is that this involves either Cu3+-Cu2+ positive hole... 

The discovery of a barium-doped lanthanum copper oxide which became superconducting at 35 K led to a flood of new high temperature superconductors some of which were superconducting above the boiling temperature of nitrogen, 77 K. Over 50 high temperature superconductors, almost all containing copper oxide layers, are now known. 

Torardi et al, 1987 David et al, 1987). The structure is closely related to that of the superconducting copper oxide YBa2Cu307, consisting of square-pyramidally coordinated copper (Cu-O sheets) and square-planar copper (Cu-O chains). Excess oxygen in the lanthanum compound is located interstitially converting partially the chain copper to square-pyramidal and/or octahedral coordination. 

Research chemists found that they could modify the conducting properties of solids by doping them, a process commonly used to control the properties of semiconductors (see Section 3.13). In 1986, a record-high Ts of 35 K was observed, surprisingly not for a metal, but for a ceramic material (Section 14.24), a lanthanum-copper oxide doped with barium. Then early in 1987, a new record T, of 93 K was set with yttrium-barium-copper and a series of related oxides. In 1988, two more oxide series of bismuth-strontium-calcium-copper and thallium-barium-calcium-copper exhibited transition temperatures of 110 and 125 K, respectively. These temperatures can be reached by cooling the materials with liquid nitrogen, which costs only about 0.20 per liter. Suddenly, superconducting devices became economically viable. 

Since 1911, scientists have been searching for materials that superconduct at higher temperatures, and more than 6000 superconductors are now known. Until 1986, however, the record value of Tc was only 23.2 K (for the compound Nb3Ge). The situation changed dramatically in 1986 when K. Alex Muller and J. Georg Bednorz of the IBM Zurich Research Laboratory reported a Tc of 35 K for the non-stoichiometric barium lanthanum copper oxide BavLa2-.vCu04, where x has a... 

Until 1986 many experiments were carried out using metals and alloys. However, in 1986 Georg Bednorz and Alex Muller, two researchers of the IBM laboratory in Ruschlikon near Zurich, published an article in the Zeitschrift fur Physik in which they announced that they had made a superconducting ceramic material. It turned out to be a compound made of barium, copper, lanthanum and oxygen, which became superconducting at 35 K. They were awarded the Nobel prize for this discovery. After this, superconductors developed rapidly, at least as far as critical temperature is concerned. 

The biggest explosion in materials chemistry and physics occurred in late 1986 when high-temperature superconductivity was discovered in a lanthanum cuprate, a material which was a ceramic and on which a few chemists had worked earlier. As stated in a report of the US National Academy of Sciences, this discovery changed the role of chemistry in the study of materials, and materials chemistry became a more significant part of materials science. It is around this time that even chemists started to consider solid state chemistry as an integral and important part of main-stream chemistry. 

In practice, both processes can occur there can be some decrease of oxygen and some conversion to Cu + ions. The concentration of holes on the copper oxide planes can be calculated when the parameter x in the formula (Lao.9,Sro.i)2Cu04 j is known. This superconductor is often called the lanthanum compound. The superconducting transition temperature depends on the hole concentration in... 

The discoveries that generated the extensive news coverage in early 1987 were the initial report of superconductivity in the lanthanum compound by Bednorz and Muller,and the observation of superconductivity in the compound YBa2Cu307 at 92-94 K, well above the boihng point (77 K) of liquid nitrogen. The latter result was the outcome of a collaboration between the research groups of C. W. Chu of the University of Houston and M. K. Wu of the University of Alabama. 

The discovery of high-temperature superconductivity in a lanthanum-based cuprate perovskite material with a transition temperature of Tc = 35 K by Bednorz and Muller... 

The now-famous formula for the first superconducting ceramic lanthanum-barium-copper-oxide. 

A few months later, at a Boston meeting on superconductivity, a somewhat different response was indicated by Shoji Tanaka, who had led the University of Tokyo team that confirmed IBM s discovery of superconductivity in the lanthanum compound. Asked to comment on recent reports that the Japanese had made great progress in firing a ceramic into a serviceable wire, Tanaka said, My work is not in that direction, adding, after a fairly long pause, but in any event that might be a secret, I think. ... 

It soon became apparent, once the structure of the yttrium compound was bared, that either or both of two central features might account for superconductivity at those record-high temperatures. One was the puckered, two-dimensional plane of copper and oxygen atoms ilar to the flat plane seen earlier in the structure of another superconductor, made of lanthanum, strontium, and copper oxide, that became superconducting at around 40° K. The other was unexpected the one-dimensional chain of copper and oxygen atoms, a sequence unknown in earlier superconductors. The challenge was fairly clear to both theorists and experimentalists. Were the planes or the chains responsible for superconductivity above 90° K ... 

IBM researchers Alex Muller and Georg Bednorz make a ceramic compound of lanthanum, barium, copper, and oxygen that superconducts at 35° K. 

Paul Ching-Wu Chu of the University of Houston and Maw-Kuen Wu of the University of Alabama substitute yttrium for lanthanum and make a ceramic that superconducts at 98° K, bringing superconductivity into the liquid nitrogen range. 

Mixed hydrides with Zr and Ti were reported several years ago. It has now been shown that substantial replacement of thorium by lanthanum is possible in Th4His. The temperature at which superconduction appears falls as the lanthanum content rises (298). 

Properties White, malleable ductile metal. Oxidizes rapidly in air. D 6.18-6.19, mp 920C, bp 3454C. Corrodes in moist air. Soluble in acids decomposes water to lanthanum hydroxide and hydrogen. Superconducting at approximately 6K. 

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