Lanthanum superconducting materials

Localisation of electron den ty over a small region in a crystal lattice occupied by 02 leads to the increased importance of electron correlation which cannot be tackled by traditional one-electron or HF theories[61]. Here we build upon our previous studies in which we used ab initio, semi-empirical and semi-classical approaches to study 0) ionic crystalline peroxides (e.g. Si and Ba02 [70,71]), (ii) point defects in the bulk and on the sui % of ionic and semi-ionic materials (e.g. corundum, silica and aluminium silicates [72,73]), and (iii) bipolaron formation in lanthanum cuprate (a superconducting material [74]). 

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. 

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. 

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... 

In 1986, the highest temperature at which any material became superconducting, i.e., the ability to conduct electricity with virtually no loss, was around —250°C, or 23 K. In that year a breakthrough came when Bednorz and Muller, shattered the record by demonstrating that a layered lanthanum, strontium copper oxide became superconducting at the relatively balmy temperature of 46 K. This discovery provoked a worldwide interest in the subject, and a few months later the record was again almost doubled, to about 90 K. The record today is in excess of 120 K. 

Perovskite A class of crystalline materials that often has the same formula and crystal structure. Two perovskites, strontium titanium trioxide and lanthanum aluminum trioxide, were central in the discovery of high-temperature superconductivity. 

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